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A Review of the South China Sea Summer Monsoon Onset
Shao Xie1,2, Huang Ping1, Huang Ronghui1
1.Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
University of Chinese Academy of Sciences, Beijing 100049, China
The onset of the South China Sea Summer Monsoon (SCSSM) indicating the arrival of the rainy season of Eastern China is a key factor for the short-term climate prediction. The advances in the research about the characteristics, interannual variation of SCSSM onset, and the influence of external forces and internal processes on the SCSSM onset are reviewed in this paper, and the impacts of Intraseasonal Oscillation (ISO) on the SCSSM onset are also summarized. Some issues, such as the monsoon wave interaction, SCSSM change under global warming, the predictability of the SCSSM onset, etc., are introduced for further study in future.
South China Sea Summer Monsoon (SCSSM); Onset; Interannual variation; Intraseasonal oscillation (ISO).
Composite of the 30~60-day filtered OLR (Unit:W/m2) of the onset-ISOs
The blue, red and green thin circles denote the monsoon onset phases and the thick circles denote the averaged onset phases for the early-onset years, the late-onset years and the normal-onset years respectively. The shadings indicate the standard deviations of the 30-60-day filtered OLR 
Composite of the 30~60-day filtered OLR (Unit:W/m2) of the onset-ISOsThe blue, red and green thin circles denote the monsoon onset phases and the thick circles denote the averaged onset phases for the early-onset years, the late-onset years and the normal-onset years respectively. The shadings indicate the standard deviations of the 30-60-day filtered OLR 
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Nanjing Institute of Meteorology, Nanjing,Nanjing Institute of Meteorology, Nanjing,Nanjing Institute of Meteorology, Nanjing
Primarily based on the 1979 FGGE data an analysis is made of the circulation differences between the East-Asian and Indian summer monsoons together with their oscillation features and also the interplay be-tween various monsoon systems originating from the fact that the Asian monsoon area is divided into the East-Asian and Indian regions, of which the former is demarcated into the Nanhai (the South China Sea) and the Mainland subregions.
Using the NECP data from 1983 to 1992, the spatial and the temporal variation characteristics of main models of the summer monsoon onset process in the South China Sea(SCS) have been studied by the method of EOF expansion. It hasbeen found that the northward shift of solar radiation is the most important factor of the monsoon onset. There is very evident adjustment of wind field at 850hPa associated with the Asian summer monsoon onset in typical monsoon area. Amplification of westerlies trough and ridge in Southern Hemisphere mid-high latitude and augmentation of subtropical system meridional circulation in Northern Hemisphere are significant role of the Asian summer monsoon onset. The western Pacific Ocean high pressure abated rapidly and retreated east about 5 days before themonsoon erupts, which caused the southwest monsoon onset.
By using the NCEP/NCAR reanalysis data of 1958—1997 and choosing the onset date of the South China sea as a critical day, we do some composite analysis to illustrate the evolvement of the atmospheric circulation around the onset of the monsoon.The result shows that the climatic average circulations of the atmosphere before and after the onset of the monsoon differs a lot, which is a global-scale phenomenon. Through observing the evolvement of the perturbation field of atmospheric circulation, we find that the variety of the perturbation field is not only global-scale but also quite abrupt. Besides, this paper concerns the source of the southwest monsoon in the South China sea, the function of three branches of cross-equatorial airflow as well as the change of the circulation situation in the Southern Hemisphere and its influence on the onset of the South China sea monsoon.
