版权声明: 2018 地球科学进展 编辑部
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.
大西洋经向模态(Atlantic Meridional Mode, AMM)是热带大西洋最重要的气候模态之一,呈现出南北半球海表面温度(Sea Surface Temperature, SST)异常偶极子的空间形态(图1a)[1,2,3]。而北半球的这一极子(图1a,黑色框)是热带北大西洋(0°~30°N, 60°W~20°E)春季SST经验正交分解(Empirical Nrthogonal Function, EOF)的第一模态(图1b),我们称之为热带北大西洋模态(North Tropical Atlantic Mode,NTAM)。NTAM表现出季节锁相的特征,并在春季达到最大值(图1c)。尽管如此,NTAM在其他季节仍然活跃并表现出较强的可持续性。
图1 HadISST 1870—2017年春季SST
(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。
Chang等首先利用风—蒸发—SST(Wind-Evaporation-SST, WES)正反馈机制解释了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]。
关于热带大西洋局地海气耦合过程是否为NTAM产生的主要原因,科学家们产生了分歧。一些研究结果表明, NTAM的产生可以仅依靠WES作用维持而不需要外部强迫[3,18,20]。然而另一些研究强调WES机制的发展只限于靠近赤道的热带地区 (0°~10°N),而在这以北的海域(10°~30°N)则是以大气强迫海洋为主,因此外部强迫机制是维持NTAM年际变率的关键[19,21~23]。相关研究表明3月之后,尽管SST异常仍然显著,10°N以北海域的风异常却迅速减弱并导致了NTAM的衰减。证明了此海域SST异常主要受到大气强迫引起的蒸发减弱影响,而局地的海气相互作用则影响较弱[23,24]。Amaya等利用超前滞后相关分析指出,10°N以北海洋超前大气信号不显著,进一步证明了WES机制(局地海气耦合作用)只存在于靠近赤道的热带地区。
厄尔尼诺发生时赤道中东太平洋SST异常增暖可以激发出球面Rossby波,即太平洋—北美(Pacific North American pattern,PNA)遥相关波列,将信号传到北大西洋。这会引起东北信风的异常,进而通过减弱蒸发导致SST异常增暖[30,31,32,33]。然而,PNA强迫引起的北大西洋SLP异常位于在美国东南部,在NTAM的西北方向并且距离较远。同时前文中指出WES过程只在靠近赤道的热带地区(0°~10°N)起作用,那么SLP异常是如何传到热带北大西洋的,这一问题仍有待后续研究。
Chang等发现,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,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。
近年来,随着ENSO研究的逐步深入,科学家们进一步将厄尔尼诺分为东太平洋型(Eastern Pacific, EP)厄尔尼诺和中太平洋型(Central Pacific, CP)厄尔尼诺[43,44,45,46]两大类。上文所述属于EP厄尔尼诺的影响。Amaya等指出,当CP厄尔尼诺发生时只有热带北大西洋东北部的小范围海域SST变冷,而其他海域则没有明显的变化。这一空间结构明显不同于EP厄尔尼诺所引起的热带北大西洋整体变暖。这主要是由于CP厄尔尼诺产生的PNA遥相关波列以及Gill响应的Kelvin波都不如EP厄尔尼诺强。而过去的研究表明,CP厄尔尼诺较EP厄尔尼诺表现出更明显的年代际变率[46,47]。由此猜测未来ENSO对NTAM的影响也会在不同的年代中有所差异。这一猜测尚需要在未来的研究中进行验证。
Chang等用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]。
与此同时,观测数据也表明NTAM-STAM这一对偶极子在有些年份里同时出现,而另一些年份里则没有。这引出一个科学问题:NTAM和STAM是对方的触发因子吗?如果不是,什么情况下两者会同时出现呢?Yang等利用太平洋—全球大气(Pacific Ocean-Global Atmosphere, POGA)实验分离ENSO影响。实验结果发现,ENSO发生时,在PNA、沃克环流等作用下NTAM和STAM往往成对出现;而去除ENSO影响后,NTAM无法激发出STAM。这一结果再次证实了NTAM和STAM是相互独立的。
北大西洋涛动(North Atlantic Oscillation, NAO)表现为冰岛低压和副热带高压之间跷跷板式的异常活动[54,55,56,57]。当NAO处于负位相时,副热带高压减弱,热带北大西洋出现西南风异常。这一风异常将减弱背景东北信风,减少蒸发引起的向上的潜热通量,并最终导致SST异常增暖[24,58]。NAO不仅是NTAM的触发机制,而且对NTAM的季节锁相有着至关重要的作用。Yang等研究发现,当耦合模式中NAO的发展较观测滞后1个月时,NTAM的发展和成熟期都落后于观测1个月。另外,NTAM对大气强迫不仅仅是被动响应,还可以强迫NAO[11,59]。
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)。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,两者的相关系数达到了0.64。
Table 1 Correlation coefficients between NTAM and other variability
|NAT||NAH||Atlantic Niño||AMO (年代际)|
尽管前人很多研究评估了耦合模式对于热带太平洋和热带印度洋年际变率的模拟能力[77,78,79,80],但是关于热带大西洋的研究仍然较少。前人利用政府间气候变化专门委员会(Intergovernment Panel on Climate Change, IPCC)第五次耦合模式比较计划(The fifth phase of the Coupled Model Intercomparison Project, CMIP5)研究了模式对于NTAM的模拟情况[24,81,82]。耦合模式对于NTAM的模拟能力要分为2个区域,离赤道较远的热带地区(10°~30°N)以及靠近赤道的热带地区(0°~10°N)。我们针对2个区域分别进行总结。
这些平均态误差导致模式模拟的NTAM在此区域与观测相比有较大的偏离。在半数CMIP5模式中,NTAM除了在达喀尔沿岸SST异常达到最大值之外,在赤道以北的一个狭长海域存在另外一个极大值中心(2°~8°N, 30°~15°W),我们将其称之为SBEV(Spurious Band of Enhanced SST Variance)。SBEV的产生主要与赤道东风异常有关,这与观测中由WES机制产生的西南风异常不同,反映了模式在此海域对海气耦合作用模拟存在着较大的误差。不仅如此,SBEV误差的存在也使得模式中NTAM和大西洋尼诺联系过于紧密。观测中NTAM与大西洋尼诺没有明显的相关关系。而模式中,SBEV在春季出现时伴随着赤道东风异常,会通过Bjerknes正反馈机制激发出夏季大西洋尼诺。
The authors have declared that no competing interests exist.
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Free and forced variability of the tropical Atlantic Ocean: Role of the wind-evaporation-sea surface temperature feedback[J]. ,
