# Progresses and Prospects for North Tropical Atlantic Mode Interannual Variability

Yang Yun, Li Jianping, Xie Fei, Feng Juan, Sun Cheng

College of Global Change and Earth System Science,Beijing Normal University,Beijing 100875,China

First author:Yang Yun(1988-),female,Anshan City,Liaoning Province,Lecturer. Research areas include ocean-atmosphere interaction and climate change. E-mail:yunyang@bnu.edu.cn

Abstract

North Tropical Atlantic Mode (NTAM) is the leading variability of the boreal spring sea surface temperature anomalies over the North Tropical Atlantic at interannual timescale. It is also known as the northern pole of the Atlantic Meridional Mode (AMM). NTAM shows significant impact on the shift of Intertropical Convergence Zone, the precipitation of the surrounding countries, the quasi-biennial oscillation of El Nino-Southern Oscillation (ENSO), and the recent global warming hiatus. Despite its distinct influence on global climate, NTAM has not received equivalent attention as other tropical variability (e.g. ENSO). By revisiting previous studies, this paper summarized the triggers and mechanisms responsible for the evolution and development of NTAM, including remote forcing from ENSO, south tropical Atlantic as well as North Atlantic Oscillation (NAO), local air-sea coupling, and the interactions among different triggers. Also, this paper detailedly introduced the ability of CMIP5 (The fifth phase of the Coupled Model Intercomparison Project) model simulation. The prominent model biases over the equatorial Atlantic significantly limit the study of NTAM. Finally, a future prospective of NTAM interannual variability was presented.

Keywords： North Tropical Atlantic Mode ; Interannual variability ; Ocean-atmosphere interaction.

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Yang Yun, Li Jianping, Xie Fei, Feng Juan, Sun Cheng. Progresses and Prospects for North Tropical Atlantic Mode Interannual Variability[J]. Advances in Earth Science, 2018, 33(8): 808-817 https://doi.org/10.11867/j.issn.1001-8166.2018.08.0808

## 1 引 言

(a)热带大西洋(30°S~30°N, 70°W~20°E) EOF分解的第二模态,黑框代表AMM北部的一支;(b)热带北大西洋(0°~30°N, 100°W~20°E)EOF分解的第一模态,单位为℃;(c)NTAM指数(SST在5°~25°N, 60°~20°W的均值)在不同月份的方差(单位:℃2)

Fig.1   HadISST boreal spring SST during 1870-2017
(a) 2nd EOF mode over the tropical Atlantic (30°S~30°N, 70°W~20°E), the black box represents the northern pole of the AMM;(b) 1st EOF mode over the northern tropical Atlantic (0°~30°N, 100°W~20°E) (unit: ℃); (c) The variance of NTAM index (SST averaged over 5°~25°N, 60°~20°W) in calendar months (unit: ℃2)

NTAM对大西洋上热带辐合带(Intertropical Convergence Zone, ITCZ)有着明显的调节作用。当它处于正位相(SST异常增暖)时,ITCZ位置较平均态偏北;与之相对,负位相时ITCZ位置偏南[1,2,4]。这一影响使得NTAM对沿岸国家的降水有着很强的影响[1,5~7]。此外,NTAM对飓风的数量、强度、持续时间以及移动路线等都有着极大的调控作用[8,9,10]

NTAM年际变率研究的开展主要集中在20世纪末和21世纪初,而最近十几年科学家们将注意力主要集中在热带太平洋和热带印度洋地区,相比之下对NTAM的研究较少。最近的研究发现,NTAM可以通过“电容器效应”对厄尔尼诺—南方涛动(El Niño-Southern Oscillation,ENSO)准2年振荡的位相转换起到辅助作用[11,12,13]。此外,在年代际时间尺度上,NTAM的异常增暖可以通过引发太平洋多年代际变率(Interdecadal Pacific Oscillation, IPO)的负位相,进而解释21世纪初的全球变暖“停滞”现象[14,15,16]。这些新发现为NTAM的研究注入了新鲜血液,也重新引起了科学家们对热带大西洋的关注。

## 2 生成机制

NTAM的生成机制主要可以归纳为4种(图2)。在下文中,我们将对各种机制分别进行回顾。

Fig.2   Schematic diagram of NTAM triggers

### 2.1 局地海气相互作用

Chang等[2]首先利用风—蒸发—SST(Wind-Evaporation-SST, WES)正反馈机制[17]解释了NTAM的生长过程。当热带北大西洋上空出现西南风异常时,它与背景东北信风方向相反,使得风速减弱,蒸发引起的潜热通量减小,SST升高。而海温的升高将进一步降低局地海表气压(Sea Level Pressure,SLP),增强西南风异常,进而形成一个正反馈过程。在WES机制的作用下,SST暖异常逐渐增大,向赤道方向(背景风的下游方向)发展,并且在春季(4月)达到鼎盛。这时,SST异常的最大值在达喀尔沿岸并且向海盆西部逐渐减小。这一季节,ITCZ位于全年的最南端——赤道附近。在SST异常增暖的作用下,ITCZ产生向北的异常移动[18,19]

### 2.2 ENSO强迫

ENSO可以通过以下4种机制激发出NTAM事件。这里我们以厄尔尼诺事件为例进行说明。

(1)太平洋—北美遥相关

(2)对流层加热

Chang等[34]发现,厄尔尼诺发生时SST异常增暖可以加热大气,引起对流层异常增暖。大气呈现出典型的Gill(Gill-Matsuno pattern)响应[35],并在大气Kelvin波的作用下将暖异常向东传播,到达大西洋。对流层的异常增暖导致热带大西洋大气稳定度增加,减弱大气热对流和蒸发,进而导致SST的异常增暖。我们将这一过程称为对流层加热(Tropospheric Temperature, TT)机制。

Saravanan等[36]指出,厄尔尼诺事件之后海洋—大气之间的温度差以及湿度差也是NTAM增暖的重要原因。Chikamoto[37]利用历史的船测资料证实了这一观点。他们的研究指出,厄尔尼诺年时大气温度异常高于并且超前于SST异常,抑制了海洋向上的感热通量,导致SST升温。这与正常年份正好相反。

(3)沃克—哈德来环流异常

(4)亚马逊加热的影响

García-Serrano等[40]近期的研究指出ENSO可以通过影响亚马逊地区温度而间接激发出NTAM事件。厄尔尼诺发生时伴随着沃克环流异常,亚马逊地区出现了异常的下沉气流。它抑制了当地强烈的蒸发作用,减少了向上的潜热通量,使得亚马逊地区温度升高。这一温度异常可以激发Gill响应,并在热带大西洋赤道两侧产生一对反气旋。位于北半球的反气旋则会减弱东北信风,减少蒸发引起的潜热通量,并最终导致热带北大西洋增温。然而这一对反气旋异常位于亚马逊地区的东侧,是什么样的机制导致了这对反气旋的向东移动,这一问题尚不清楚。

### 2.3 热带南大西洋的影响

Chang等[2]用WES正反馈解释NTAM的形成过程中指出AMM总是成对出现。我们将南半球的这一极子称为热带南大西洋模态(South Tropical Atlantic Mode, STAM)。热带北大西洋的异常增暖会引起跨赤道的南风异常。在南半球,风异常在科氏力的作用下形成东南风异常,增强了背景的东南信风,潜热通量增加,SST变冷。南北半球SST经向梯度进一步加强跨赤道气流,并在WES机制下发展成一对偶极子。根据这一WES机制,NTAM正位相可以激发出STAM的负位相;反之,STAM应该也可以触发NTAM。这种NTAM-STAM偶极子结构在热带大西洋(30°S~30°N,60°W~20°E)EOF分解中可以得到(图1a)[48,49]

### 2.4 北大西洋涛动的影响

NTAM与北大西洋的其他几种海温模态联系紧密。在北大西洋(20°~70°N),最重要的2个海洋模态是北大西洋三极子(North Atlantic Tripole, NAT)以及北大西洋多年代际振荡(Atlantic Multidecadal Oscillation, AMO)[56,57]。NAT与NAO联系紧密,并在冬季达到最大值;随着时间的推移,NAT在热通量的作用下演化为夏季的北大西洋马蹄型模态(North Atlantic Horseshoe,NAH)[60]。尽管发生的季节不同,NTAM与NAT和NAH联系紧密,这是由于三者均受到大气变率NAO的影响(表1)。NAT的热带极子与NTAM空间结构非常相似(相关系数为-0.88),并且2个序列显著相关(相关系数为-0.24)。NAH与NTAM相关系数则为-0.16(通过95%信度检验)。这种相关关系也是导致 NTAM海温持续性的原因。另外,NTAM也表现出很强的年代际特征,这主要与AMO有关[9],两者的相关系数达到了0.64。

Table 1   Correlation coefficients between NTAM and other variability

NATNAHAtlantic NiñoAMO (年代际)

NTAM与大西洋尼诺没有明显相关(r=-0.08)。Servain等[61]指出,春季NTAM的正位相能够激发出赤道及其以南海域的东风异常,并通过Bjerknes机制和WES机制发展成为夏季大西洋尼娜(大西洋尼诺的负位向)。Foltz等[62]进一步研究两者的关系,提出了NTAM通过Rossby波反射来影响大西洋尼诺的正相关机制,并解释了2009年的大西洋尼娜现象[63]。值得注意的是,这一延迟的Rossby波反射机制与Servain等[61]提出的负相关机制相互抵消。在这2种机制的共同作用下,NTAM和大西洋尼诺相关不显著。然而,Richter等[64]反驳说,海洋上层100 m热含量数据显示Rossby波并没有出现西边界反射现象。他们将NTAM和大西洋尼诺的正相关机制归因为海流的输运作用。

