Intraseasonal oscillation of the southwest monsoon over Sri Lanka and evaluation of its subseasonal forecast skill

Sri Lanka,a small island country located near the southernmost end of the Indian subcontinent,is controlled by the southwest monsoon(SWM)during May to September,when it suffers the most accumulated rainfall in a year.Compared with extensive studies on the intraseasonal oscillation(ISO)of the Indian monsoon,less attention has been paid to the ISO of the SWM over Sri Lanka.Based on observational data,this study reveals that the leading mode of SWM rainfall shows a significant variability on a 10-35-day time scale,and it accounts for 66%of the fractional variance.The development of the intraseasonal rainfall anomaly is associated with a westward propagating anomalous cyclonic circulation.Furthermore,the skill of current dynamic models in simulating the SWM on the subseasonal time scale was evaluated by using the ECMWF(European center for Medium-Range Weather Forecasts)reforecast data from S2S(the Subseasonal to Seasonal Prediction project).In general,the model is more skillful in predicting the monsoonal wind index than the monsoonal rainfall index,with the skill for the former being beyond 30 days and the latter about two weeks.The forecast skills exhibit prominent interannual differences for both indices.It is suggested that a correct simulation of the large-scale circulation response to tropical convection is crucial for the subseasonal prediction of monsoonal rainfall over Sri Lanka.

THE RELATIONSHIP BETWEEN SOUTH CHINA SEA SOUTHWEST MONSOON ANOMALIES AND IMPORTANT WEATHER IN GUANGDONG PROVINCE DURING THE RAINING SEASONS

The activity of South China Sea southwest monsoon (SCSSM) has direct impacts on the anomalies of important weather in Guangdong province during the raining seasons. So it is necessary to explore thoroughly the activity pattern of SCSSM and its relationship with important weather anomalies in the province. In this paper, the methods of composite analysis and correlation statistics are used to study the relationship between the onset date and intensity of SCSSM and the important weather, such as precipitation trends in Guangdong province during the annually first and second raining seasons, the timing of the annually first and last typhoon and the number of typhoons landing in Guangdong province. The results show that the rainfall is less than normal during the first raining season, but more than normal during the second one and there are more tropical cyclones landing in Guangdong province in the years of early SCSSM onset. The rainfall is more than normal during the second raining season and there are more tropical cyclones landing in Guangdong province in the years of strong SCSSM. The relationship between the SST of April - June, July - September and previous winter (December - February) and 500 hPa geopotential height and the onset date and intensity of SCSSM is analyzed. Some mechanisms between the onset dates and intensity of SCSSM and the important weather anomalies in Guangdong province are preliminarily explored. The results can be used for reference in short-term climate forecast.

Solar insolation driven periodicities in southwest monsoon and its impact on NE Arabian Sea paleoceanography

In the northeastern(NE) Arabian Sea,the fluctuation in terrestrial and freshwater runoff directly depends on southwest monsoon(SWM) precipitation as well as the meltwater flux provided by Indus River.Therefore,analysis of multi-proxy spectral signatures was carried out to trace the high-resolution SWM periodicities and their influence on the productivity,regional sea level fluctuations and depositional processes in the NE Arabian Sea.The time series data of stable isotopes of oxygen(δ^18 O(G.ruber)).carbon(δ^13 C(G.ruber) and δ^13 C(org)) and nitrogen(δ^15 N).Total Organic Carbon(TOC),planktic-benthic foraminiferal ratio(P/B ratio) and>63 μm coarse fraction(CF) were used from two coastal sedimentary cores located offshore Saurashtra,NE Arabian Sea(Core SK-240/485 having 88 m water depth;Core GC/SK-240/496 having 174 m water depth).The REDFIT based spectral analysis recorded significant periodicities(>90% significance) in δ^18 0 time series centered at^1609,~667,~525,~296,~290 and^256 years.Further,the significant periodicities recorded in carbon isotopes time series(δ^13 C(G.ruber) and δ^13 C(org))centered at^681,~512,~471,~452,~438,~360,~292,~275,~269,~245 and^209 years.The significant periodicities in TOC include^471 and-322 years whereas δ^15 N time series recorded significant periodicity centered at-360 years.The significant periodicities in P/B ratio time series centered at^512,~388,~304,~250,~235,~217,~152,~139 and^135 years while CF recorded^268,~216,~209,~198,~188,~173 and^140 years significant periodicities.The observed periodicities in the multi-proxy record consist of similar cycles(within the radiocarbon dating error) which also natch with previously reported solar insolation influenced SWM and other global and regional cycles.Further,the stationarity of the data has been verified using wavelet analysis and shows similar periodicities as observed in REDFIT analysis.Thereafter,the depositional behaviour was studied using correlation analysis of the common periods of δ^18 0 time series of both the cores.The result suggests that the depositional behaviour was different for both the core sites during the early Holocene and became similar during the middle Holocene.The correlation analysis of Total Solar Index(TSI) with δ^18 O time series reveals a significant correlation with the core SK-240/485 whereas an insignificant correlation with the core GC/SK-240/496.These observations suggest that the solar insolation has been a leading factor responsible for the SWM trends during the Holocene which may have further influenced the productivity.regional sea level fluctuations and depositional conditions in the NE Arabian Sea.However,these trends are better preserved in shallow marine sediments as compared to the deeper marine sediments.