A dynamic ocean-atmosphere model of the tropical Atlantic decadal variability[J]. ,
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.
A hybrid coupled model study of tropical Atlantic variability[J]. ,
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 the`noise' 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.
A diagnostic study of the role of remote forcing in Tropical Atlantic variability[J]. ,
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
Causes of enhanced SST variability over the equatorial Atlantic and its relationship to the Atlantic zonal mode in CMIP5[J]. ,
WES feedback and the Atlantic Meridional Mode: Observations and CMIP5 comparisons[J]. ,
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.
The Atlantic meridional mode and its coupled variability with the Guinea Dome[J]. ,
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.
Interannual variability of the Guinea Dome and its possible link with the Atlantic Meridional Mode[J]. ,
Campagnes oceanographiques du Gerad-Treca, Cent. Oceanogr[M]. ,
A regional climate mode discovered in the North Atlantic: Dakar Niño/Niña[J]. ,
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.
Tropical Atlantic SST variability and its relation to El Niño-Southern Oscillation[J]. ,
Interannual variability of caribbean rainfall, ENSO, and the Atlantic Ocean[J]. ,
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.
Remotely forced variability in the tropical Atlantic Ocean[J]. ,
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.
Impacts of canonical and Modoki El Niño on tropical Atlantic SST[J]. ,
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.
The cause of the fragile relationship between the Pacific El Niño and the Atlantic Niño[J]. ,
Some simple solutions for heat-induced tropical circulation[J]. ,
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.
Interaction between Tropical Atlantic variability and El Niño-Southern Oscillation[J]. ,
Tropical Atlantic Ocean-Atmospheric Response to Tropical Pacific SST Variations[D]. ,
Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge[J]. ,
ENSO, Atlantic climate variability, and the walker and hadley circulations[M]∥,
Revisiting the ENSO teleconnection to the tropical North Atlantic[J]. ,
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.
The ENSO teleconnection to the Tropical Atlantic Ocean: Contributions of the remote and local SSTs to rainfall variability in the Tropical Americas[J]. ,
Why do some El Niños have no impact on tropical North Atlantic SST?[J]. ,
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.
El Niño in a changing climate[J]. ,
El Niño Modoki and its possible teleconnection[J]. ,
 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.
Two types of El Niño events: Cold Tongue El Niño and Warm Pool El Niño[J]. ,
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.
El Niño and its relationship to changing background conditions in the tropical Pacific Ocean[J]. ,
Natural variability of the central Pacific El Niño event on multi-centennial timescales[J]. ,
Variations of sea surface temperature, wind stress, and rainfall over the Tropical Atlantic and South America[J]. ,
Structure of interannual-to-decadal climate variability in the Tropical Atlantic Sector[J]. ,
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.