## 3 对气候的影响

Fig.3   Schematic diagrams of NTAM impacts on global climate

### 3.1 对周围陆地的影响

NTAM模态对热带大西洋沿岸国家的降水有着很强的影响。它最早受到关注是因为其对巴西东北部降雨的显著影响[4,5]。巴西东北部是典型的半干旱地区,降水具有显著的季节性,全年的降水主要集中在春季[6,7]。这一地区人口稠密,且农业是其主要产业,受气象条件影响严重。1958年发生的NTAM暖事件使这一地区发生严重干旱,导致居住在这里的1 000万民众被迫离开家园,经济损失超过8亿美元[5,65,66]。类似的旱灾在1993年再次上演[67]。相反,当NTAM处于负位相时巴西东北部降雨偏多,例如:2009年的洪涝[68]。此外,NTAM对非洲西部的降水也有很强的调节作用,但是这一影响要弱于巴西东北部[69,70]。研究表明,西非夏季降水受到ITCZ南北移动的强烈影响。当NTAM处于正位相时,热带北大西洋SST变暖导致ITCZ向北移动,西非降水增多。这一结果在年际和年代际时间尺度上都有显示[69,71]

### 3.2 对全球气候的影响

NTAM不仅可以影响局地的气候,更能进一步影响全球气候系统[11,12,13]。Ham等[12]发现NTAM正位相发生时会引发西太平洋低层反气旋,导致赤道东风爆发,并引发当年冬季的拉尼娜事件。这一机制为ENSO的准两年振荡的位相转换起到了辅助作用。冬季的拉尼娜事件进一步对东亚气候产生剧烈影响,包括海洋大陆的同期暴雨和次年春季亚洲北部的热浪现象[24]。在年代际时间尺度上,NTAM与AMO联系紧密[9]。正位相发生时SST增暖可以通过TT机制导致印度洋增暖,并进一步引起沃克环流异常,触发太平洋多年代际变率IPO负位相[16]。而IPO与全球平均温度联系紧密[72],其负位相发生时全球平均温度降低。这一降温作用与人类活动引起的全球变暖相叠加,进而导致了21世纪初的全球变暖“停滞”现象[14,15,16]

## 5 结论与展望

(1)ENSO与NTAM的季节差。ENSO在冬季达到鼎盛,但其对NTAM的影响在冬季开始发展并且在春季达到最大值。前人研究表明,热带地区大气对海洋SST异常的响应几乎是同时的[51],那么是什么作用导致ENSO的影响出现了延迟,又或者是什么过程储存了ENSO的能量并在春季释放,这些问题仍有待深入研究。

(2)PNA强迫。观测数据显示,PNA强迫引起的北大西洋SLP异常位于美国东南部,在NTAM的西北方向并且距离较远。而WES过程只在靠近赤道的热带地区(0°~10°N)起作用,那么SLP异常是如何激发出NTAM的,中间有哪些物理过程被遗漏掉了,值得深思。

(3)沃克与哈德来环流异常。热带地区的SST异常升温(降温)会引起沃克与哈德来环流异常。而它的水平尺度由什么因素决定,如何确定异常的上升支和下沉支的位置,其影响为什么会有南北半球不对称的现象?这一系列问题仍有待进一步探索。

(4)提高耦合模式对于热带大西洋的模拟能力。CMIP5对于热带大西洋平均态的模拟仍然存在较大的问题,如前面提到的春季赤道东风以及ITCZ位置的模拟。这些平均态误差极大程度影响了模式中的海气相互作用,导致NTAM靠近赤道部分的模拟误差。不仅如此,模式中NTAM与其他变率的相互作用以及NTAM对陆地降雨的影响也产生了极大的偏差。模式的严重误差极大程度地限制了科学家们对于NTAM的探索。

(5)关于NTAM在全球变暖后变化的研究较少。全球变暖后,热带北大西洋气候平均态发生变化(例如:SST变暖)。NTAM的年际变率产生什么样的影响,这一方面的研究仍然较少。这可能与气候模式在热带大西洋较强的模式误差有关。

The authors have declared that no competing interests exist.