Evidence of Upwelling along Peninsular Malaysia during Southwest Monsoon

Upwelling off the east coast of Peninsular Malaysia (PM) was detected from recent cruise data collected during the southwest monsoon. Thermocline lifting was observed at 104?E from a number of parallel transects. To confirm the presence of upwelling, satellite remote sensing data were used, and numerical model experiments were conducted. A cooler sea-surface temperature along the coast was spotted from both in-situ and satellite data while upward movement from the model agreed with field data. The southwesterly wind that blows along PM from June to September is believed to be the important mechanism that contributed to this upwelling through an Ekman dynamics process.

Observational Study on the Onset of the South China Sea Southwest 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 elements 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 precipitation 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.

Variability of the Southwest Monsoon since the Last 25 000 Years and Their Possible Causes: Role of Northern Hemisphere versus Tibetan Plateau

High-precision, clay sediment oxygen and hydrogen isotopes analyses of Pleistocene-Holocene deep-sea sediments from the Bay of Bengal （BOB） are presented for the first time. Our study shows that the major source of sediments in the study area, since the last-25 000 years, is likely to be the Higher Himalayan crystalline rocks. Further, the study of these stable isotope data displays the variation of southwest monsoon （SWM） in the BOB region since the last-25 000 years and the cause behind the variation has been interpreted. The δ 18 O values of the clay sediments are compared with δ 18 O values of the BOB seawater. This comparison shows that the clay sediment δ 18 O values of the studied sediment cores temporally vary along with the changes in strength of the SWM. Based on the changes in the clay sediment δ 18 O values of the studied sediment, we evaluate the variance in the SWM since the last 25 000 years in the BOB. Our results are consistent with previous work in the region based on other proxies. To evaluate the factors influencing the intensity of the SWM since the last glacial maxima, we conducted comparative analyses of the studied clay sediment δ 18 O values with δ 18 O values in the Greenland ice cores （GISP2） and Tibetan ice cores （Guliya）. The results from this comparative study show that large-scale changes in the intensity of the SWM since 25 000 years are affected by the climate oscillations of the Northern Hemisphere, but rapid and abrupt fluctuations in the SWM seem to be controlled by the amount of snow cover in the Tibetan Plateau.