Characteristics of low-frequency sea surface temperature fluctuations in the Tropical Atlantic[J]. ,
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.
Variability of the tropical ocean surface temperatures at decadal-multidecadal time scales. Part I: The Atlantic Ocean[J]. ,
Decadal variability of the Tropical Atlantic Ocean surface temperature in shipboard measurements and in a Global Ocean-Atmosphere Model[J]. ,
How ubiquitous is the dipole relationship in tropical Atlantic sea surface temperatures?[J]. ,
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%).
North Atlantic climate variability: Phenomena, impacts and mechanisms[J]. ,
The North Atlantic Oscillation (NAO) and Europe Blocking and their impacts on extreme snowstorms: A review[J]. ,
North Atlantic decadal variability: Air-sea coupling, oceanic memory, and potential Northern Hemisphere resonance[J]. ,
Will global warming Suppress North Atlantic Tripole decadal variability?[J]. ,
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.
The role of Atlantic Ocean-atmosphere coupling in affecting North Atlantic oscillation variability[J]. ,
Dynamical nonlinearity in the atmospheric response to Atlantic sea surface temperature anomalies[J]. ,
Abstract Top of page Abstract 1.Introduction 2.Experiments and Results 3.Diagnoses 4.Discussion Acknowledgments References  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.
Changes of air-sea coupling in the North Atlantic over the 20th century[J]. ,
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.
Relationship between the equatorial and meridional modes of climatic variability in the tropical Atlantic[J]. ,
Abrupt equatorial wave-induced cooling of the Atlantic cold tongue in 2009[J]. ,
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.
McPhaden Michael J. Interaction between the Atlantic meridional and Niño modes[J]. ,
Multiple causes of interannual sea surface temperature variability in the equatorial Atlantic Ocean[J]. ,
Influence of northern hemisphere general circulation on drought in northeast Brazil1[J]. ,
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.
Drought in Northeast Brazil—Past, present, and future[J]. ,
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.
On the severe drought of 1993 in north-east Brazil[J]. ,
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.
A strong Atlantic Meridional Mode Event in 2009: The role of Mixed Layer dynamics[J]. ,
Sahel rainfall and worldwide sea temperatures, 1901-85[J]. ,
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.
Further case studies of Tropical Atlantic surface atmospheric and oceanic patterns associated with Sub-Saharan Drought[J]. ,
Changes in the latitude of the climatic zones of the Northern Hemisphere[J]. ,
No Abstract available for this article.
The tropical Pacific as a key pacemaker of the variable rates of global warming[J]. ,
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.
Wind-driven trends in Antarctic sea-ice drift[J]. ,
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.
Solve Antarctica's sea-ice puzzle[J]. ,
John Turner and Josefino Comiso call for a coordinated push to crack the baffling rise and fall of sea ice around Antarctica.
Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice[J]. ,
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.
A Rossby Wave Bridge from the Tropical Atlantic to West Antarctica[J]. ,
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.
Indian Ocean Dipole response to global warming in the CMIP5 multimodel Ensemble[J]. ,
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.
Realism of the Indian Ocean Dipole in CMIP5 models, and the implication for climate projections[J]. ,
CMIP5 projections of two types of El Niño and their related tropical precipitation[J]. ,
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.
Diagnosing relationships between mean state biases and El Niño shortwave feedback in CMIP5 Models[J]. ,
Causes of enhanced SST variability over the equatorial atlantic and its relationship to the Atlantic Zonal Mode in CMIP5[J]. ,
Atlantic Warm Pool Variability in the CMIP5 simulations[J]. ,
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.
Seasonal climate of the Tropical Atlantic Sector in the NCAR community climate system Model 3: Error structure and probable causes of errors[J]. ,
On the origin of equatorial Atlantic biases in coupled general circulation models[J]. ,
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.
Key factors in simulating the equatorial Atlantic zonal sea surface temperature gradient in a coupled general circulation model[J]. ,
Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation[J]. ,
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.
Multiple causes of interannual sea surface temperature variability in the equatorial Atlantic Ocean[J]. ,
Tropical Atlantic SST prediction with Coupled Ocean Atmosphere GCMs[J]. ,
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.
STOIC: A study of coupled model climatology and variability in tropical ocean regions[J]. ,
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.
Atmospheric teleconnections from the equatorial Pacific[J]. ,
Thermodynamic controls of the Atlantic Niño[J]. ,
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.
An equatorial-extratropical dipole structure of the Atlantic Niño[J]. ,
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...
Equatorial Atlantic variability and its relation to mean state biases in CMIP5[J]. ,