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Specifically, it was found that Atlantic SST DM is the only index that is associated with all top three empirical orthogonal function (EOF) modes of the Atlantic HTDF. ENSO and tropical Atlantic VWS are significantly correlated with the first and the third EOF of the HTDF over the North Atlantic Ocean. The second EOF of North Atlantic HTDF, which represents the 鈥渮onal gradient鈥 of North Atlantic hurricane track density, showed no significant correlation with ENSO or with tropical Atlantic VWS. Instead, it is associated with the Atlantic SST DM, and extratropical processes including NAO and AO. Since for a given hurricane season, the preferred hurricane track pattern, together with the overall basinwide hurricane activity, collectively determines the hurricane landfall frequency, the results provide a foundation for the construction of a statistical model that projects the annual number of hurricanes striking the eastern seaboard of the United States. [9] Vimont D J, Kossin J P.The Atlantic Meridional Mode and hurricane activity[J]. Geophysical Research Letters, 2007, 34(7):248-265. Connections between the Atlantic Meridional Mode (AMM) and seasonal hurricane activity are investigated. The AMM, a dynamical mode'' of variability intrinsic to the tropical coupled ocean-atmosphere system, is strongly related to seasonal hurricane activity on both decadal and interannual time scales. The connection arises due to the AMM's relationship with a number of local climatic conditions that all cooperate in their influence on hurricane activity. Further analysis indicates that the Atlantic Multi-decadal Oscillation (AMO) can excite the AMM on decadal time scales. As such, it is suggested that the AMO's influence on seasonal hurricane activity manifests itself through the AMM. This relationship between the AMM, AMO, and seasonal hurricane activity refocuses our understanding of how climate variations relate to seasonal hurricane activity in the Atlantic, and offers an improved framework beyond purely thermodynamic arguments that relates hurricanes to large-scale climate variations. [10] Vimont D J.Analysis of the atlantic meridional mode using linear inverse modeling: Seasonality and regional influences[J]. Journal of Climate, 2012, 25(4):1 194-1 212. [11] Wu Lixin, He Feng, Liu Zhengyu, et al. Atmospheric teleconnections of tropical atlantic nariability: Interhemispheric, tropical extratropical, and cross-basin interactions[J]. Journal of Climate, 2007, 20(20):856-870. [12] Ham Y G, Kug J S, Park J Y, et al. Sea surface temperature in the north tropical Atlantic as a trigger for El Niño/Southern Oscillation events[J]. Nature Geoscience, 2013, 6(2):112-116. [13] Yu Jinhua, Li T, Tan Zhemin, et al. Effects of tropical North Atlantic SST on tropical cyclone genesis in the western North Pacific[J]. Climate Dynamics, 2016, 46(3/4):1-13. This study analyzed the correlation between tropical cyclone (TC) frequency and the Western North Pacific monsoon index (WNPMI), which have both been influential in East Asia’s mid-latitude regions during the summer season over the past 3702years (1977–2013). A high positive correlation existed between these two variables, which was not reduced even if El Ni09o-Southern Oscillation (ENSO) years were excluded. To determine the cause of this positive correlation, the highest (positive WNPMI phase) and lowest WNPMIs (negative WNPMI phase) during a nine-year period were selected to analyze the mean difference between them, excluding ENSO years. In the positive WNPMI phase, TCs were mainly generated in the eastern seas of the tropical and subtropical western North Pacific, passing through the East China Sea and moving northward toward Korea and Japan. In the negative phase, TCs were mainly generated in the western seas of the tropical and subtropical western North Pacific, passing through the South China Sea and moving westward toward China’s southern regions. Therefore, TC intensity in the positive phase was stronger due to the acquisition of sufficient energy from the sea while moving a long distance up to East Asia’s mid-latitude. Additionally, TCs occurred more in the positive phase. Regarding the difference of the two phases between the 850 and 500-hPa streamlines, anomalous cyclones were strengthened in the tropical and subtropical western North Pacific, whereas anomalous anticyclones were strengthened in East Asia’s mid-latitude regions. Due to these two anomalous pressure systems, anomalous southeasterlies developed in East Asia’s mid-latitude regions, which played a role in the anomalous steering flows that moved TCs into these regions. Furthermore, due to the anomalous cyclones that developed in the tropical and subtropical western North Pacific, more TCs could be generated in the positive phase. Both the lower and upper tropospheric layers had warm anomalies in most regions of the Western North Pacific, while relative humidity in the middle tropospheric layer showed a positive anomaly in the tropical and subtropical western North Pacific, which provided a better environment to strengthen TC intensity in the positive WNPMI phase. Furthermore, a negative anomaly was manifested not only in the tropical and subtropical western North Pacific, but also in East Asia’s mid-latitude regions, with 200–850-hPa vertical wind shear, while a warm sea surface temperature anomaly was shown in East Asia’s mid-latitude seas, which further strengthened TC intensity in the positive phase. The analysis on the global-scale atmospheric circulations showed that converged air in the lower layer of the subtropical western Pacific during the positive phase diverged in the upper layer, which moved westward and converged in the upper layer of the equatorial Indian Ocean and then diverged in its lower layer. [14] Lin Xiaopei, Xu Lixiao, Li Jianping, et al. Research on the global warming hiatus[J]. Advances in Earth Science, 2016, 31(10): 995-1 000. [林霄沛, 许丽晓, 李建平, 等. 全球变暖“停滞”现象辨识与机理研究[J]. 地球科学进展, 2016, 31(10): 995-1 000.] 观测表明全球温室气体浓度持续快速增加,但21世纪以来全球表面平均温度升高有减缓趋势,呈现变暖“停滞”现象,这对已有的全球变暖认识带来挑战。围绕“变暖‘停滞’机理及其可预测性”这一国际前沿科学问题,国家重点研发计划“全球变暖‘停滞’现象辨识与机理研究”主要研究内容有:①辨识变暖“停滞”的时空特征,阐明外部强迫和内部自然变率的相对贡献;②阐明全球变暖停滞背景下,大气在气候系统能量热量再分配过程中的作用及机理;③阐明全球变暖“停滞”背景下,海洋动力热力过程对能量热量再分配的调制机理;④探讨全球变暖“停滞”现象的可预测性,对其未来变化及重要区域气候影响进行预测预估。以期通过变暖“停滞”研究回答人们所关心的目前变暖停滞现象未来发展及其对我国及周边的“一带一路”核心区和南北极重要区域的影响,为我国未来气候政策的制定提供参考依据,为国家参与全球气候治理及国际气候谈判提供科学支撑。 [15] Chikamoto Y, Timmermann A, Luo J J, et al. Skilful multi-year predictions of tropical trans-basin climate variability[J]. Nature Communications, 2015, 6:6 869. Abstract Tropical Pacific sea surface temperature anomalies influence the atmospheric circulation, impacting climate far beyond the tropics. The predictability of the corresponding atmospheric signals is typically limited to less than 1 year lead time. Here we present observational and modelling evidence for multi-year predictability of coherent trans-basin climate variations that are characterized by a zonal seesaw in tropical sea surface temperature and sea-level pressure between the Pacific and the other two ocean basins. State-of-the-art climate model forecasts initialized from a realistic ocean state show that the low-frequency trans-basin climate variability, which explains part of the El Ni o Southern Oscillation flavours, can be predicted up to 3 years ahead, thus exceeding the predictive skill of current tropical climate forecasts for natural variability. This low-frequency variability emerges from the synchronization of ocean anomalies in all basins via global reorganizations of the atmospheric Walker Circulation. [16] Li Xichen, Xie Shangping, Gille S T, et al. Atlantic-induced pan-tropical climate change over the past three decades[J]. Nature Climate Change, 2016, 6(3):275-279. The dipole-like trend of tropical sea surface temperature is investigated and this study finds it to be initiated in the Atlantic Ocean. Atlantic warming drives wind and circulation changes and influences Pacific Ocean surface temperatures. [17] Xie Shangping, Philander S G H. A coupled ocean-atmosphere model of relevance to the ITCZ in the eastern Pacific[J]. Tellus, 1994, 46(4):340-350. [18] Wu Lixin, Zhang Qiong, Liu Zhengyu.Toward understanding tropical Atlantic variability using coupled modeling surgery[M]∥Earth's Climate: The Ocean-Atmosphere Interaction. American Geophysical Union, 2004:157-170. [19] Chiang J C H, Sobel A H. Tropical tropospheric temperature variations caused by ENSO and their influence on the remote tropical climate[J]. Journal of Climate, 2002, 15(18):2 616-2 631. [20] Mahajan S, Saravanan R, Chang P.Free and forced variability of the tropical Atlantic Ocean: Role of the wind-evaporation-sea surface temperature feedback[J]. Journal of Climate, 2010, 23(23):5 958-5 977. [21] Xie Shangping.A dynamic ocean-atmosphere model of the tropical Atlantic decadal variability[J]. Journal of Climate, 1999, 12(12):64-70. A linear model that couples an ocean mixed layer with a simple dynamic atmosphere is used to study the mechanism for decadal variability over the tropical Atlantic. An unstable mode with a dipole sea surface temperature (SST) pattern similar to observed decadal variability in the tropical Atlantic emerges in the time integration of the model. A wind-evaporation-SST feedback is responsible for the growth and oscillation of the unstable mode whereas the mean state of the Atlantic climate is essential for maintaining the spatially quasi-standing dipole structure. The oscillation period ranges from several to a few tens of years and is sensitive to coupling strength. The oscillation is not self-sustainable as the realistic damping rate exceeds the growth rate. In response to white noise forcing, the model produces a red SST spectrum without a peak at finite frequencies. Therefore it is suggested that the tropical dipole's preferred timescales, if any, arise from the forcing by or interaction with the extratropics. In a model run where the forcing is confined to the extratropics, a dipole SST pattern still dominates the forcing-free Tropics, in support of the proposed linkage between the Tropics and extratropics. [22] Chang Ping, Ji L, Saravanan R.A hybrid coupled model study of tropical Atlantic variability[J]. Journal of Climate, 2001, 14(3):361-390.           摘要 A hybrid coupled model (HCM) is used to explore the underlying dynamics governing tropical Atlantic variability (TAV) and the dynamic regime that may be most relevant to TAV. By coupling an empirical atmospheric feedback model to an ocean GCM, the authors have conducted a detailed investigation on the potential importance of an unstable ocean-atmosphere interaction between wind-induced heat flux and sea surface temperature (SST) in driving decadal climate variability in the tropical Atlantic basin. The investigation consists of a systematic parameter sensitivity study of the hybrid coupled model. It is shown that in a strong coupling regime the local air-sea feedbacks can support a self-sustained decadal oscillation that exhibits strong cross-equatorial SST gradient and meridional wind variability. An upper-ocean heat budget analysis suggests that the oscillation results from an imbalance between the positive and negative feedbacks in the model. The dominant negative feedback that counteracts the positive feedback between surface heat flux and SST appears to be the advection of heat by ocean currents. The major imbalance in the model occurs in the north tropical Atlantic between 5掳 and 15掳N, caused by a phase delay between the surface heat flux forcing and horizontal heat advection. It is suggested that this may be one of the crucial regions of ocean-atmosphere interactions for TAV.Based on the HCM results, a simple 1D model is derived to further elucidate key coupled dynamics. The model assumes that air-sea coupling takes place in a limited area within the deep Tropics of the Atlantic sector and the change of upper-ocean heat transport is regulated by the advection of anomalous temperatures by the mean meridional current and equatorial upwelling. The analysis shows that the simple model captures many of the salient features of the decadal SST cycle in the HCM, suggesting that the decadal oscillations simulated by the HCM are primarily controlled by the coupled dynamics local to the deep Tropics.The parameter sensitivity study further suggests that in reality the