Summer temperature in the eastern part of southern South America: its variability in the twentieth century and a teleconnection with Oceania

The 1907–2001 summer-to-summer surface air temperature variability in the eastern part of southern South America (SSA, partly including Patagonia) is analysed. Based on records from instruments located next to the Atlantic Ocean (36°S–55°S), we define indices for the interannual and interdecadal timescales. The main interdecadal mode reflects the late-1970s cold-to-warm climate shift in the region and a warm-to-cold transition during early 1930s. Although it has been in phase with the Pacific Decadal Oscillation (PDO) index since the 1960s, they diverged in the preceding decades. The main interannual variability index exhibits high spectral power at ~3.4 years and is representative of temperature variability in a broad area in the southern half of the continent. Eleven-years running correlation coefficients between this index and December-to-February (DJF) Niño3.4 show significant decadal fluctuations, out-of-phase with the running correlation with a DJF index of the Southern Annular Mode. The main interannual variability index is associated with a barotropic wavetrain-like pattern extending over the South Pacific from Oceania to SSA. During warm (cold) summers in SSA, significant anticyclonic (cyclonic) anomalies tend to predominate over eastern Australia, to the north of the Ross Sea, and to the east of SSA, whereas anomalous cyclonic (anticyclonic) circulation is observed over New Zealand and west of SSA. This teleconnection links warm (cold) SSA anomalies with dry (wet) summers in eastern Australia. The covariability seems to be influenced by the characteristics of tropical forcing; indeed, a disruption has been observed since late 1970s, presumably due to the PDO warm phase.     

Multidecadal changes in the relationship between extreme temperature events in Uruguay and the general atmospheric circulation

We analyze changes in the relationship between extreme temperature events and the large scale atmospheric circulation before and after the 1976 climate shift. To do so we first constructed a set of two temperature indices that describe the occurrence of warm nights (TN90) and cold nights (TN10) based on a long daily observed minimum temperature database that spans the period 1946?C2005, and then divided the period into two subperiods of 30?years each (1946?C1975 and 1976?C2005). We focus on summer (TN10) and winter (TN90) seasons. During austral summer before 1976 the interannual variability of cold nights was characterized by a negative phase of the Southern Annular Mode (SAM) with a cyclonic anomaly centered off Uruguay that favoured the entrance of cold air from the south. After 1976 cold nights are associated not with the SAM, but with an isolated vortex at upper levels over South Eastern South America. During austral winter before 1976, the El Ni?o phenomenon dominated the interannual variability of warm nights through an increase in the northerly warm flow into Uruguay. However, after 1976 the El Ni?o connection weakened and the variability of warm nights is dominated by a barotropic anticyclonic anomaly located in the South Atlantic and a low pressure center over South America. This configuration also strengthens the northward flow of warm air into Uruguay. Our results suggest that changes in El Ni?o evolution after 1976 may have played a role in altering the relationship between temperature extreme events in Uruguay and the atmospheric circulation.     

Intraseasonal variability in South America during the cold season

Intraseasonal (IS) variability in South America is analyzed during the cold season using 10–90 day bandpass filtered OLR anomalies (FOLR). IS variability explains a large percentage of variance with maximum values over Paraguay, northeastern Argentina, and southern Brazil. The leading pattern of FOLR, as isolated from an EOF analysis, (Cold Season IS pattern, CSIS), is characterized by a monopole centered over southeastern South America (SESA) with a northwest-southeast orientation. CSIS induces a large modulation on daily precipitation anomalies, especially on both wet spells and daily precipitation extremes, which are favored during positive (wet) CSIS phases. Large-Scale OLR anomalies over the tropical Indian and west Pacific Oceans associated with CSIS exhibit eastward propagation along tropical latitudes. In addition, circulation anomalies in the Southern Hemisphere reveal the presence of an anticyclonic anomaly over Antarctica with opposite-sign anomalies in middle latitudes 10 days before CSIS is maximum as well as evidence of Rossby wave-like patterns. Positive precipitation anomalies in SESA are favored during wet CSIS phases by the intensification of a cyclonic anomaly located further south, which is discernible over the southeastern Pacific for at least 14 days before CSIS peaks. The cyclonic anomaly evolution is accompanied by the intensification of an upstream anticyclonic anomaly, which remains quasi-stationary near the Antarctica Peninsula before the CSIS peak. We speculate that the stationary behavior of the anticyclonic center is favored by a hemispheric circulation anomaly pattern resembling that associated with a negative southern annular mode phase and a wavenumber 3–4 pattern at middle latitudes.     