Based on the long-term marine ship observation data, records of meteorological stations and High-Reflective Cloud (HRC) data by satellite remote sensing, this paper has studied the circulation patterns and variability in ele-ments during onset and the established periods of the South China Sea (SCS) southwest (SW) monsoon, The averaged date of the onset SW monsoon in the SCS occurs in the middle of May climatologically. The corresponding date for the northern part is little earlier (May 12) and those for the southern parts are little later (May 20), The interannual range of the onset dates is about one month. Following the onset of the SW monsoon, the cloud amount and the pre-cipitation increase while the convection activities enhance over the SCS. But there is a strong spatial heterogeneity within the domain. After onset of the SW monsoon the strong convective area moves northwards, while the SCS rain band moves to the center and north. Sea surface temperature (SST) increases rapidly before the onset and the leading time is about one month. The increment of SST supplies heat and vapor for the onset. From April to May the surface heat fluxes display obvious changes, e.g., latent heat exchange and evaporation enhancement. It is one of the reasons why the SW monsoon bursts firstly in the SCS.
According to the definition criterion of this paper,the mean onset time of the South China Sea Summer Monsoon(SCSSM) from 1979 to 1994 is the Fourth pentad of MAY. The composite analyses of 16 years of OLR,wind field and θ se field show that the onset of SCSSM is caused by tthe SCS Western Pacific System,In the lower troposphere,the weakening and retreating eastward of the Subtropical High in SCS region is the outstanding feature of SCS circulation system around the onset time of SCSSM.At the same time,the moving northwestward of the SCS High is the characteristics of SCS circulation systems in the upper troposphere.The evolution of potential instability and tropical deep convection develops earlier over the continent and island areas than over the sea,and the developing of potential instability is earlier than that of deep convection.The strengthening and the stretching eastward of 70-90° E equatorial westerly at the upper reaches of SCS region has direct effect on the onset of SCSSM.Meanwhile the enhancement of Somali cross equatorial flow and the retreating northward of Arabian High affect the onset of SCSSM indirectly through producing effect on the tropical westerly.
首先以南海区域平均的候平均OLR值下降至235W/m 2 ,同时纬向风由东转成西为定义标准确定南海季风从1979到1994年间爆发的平均日期是5月第4候,然后对16年(1979-1994年)的OLR场、风场及θ se 场的时段叠加合成图进行了分析,认为南海夏季风的建立是南海-西太平洋系统所造成。南海夏季风爆发前后,南海地区环流系统最显着的特征是低层西太平洋副高脊减弱东撤;高层南亚高压从10°N的菲律宾北跳到15°N以北的中南半岛;位势不稳定和热带对流的演变呈现海洋上的爆发晚于大陆和岛屿地区的特征,只是位势不稳定的变化超前深对流的发展。而爆发前后,周围地区对南海的最直接影响来自南海上游70-90°E赤道西风的加强和北抬东伸,同时索马里越赤道气流的加强和阿拉伯海高压的北撤,通过影响赤道西风也间接影响到南海。
Employing OLR and TBB data,we presented an index and decided the date of summer monsoon onset over South China Sea(SCS) during the period from 1975-1993, which is consistent with the date ofrecurvature of wind.On the basis of this,we discussed the pro cesses of summer monsoon over SCS.Furthermore,the relationship between summer monsoo nonsetover SCS and sea temperature anomaly,thermal regime over Tibetan Plateau as wellas variation of temperature difference between land and sea was discussed.we found it relates to SSTA in SCS,equatorial east Pacific and region between 30-40°N in Apriland it relates to changing of temperature difference land and sea from status of cool land and warm sea into status of warm land and cool sea in winter.