local air-sea coupling in the tropical Atlantic is most likely to be too weak to maintain a self-sustained oscillation, and stochastic forcing may be necessary to excite the coupled variability. Using a realistic representation of external noise' derived from a 145-yr simulation of the National Center for Atmospheric Research atmospheric GCM (CCM3) forced with the observed SST annual cycle, the effect of stochastic forcing on TAV when the coupled system resides in a stable dynamical regime is examined. It is found that the local air-sea feedback and the North Atlantic oscillation-(NAO) dominated noise' forcing are both required to simulate a realistic TAV. In the absence of the local air-sea feedback, the noise' forcing can produce substantial SST anomalies in the subtropical Atlantic up to about 15掳N, particularly off the coast of North Africa. The local air-sea feedback appears to be particularly important for generating the covarying pattern of interhemipheric SST gradient and cross-equatorial atmospheric flow within the deep Tropics. However, too-strong local coupling can lead to an exaggerated tropical response. It is therefore conjectured that TAV may best fit into a weakly coupled scenario in which at minimum the air-sea feedback plays a role in enhancing the persistence of the cross-equatorial gradient of SST and the circulation anomalies, while the NAO provides an important source of external forcing to excite the coupled variability in the Tropics. Furthermore, it is argued that thenoise' forcing can significantly weaken the correlation between the SST variability on either side of the equator, thus hiding any underlying weak `dipole' structure in the SST. [23] Czaja A, Van Der Vaart P, Marshall J. A diagnostic study of the role of remote forcing in Tropical Atlantic variability[J]. Journal of Climate, 2002, 15(22):3 280-3 290. This observational study focuses on remote forcing of the dominant pattern of north tropical Atlantic sea surface temperature (SST) anomalies by ENSO and NAO. Based on a spring SST index of the north tropical Atlantic (NTA) SST (5 -25 N), it is shown that almost all NTA-SST extreme events from 1950 to the present can be related to either ENSO or NAO. Since the SST NTA events lag NAO and ENSO events, NTA variability is interpreted as being largely a response to remote NAO or ENSO forcing. The local response of the tropical Atlantic to these external sources-whether it be ENSO or the NAO-is observed to be rather similar: changes in surface winds induce changes in latent heating that, in turn, generate SST anomalies. Once generated, the latter are damped through local air-sea interaction, at a rate estimated to be 10 W m [24] Yang Y, Xie S P, Wu L, et al. Causes of enhanced SST variability over the equatorial Atlantic and its relationship to the Atlantic zonal mode in CMIP5[J]. Journal of Climate, 2017, 30(16): 6 171-6 182. [25] Amaya D J, Deflorio M J, Miller A J, et al. WES feedback and the Atlantic Meridional Mode: Observations and CMIP5 comparisons[J]. Climate Dynamics, 2017,49(5/6):1 665-1 679. Abstract The Atlantic Meridional Mode (AMM) is the dominant mode of tropical SST/wind coupled variability. Modeling studies have implicated wind-evaporation-SST (WES) feedback as the primary driver of the AMM evolution across the Atlantic basin; however, a robust coupling of the SST and winds has not been shown in observations. This study examines observed AMM growth, propagation, and decay as a result of WES interactions. Investigation of an extended maximum covariance analysis shows that boreal wintertime atmospheric forcing generates positive SST anomalies (SSTA) through a reduction of surface evaporative cooling. When the AMM peaks in magnitude during spring and summer, upward latent heat flux anomalies occur over the warmest SSTs and act to dampen the initial forcing. In contrast, on the southwestern edge of the SSTA, SST-forced cross-equatorial ow reduces the strength of the climatological trade winds and provides an anomalous latent heat flux into the ocean, which causes southwestward propagation of the initial atmosphere-forced SSTA through WES dynamics. Additionally, the lead-lag relationship of the ocean and atmosphere indicates a transition from an atmosphere-forcing-ocean regime in the northern subtropics to a highly coupled regime in the northern tropics that is not observed in the southern hemisphere. CMIP5 models poorly simulate the latitudinal transition from a one-way interaction to a two-way feedback, which may explain why they also struggle to reproduce spatially coherent interactions between tropical Atlantic SST and winds. This analysis provides valuable insight on how meridional modes act as links between extratropical and tropical variability and focuses future research aimed at improving climate model simulations. [26] Doi T, Tozuka T, Yamagata T.The Atlantic meridional mode and its coupled variability with the Guinea Dome[J]. Journal of Climate, 2010, 23(2):455. During the preconditioning phase of the positive (negative) Atlantic meridional mode, the dome is anomalously weak (strong) and the mixed layer is anomalously deep (shallow) there in late fall. This condition reduces (enhances) the sensitivity of the mixed layer temperature to the climatological atmospheric cooling. As a result, the positive (negative) SST anomaly appears there in early winter. Then, it develops in the following spring through the wind–evaporation–SST (WES) positive feedback associated with the anomalous northward (southward) migration of the ITCZ. This, in turn, leads to the stronger (weaker) Ekman upwelling and colder (warmer) subsurface temperature in the dome region during summer. It plays an important role on the decay of the warm (cold) SST anomaly through entrainment as a negative feedback. Therefore, simulating this interesting air–sea interaction in the Guinea Dome region is critical in improving prediction skill for the Atlantic meridional mode. [27] Doi T, Tozuka T, Yamagata T.Interannual variability of the Guinea Dome and its possible link with the Atlantic Meridional Mode[J]. Climate Dynamics, 2009, 33(7/8):985-998. [28] Rossignol M, Meyrueis A M.Campagnes oceanographiques du Gerad-Treca, Cent. Oceanogr[M]. Dakar, Senegal: Dakar-Thiaroye, ORSTOM, 1964: 53. [29] Oettli P, Yushi M, Toshio Y.A regional climate mode discovered in the North Atlantic: Dakar Niño/Niña[J]. Scientific Reports, 2016, 6:18 782. The interrannual variability of coastal sea surface temperature (SST) anomalies confined off Senegal is explored from a new viewpoint of the ocean-land-atmosphere interaction. The phenomenon may be classified into “coastal Ni09o/Ni09a” in the North Atlantic as discussed recently in the Northeastern Pacific and Southeastern Indian Oceans. The interannual variability of the regional mixed-layer temperature anomaly that evolves in boreal late fall and peaks in spring is associated with the alongshore wind anomaly, mixed-layer depth anomaly and cross-shore atmospheric pressure gradient anomaly, suggesting the existence of ocean-land-atmosphere coupled processes. The coupled warm (cold) event is named Dakar Ni09o (Ni09a). The oceanic aspect of the Dakar Ni09o (Ni09a) may be basically explained by anomalous warming (cooling) of the anomalously thin (thick) mixed-layer, which absorbs shortwave surface heat flux. In the case of Dakar Ni09a, however, enhancement of the entrainment at the bottom of the mixed-layer is not negligible. [30] Enfield D B.Tropical Atlantic SST variability and its relation to El Niño-Southern Oscillation[J]. Journal of Geophysical Research: Oceans Banner, 1997, 102(C1):929-945. [31] Giannini A, Kushnir Y, Cane M A.Interannual variability of caribbean rainfall, ENSO, and the Atlantic Ocean[J]. Journal of Climate, 2000, 13(13):297-311. The large-scale ocean-atmosphere patterns that influence the interannual variability of Caribbean-Central American rainfall are examined. The atmospheric circulation over this region is shaped by the competition between the North Atlantic subtropical high sea level pressure system and the eastern Pacific ITCZ, which influence the convergence patterns on seasonal and interannual timescales.The authors find the leading modes of interannual sea level pressure (SLP) and SST variability associated with Caribbean rainfall, as selected by canonical correlation analysis, to be an interbasin mode, linking the eastern Pacific with the tropical Atlantic, and an Atlantic mode. North Atlantic SLP affects Caribbean rainfall directly, by changing the patterns of surface flow over the region, and indirectly, through SST anomalies. Anomalously high SLP in the region of the North Atlantic high translates into stronger trade winds, hence cooler SSTs, and less Caribbean rainfall. The interbasin mode, which manifests itself as a zonal seesaw in SLP between the tropical Atlantic and the eastern equatorial Pacific, is correlated with ENSO. When SLP is low in the eastern equatorial Pacific, it is high in the tropical Atlantic: the surface atmospheric flow over the basin is divergent, to the west, toward the eastern Pacific ITCZ, and to the east, toward the tropical North Atlantic. A weakened meridional SLP gradient in the tropical North Atlantic signifies weaker trade winds and the opportunity for SSTs to warm up, reaching peak intensity 2-4 months after the mature phase of an ENSO event. This SST anomaly is particularly evident in the Caribbean-western Atlantic basin.The tendency is for drier-than-average conditions when the divergent atmospheric flow dominates, during the rainy season preceding the mature phase of a warm ENSO event. The dry season that coincides with the mature phase of ENSO is wetter than average over the northwestern section of the basin, that is, Yucatan, the Caribbean coast of Honduras, and Cuba, and drier than average over the rest of the basin, that is, Costa Rica and northern South America. The following spring, as the atmospheric circulation transitions to normal conditions, the positive SST anomaly that has built up in the basin takes over, favoring convection. The positive precipitation anomaly spreads southeastward, from the northwest to the entire basin. At the start of a new rainy season, it is especially strong over the Greater Antilles. [32] Huang Bohua.Remotely forced variability in the tropical Atlantic Ocean[J]. Climate Dynamics, 2004, 23(2):133-152. An ensemble of eight hindcasts has been conducted using an ocean-atmosphere general circulation model fully coupled only within the Atlantic basin, with prescribed observational sea surface temperature (SST) for 1950-1998 in the global ocean outside the Atlantic basin. The purpose of these experiments is to understand the influence of the external SST anomalies on the interannual variability in the tropical Atlantic Ocean. Statistical methods, including empirical orthogonal function analysis with maximized signal-to-noise ratio, have been used to extract the remotely forced Atlantic signals from the ensemble of simulations. It is found that the leading external source on the interannual time scales is the El Ni o/Southern Oscillation (ENSO) in the Pacific Ocean. The ENSO signal in the tropical Atlantic shows a distinct progression from season to season. During the boreal winter of a maturing El Ni o event, the model shows a major warm center in the southern subtropical Atlantic together with warm anomalies in the northern subtropical Atlantic. The southern subtropical SST anomalies is caused by a weakening of the southeast trade winds, which are partly associated with the influence of an atmospheric wave train generated in the western Pacific Ocean and propagating into the Atlantic basin in the Southern Hemisphere during boreal fall. In the boreal spring, the northern tropical Atlantic Ocean is warmed up by a weakening of the northeast trade winds, which is also associated with a wave train generated in the central tropical Pacific during the winter season of an El Ni o event. Apart from the atmospheric planetary waves, these SST anomalies are also related to the sea level pressure (SLP) increase in the eastern tropical Atlantic due to the global adjustment to the maturing El Ni o in the tropical Pacific. The tropical SLP anomalies are further enhanced in boreal spring, which induce anomalous easterlies on and to the south of the equator and lead to a dynamical oceanic response that causes cold SST