Interannual heat content variability in the South China Sea and its response to ENSO

The interannual variability of upper ocean heat content (OHC) in the South China Sea (SCS) for the period of 1987–2006 and its response to ENSO events are investigated. It is found that the variability has a good correspondence with ENSO events, but with opposite phase. Negative OHC anomalies appear during ENSO warm phases, while positive OHC anomalies occur during ENSO cool phases. In addition, negative (positive) OHC anomalies propagate westward obviously during ENSO warm (cool) phases in the northern SCS. In contrast, OHC anomalies in the southern SCS do not exhibit distinct westward propagation during ENSO events. To explore why the OHC anomalies cannot propagate westward in the southern SCS, the interannual variability of oceanic and atmospheric anomaly fields including wind stress curl (WSC), horizontal wind stress, latent heat flux (LHF) and sea level pressure (SLP) is investigated. The results show that after a mature phase of ENSO warm (cool) event, negative (positive) OHC anomalies first appear in the northern SCS, which comes from the western Pacific through Luzon Strait. Then cyclonic (anticyclonic) wind stress anomalies occur in the northern SCS, which leads to positive (negative) WSC anomalies. Meanwhile, positive (negative) LHF anomalies which correspond to oceanic heat loss (gain) occur in this region. The effects of WSC and LHF, combined with the westward propagating negative (positive) OHC anomalies from the western Pacific, may contribute to rapid growth and propagation of the OHC anomalies in the northern SCS. On the contrary, the negative (positive) WSC and LHF anomalies associated with positive (negative) SLP in the southern SCS seem to be the important processes responsible for the weakening and non-propagation of the OHC anomalies in the southern SCS after a mature phase of ENSO warm (cool) event.     

The relationship between South Atlantic SST and SACZ intensity and positioning

This study explores the ocean–atmosphere interaction in the formation and dynamics of the South Atlantic Convergence Zone (SACZ), through the analysis of the heat sources estimated through the outgoing longwave radiation. The results obtained with this study show that the coupled variability between SACZ and the South Atlantic Ocean indicates that in northern positioned SACZ cases (over Southeastern Brazil), westerly anomalies are verified in the low level continental tropical circulation, consistent with the active phase of the South America Monsoon System (SAMS). In these cases, cold anomalies in the subtropical Atlantic Ocean cause an increase in the continent–ocean temperature gradient, favoring an easterly flow in this region, and blocking the SACZ at a northerly position. Easterly anomalies in the tropical continent were verified in the low level circulation in southern positioned cases (over Southern Brazil), consistent with the SAMS break phase. The SST anomaly patterns indicate cold anomalies in the tropics and warm anomalies in the subtropics, which do not favor the development of an easterly flow at low levels over the western tropical Atlantic. In these cases, two situations may occur: the strengthening of the Low Level Jet (LLJ), which prevails in the eastern subtropical South America and convergence with the South Atlantic Subtropical High at its southern position; or the atmospheric unstable conditions caused by ocean warm SST anomalies (in this case the LLJ may be weaker than its climatological intensity).     

Seasonal prediction skill of ECMWF System 4 and NCEP CFSv2 retrospective forecast for the Northern Hemisphere Winter

The seasonal prediction skill for the Northern Hemisphere winter is assessed using retrospective predictions (1982–2010) from the ECMWF System 4 (Sys4) and National Center for Environmental Prediction (NCEP) CFS version 2 (CFSv2) coupled atmosphere–ocean seasonal climate prediction systems. Sys4 shows a cold bias in the equatorial Pacific but a warm bias is found in the North Pacific and part of the North Atlantic. The CFSv2 has strong warm bias from the cold tongue region of the eastern Pacific to the equatorial central Pacific and cold bias in broad areas over the North Pacific and the North Atlantic. A cold bias in the Southern Hemisphere is common in both reforecasts. In addition, excessive precipitation is found in the equatorial Pacific, the equatorial Indian Ocean and the western Pacific in Sys4, and in the South Pacific, the southern Indian Ocean and the western Pacific in CFSv2. A dry bias is found for both modeling systems over South America and northern Australia. The mean prediction skill of 2 meter temperature (2mT) and precipitation anomalies are greater over the tropics than the extra-tropics and also greater over ocean than land. The prediction skill of tropical 2mT and precipitation is greater in strong El Nino Southern Oscillation (ENSO) winters than in weak ENSO winters. Both models predict the year-to-year ENSO variation quite accurately, although sea surface temperature trend bias in CFSv2 over the tropical Pacific results in lower prediction skill for the CFSv2 relative to the Sys4. Both models capture the main ENSO teleconnection pattern of strong anomalies over the tropics, the North Pacific and the North America. However, both models have difficulty in forecasting the year-to-year winter temperature variability over the US and northern Europe.     