A STUDY OF THE TURBULENCE STRUCTURE AND FLUX TRANSFER AT SCS SURFACE DURING THE ONSET OF SOUTHWEST MONSOON

Turbulent fluxes were measured on the Xisha air-sea flux research tower in May 14-June 22, 1998 and May 7-June 17,2000.The observational period can be divided into some synoptic stages:prior to onset,onset period,monsoon break,cold air and southwesterly period.The observational results show that corresponding to the onset of SCS monsoon the air-sea fluxes indicate considerable changes.This paper presented the preliminary analysis results of fluctuation and gradient measurements of wind velocity,air temperature and moisture at the air-sea flux tower.The wind spectrum and some parameters including latent and sensible heat obtained from the eddy correlation or profile method are shown with various statistical quantities of scaling parameters such as intensities of turbulence and friction velocity u~*.Intensities of turbulence are estimated to the values of σ_u/■=0.096,■=0.066 and ■=0.045 respectively at about 10 m high during the measured period in 1998.Momentum flux is about 0.1 N/m^2.The drag coefficient C_D of momentum transfer is approximated by the statistical relationship 10~3C_D=0.003u_(10)~2+ 0.020u_(10)+0.836.Furthermore,the analyzed results of four synoptic stages are compared and the special characteristics of flux transfer during the different stages around onset of SCS monsoon are discussed.Finally,through comparison of the Xisha fluxes with those obtained from Indian Ocean and western Pacific Ocean,we can see their differences and a possible link between the moisture fluxes and rainfall in continent areas.

Southwest monsoon changes indicated by oxygen isotope of ostracode shells from sediments in Qinghai Lake since the late Glacial

The δ 18O records of ostracode shells in sediments of core QH-2000 from Qinghai Lake can be used as a better proxy to reflect monsoon changes. Low monsoon precipitation between 17.5 and 11 cal. ka BP is indicated by positive δ 18O values averaging 2.37‰. A fast shift in δ 18O from positive at 11 cal. ka BP to negative at 10 cal. ka BP indicates sharp increase of monsoon precipitation. An interval of generally high monsoon precipitation is observed between 10 and 6 cal. ka BP with δ 18O values averaging -2.15‰. Decrease of monsoon precipitation between 6 and 2.5 cal. ka BP is indicated by positive δ 18O values. δ 18O keeps positive values averaging 3.0‰ between 2.5 and 0 cal. ka BP suggesting low high monsoon precipitation. The climatic changes indicated by δ 18O records of ostracode shells in sedi- ments of core QH-2000 from Qinghai Lake and our broader regional comparison show that the climate in Qinghai Lake since the late Glacial is probably controlled by southwest monsoon other than southeast monsoon.

Environmental evolution and southwest monsoon changes in mid-Holocene recorded by lake sediments in Erhai Lake

IN recent years, much progress has been made in the research of the temporal and spatial changes of East Asian monsoon. To research the environmental evolution and monsoon variations in Holocene in the region of Southwest China, a 310-cm continuous lake sediments core in Erhai Lake, Yunnan Province was sampled with Livingston sampler on July 6, 1996. The lake sediments were separated at 2-cm intervals and the core samples were analyzed with chronology, stable isotope ratios of organic carbon and diatoms determination. Three 14C ages are （4 473±40）a B.P. （98—102 cm）, （5 825±85） a B. P. （144—148 cm） and （7 754±45） a B.P. （198—202 cm）. By 137Cs dating, the sedimentation rate is about 0.3 mm/a and this result is close to the 14C dating results. The average sedimentation rates

Relationships among the Monsoon-like Southwest Australian Circulation,the Southern Annular Mode, and Winter Rainfall over Southwest Western Australia

This study examines the relationships among the monsoon-like southwest Australian circulation （SWAC）, the South- ern Annular Mode （SAM）, and southwest Western Australia winter rainfall （SWR）, based on observed rainfall, reanalysis datasets, and the results of numerical modeling. By decomposing the SWAC into two components using a linear model, i.e. the component related to SAM （RSAM） and the component unrelated to SAM （SWACI＊）, we find it is the SWACI＊ that shows a significant influence on SWR. Similarly, it is the component of SAM associated with SWAC that exhibits an impact on SWR, whereas the component unrelated to SAM. A similar result is obtained in terms of the circulation associated with SWAC and the SAM. These facts suggest the SAM plays an indirect role in influencing SWR, and raise the possibility that SWAC acts as a bridge between the SAM and SWR, by which the SAM passes its influences onto SWR. This is due to the fact that the variations of SWAC are closely linked to the thermal contrast between land and sea across the southern Indian Ocean and southwest Australia. By contrast, the SAM does not significantly relate to this thermal structure, particularly for the component unrelated to SWAC. The variations of surface sea temperature over the southern Indian Ocean contribute to the favored rainfall circulation patterns. This finding is supported by the numerical modeling results. The strong coupling between SWAC and SWR may be instrumental for understanding the interactions between SWR and the southern Indian Ocean, and provides another perspective in examining the variations in SWR.