Spatio-temporal variations of the sea temperature anomaly in the South China Sea (SCS) were investigated based on the reanalysis dataset of China seas from 1986 to 2003. With focus on the evolution of the sea surface temperature anomaly(SSTA) before and after (during April to Jun) the onset of the SCS summer monsoon (SCSSM), its relation with the occasion of the SCSSM onsets was discussed. The results show that, there exists a significant evolution mode on seasonal scale, in which the positive SSTA develops firstly near the sea area west of Palawan in April, and extends rapidly westward and northward in May when the positive SSTA controls the most part of the SCS, and then the SSTA in the SCS achieves a negative-positive phase transition in June. The analysis indicates that, the interannual difference of the SSTA evolution mode was associated significantly with the earlier or later onset of the SCSSM. Synthesizing with the previous studies, it is considered that the anomalous thermal evolution (including trends,speed,magnitude) on intra-seasonal scale in the sea surface of the SCS could be one of important factors to impact earlier and later onset of the SCSSM. According to this, the author established a temperature difference index based on the SSTA evolution in the SCS, which reflects well to the earlier and later onset of the SCSSM. In addition, the evolution of the SSTA can be related closely with the variation of the SCS warm pool (SCSWP). It is found that the SST difference (April-May) anomaly of the SCSWP during the period before the onset of the SCSSM was striking of positive correlation with that of the Indian Ocean warm pool (IOWP), while evident negative relation with that of the Western Pacific warm pool (WPWP). Under the background of formation of the surface thermal pattern and contrast in Indio-Pacific warm pool regions, the anomalous intra-seasonal evolution of the SSTA in the SCS is expected to give rise to the interannual difference of the SCSSM onset (earlier or later) through the thermal effect on the large scale of meridional and zonal atmospheric current anomalies.
The seasonal and inter-annual variability of the South China Sea warm pool and its relations to the South China Sea monsoon onset were analyzed using Levitus and NCEP/NCAR OISST data. The results show that, there are obvious seasonal variability of the South China Sea warm pool, very weak in winter, rapid development in spring, strong and large in summer and earlier autumn, quick decay from mid-autumn. the thickness of South China Sea warm pool reaches 55 m in its strong period, its main axis is from southwest to northeast and locates alone the steep slope area of west off shore of North Kalimantan Palawan island; the South China Sea, Western Pacific and Indian Ocean warm pools have about 5 years period oscillation which is the same as ENSO cycle, the South China Sea and Indian Ocean warm pools have about 2.5 years period oscillation and similar inter-annual variation, and the South China Sea warm pool has best correlation with 5 months lag to the Western Pacific warm pool. The results also show that there are close relations of the South China Sea warm pool strength index to the South China Sea monsoon onset, when the South China Sea warm pool is continuously stronger (weaker) than normal in previous winter and spring, and the South China Sea monsoon will begin later (earlier) than the normal date, and this relation is connected with the activity of the South China Sea High in early summer.
There are two major sea surface temperature anomaly (SSTA) patterns over the tropical Indian Ocean (TIO): basin-wide SSTA variation which peaks in the boreal spring, and the dipole mode that is phase-locked in the boreal autumn. This paper explores possible impacts of these two SSTA patterns on the South China Sea (SCS) summer monsoon onset. Using composite analyses, it is revealed that basin-wide warming (cooling) in the TIO apparently induces an anomalous reversed (intensified) Walker circulation over the tropical Indo-Pacific region, leading to anomalous descending (ascending) motion, and hence suppressed (increased) convection over the western Pacific. The intensified (weakened) western Pacific anticyclone in April and May prevents (favors) the extension of the Indian Ocean westerly flow into the SCS region, thereby causing a late (an early) SCS summer monsoon onset. The results suggest that the basin-wide SSTA mode in the Indian Ocean should play an important role in prolonging the El Niño-Southern Oscillation (ENSO) effects on the subsequent Asian summer monsoon, mainly through modifying the strength of the western Pacific anticyclone. Further, impacts of the Indian Ocean dipole (IOD) case on the SCS summer monsoon onset in 1994 are carried out. Coupled to the exceptionally strong positive IOD over the tropical Indian Ocean in early May, the Asian summer monsoon circulation around the Indian Ocean is significantly weakened, especially the Somali jet along the east coast of Africa and the low-level westerlies across the equatorial Indian Ocean. Anomalous anticyclones dominate the 850-hPa wind field in the tropical Indian Ocean, accompanied by easterly anomalies along the equatorial Indian Ocean. These features, induced apparently by the positive IOD event, are unfavorable for the establishment and advancement of the Asian monsoon westerlies, and lead to a delay of the SCS summer monsoon onset in 1994.
LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijng 100080,Center for Climate System Research, University of Tokyo, Tokyo 153, Japan,Center for Climate System Research, University of Tokyo, Tokyo 153, Japan
The impact of El Ni?o on the precipitation in China for different seasons are investigated diagnostically. It is found that El Ni?o can influence the precipitation in China significantly during its mature phase. In the Northern winter, spring and autumn, the positive precipitation anomalies are found in the southern part of China during the El Ni?o mature phase. In the Northern summer, the patterns of the precipitation anomalies in the El Ni?o mature phase are different from those in the other seasons. The negative precipitation anomalies appear in both southern and northern parts of China, while in between around the lower reaches of the Yangtze River and the Huaihe River valleys the precipitation anomalies tend to be positive. In the Northern winter, spring and autumn, the physical process by which El Ni?o affects the precipitation in the southern part of China can be explained by the features of the circulation anomalies over East Asia during the El Ni?o mature phase (Zhang el al., 1996). The appearance of an anticyclonic anomaly to the north of the maritime continent in the lower troposphere during the El Ni?o mature phase intensifies the subtropical high in the western Pacific and makes it shift westward. The associated southwesterly flow is responsible for the positive precipitation anomalies in the southern part of China, In the Northern summer, the intensified western Pacific subtropical high covers the southeastern periphery of China so that the precipitation there becomes less. In ad?dition, the weakening of the Indian monsoon provides less moisture inflow to the northern part of China.
Center for Monsoon System Research, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100190;Center for Monsoon System Research, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100190;Center for Monsoon System Research, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100190;Center for Monsoon System Research, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100190
Recent advances in the study of the characteristics, processes, and causes of spatio-temporal variabilities of the East Asian monsoon (EAM) system are reviewed in this paper. The understanding of the EAM system has improved in many aspects: the basic characteristics of horizontal and vertical structures, the annual cycle of the East Asian summer monsoon (EASM) system and the East Asian winter monsoon (EAWM) system, the characteristics of the spatio-temporal variabilities of the EASM system and the EAWM system, and especially the multiple modes of the EAM system and their spatio-temporal variabilities. Some new results have also been achieved in understanding the atmosphere--ocean interaction and atmosphere--land interaction processes that affect the variability of the EAM system. Based on recent studies, the EAM system can be seen as more than a circulation system, it can be viewed as an atmosphere--ocean--land coupled system, namely, the EAM climate system. In addition, further progress has been made in diagnosing the internal physical mechanisms of EAM climate system variability, especially regarding the characteristics and properties of the East Asia-Pacific (EAP) teleconnection over East Asia and the North Pacific, the Silk Road teleconnection along the westerly jet stream in the upper troposphere over the Asian continent, and the dynamical effects of quasi-stationary planetary wave activity on EAM system variability. At the end of the paper, some scientific problems regarding understanding the EAM system variability are proposed for further study.
Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100080;Graduate School of the Chinese Academy of Sciences, Beijing 100039,Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100080
On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS's meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-JulyAugust, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.
Center for Monsoon System Research, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100080,Center for Monsoon System Research, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100080,Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100080
Recent advances in studies of the structural characteristics and temporal-spatial variations of the East Asian monsoon (EAM) system and the impact of this system on severe climate disasters in China are reviewed. Previous studies have improved our understanding of the basic characteristics of horizontal and vertical structures and the annual cycle of the EAM system and the water vapor transports in the EAM region. Many studies have shown that the EAM system is a relatively independent subsystem of the Asian-Australian monsoon system, and that there exists an obvious quasi-biennial oscillation with a meridional tripole pattern distribution in the interannual variations of the EAM system. Further analyses of the basic physical processes, both internal and external, that influence the variability of the EAM system indicate that the EAM system may be viewed as an atmosphere-ocean-land coupled system, referred to the EAM climate system in this paper. Further, the paper discusses how the interaction and relationships among various components of this system can be described through the East Asia Pacific (EAP) teleconnection pattern and the teleconnection pattern of meridional upper-tropospheric wind anomalies along the westerly jet over East Asia. Such reasoning suggests that the occurrence of severe floods in the Yangtze and Huaihe River valleys and prolonged droughts in North China are linked, respectively, to the background interannual and interdecadal variability of the EAM climate system. Besides, outstanding scientific issues related to the EAM system and its impact on climate disasters in China are also discussed.