anomalies in the eastern and equatorial Atlantic from boreal spring to summer. Most of these SST anomalies persist into the boreal fall season. [33] Amaya D J, Foltz G R.Impacts of canonical and Modoki El Niño on tropical Atlantic SST[J]. Journal of Geophysical Research Oceans, 2014, 119(2):777-789. impacts of canonical and Modoki El Ni o on tropical Atlantic sea surface temperature (SST) are quantified using composite analysis. Results show that El Ni o Modoki fails to produce significant warming in the tropical Atlantic, in contrast to the well known warming following canonical El Ni o events. El Ni o Modoki instead induces significant cooling in the northeastern tropical Atlantic and near-neutral conditions elsewhere in the tropical Atlantic. It is shown that the difference in SST response stems primarily from a much stronger Pacific/North American (PNA) teleconnection pattern and stronger atmospheric Kelvin wave response during canonical events compared to Modoki. The stronger PNA pattern and Kelvin waves during canonical events generate anomalously weak surface winds in the tropical North Atlantic, driving anomalously weak evaporative cooling and warmer SSTs. Past research has shown significant decadal variability in the frequency of noncanonical El Ni os relative to canonical events. If such variability continues, it is likely that the impact of El Ni o on tropical Atlantic SST will also fluctuate from one decade to the next. [34] Chang Ping, Fang Y, Saravanan R, et al. The cause of the fragile relationship between the Pacific El Niño and the Atlantic Niño[J]. Nature, 2006, 443(7 109): 324-328. [35] Gill A E.Some simple solutions for heat-induced tropical circulation[J]. Quarterly Journal of the Royal Meteorological Society, 1980, 106(449):447-462. A simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating. In particular, there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent. This is of more than academic interest because heating in practice tends to be concentrated in specific areas. For instance, a model with heating symmetric about the equator at Indonesian longitudes produces low-level easterly flow over the Pacific through propagation of Kelvin waves into the region. It also produces low-level westerly inflow over the Indian Ocean (but in a smaller region) because planetary waves propagate there. In the heating region itself the low-level flow is away from the equator as required by the vorticity equation. The return flow toward the equator is farther west because of planetary wave propagation, and so cyclonic flow is obtained around lows which form on the western margins of the heating zone. Another model solution with the heating displaced north of the equator provides a flow similar to the monsoon circulation of July and a simple model solution can also be found for heating concentrated along an inter-tropical convergence line. [36] Saravanan R, Chang P.Interaction between Tropical Atlantic variability and El Niño-Southern Oscillation[J]. Journal of Climate, 2000, 13(D14):2 177-2 194. [37] Chikamoto Y.Tropical Atlantic Ocean-Atmospheric Response to Tropical Pacific SST Variations[D]. Sapporo, Japan: Hokkaido University, 2002. [38] Klein S A.Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge[J]. Journal of Climate, 1999, 12(12):917-932. [39] Wang Chunzai.ENSO, Atlantic climate variability, and the walker and hadley circulations[M]∥The Hadley Circulation: Present, Past and Future Netherlands. Springer Netherlands, 2004:173-202. [40] García-Serrano J, Cassou C, Douville H, et al. Revisiting the ENSO teleconnection to the tropical North Atlantic[J]. Journal of Climate, 2017, 30(17).DOI:10.1175/JCLI-D-16-0641. One of the most robust remote impacts of El Ni09o–Southern Oscillation (ENSO) is the teleconnection to tropical North Atlantic (TNA) sea surface temperature (SST) in boreal spring. However, important questions still remain open. In particular, the timing of the ENSO–TNA relationship lacks understanding. The three previously proposed mechanisms rely on teleconnection dynamics involving a time lag of one season with respect to the ENSO mature phase in winter, but recent results have shown that the persistence of ENSO into spring is necessary for the development of the TNA SST anomalies. Likewise, the identification of the effective atmospheric forcing in the deep TNA to drive the regional air–sea interaction is also lacking. In this manuscript a new dynamical framework to understand the ENSO–TNA teleconnection is proposed, in which a continuous atmospheric forcing is present throughout the ENSO decaying phase. Observational datasets in the satellite era, which include reliable estimates over the ocean, are used to illustrate the mechanism at play. The dynamics rely on the remote Gill-type response to the ENSO zonally compensated heat source over the Amazon basin, associated with perturbations in the Walker circulation. For El Ni09o conditions, the anomalous diabatic heating in the tropical Pacific is compensated by anomalous diabatic cooling, in association with negative rainfall anomalies and descending motion over northern South America. A pair of anomalous cyclonic circulations is established at upper-tropospheric levels in the tropical Atlantic straddling the equator, displaying a characteristic baroclinic structure with height. In the TNA region, the mirrored anomalous anticyclonic circulation at lower-tropospheric levels weakens the northeasterly trade winds, leading to a reduction in evaporation and of the ocean mixed layer depth, hence to positive SST anomalies. Apart from the dominance of latent heat flux anomalies in the remote response, sensible heat flux and shortwave radiation anomalies also appear to contribute. The “lagged” relationship between mature ENSO in winter and peaking TNA SSTs in spring seems to be phase locked with the seasonal cycle in both the location of the mechanism’s centers of action and regional SST variance. [41] Giannini A, Chiang J C H, Cane M A, et al. The ENSO teleconnection to the Tropical Atlantic Ocean: Contributions of the remote and local SSTs to rainfall variability in the Tropical Americas[J]. Journal of Climate, 2001, 14(24):4 530-4 544. [42] Lee S, Enfield D B, Wang C.Why do some El Niños have no impact on tropical North Atlantic SST?[J]. Geophysical Research Letters, 2008, 35:537-537. Warming of the Tropical North Atlantic (TNA) in boreal spring and early summer (April-June) following El Ni09o peaks in boreal winter is a well-known phenomenon that involves formation of the so-called atmospheric bridge (or teleconnection) from the Pacific. However, the existence of an El Ni09o in boreal winter does not guarantee a warm TNA in the following April-June (AMJ): for sixteen observed El Ni09o events that occurred during 1950-2005, the TNA (AMJ) remained neutral in six of them. A careful examination of the sixteen El Ni09o events leads to a hypothesis that if an El Ni09o ends before April, the TNA remains neutral. Here, we test this working hypothesis by performing multiple sets of ensemble model experiments using the NCAR atmospheric general circulation model coupled to a slab mixed layer ocean model. Analysis of the model experiments indicates that January-March (JFM) are the crucial months for the El Ni09o-induced warming of TNA. Therefore, if an El Ni09o does not continue throughout JFM, the atmospheric bridge connecting the tropical Pacific to the TNA is not persistent enough to force the TNA, thus the TNA remains neutral. Finally, our model experiments indicate even if an El Ni09o continues beyond JFM, the El Ni09o-induced warming of TNA in AMJ can be greatly reduced by Atlantic internal variability, and vice versa. [43] Yeh S W, Kug J S, Dewitte B, et al. El Niño in a changing climate[J]. Nature, 2009, 461: 511-514. DOI:10.1038/nature08316. [44] Ashok K, Behera S K, Rao S A, et al. El Niño Modoki and its possible teleconnection[J]. Journal of Geophysical Research Oceans, 2007, 112(C11):C11007.DOI:10.1029/2006JC003798. [1] Using observed data sets mainly for the period 19790900092005, we find that anomalous warming events different from conventional El Ni01±o events occur in the central equatorial Pacific. This unique warming in the central equatorial Pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly (SSTA) on both sides along the equator. empirical orthogonal function (EOF) analysis of monthly tropical Pacific SSTA shows that these events are represented by the second mode that explains 12% of the variance. Since a majority of such events are not part of El Ni01±o evolution, the phenomenon is named as El Ni01±o Modoki (pseudo-El Ni01±o) (090008Modoki090009 is a classical Japanese word, which means 090008a similar but different thing090009). The El Ni01±o Modoki involves ocean-atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution, analogous to the Southern Oscillation in the case of El Ni01±o. Hence the total entity is named as El Ni01±o090009Southern Oscillation (ENSO) Modoki. The ENSO Modoki events significantly influence the temperature and precipitation over many parts of the globe. Depending on the season, the impacts over regions such as the Far East including Japan, New Zealand, western coast of United States, etc., are opposite to those of the conventional ENSO. The difference maps between the two periods of 19790900092004 and 19580900091978 for various oceanic/atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline. This appears to be a cause of more frequent and persistent occurrence of the ENSO Modoki event during recent decades. [45] Kug J S, Jin Feifei, An S I.Two types of El Niño events: Cold Tongue El Niño and Warm Pool El Niño[J]. Journal of Climate, 2009, 22(22):1 499-1 515. In this study, two types of El Ni09o events are classified based on spatial patterns of the sea surface temperature (SST) anomaly. One is the cold tongue (CT) El Ni09o, which can be regarded as the conventional El Ni09o, and the other the warm pool (WP) El Ni09o. The CT El Ni09o is characterized by relatively large SST anomalies in the Ni09o-3 region (58S-58N, 1508-908W), while the WP El Ni09o is associated with SST anomalies mostly confined to the Ni09o-4 region (58S-58N, 1608E-1508W). In addition, spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of El Ni09o events. Furthermore, the difference in the transition mechanism between the two types of El Ni09o is clearly identified. That is, the discharge process of the equatorial heat content associated with the WP El Ni09o is not efficient owing to the spatial structure of SST anomaly; as a result, it cannot trigger a cold event. It is also demonstrated that zonal advective feedback (i.e., zonal advection of mean SST by anomalous zonal currents) plays a crucial role in the development of a decaying SST anomaly associated with theWP El Ni09o, while thermocline feedback is a key process during the CT El Ni09o. [46] Mcphaden M J, Lee T, Mcclurg D.El Niño and its relationship to changing background conditions in the tropical Pacific Ocean[J]. Geophysical Research Letters, 2011, 38(15):175-188. [47] Yeh S, Kirtman B P, Kug J, et al. Natural variability of the central Pacific El Niño event on multi-centennial timescales[J]. Geophysical Research Letters, 2011, 38(2):79-89. [48] Nobre P, Srukla J.Variations of sea surface temperature, wind stress, and rainfall over the Tropical Atlantic and South America[J]. Journal of Climate, 1996, 18(1):73-84. [49] Ruizbarradas A, Carton J A, Nigam S.Structure of interannual-to-decadal climate variability in the Tropical Atlantic Sector[J]. Journal of Climate, 2000, 13(18):3 285-3 297. A search for coupled modes of atmosphere--ocean interaction in the tropical Atlantic sector is presented. Previous studies have provided conflicting indications of the existence of coupled modes in this region. The subject is revisited through a rotated principal component analysis performed on datasets spanning the 36-yr period 1958--93. The analysis includes four variables, sea surface temperature, oceanic heat content, wind stress, and atmospheric diabatic heating. The authors find that the first rotated principal component is associated with fluctuations in the subtropical wind system and correlates with the North Atlantic oscillation (NAO), while the second and third modes, which are the focus of interest, are related to tropical variability. The second mode is the Atlantic Nino mode with