中国南方夏季降水的年代际变率主模态特征及机理研究

在气候变暖背景下,中国南方夏季降水存在明显的年代际变化特征。本文利用1920~2014年的逐月降水,以分析南方夏季降水年代际变率主模态为切入点,以研究南方夏季降水年代际变率空间分布型的年代际变化特征为重点,进一步研究了印度洋、北太平洋及北大西洋海温的年代际变率对南方夏季降水主模态年代际变率的可能影响机制。得到的主要结论包括:（1）指出中国南方夏季降水年代际变率的两个主模态为全区一致型和东西反相型降水模态。两个主导模态在1971/1972年发生了显著的年代际转变,在1925~1971年的第一主模态为东西反相型降水;在1972~2009年的第一主模态为一致型降水。不同主模态对应的海温异常关键区也在1971/1972年发生了相应的年代际变化。（2）揭示了全区一致型和东西反相型降水模态对应的环流场异常特征。一致多（少）型降水对应着中国南海及西北太平洋低空的反气旋（气旋）性异常,有（不）利于水汽自南海向南方地区输送。而贝加尔湖东侧低空的反气旋（气旋）性异常,有（不）利于冷空气向南方输送,并与来自南海地区的水汽在南方地区辐合,有利于南方地区降水一致偏多（少）。东多西少（西多东少）型降水对应着中国东南地区高空的正（负）异常中心,有利于高空辐散（辐合）及异常的上升（下沉）运动,其与南方地区东部低空的气旋（反气旋）性异常共同作用,有利于东部降水偏多（偏少）。与此同时,低空中南半岛反气旋（气旋）性异常及菲律宾地区反气旋（气旋）性异常,不（有）利于水汽自孟加拉湾及南海地区输送向南方地区西部,有利于形成东多西少（西多东少）的降水型。（3）揭示了印度洋海温、北太平洋海温和北大西洋海温协同影响南方地区东西反相型降水和一致型降水的机制。     

Indian summer monsoon rainfall predictability and variability associated with Northwest Pacific circulation in a suit of coupled model hindcasts

The Northwest Pacific (NWP) circulation (subtropical high) is an important component of the East Asian summer monsoon system. During summer (June–August), anomalous lower tropospheric anticyclonic (cyclonic) circulation appears over NWP in some years, which is an indicative of stronger (weaker) than normal subtropical high. The anomalous NWP cyclonic (anticyclonic) circulation years are associated with negative (positive) precipitation anomalies over most of Indian summer monsoon rainfall (ISMR) region. This indicates concurrent relationship between NWP circulation and convection over the ISMR region. Dry wind advection from subtropical land regions and moisture divergence over the southern peninsular India during the NWP cyclonic circulation years are mainly responsible for the negative rainfall anomalies over the ISMR region. In contrast, during anticyclonic years, warm north Indian Ocean and moisture divergence over the head Bay of Bengal-Gangetic Plain region support moisture instability and convergence in the southern flank of ridge region, which favors positive rainfall over most of the ISMR region. The interaction between NWP circulation (anticyclonic or cyclonic) and ISMR and their predictability during these anomalous years are examined in the present study. Seven coupled ocean–atmosphere general circulation models from the Asia-Pacific Economic Cooperation Climate Center and their multimodel ensemble mean skills in predicting the seasonal rainfall and circulation anomalies over the ISMR region and NWP for the period 1982–2004 are assessed. Analysis reveals that three (two) out of seven models are unable to predict negative (positive) precipitation anomalies over the Indian subcontinent during the NWP cyclonic (anticyclonic) circulation years at 1-month lead (model is initialized on 1 May). The limited westward extension of the NWP circulation and misrepresentation of SST anomalies over the north Indian Ocean are found to be the main reasons for the poor skill (of some models) in rainfall prediction over the Indian subcontinent. This study demonstrates the importance of the NWP circulation variability in predicting summer monsoon precipitation over South Asia. Considering the predictability of the NWP circulation, the current study provides an insight into the predictability of ISMR. Long lead prediction of the ISMR associated with anomalous NWP circulation is also discussed.     