季风降雨前后深圳地区地表通量差异分析

为进一步认识夏季西南季风对深圳地区地表通量的影响,采用深圳气象梯度塔的辐射和涡动协方差系统观测资料,研究该地区季风强降水过程中地表通量的变化特征。结果显示:在季风降水期间和降水后,深圳地区近地面太阳辐射明显减弱,同时反照率减小。在降水期间,深圳地区地表感热和潜热通量均明显低于降水前和降水后,而且由碳汇转为碳源。

青藏高原东南缘攀枝花市降水特征及其成因初探

为更好地认识青藏高原东南缘地形复杂区降水特征及其成因,利用位于青藏高原东南缘的攀枝花市2015—2020年72个国家、区域气象观测站资料和欧洲中心0.25°×0.25°分辨率的ERA5再分析资料,对攀枝花降水特征及成因进行分析。结果表明:(1)攀枝花降水具有地形作用突出、北多南少的特点,降水日数是造成降水空间分布差异的主要原因之一。(2)攀枝花夜雨特征显著,呈单峰型,降水峰值出现在03时(北京时),因攀枝花位于干热河谷区,日间湿度小、夜间湿度大,夜间较饱和的大气更容易凝结,触发降水,湿度的日变化是攀枝花易发生夜雨的原因之一。(3)攀枝花干湿季分明,6—10月为攀枝花湿季,11月至次年5月为攀枝花干季,6月和10月是干湿转换的过渡期。6月孟加拉湾西南季风爆发,攀枝花雨季开始,干季逐渐结束;10月干燥的高原南支西风气流加强,雨季趋于结束,干季开始。

IMPACT OF LATENT HEAT FLUX ON INDIAN SUMMER MONSOON DURING EL NIO/LA NIA YEARS

El Nio or La Nia manifest in December over the Pacific and will serve as an index for the forecasting of subsequent Indian summer monsoon,which occurs from June to mid-September.In the present article,an attempt is made to study the variation of latent heat flux (LHF) over the north Indian Ocean during strong El Nio and strong La Nia and relate it with Indian monsoon rainfall.During strong El Nio the LHF intensity is higher and associated with higher wind speed and lower cloud amount.During El Nio all India rainfall is having an inverse relation with LHF.Seasonal rainfall is higher in YY+1 (subsequent year) than YY (year of occurrence).However there is a lag in rainfall during El Nio YY+1 from June to July when compared with the monthly rainfall.

降水微物理过程的C波段垂直指向雷达反演研究

降水微物理过程是对空中雨滴的蒸发、碰并和凝结等演化过程的最直接描述。对降水过程中微物理特征的分析,有利于提高对云体的认识,为降水估计与数值模式参数化提供技术支撑。研究依托中国气象科学研究院灾害天气国家重点实验室在广东龙门建立的超级观测站,基于C波段调频连续波雷达高时空分辨率的多普勒功率谱数据。分析了2020年6月6-9日(前汛期),受西南季风影响,形成的一次持续时间长、累积雨量大、降水分布不均的降水过程中的几种典型对流单体的空中微物理特征。

重庆金佛洞石笋记录的410 ka弱季风事件

冰消期或冰期由于冰盖消融引起淡水排放,容易造成不同纬度之间海洋-大气传输的异常,由此引发一系列或明显或不明显的千年级气候突变事件,如Younger Dryas(YD,新仙女木)和类似YD事件。MIS11c(深海氧同位素11阶段)作为当前全新世的最佳参照物之一,对期间可能发生的类YD事件及触发机制的研究有助于认识极端气候事件的发生规律。文章通过对重庆金佛洞石笋J33δ^(18)O序列记录的研究结果显示:(1)在MIS11间冰期盛期之前,亚洲季风气候区石笋揭示了一次发生于410 ka BP左右的千年尺度弱季风事件;(2)410 ka弱季风事件与YD事件均发生于间冰期盛期之前季风逐渐增强过程中以及北半球夏季太阳辐射上升阶段,期间都发生了AMOC(大西洋经向翻转环流)扰动,除了在变化幅度、冰量条件等方面有些差异,事件的持续时间、内部结构、变化模式相似;(3)410 ka弱季风事件主要受太阳辐射和AMOC共同驱动主导,持续较强的变暖进程加速了格陵兰冰盖融化并导致了冰盖的不稳定,淡水持续注入北大西洋,造成短暂的AMOC振荡。AMOC的减弱使得北大西洋上空产生了冷异常,通过大气遥相关作用导致了较弱的ASM(亚洲夏季风)。