anomalous sea surface temperature and anomalous heat content in the eastern equatorial basin. Wind stress weakens to the west of anomalously warm water, while convection is shifted south and eastward. Surface and upper-level wind anomalies of this mode resemble those of El Nino--Southern Oscillation (ENSO) events. When the analysis is limited to boreal summer, the season of maximum amplitude, the Atlantic Nino mode explains 7.5% of the variance of the five variables. Thermodynamic air--sea interactions do not seem to play a role for this mode. The third mode is associated with an interhemispheric gradient of anomalous sea surface temperature and a dipole pattern of atmospheric heating. In its positive phase anomalous heating occurs over the warmer Northern Hemisphere with divergence aloft shifting convection to the north and west of the equator and intensifying the subtropical jet stream, while descending motion occurs on the western side of the Southern Hemisphere. Surface and subsurface structures in the ocean are controlled by surface winds. This interhemispheric mode is strongest in boreal spring when it explains 9.1% of the combined variance of the five. [50] Houghton R W, Tourre Y M.Characteristics of low-frequency sea surface temperature fluctuations in the Tropical Atlantic[J]. Journal of Climate, 1992, 5(7):765-772. Sea surface temperature anomalies in the tropical Atlantic Ocean are reexamined to investigate an apparent low-frequency oscillation that has been described as a fluctuating dipole structure with poles north and south of the equator and a node near the ITCZ. Using principal components rotated by the varimax method and simple correlations of area-averaged temperatures, we show that during the 1964–88 interval SST anomalies north and south of the ITCZ are not significantly correlated. Therefore, the low-frequency variation, with an apparent decadal period observed in the SST gradient across the ITCZ during 1964–88, does not arise from temporally coherent and out-of-phase fluctuations in each hemisphere and cannot be characterized as a dipole. [51] Mehta V M.Variability of the tropical ocean surface temperatures at decadal-multidecadal time scales. Part I: The Atlantic Ocean[J]. Journal of Climate, 1998,11(9): 2 351-2 375. [52] Mehta V M, Delworth T.Decadal variability of the Tropical Atlantic Ocean surface temperature in shipboard measurements and in a Global Ocean-Atmosphere Model[J]. Journal of Climate, 1995, 8(8):172-190. [53] Enfield D B, Mestas-Nuñez A M, Mayer D A, et al. How ubiquitous is the dipole relationship in tropical Atlantic sea surface temperatures?[J]. Journal of Geophysical Research, 1999, 104:7 841-7 848. Several kinds of analysis are applied to sea surface temperature anomalies (SSTA) (1856-1991) to determine the degree to which SSTA of opposite sign in the tropical North and South Atlantic occur. Antisymmetric ("dipole") configurations of SSTA on basin scales are not ubiquitous in the tropical Atlantic. Unless the data are stratified by both season and frequency, inherent dipole behavior cannot be demonstrated. Upon removing the global El Ni o-Southern Oscillation signal in SSTA (which is symmetric between the North and South Atlantic) from the data, the regions north or south of the Intertropical Convergence Zone have qualitatively different temporal variabilities and are poorly correlated. Dipole configurations do occur infrequently (12-15% of the time), but no more so than expected by chance for stochastically independent variables. Nondipole configurations that imply significant meridional SSTA gradients occur much more frequently, nearly half of the time. Cross-spectral analysis of seasonally averaged SSTA indices for the North and South Atlantic show marginally significant coherence with antisymmetric phase in two period bands: 8-12 years for the boreal winter-spring and 2.3 years for the boreal summer-fall. Antisymmetric coherence is optimal for a small subregion west of Angola in the South Atlantic, with respect to SSTA of basin scale in the tropical North Atlantic. Dipole variability, even where optimal, explains only a small fraction of the total variance in tropical Atlantic SSTA (<7%). [54] Marshall J, Kushnir Y, Battisti D, et al. North Atlantic climate variability: Phenomena, impacts and mechanisms[J]. International Journal of Climatology, 2001, 21(15):1 863-1 898. [55] Yao Yao, Luo Dehai.The North Atlantic Oscillation (NAO) and Europe Blocking and their impacts on extreme snowstorms: A review[J]. Advances in Earth Science, 2016, 31(6): 581-594. [姚遥, 罗德海. 北大西洋涛动—欧洲阻塞及其对极端暴雪影响的研究进展[J]. 地球科学进展, 2016, 31(6): 581-594.] 北大西洋涛动(NAO)和阻塞等大气大尺度低频模态对北半球天气气候起着重要的调控作用。首先回顾了NAO年代际变率物理机制的研究进展,并从季节内尺度NAO位相转换的角度讨论了其对NAO年代际变率的影响。介绍了NAO与阻塞时空关系的研究进展,讨论了年代际NAO变率对阻塞时空分布的可能影响。另外,以2次极端暴雪天气事件为例,从观测事实方面讨论了NAO和阻塞对极端暴雪天气的影响机制,同时从理论模型方面部分解释了其可能的物理机制。总结了有关NAO和阻塞理论模式的研究进展,介绍了非线性多尺度相互作用模型的发展过程和应用。最后,基于对以上研究进展的总结,给出了NAO-阻塞—极端暴雪天气的机制示意图,概括部分研究的特点和不同之处,并提出了该领域有待解决的几个问题和未来可能的研究方向,为相关的研究提供参考。 [56] Wu Lixin, Liu Zhengyu.North Atlantic decadal variability: Air-sea coupling, oceanic memory, and potential Northern Hemisphere resonance[J]. Journal of Climate, 2005, 18(2):331-349. [57] Yang Yun, Wu Lixin, Fang Changfang.Will global warming Suppress North Atlantic Tripole decadal variability?[J]. Journal of Climate, 2012, 25(6):2 040-2 055. ABSTRACT In this paper, the modulations of the North Atlantic tripole (NAT) decadal variability from global warming are studied by conducting a series of coupled ocean-atmosphere experiments using the Fast Ocean Atmosphere Model (FOAM). The model reasonably captures the observed NAT decadal variability with a preferred time scale of about 11 years. With the aid of partial-blocking and partial-coupling experiments, it is found that the NAT decadal cycle can be attributed to oceanic planetary wave adjustment in the subtropical basin and ocean-atmosphere coupling over the North Atlantic. In a doubled CO 2 experiment, the spatial pattern of the NAT is preserved; however, the decadal cycle is significantly suppressed. This suppression appears to be associated with the acceleration of oceanic planetary waves due to an increase of buoyancy frequency in global warming. This shortens the time from a decadal to an interannual time scale for the first-mode baroclinic Rossby waves to cross the subtropical North Atlantic basin, the primarymemory for theNATdecadal variability in themodel. Themodeling study also found that the global warming does not modulate the North Atlantic air-sea coupling significantly, but it may be model dependent. [58] Czaja A, Robertson A W, Huck T.The role of Atlantic Ocean-atmosphere coupling in affecting North Atlantic oscillation variability[J]. North Atlantic Oscillation Climatic Significance & Environmental Impact, 2003, 134:147-172. [59] Robinson W A, Li Shuanglin, Peng S.Dynamical nonlinearity in the atmospheric response to Atlantic sea surface temperature anomalies[J]. Geophysical Research Letters, 2003, 30(20):315-331. Abstract Top of page Abstract 1.Introduction 2.Experiments and Results 3.Diagnoses 4.Discussion Acknowledgments References [1] Large ensembles (100 members) of atmospheric general circulation model experiments are forced throughout the Northern Hemisphere cold season by four different sea surface temperature (SST) fields: the observed climatology, the so-called SST tripole pattern, and its tropical and its extratropical subcomponents. Late winter responses to these anomalies are of modest amplitude, in comparison with the amplitudes of climatological stationary waves, but are, because of the large ensemble, significant. Despite their modest amplitudes, the responses display additive nonlinearity, in that the sum of the separate responses to the component anomalies differs significantly from the response to the tripole. Neither the heating field nor the basin averaged zonal winds display this nonlinearity. It is most evident in a sub-basin scale wave train, and most significant in its impact on the amplitude of the geopotential response. These results indicate that even for modest forcing, responses to patterns of SST anomalies cannot necessarily be understood as the sums of responses to constituent anomalies. [60] Yang Yun, Wu Lixin.Changes of air-sea coupling in the North Atlantic over the 20th century[J]. Advances in Atmospheric Sciences, 2015, 32(4):445-456. Changes of air-sea coupling in the North Atlantic Ocean over the 20th century are investigated using reanalysis data, climate model simulations, and observational data. It is found that the ocean-to-atmosphere feedback over the North Atlantic is significantly intensified in the second half of the 20th century. This coupled feedback is characterized by the association between the summer North Atlantic Horseshoe (NAH) SST anomalies and the following winter North Atlantic Oscillation (NAO). The intensification is likely associated with the enhancement of the North Atlantic storm tracks as well as the NAH SST anomalies. Our study also reveals that most IPCC AR4 climate models fail to capture the observed NAO/NAH coupled feedback. [61] Servain J, Wainer I, Jr M C, et al. Relationship between the equatorial and meridional modes of climatic variability in the tropical Atlantic[J]. Geophysical Research Letters, 1999, 26(4):485-488. [62] Foltz G R, Mcphaden M J.Abrupt equatorial wave-induced cooling of the Atlantic cold tongue in 2009[J]. Geophysical Research Letters, 2010, 37(24):701-719. Between May and August 2009 sea surface temperatures (SSTs) in the eastern equatorial Atlantic dropped 5掳C, from 1掳C above normal to 1掳C below normal. The magnitude of this cooling is unprecedented since satellite SST measurements began in 1982. In this study, observations and a linear equatorial wave model are used to examine the causes of the sharp decrease in SST. It is found that the anomalous cooling along the equator can be traced to an anomalous meridional gradient of SST and associated northwesterly anomalous winds that developed in the equatorial Atlantic the preceding spring. The anomalous winds forced upwelling equatorial Rossby waves that propagated westward during boreal spring and reflected at the western boundary into upwelling Kelvin waves during late spring and summer. The upwelling Kelvin waves propagated eastward along the equator, anomalously decreasing sea level and SST during May-August. [63] Foltz Gregory R, McPhaden Michael J. Interaction between the Atlantic meridional and Niño modes[J]. Geophysical Research Letters, 2010, 37:44 727-44 734. [64] Richter I, Behera S K, Masumoto Y, et al. Multiple causes of interannual sea surface temperature variability in the equatorial Atlantic Ocean[J]. Nature Geoscience, 2013, 6(1):43-47. [65] Namias J.Influence of northern hemisphere general circulation on drought in northeast Brazil1[J]. Tellus, 1972, 24(4):336-343. Evidence is presented to show that the highly variable interannual rainfall over Northeast Brazil is dependent on the degree of cyclonic activity or blocking in the Newfoundland-Greenland area during the Northern Hemisphere winter and spring. The linkage is traced through variations in the subtropical Atlantic anticyclone, the northeast trades and the responding Hadley cell which is forced to vary in position and strength. Intense blocking over North America and the North Atlantic is usually associated with devastating drought over Northeast Brazil. [66] Marengo J A, Torres R R, Alves L M.Drought in Northeast Brazil—Past, present, and future[J]. Theoretical & Applied Climatology, 2017, 129:1-12. This study provides an overview of the drought situation in Northeast Brazil for the past, present, and future. Droughts affect more people than any other natural hazard owing to their large scale and long-lasting nature. They are recurrent in the region and while some measures have been taken by the governments to mitigate their impacts, there is still a perception that residents, mainly in rural areas, are not yet adapted to these hazards. The drought affecting the Northeast from 2012 to 2015, however, has had an intensity and impact not seen in several decades and has already destroyed large swaths of cropland, affecting hundreds of cities and towns across the region, and leaving ranchers struggling to feed and water cattle. Future climate projections for the area show large temperature increases and rainfall reductions, which, together with a tendency for