Influence of the intraseasonal variability on heat waves in subtropical South America

The influence of the intraseasonal variability on heat wave development over subtropical South America during austral summer is analyzed. The role of the South Atlantic Convergence Zone (SACZ) on this development is documented. Results show that intraseasonal variability can explain on average at least 32% of summer temperature variance. Moreover, 73% of the heat waves in subtropical South America develop in association with an active SACZ. The analysis of pentad maps shows that warm conditions in the region under study develop in association with the strengthening of an anticyclonic anomaly, which is discernible over the subtropical regions at least 15?days before temperature peak occurrence. That circulation anomaly is embedded in a large-scale Rossby wave train extending along the South Pacific Ocean that is linked to convection anomalies at the equatorial western and central Pacific Ocean. In addition, the development of the anticyclonic circulation over subtropical South America appears to be strengthened by the subsidence conditions promoted by the active SACZ, which result in temperature rise in the subtropical region under relatively dry conditions. On the other hand, during the last 2?days of evolution, SACZ activity weakens and the progressive temperature rise in the region is dominated by warmer and moister air being anomalously advected from the north. Results confirm the important role that SACZ activity on intraseasonal time scales has in inducing persistent circulation anomalies at the subtropical regions that can result in the development of persistent heat waves, and very extreme daily temperature.     

中国冬季气温不同年代际的季节内变化特征及成因分析

利用1951～2020年中国观测站气温资料、NCEP/NCAR再分析资料和统计方法,分析了不同年代际时间尺度背景下我国冬季气温的季节内变化特征及相联系的大气环流异常。结果表明,1986年前、后为两个年代际时间尺度阶段,各阶段内前冬（12月）与后冬（1～2月）气温异常反位相年的比例均高于同位相年。1986年之前,季节内的优势空间模态为前冬全国冷（暖）转为后冬南方暖（冷）的可能性大,即南方地区季节内变率大;而1986年之后的优势空间模态为前冬北方冷（暖）转为后冬全国明显暖（冷）的可能性大,即北方地区季节内变率大。冬季气温的季节内变化显著受到冬季风系统关键环流季节内变化的影响。对应优势模态的正异常年份,1986年之前,欧亚中高纬地区对流层环流异常信号从前冬到后冬显著性减弱,其中西北太平洋地区对流层中高层的环流调整更明显,副热带高度场增强,热带东风急流北扩,前冬到后冬的环流调整有利于前冬全国大范围偏冷而后冬我国南方地区气温升高,造成南方地区季节内反位相变率增大。1986年之后,欧亚高中低纬地区的环流异常从前冬到后冬显著性增强,欧亚中高纬度环流发生较大调整,而低纬度的环流变化不大,北方地区前冬冷到后冬全国明显转暖,造成北方地区季节内反位相变率大。即副热带环流和中高纬度环流分别在两个年代际尺度阶段南方和北方的冬季气温季节内变率中起到主导作用。     

我国华南4、5月份降水年代际变化的特征及其与中西太平洋海温的可能关系

利用1951～2005年华南4、5月份降水资料、NOAA海温资料以及NCEP再分析资料,对华南4、5月份降水年代际变化的特征、及其所对应的大尺度环流以及与中西太平洋海温的关系作了分析。结果表明,华南4、5月份降水均在1970年代初期发生显著的年代际转变,从之前的降水偏少转变为降水偏多。华南4月份降水与前一年7～11月份的中西太平洋海温、华南5月份降水与当年2～5月份的中西太平洋海温有显著的负相关。在4、5月份年代际降水偏少(多)时期,前期中西太平洋海温偏暖(冷);同期亚洲大陆南部及非洲大陆的海平面气压显著偏低(高),北太平洋海区海平面气压偏高(低);我国华南上空存在反气旋性(气旋性)环流异常,我国华南地区北边界存在显著的南(北)风异常,造成华南地区北边界异常水汽输出增强(减弱)。同时,我国大陆对流层中上层大气显著偏暖(冷),东亚高空急流显著偏北(南),副热带高压偏弱(强)偏东(西),向华南地区输送的水汽减少(增加),从而在华南地区形成异常的水汽辐散(辐合),最终导致华南地区4、5月份降水的减少(增加)。     