孟加拉湾南部冷池的极端事件及其机理初析

本研究利用多源卫星遥感数据对比分析了2018年和2015年孟加拉湾南部冷池的极端异常事件。孟加拉湾南部冷池早期出现在5月,8月和9月强度达到最大,10月开始减弱,随后逐渐消失。数据显示2018年和2015年冷池海表温度距平值分别为-0.55℃和0.43℃,是1993—2018年26年中冷池降温最强和最弱的年份。2018年孟加拉湾南部冷池风应力距平值为0.02 N/m2,而2015年距平值为-0.01 N/m2。2018年Ekman抽吸速度距平在冷池区域为正,2015年为负。经混合层热收支计算得到,造成冷池产生的主要原因是平流效应,且2018年与2015年的夏季(6—8月)降温平流项分别占49.2%和80.7%。冷池极端事件主要发生在6月,2018年6月冷池降温是2015年的2.76倍,并且海表净热通量项和平流项的异常对冷池极端事件的产生起了重要作用。

Monsoonal impact on circulation pathways in the Indian Ocean

Monsoon driven water mass exchange between the Bay of Bengal(Bo B) and Arabian Sea(AS) is the common experience. However, it is not yet firmly confirmed that the exchange pathway is either passing through southern tip of Sri Lanka or Palk Strait. Local circulation patterns impact the pathways followed by the East Indian Coastal Currents(EICC) that drive exchange, thereby modulating mixing and water mass transformation in the Bay of Bengal around Sri Lanka. In this study, observations from surface drifters were incorporated with the satellite derived data to understand the monsoonal impact on circulation patterns in the Indian Ocean. This was the first multi-national scientific effort which was conducted in the Bo B and AS during 2013 to 2015 to understand the monsoonal impact on circulation patterns in the complex region. The results indicated that seasonally reversing monsoonal currents of southern Sri Lanka, traced by the wintertime freshwater export pathways of the EICC. The deflection of monsoon currents running along the east coast of Sri Lanka by forming cyclonic and anti-cyclonic eddies, which influence the mixing and stirring associated with these flows. Results further indicate the low salinity cold water flows from the Bo B to AS along the western boundary of the Bo B during northeast monsoon. In the same way, reverses the phenomena during southwest monsoon, transporting high salinity warm water from AS to the Bo B. This maintain the bay status which occurred due to freshwater influx from large rivers and high saline water from AS. However, no evidences were observed for the exchange through Palk Strait during the study.Also, there are some mis-matches in in-situ and remotely sensed measurements which imply the necessity of systematic observation system for the complex region as an alternative approach.