Interdecadal variation of the leading modes of summertime precipitation anomalies in the monsoon regions of eastern China and its association with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia are analyzed by using the daily data of the ERA-40 reanalysis and precipitation data at 516 observational stations of China for 1958－2000 and the EOF analysis method. The analysis results show that there are two leading modes in the spatio-temporal variations of summertime precipitation anomalies over the monsoon region of eastern China: The first leading mode exhibits not only a characteristic of obvious interannual variation with a quasi-biennial oscillation, but also a feature of interdecadal variability, and its spatial distribution is of a meridional tripole pattern. And the second leading mode exhibits a characteristic of obvious interdecadal variability, and its spatial distribution is of a meridional dipole pattern. This shows that these two leading modes have a significant interdecadal variability. During the period of 1958－1977, the distribution of summertime precipitation anomalies in eastern China exhibited a “＋－＋” meridional tripole pattern from the south to the north, and the distribution of precipitation anomalies for 1978－1992 showed a “－＋－” meridional tripole pattern in the region, which was opposite to that for 1958－1977, but during the period of 1993－1998, since the role of the second leading mode in summertime precipitation anomalies in eastern China was intensified, the distribution of summertime precipitation in this region showed a combination of “＋－＋” meridional tripole pattern and “＋－” meridional pattern, which caused the increase of summertime precipitation in South China. Moreover, the analysis results also show that the interdecadal variation of these two leading modes is closely associated with the spatio-temporal variations of summertime water vapor transport fluxes over East Asia, which is associated not only with the interdecadal variation of the EAP pattern teleconnection-like wave-train distribution of summertime water vapor transport flux anomalies over East Asia and the western North Pacific, but also with the interdecadal variation of the EU pattern teleconnection-like wave-train distribution of summertime waver vapor transport flux anomalies in the westerly zone over middle and high latitudes of Eurasia.
Institute of Atmospheric Physics, Academia Sinica, Beijing
The analyzed wind field during MONEX shows that the monsoon troughs (or ridges) over South Asia have 30—50 day oscillation and slowly propagatu northward. Simultaneously, the tropical cyclones in South Asia could disperse energy to the east. Considering the feedback effect of the cumulus convection and the influence of the shearing basic flow, we introduce the vertical shearing basic flow into the conditional instability of the second kind (CISK) model and obtain a moving CISK mode. Under the Summer Monsoon in South Asia, the meridional propagation speed (northward) is about 0.6 latitude per day and the oscillation period is about 34 days for this CISK mode. And it has an easterly group velocity. The above-mentioned characteristics of this mode are similar to the analyzed results of the data during MONEX. Therefore, it can be initially concluded that the CISK mode caused by the shearing basic flow and the feedback effect of the cumulus convection seems to be an important mechanism driving the monsoon troughs (ridges) and the tropical cyclones over South Asia in summer.