我国华南3月份降水异常的可能影响因子分析

利用1951～2005年华南地区3月份的降水资料、NOAA海温资料、Ni?o3.4指数和NCEP再分析资料,分析了华南3月份降水异常与同期环流场、全球海温场的关系,从环流和海温的角度揭示了华南3月份降水异常的可能原因。结果表明,当华南3月份降水偏多(少)时,在对流层中低层,北太平洋海区存在气旋(反气旋)性环流异常,西太平洋及南海海面上存在反气旋(气旋)性环流异常,这样的环流异常有利(不利)于东南暖湿气流与北方东部异常冷空气在华南地区形成水汽辐合,导致降水显著增多(减少)。进一步的分析表明,ENSO和北印度洋及南海附近海温是影响华南3月份降水异常的重要外强迫因子,ENSO对华南3月降水异常的影响是通过影响春季西太平洋副热带高压和低层风场异常实现的,而北印度洋及南海附近海温对华南3月降水异常的影响则是通过垂直环流场异常和低层风场以及西太平洋副热带高压异常来实现的。     

Cross-season relation of the South China Sea precipitation variability between winter and summer

The present study reveals cross-season connections of rainfall variability in the South China Sea (SCS) region between winter and summer. Rainfall anomalies over northern South China Sea in boreal summer tend to be preceded by the same sign rainfall anomalies over southern South China Sea in boreal winter (denoted as in-phase relation) and succeeded by opposite sign rainfall anomalies over southern South China Sea in the following winter (denoted as out-of-phase relation). Analysis shows that the in-phase relation from winter to summer occurs more often in El Niño/La Niña decaying years and the out-of-phase relation from summer to winter appears more frequently in El Niño/La Niña developing years. In the summer during the El Niño/La Niña decaying years, cold/warm and warm/cold sea surface temperature (SST) anomalies develop in tropical central North Pacific and the North Indian Ocean, respectively, forming an east–west contrast pattern. The in-phase relation is associated with the influence of anomalous heating/cooling over the equatorial central Pacific during the mature phase of El Niño/La Niña events that suppresses/enhances precipitation over southern South China Sea and the impact of the above east–west SST anomaly pattern that reduces/increases precipitation over northern South China Sea during the following summer. The impact of the east–west contrast SST anomaly pattern is confirmed by numerical experiments with specified SST anomalies. In the El Niño/La Niña developing years, regional air-sea interactions induce cold/warm SST anomalies in the equatorial western North Pacific. The out-of-phase relation is associated with a Rossby wave type response to anomalous heating/cooling over the equatorial central Pacific during summer and the combined effect of warm/cold SST anomalies in the equatorial central Pacific and cold/warm SST anomalies in the western North Pacific during the mature phase of El Niño/La Niña events.     

不同气候背景下新疆冬季极端冷(暖)事件的变化特征

利用1961—2013年新疆89站逐日气温和NCEP再分析高度场资料,分析了不同气候背景下新疆1961年以来冬季(12月—翌年2月)出现的极端冷(暖)事件年代际变化及与其相联系的环流特征。根据对新疆冬季极端冷(暖)事件的气候背景划分,认为新疆冬季极端冷(暖)事件在不同气候背景中都有明显的不同,全疆冬季极端冷事件存在随气候背景转变而发生区域一致变化的特征,但冬季极端暖事件的变化则有南北反相的区域差异。总体而言,新疆极端冷暖事件发生的日数趋于减少,极端冷暖事件强度也具有显著减小的趋势;北疆西部和天山两侧是气候极端性变化最显著的区域。从冷暖期环流特征的差异来看,北疆型极端冷事件减少的主要原因来自于突变后极涡减弱,而南疆型极端冷(暖)事件减少(增加)则主要受欧亚范围内大片正变高区的影响。     

500 hPa Vorticity Analyses over Argentina: Their Climatologyand Capacity to Distinguish Synoptic-Scale Precipitation