贵州黔西县水西洞石笋记录的末次冰期Heinrich Stadial 4气候突变事件

末次冰期Heinrich Stadial 4气候突变事件(HS4事件)是发生于约40 ka B.P.(B.P.表示Before Present, Present为公元1950年)最为显著的一次海因里希冰阶事件,对其转型特征和精细结构的刻画有助于深入理解千年尺度气候突变事件的机制。本研究基于贵州黔西县水西洞SXG-3石笋的11个高精度230Th年龄和277个δ^(18)O数据,重建了40.77~37.17 ka B.P.时段平均分辨率为13 a的亚洲夏季风强度演变序列。该石笋氧同位素记录清晰地捕捉到了HS4弱季风事件,呈现出三阶段变化的特征,即:第1阶段(39.97~39.13 ka B.P.),石笋δ^(18)O在840±90 a内偏正1.32‰,夏季风缓慢减弱,对应于热带辐合带(Intertropical Convergence Zone,简称ITCZ)的南移和格陵兰气候快速变冷;第2阶段(39.13~38.35 ka B.P.),石笋δ^(18)O整体偏正,平均为-8.34‰,夏季风强度达到最弱,而南美季风达到最强,对应于ITCZ移动至最南端;第3阶段(38.35~37.59 ka B.P.),石笋δ^(18)O在760±89 a的时间内偏负至-9.25‰,对应于ITCZ的向北移动和格陵兰气候快速变暖。水西洞石笋记录的HS4事件三阶段变化特征与福建仙云洞记录十分相似,对应于NEEM(Northern Greenland Eemian Ice Drilling)冰心^(17)O-excess所反映的低纬水文循环过程变化,同时与南美Toca da Boa Vista(TBV)和Toca da Barrigude(TBR)洞穴石笋记录呈“镜像关系”。分析结果表明,在北半球高纬气候触发后,热带海洋和南半球热量的不断积聚及其随后的释放所引起的ITCZ的南北移动是造成这种三阶段变化的主要原因。

斯里兰卡穹顶的演变过程及其能量学特征

文章基于混合坐标海洋模式(hybrid coordinate ocean model,HYCOM)等多套再分析资料,研究了气候态斯里兰卡穹顶(Sri Lanka Dome,SLD)的演变过程及其能量学特征。研究显示,SLD有两次从发展、成熟至减弱的过程,相伴随的是其涡动能(eddy kinetic energy,EKE)也出现两次峰值。在第一次发展阶段(5月23日—6月10日),SLD在斯里兰卡的东南部开始发展,并逐渐移向东部,伴随着面积和强度逐渐增大。在此过程中,风应力持续输入EKE,海洋不稳定过程使得平均流能量转化为EKE和涡势能(eddy available potential energy,EPE),以及西南季风流(southwest monsoon current,SMC)的平流作用,均使得SLD迅速加强。在成熟阶段(6月11—22日),SLD位于斯里兰卡东部,风应力做功和涡流相互作用的增强使得SLD区域内的EKE和EPE达到第一个峰值。在减弱阶段(6月23日—7月20日),SLD向西北移动,由于平流项引起EKE和EPE的耗散,加上风应力做功和斜压不稳定显著减小,使得SLD区域的EKE和EPE衰减,强度显著减小。而在稳定阶段(7月21日—8月14日),SLD移至斯里兰卡东北部,风应力做功,压强做功和涡流相互作用较弱,使得SLD的强度始终维持一个较弱的水平。在第二次发展阶段(8月15—25日),SLD北移,伴随着风应力做功和压强做功的增强,其强度增大。在消亡阶段(8月26日—9月5日),海洋不稳定过程使得EKE和EPE转化为平均流能量,导致SLD逐渐消亡。因此,风应力做功、涡流相互作用、压强做功以及源自SMC的平流作用是控制SLD演变的主要因子。

重庆金佛洞石笋δ13C记录的Heinrich6期间气候环境变化

在末次冰期发生的6次海因里希事件(Heinrich事件,简称H事件)中,H6事件发生年代最早,对其研究较少,利用高分辨率石笋记录研究H6事件期间的气候环境变化,有助于理解高低纬度气候变化对H事件的响应过程。本文基于重庆市金佛洞石笋JF2017铀系测年数据和碳同位素数据,重建H6事件期间中国西南地区季风气候环境的演化过程。结果显示:石笋JF2017的δ13C值在61811±204 a B.P.~59716±159 a B.P.时段显著偏重,持续大约2095 a,暗示该时段季风减弱和生态植被退化,对应北大西洋H6事件。H6事件期间,石笋JF2017的δ13C记录在61811~60848 a B.P.期间开始先逐渐偏重至最大值,随后发生短时间尺度的波动变化;最后在60848~59716 a B.P.期间缓慢偏轻至H6事件结束,整体呈现先逐渐偏重后又缓慢偏轻的趋势,内部存在百年际尺度的气候震荡,表明H6事件期间当地气候不稳定。石笋JF2017记录的百年际尺度的季风气候变化可能与热带辐合带和北大西洋经向翻转环流密切相关。