The interaction between tropical intraseasonal oscillation(ISO) and El Nino has been paid more attentions.But El Nino events just occur after an interval of several years,which is different from that tropical intraseasonal oscillations always exist in the atmosphere.Therefore,it is important to know how tropical intraseasonal oscillation excite the El Nino event.The data analyses show that not only the kinetic energy of tropical ISO decreases abruptly in association with the occurrence of El Nino,but also that the interannual anomalous variation of tropical ISO intensity is closely related to El Nino event.These mean that the tropical intraseasonal oscillation plays an important role in exciting El Nino through its interannual anomalous variation.A simple air-sea coupled model study shows that the periodical self-excited oscillation can be produced in air sea coupled system,but the pattern is different from ENSO mode.When there is external (atmospheric)forcing with interannual time scale,a coupled mode which likes ENSO mode will be excited in the air-sea coupled system.Therefore,the dynamical analysis has consistent result with the data nanlyses:The interannual anomalies of intraseasonal oscillation in the tropical atmosphere play an important role in exciting El Nino event.
1.City University of Hong Kong Guy Carpenter Asia-Pacific Climate Impact Centre Tat Chee Ave., Kowloon Hong Kong China
Previous studies have suggested that the South China Sea (SCS) summer monsoon onset is concurrent with the arrival of a 30–60-day northward-propagating trough. On the other hand, from a synoptic viewpoint, some studies pointed out that the arrival of a mid-latitude front may be the triggering mechanism of the SCSSM onset. This study attempts to link these two viewpoints and to investigate their relative role in inducing the SCSSM onset. Composites of low-level zonal winds, geopotential heights and temperatures during the 1991–1999 SCSSM onsets based on the European Centre for Medium Range Weather Forecast ERA-40 data indicate that both the Madden and Julian Oscillation (MJO)/Kelvin waves and mid-latitude trough are apparently involved in the onset. The MJO/Kelvin waves play a major role in inducing the large-scale easterly-westerly shift over the central SCS, while the effect of the acceleration of westerlies ahead of the mid-latitude trough is limited to the northern SCS only. Numerical experiments using a regional climate model further demonstrate that the MJO/Kelvin waves control the timing of the onset by changing the background meridional geopotential height gradient over the SCS. When the MJO is at its peak phase over the Maritime continent, it imposes a positive meridional geopotential height gradient over the SCS such that easterly winds are induced, which significantly reduces the strength of a mid-latitude trough. After the equatorial convection has dissipated, a Rossby-wave response is induced, leading to the formation of a northward-moving trough. When this trough moves northward, the meridional geopotential height gradient is reversed and westerly winds are induced. At the same time, if a mid-latitude trough arrives in south China, the westerlies associated with the mid-latitude trough will strengthen because of the background meridional geopotential height gradient, which gives the impression that both the northward-moving trough and mid-latitude trough are in phase and work together to induce the onset.
Based on the long time series of mean Sea Surface Temperature (SST) and high-resolution wind field reanalysis data such as HadISST and ERA-40 reanalysis data, the variations of the SST in the offshore area of China and their relationship with the East Asian Monsoon (EAM) in winter (December to the next February) and summer (June to August) are analyzed using the Empirical Orthogonal Function (EOF) and linear regression analysis methods. The results show that: 1) The SST in the offshore area of China in winter or summer exhibited significant interannual and interdecadal variations, and experienced a climate shift in the mid-1980s. The areas with the strongest increase in SST are located in the East China Sea (ECS) in winter and in the Yellow Sea in summer. The SST increased by 1.96°C in winter for the period of 1955-2005 and 1.10 °C in summer for the period of 1971-2006. 2）The EAM has displayed distinct interannual and interdecadal variations with a weakening trend since the end of the 1980s in winter, and since the end of the 1970s in summer. In addition, the linear regression analysis indicates the relationship of the SST to EAM in winter on interdecadal timescale is closer than that on interannual timescale. The interdecadal weakening trend of EAM in winter contributes to the rise in SST in the offshore areas of China, particularly significant in the ECS. Moreover, the related areas of winter or summer mean SST on the interannual timescale in the offshore area of China to the EAM are located in the South China Sea (SCS), and the relationship in winter is much more obvious than that in summer. It is found that the interannual variation of SST in the SCS has obvious relation to the anomalies of the meridional southward and northward winds over the SCS and zonal migration of the subtropical anticyclone over the western Pacific.