Summary The Southern South America climatological 500 hPa relative vorticity mean state was examined using regional objective analyses of 500 hPa geopotential heights provided by the Servicio Meteorológico Nacional of Argentina. The dataset, covering the period June 1983 to July 1987, was stratified into two samples: the cold and warm seasons. Mean cyclonic vorticity south of 40° S results in a climatological trough over Patagonia with a northwest-southeast tilt. North of this latitude, mean anticyclonic circulation dominates with the exception of a centre of cyclonic vorticity over the Río de la Plata (35° S, 56° W). Seasonal changes appear to be small. Relative vorticity frequency distributions were also analysed. The association between precipitation and synoptic-scale features of the mid-troposphere circulation was investigated through vorticity fields. A particular distribution of vorticity anomalies associated with daily precipitation in Buenos Aires is revealed by biserial correlation coefficient fields. In winter, the strongest relationships are found between 35° S and 40° S over the Andes Mountains (minimum significant correlation coefficients indicating a cyclonic vorticity anomaly), and in the south of Brazil and east of Buenos Aires over the Atlantic Ocean down to a latitude of 40° S (maximum correlation coefficients related to anomalously anticyclonic circulation). This shows the preferential position of troughs and ridges that produce precipitation in Buenos Aires on the time scale of a day. In summer, centres of anomalously cyclonic and anticyclonic vorticity associated with precipitation shift slightly southward. For moderate or intense precipitation in Buenos Aires, advection of warm and wet air southwards appears to be more important in winter, while in summer the strong anomalous vorticity gradient north of the negative centre over the Andes Cordillera favours rainfall in Buenos Aires. Received April 17, 1997     

Influence of the Madden Julian Oscillation on precipitation and surface air temperature in South America

The regional influence of the Madden–Julian oscillation (MJO) on South America is described. Maps of probability of weekly-averaged rainfall exceeding the upper tercile were computed for all seasons and related statistically with the phase of the MJO as characterized by the Wheeler–Hendon real-time multivariate MJO (RMM) index and with the OLR MJO Index. The accompanying surface air temperature and circulation anomalies were also calculated. The influence of the MJO on regional scales along with their marked seasonal variations was documented. During December–February when the South American monsoon system is active, chances of enhanced rainfall are observed in southeastern South America (SESA) region mainly during RMM phases 3 and 4, accompanied by cold anomalies in the extratropics, while enhanced rainfall in the South Atlantic Convergence Zone (SACZ) region is observed in phases 8 and 1. The SESA (SACZ) signal is characterized by upper-level convergence (divergence) over tropical South America and a cyclonic (anticyclonic) anomaly near the southern tip of the continent. Impacts during March–May are similar, but attenuated in the extratropics. Conversely, in June–November, reduced rainfall and cold anomalies are observed near the coast of the SACZ region during phases 4 and 5, favored by upper-level convergence over tropical South America and an anticyclonic anomaly over southern South America. In September–November, enhanced rainfall and upper-level divergence are observed in the SACZ region during phases 7 and 8. These signals are generated primarily through the propagation of Rossby wave energy generated in the region of anomalous heating associated with the MJO.     

Analysis on Anomalous Precipitation in Southern China During Winter Monsoons

The winters of 1997/1998 and 1998/1999,corresponding to El Ni(?)o and La Ni(?)a episodes,respectively, were two typical rain-abundant and-scarce seasons for the southern China.In order to understand the cause of the anomalous precipitation during the two winters,a comparative analysis technique has been employed to investigate the differences in general circulation and moisture transportation between the two seasons. The results show that the abundant rainfall during the winter of 1997/1998 was associated with the ENSO warm episode event,eastward shifted weak westerly trough/ridge,weakened East Asian winter monsoon (EAWM),strengthened subtropical high,and presented two anti-cyclonic circulations over Hokkaido and the Philippine Sea,respectively,as well as one cyclonic circulation over the Yangtze River Basin in the anomalous wind fields of the lower troposphere.During the rain-scarce winter,however,the patterns of equatorial sea surface temperature anomalies and the circulation systems both in upper and lower levels were nearly the opposite of those during the rain-abundant winter.It has also been discovered that the water vapor over southern China during the winters came mainly from the southwesterly flow ahead of troughs in the southern branch of westerlies and the turning flow over the South China Sea-Indo-China Peninsula area;and the moisture transportation channels varied significantly with regard to height.The intensified flow in the southern branch of westerlies and the anti-cyclonic circulation anomaly over the Philippine Sea during the winter of 1997/1998 were favorable for moisture transportation to mainland China,however the two moisture transportation streams were dramatically weakened during the winter of 1998/1999 due to weak westerly flow and the dominance of a cold high system in the lower level over the southeast coast of China.Such a significant inter-annual change of moisture transportation is a key factor resulting in the obvious difference in precipitation between the two winters.     