longer periods with consecutive dry days, suggest the occurrence of more frequent/intense dry spells and droughts and a tendency toward aridification in the region. All these conditions lead to an increase in evaporation from reservoirs and lakes, affecting irrigation and agriculture as well as key water uses including hydropower and industry, and thus, the welfare of the residents. Integrating drought monitoring and seasonal forecasting provides efficient means of assessing impacts of climate variability and change, identifying vulnerabilities, and allowing for better adaptation measures not only for medium- and long-term climate change but also for extremes of the interannual climate variability, particularly droughts. [67] Rao V B, Hada K, Herdies D L.On the severe drought of 1993 in north-east Brazil[J]. International Journal of Climatology, 1995, 15(6):697-704. During the period 1990 through to 1993 a dry spell occurred over north-east Brazil. The drought of 1993 was very severe. Rainfall series for north-east Brazil are updated to 1993 and the drought conditions during 1993 are discussed. The 1993 drought seems to be connected at least partially to the unusual ENSO conditions during that year. [68] Foltz G R, Mcphaden M J, Lumpkin R.A strong Atlantic Meridional Mode Event in 2009: The role of Mixed Layer dynamics[J]. Journal of Climate, 2011, 25(1):363-380. Not Available [69] Folland C K, Palmer T N, Parker D E.Sahel rainfall and worldwide sea temperatures, 1901-85[J]. Nature, 1986, 320(6 063):602-607. Using the comprehensively quality-controlled Meteorological Office Historical Sea Surface Temperature data set (MOHSST) 1,2 we show for the first time that persistently wet and dry periods in the Sahel region of Africa are strongly related to contrasting patterns of sea-surface temperature (SST) anomalies on a near-global scale. The anomalies include relative changes in SST between the hemispheres, on timescales of years to tens of years, which are most pronounced in the Atlantic. Experiments with an 11-level global atmospheric general circulation model (AGCM) support the idea that the worldwide SST anomalies modulate summer Sahel rainfall through changes in tropical atmospheric circulation 3鈥6 . El Ni帽o events may also play a part. We do not discount the effects of soil moisture and albedo changes in the Sahel 7,8 , although Courel et al. 9 have questioned the importance of albedo changes, but we do suggest that worldwide SST anomalies may have a more fundamental influence on Sahel rainfall. [70] Lamb P J, Peppler R A.Further case studies of Tropical Atlantic surface atmospheric and oceanic patterns associated with Sub-Saharan Drought[J]. Journal of Climate, 1992, 5(5):476-488. [71] Miles M K, Follard C K.Changes in the latitude of the climatic zones of the Northern Hemisphere[J]. Nature, 1974, 252(5 484):616. No Abstract available for this article. [72] Yu K, Xie Shangping.The tropical Pacific as a key pacemaker of the variable rates of global warming[J]. Nature Geoscience, 2016, 9(9):669-673. Global mean surface temperature change over the past 120 years resembles a rising staircase: the overall warming trend was interrupted by the mid-twentieth-century big hiatus and the warming slowdown since about 1998. The Interdecadal Pacific Oscillation has been implicated in modulations of global mean surface temperatures, but which part of the mode drives the variability in warming rates is unclear. Here we present a successful simulation of the global warming staircase since 1900 with a global ocean-atmosphere coupled model where tropical Pacific sea surface temperatures are forced to follow the observed evolution. Without prescribed tropical Pacific variability, the same model, on average, produces a continual warming trend that accelerates after the 1960s. We identify four events where the tropical Pacific decadal cooling markedly slowed down the warming trend. Matching the observed spatial and seasonal fingerprints we identify the tropical Pacific as a key pacemaker of the warming staircase, with radiative forcing driving the overall warming trend. Specifically, tropical Pacific variability amplifies the first warming epoch of the 1910s-1940s and determines the timing when the big hiatus starts and ends. Our method of removing internal variability from the observed record can be used for real-time monitoring of anthropogenic warming. [73] Holland P R, Kwok R.Wind-driven trends in Antarctic sea-ice drift[J]. Nature Geoscience, 2012, 5(12):872-875. The sea-ice cover around Antarctica has experienced a slight expansion in area over the past decades. This small overall increase is the sum of much larger opposing trends in different sectors that have been proposed to result from changes in atmospheric temperature or wind stress, precipitation, ocean temperature, and atmosphere or ocean feedbacks. However, climate models have failed to reproduce the overall increase in sea ice. Here we present a data set of satellite-tracked sea-ice motion for the period of 1992-2010 that reveals large and statistically significant trends in Antarctic ice drift, which, in most sectors, can be linked to local winds. We quantify dynamic and thermodynamic processes in the internal ice pack and show that wind-driven changes in ice advection are the dominant driver of ice-concentration trends around much of West Antarctica, whereas wind-driven thermodynamic changes dominate elsewhere. The ice-drift trends also imply large changes in the surface stress that drives the Antarctic ocean gyres, and in the fluxes of heat and salt responsible for the production of Antarctic bottom and intermediate waters. [74] Turner J, Comiso J.Solve Antarctica's sea-ice puzzle[J]. Nature, 2017, 547(7 663):275. John Turner and Josefino Comiso call for a coordinated push to crack the baffling rise and fall of sea ice around Antarctica. [75] Li Xichen, Holland D M, Gerber E P, et al. Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice[J]. Nature, 2014, 505(7 484):538-542. In recent decades, Antarctica has experienced pronounced climate changes. The Antarctic Peninsula exhibited the strongest warming of any region on the planet, causing rapid changes in land ice. Additionally, in contrast to the sea-ice decline over the Arctic, Antarctic sea ice has not declined, but has instead undergone a perplexing redistribution. Antarctic climate is influenced by, among other factors, changes in radiative forcing and remote Pacific climate variability, but none explains the observed Antarctic Peninsula warming or the sea-ice redistribution in austral winter. However, in the north and tropical Atlantic Ocean, the Atlantic Multidecadal Oscillation (a leading mode of sea surface temperature variability) has been overlooked in this context. Here we show that sea surface warming related to the Atlantic Multidecadal Oscillation reduces the surface pressure in the Amundsen Sea and contributes to the observed dipole-like sea-ice redistribution between the Ross and Amundsen-Bellingshausen-Weddell seas and to the Antarctic Peninsula warming. Support for these findings comes from analysis of observational and reanalysis data, and independently from both comprehensive and idealized atmospheric model simulations. We suggest that the north and tropical Atlantic is important for projections of future climate change in Antarctica, and has the potential to affect the global thermohaline circulation and sea-level change. [76] Li Xichen, Gerber E P, Holland D M, et al. A Rossby Wave Bridge from the Tropical Atlantic to West Antarctica[J]. Journal of Climate, 2015, 28(6):2 256-2 273. ABSTRACT Tropical Atlantic sea surface temperature changes have recently been linked to circulation anomalies around Antarctica during austral winter. Warming in the tropical Atlantic associated with the Atlantic multidecadal oscillation forces a positive response in the southern annular mode, strengthening the Amundsen-Bellingshausen Sea low in particular. In this study, observational and reanalysis datasets and a hierarchy of atmospheric models are used to assess the seasonality and dynamical mechanism of this teleconnection. Both the reanalyses and models reveal a robust link between tropical Atlantic SSTs and the Amundsen-Bellingshausen Sea low in all seasons except austral summer. A Rossby wave mechanism is then shown to both explain the teleconnection and its seasonality. The mechanism involves both changes in the excitation of Rossby wave activity with season and the formation of a Rossby wave guide across the Pacific, which depends critically on the strength and extension of the subtropical jet over the west Pacific. Strong anticyclonic curvature on the poleward flank of the jet creates a reflecting surface, channeling quasi-stationary Rossby waves from the subtropical Atlantic to the Amundsen-Bellingshausen Sea region. In summer, however, the jet is weaker than in other seasons and no longer able to keep Rossby wave activity trapped in the Southern Hemisphere. The mechanism is supported by integrations with a comprehensive atmospheric model, initial-value calculations with a primitive equation model on the sphere, and Rossby wave ray tracing analysis. [77] Zheng Xiaotong, Xie Shangping, Du Yan, et al. Indian Ocean Dipole response to global warming in the CMIP5 multimodel Ensemble[J]. Journal of Climate, 2013, 26(16):6 067-6 080. The response of the Indian Ocean dipole (IOD) mode to global warming is investigated based on simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). In response to increased greenhouse gases, an IOD-like warming pattern appears in the equatorial Indian Ocean, with reduced (enhanced) warming in the east (west), an easterly wind trend, and thermocline shoaling in the east. Despite a shoaling thermocline and strengthened thermocline feedback in the eastern equatorial Indian Ocean, the interannual variance of the IOD mode remains largely unchanged in sea surface temperature (SST) as atmospheric feedback and zonal wind variance weaken under global warming. The negative skewness in eastern Indian Ocean SST is reduced as a result of the shoaling thermocline. The change in interannual IOD variance exhibits some variability among models, and this intermodel variability is correlated with the change in thermocline feedback. The results herein illustrate that mean state changes modulate interannual modes, and suggest that recent changes in the IOD mode are likely due to natural variations. [78] Weller E, Cai Wenju, Cowan T.Realism of the Indian Ocean Dipole in CMIP5 models, and the implication for climate projections[J]. Journal of Climate, 2013, 26(17):6 649-6 659. [79] Xu Kang, Tam C Y.CMIP5 projections of two types of El Niño and their related tropical precipitation[J]. Journal of Climate, 2017, 30(3):849-864. Future projections of the eastern-Pacific (EP) and central-Pacific (CP) types of El Nino in the twenty-first century, as well as their associated tropical circulation and precipitation variability, are investigated using historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). As inferred from CMIP5 models that best capture both El Nino flavors, EP El Nino sea surface temperature (SST) variability will become weaker in the future climate, while no robust change of CP El Nino SST is found. Models also reach no consensus on the future change of relative frequency from CP to EP El Nino. However, there are robust changes in the tropical overturning circulation and precipitation associated with both types of El Nino. Under a warmer climate, magnitudes of precipitation anomalies during EP El Nino are projected to increase, presenting significant enhancement of the dry (wet) signal over the western (central-eastern) Pacific. This is consistent with an accelerated hydrological cycle in the deep tropics; hence, a "wet get wetter'' picture appears under global warming, accompanied by a weakened anomalous Walker circulation. For CP El Nino, drier-than-normal conditions will be intensified over the tropical central-eastern Pacific in the future climate, with stronger anomalous sinking related to the strengthened North Pacific local Hadley cell. These results suggest that, besides the enhanced basic-state hydrological cycle over the tropics, other elements, such as the anomalous overturning circulation, might also play a role in determining the ENSO precipitation response to a warmer background climate. [80] Ferrett S, Collins M.Diagnosing relationships between mean state biases and El Niño shortwave feedback in CMIP5 Models[J]. Journal of Climate, 2018, 31: 1 315-1 335. [81] Yang Yun, Xie Shangping, Wu Lixin, et al. Causes of enhanced SST variability over the equatorial atlantic and its relationship to the Atlantic Zonal Mode in CMIP5[J]. Journal of Climate, 2017, 30(16):6 171-6 182. [82] Liu Hailong, Wang Chunzai, Lee S K, et al. Atlantic Warm Pool Variability in the CMIP5 simulations[J]. Journal of Climate, 2013, 26(15):5 315-5 336. This study investigates Atlantic warm pool (AWP) variability in the historical run of 19 coupled general circulation models (CGCMs) submitted to phase 5 of the Coupled Model Intercomparison Project (CMIP5). As with the CGCMs in phase 3 (CMIP3), most models suffer from the cold SST bias in the AWP region and also show very weak AWP variability as represented by the AWP area index. However, for the seasonal cycle the AWP SST bias of model ensemble and model sensitivities are decreased compared with CMIP3, indicating that the CGCMs are improved. The origin of the cold SST bias in the AWP region remains unknown, but among the CGCMs in CMIP5 excess (insufficient) high-level cloud simulation decreases (enhances) the cold SST bias in the AWP region through the warming effect of the high-level cloud radiative forcing. Thus, the AWP SST bias in CMIP5 is more modulated by an erroneous radiation balance due to misrepresentation of high-level clouds rather than low-level clouds as in CMIP3. AWP variability is assessed as in the authors' previous study in the aspects of spectral analysis, interannual variability, multidecadal variability, and comparison of the remote connections with ENSO and the North Atlantic Oscillation (NAO) against observations. In observations the maximum influences of the NAO and ENSO on the AWP take place in boreal spring. For some CGCMs these influences erroneously last to late summer. The effect of this overestimated remote forcing can be seen in the variability statistics as shown in the rotated EOF patterns from the models. It is concluded that the NCAR Community Climate System Model, version 4 (CCSM4), the Goddard Institute for Space Studies (GISS) Model E, version 2, coupled with the Hybrid Coordinate Ocean Model (HYCOM) ocean model (GISS-E2H), and the GISS Model E, version 2, coupled with the Russell ocean model (GISS-E2R) are the best three models of CMIP5 in simulating AWP variability. [83] Chang C Y, Carton J A, Grodsky S A, et al. Seasonal climate of the Tropical Atlantic Sector in the NCAR community climate system Model 3: Error structure and probable causes of errors[J]. Journal of Climate, 2007, 20(6):1 053-1 070. [84] Richter I, Xie Shangping.On the origin of equatorial Atlantic biases in coupled general circulation models[J]. Climate Dynamics, 2008, 31(5):587-598. Many coupled ocean-atmosphere general circulation models (GCMs) suffer serious biases in the tropical Atlantic including a southward shift of the intertropical convergence zone (ITCZ) in the annual mean, a westerly bias in equatorial surface winds, and a failure to reproduce the eastern equatorial cold tongue in boreal summer. The present study examines an ensemble of coupled GCMs and their uncoupled atmospheric component to identify common sources of error. It is found that the westerly wind bias also exists in the atmospheric GCMs forced with observed sea surface temperature, but only in boreal spring. During this time sea-level pressure is anomalously high (low) in the western (eastern) equatorial Atlantic, which appears to be related to deficient (excessive) precipitation over tropical South America (Africa). In coupled simulations, this westerly bias leads to a deepening of the thermocline in the east, which prevents the equatorial cold tongue from developing in boreal summer. Thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical Atlantic climate. [85] Tozuka T, Doi T, Miyasaka T, et al. Key factors in simulating the equatorial Atlantic zonal sea surface temperature gradient in a coupled general circulation model[J]. Journal of Geophysical Research Oceans, 2011, 116(C6):C06010.DOI10.1029/2010JC00671. [86] Richter I, Xie Shangping, Wittenberg A T, et al. Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation[J]. Climate Dynamics, 2012, 38(5):985-1 001. Most coupled general circulation models (GCMs) perform poorly in the tropical Atlantic in terms of climatological seasonal cycle and interannual variability. The reasons for this poor performance are investigated in a suite of sensitivity experiments with the Geophysical Fluid Dynamics Laboratory (GFDL) coupled GCM. The experiments show that a significant portion of the equatorial SST biases in the model is due to weaker than observed equatorial easterlies during boreal spring. Due to these weak easterlies, the tilt of the equatorial thermocline is reduced, with shoaling in the west and deepening in the east. The erroneously deep thermocline in the east prevents cold tongue formation in the following season despite vigorous upwelling, thus inhibiting the Bjerknes feedback. It is further shown that the surface wind errors are due, in part, to deficient precipitation over equatorial South America and excessive precipitation over equatorial Africa, which already exist in the uncoupled atmospheric GCM. Additional tests indicate that the precipitation biases are highly sensitive to land surface conditions such as albedo and soil moisture. This suggests that improving the representation of land surface processes in GCMs offers a way of improving their performance in the tropical Atlantic. The weaker than observed equatorial easterlies also contribute remotely, via equatorial and coastal Kelvin waves, to the severe warm SST biases along the southwest African coast. However, the strength of the subtropical anticyclone and along-shore winds also play an important role. [87] Richter I, Behera S K, Masumoto Y, et al. Multiple causes of interannual sea surface temperature variability in the equatorial Atlantic Ocean[J]. Nature Geoscience, 2014, 6(1):43-47. [88] Stockdale T N, Balmaseda M A, Vidard A.Tropical Atlantic SST prediction with Coupled Ocean Atmosphere GCMs[J]. Journal of Climate, 2006, 19(23):6 047. Variations in tropical Atlantic SST are an important factor in seasonal forecasts in the region and beyond. An analysis is given of the capabilities of the latest generation of coupled GCM seasonal forecast systems to predict tropical Atlantic SST anomalies. Skill above that of persistence is demonstrated in both the northern tropical and equatorial Atlantic, but not farther south. The inability of the coupled models to correctly represent the mean seasonal cycle is a major problem in attempts to forecast equatorial SST anomalies in the boreal summer. Even when forced with observed SST, atmosphere models have significant failings in this area. The quality of ocean initial conditions for coupled model forecasts is also a cause for concern, and the adequacy of the near-equatorial ocean observing system is in doubt. A multimodel approach improves forecast skill only modestly, and large errors remain in the southern tropical Atlantic. There is still much scope for improving forecasts of tropical Atlantic SST. [89] Davey M, Huddleston M, Sperber K, et al. STOIC: A study of coupled model climatology and variability in tropical ocean regions[J]. Climate Dynamics, 2002, 18(5):403-420. Author(s): Davey, M; Huddleston, M; Sperber, K; Braconnot, P; Bryan, F; Chen, D; Colman, R; Cooper, C; Cubasch, U; Delecluse, P; DeWitt, D; Fairhead, L; Flato, G; Gordon, C; Hogan, T; Ji, M; Kimoto, M; Kitoh, A; Knutson, T; Latif, M; Le Treut, H; Li, T; Manabe, S; Mechoso, C; Meehl, G; Power, S; Roeckner, E; Terray, L; Vintzileos, A; Voss, R; Wang, B; Washington, W; Yoshikawa, I; Yu, J; Yukimoto, S; Zebiak, S | Abstract: We describe the behaviour of 23 dynamical ocean-atmosphere models, in the context of comparison with observations in a common framework. Fields of tropical sea surface temperature (SST), surface wind stress and upper ocean vertically averaged temperature (VAT) are assessed with regard to annual mean, seasonal cycle, and interannual variability characteristics. Of the participating models, 21 are coupled GCMs, of which 13 use no form of flux adjustment in the tropics. The models vary widely in design, components and purpose: nevertheless several common features are apparent. In most models without flux adjustment, the annual mean equatorial SST in the central Pacific is too cool and the Atlantic zonal SST gradient has the wrong sign. Annual mean wind stress is often too weak in the central Pacific and in the Atlantic, but too strong in the west Pacific. Few models have an upper ocean VAT seasonal cycle like that observed in the equatorial Pacific. Interannual variability is commonly too weak in the models: in particular, wind stress variability is low in the equatorial Pacific. Most models have difficulty in reproducing the observed Pacific 'horseshoe' pattern of negative SST correlations with internnual Nino3 SST anomalies, or the observed Indian-Pacific lag correlations. The results for the fields examined indicate that several substantial model improvements are needed, particularly with regard to surface wind stress. [90] Bjerknes J.Atmospheric teleconnections from the equatorial Pacific[J]. Monthly Weather Review, 1969, 97(3): 163-172. [91] Nnamchi H C, Li J, Kucharski F, et al. Thermodynamic controls of the Atlantic Niño[J]. Nature Communications, 2015, 6.DOI:10.1038/ncomms9895. Prevailing theories on the equatorial Atlantic Ni09o are based on the dynamical interaction between atmosphere and ocean. However, dynamical coupled ocean-atmosphere models poorly simulate and predict equatorial Atlantic climate variability. Here we use multi-model numerical experiments to show that thermodynamic feedbacks excited by stochastic atmospheric perturbations can generate Atlantic Ni09o s.d. of 650.28±0.0765K, explaining 6568±23% of the observed interannual variability. Thus, in state-of-the-art coupled models, Atlantic Ni09o variability strongly depends on the thermodynamic component (R2=0.92). Coupled dynamics acts to improve the characteristic Ni09o-like spatial structure but not necessarily the variance. Perturbations of the equatorial Atlantic trade winds (65±1.5365m65s611) can drive changes in surface latent heat flux (65±14.3565W65m612) and thus in surface temperature consistent with a first-order autoregressive process. By challenging the dynamical paradigm of equatorial Atlantic variability, our findings suggest that the current theories on its modelling and predictability must be revised. The nature of the El Ni09o-like variability in the Atlantic Ocean and its limited predictability remain unresolved. Here, via multi-model numerical experiments, the authors show that much of the variability can be explained by the interaction of stochastic atmospheric fluctuations with the ocean mixed layer. [92] Nnamchi H C, Li J, Kucharski F, et al. An equatorial-extratropical dipole structure of the Atlantic Niño[J]. Journal of Climate, 2016, 29(20):7 295-7 311. AbstractEquatorial Atlantic variability is dominated by the Atlantic Ni09o peaking during the boreal summer. Studies have shown robust links of the Atlantic Ni09o to fluctuations of the St. Helena subtropical anticyclone and Benguela Ni09o events. Furthermore, the occurrence of opposite sea surface temperature (SST) anomalies in the eastern equatorial and southwestern extratropical South Atlantic Ocean (SAO), also peaking in boreal summer, has recently been identified and termed the SAO dipole (SAOD). However, the extent to which and how the Atlantic Ni09o and SAOD are related remain unclear. Here, an analysis of historical observations reveals the Atlantic Ni09o as a possible intrinsic equatorial arm of the SAOD. Specifically, the observed sporadic equatorial warming characteristic of the Atlantic Ni09o (~0.4 K) is consistently linked to southwestern cooling (~-0.4 K) of the Atlantic Ocean during the boreal summer. Heat budget calculations show that the SAOD is largely driven by the surface net heat flux anoma... [93] Richter I, Xie Shangping, Behera S K, et al. Equatorial Atlantic variability and its relation to mean state biases in CMIP5[J]. Climate Dynamics, 2014, 42(1/2):171-188.

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