Evolution of late pleistocene and holocene climates in the circum-south pacific land areas

Paleovegetation maps were reconstructed based on a network of pollen records from Australia, New Zealand, and southern South America for 18 000, 12000, 9000, 6000, and 3000 BP and interpreted in terms of paleoclimatic patterns. These patterns permitted us to speculate on past atmospheric circulation in the South Pacific and the underlying forcing missing line mechanisms. During full glacial times, with vastly extended Australasian land area and circum-Antarctic ice-shelves, arid and cold conditions characterized all circum-South Pacific land areas, except for a narrow band in southern South America (43° to 45°S) that might have been even wetter and moister than today. This implies that ridging at subtropical and mid-latitudes must have been greatly increased and that the storm tracks were located farther south than today. At 12000 BP when precipitation had increased in southern Australia, New Zealand, and the mid-latitudes of South America, ridging was probably still as strong as before but had shifted into the eastern Pacific, leading to weaker westerlies in the western Pacific and more southerly located westerlies in the eastern Pacific. At 9000 BP when, except for northernmost Australia, precipitation reached near modern levels, the south Pacific ridges and the westerlies must have weakened. Because of the continuing land connection between New Guinea and Australia, and reduced seasonality, the monsoon pattern had still not developed. By 6000 BP, moisture levels in Australia and New Zealand reached their maximum, indicating that the monsoon pattern had become established. Ridging in the South Pacific was probably weaker than today, and the seasonal shift of the westerlies was stronger than before. By 3000 BP essentially modern conditions had been achieved, characterized by patterns of high seasonal variability.Contribution to Clima Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

青藏高原隆升对春、夏季亚洲大气环流的影响

利用全球大气环流谱模式R42L9,进行了有、无青藏高原大地形两种情况的10年积分,通过两个试验结果的比较,研究了青藏高原大地形对春、夏亚洲大气环流的影响。模拟结果表明：春季,青藏高原大地形对低层西风的阻挡引起了绕流,其北支气流加强了北方冷空气在高原东侧的南下;同时,作为一个弱热源,它的热力作用加强了高原南侧的南支西风气流,为华南地区输送了大量的暖湿空气。冷暖空气的交汇,加强了华南地区春季的降水。夏季,青藏高原强热源的存在,引起的低层气旋性环流,加强了青藏高原东侧的东亚夏季风,使其向北发展。盛夏,青藏高原“感热气泵（SHAP）”在南亚地区上空低层造成了负涡度和辐散异常,使南亚地区的夏季降水减少,南亚夏季风减弱;在对流层上层高原上宅形成负涡源,并通过遥相关加强了伊朗高压。     

How do La Niña events disturb the summer monsoon system in Brazil?

The rainy season in most of Brazil is associated with the summer monsoon regime in South America. The quality of this season is important because it rains little during the rest of the year over most of the country. In this study, the influence of La Niña events on the summer monsoon circulation, rainfall and temperature is analyzed with seasonal and monthly resolution, using data from a dense network of stations, giving a comprehensive view of the impact of these events. The expected precipitation percentiles during the monsoon season of La Niña events are calculated, as well as anomalies of surface temperature and thermodynamic parameters. This information is analyzed jointly with anomaly composites of several circulation parameters. The analysis shows that some anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. There are abrupt changes of anomalies within the summer monsoon season, suggesting the prevalence of regional processes over remote influences during part of the season. In spring there are positive precipitation anomalies in north and central-east Brazil and negative ones in south Brazil. These precipitation anomalies are favored by the perturbation in the Walker and Hadley circulation over the eastern Pacific and South America, and by perturbations in the rotational circulation over southern South America. Northerly moisture inflow from the Atlantic into northern South America is emphasized and diverted towards the mouth of the Amazon by the low-level cyclonic anomaly north of the equator. In December and January, probably triggered by anomalous surface cooling during the spring, there is an anomalous low-level divergence and an anticyclonic anomaly over southeast Brazil. This anomalous circulation directs moisture flux towards south Brazil, causing moisture convergence in part of this region and part of central-west Brazil. The thermodynamic structure in central-east Brazil does not favor precipitation over this region, and the wet anomalies in north Brazil are displaced northward. The dry anomalies in south Brazil almost disappear and even turn positive. In February, after the strongly below normal precipitation of January, the surface temperature anomalies turn positive over southeast Brazil. The low-level anticyclonic anomaly is much weaker than in January. There are positive rainfall anomalies in north Brazil and in the South Atlantic Convergence Zone, and negative ones return to south Brazil.