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1.
Moisture exchange between the South Atlantic and southern Africa is examined in this study through zonal moisture transport. Along the west coast of southern Africa, a multivariate analysis of the zonal flow of moisture computed from NCEP-DOE AMIP II Re-analyses reveals a primary mode of variability typical of variations in intensity and of the latitudinal migration of the circulation associated with the midlatitude westerlies and the South Atlantic anticyclone. In austral summer (January–February), this mode, referred to as the South Atlantic midlatitude mode, is found to be well correlated with rainfall over southern Africa (i.e. to the south of the upper lands surrounding the Congo basin). Its positive/negative phases are found to correspond with surface pressures changes over the South Atlantic region in austral summer when the South Atlantic anticyclone is shifted northward/southward respectively. Such changes are accompanied by dipole-like SST anomalies in the midlatitude South Atlantic Ocean, while simultaneous SST anomalies with a similar structure are also found over South Indian Ocean regions. In January–February, positive/negative events linked to the South Atlantic midlatitude mode are marked by meridional shifts (northward/southward) and weakening/strengthening of the ITCZ over the southern tropics, together with modulations in intensity (weakened/sustained) of the Angola low, which could act as a tropical source of moisture for Tropical Temperate Troughs (TTTs). In association with a strengthened/weakened zonal component of the southern extension of the African Easterly Jet (AEJ), this could modulate the meridional transfer of moisture south of 15°S to the advantage/detriment of Angolan coastal regions, where above/below rainfall are expected. Variations in the latitudinal position (northward/southward) of the South Atlantic anticyclone, and thus of the midlatitude westerlies, are also found to reduce/favour moisture advection towards southern Africa subtropics allowing the southern Indian trades to penetrate less/more over the subcontinent south of 25°S. This would create a situation where convection processes are inhibited/supported within the SICZ/TTTs region resulting in drier/wetter conditions locally for positive/negative events respectively.  相似文献   

2.
The positive phase of the subtropical Indian Ocean dipole(SIOD) is one of the climatic modes in the subtropical southern Indian Ocean that influences the austral summer inter-annual rainfall variability in parts of southern Africa. This paper examines austral summer rain-bearing circulation types(CTs) in Africa south of the equator that are related to the positive SIOD and the dynamics through which specific rainfall regions in southern Africa can be influenced by this relationship. Four austral...  相似文献   

3.
Teleconnections between equatorial African climate and the surrounding circulation are examined using a convective index over the Congo River Basin in March to May (MAM) and July to September (JAS) seasons. Its influence on the wider region is determined through lag correlation and cross-wavelet analysis. During seasons of deeper convection, easterly winds weaken over the tropical Atlantic (anomalous flow toward Africa), whilst upper westerly winds weaken over southern Africa (in JAS). We view this as zonal overturning with ascent over the equatorial African lowlands and Congo River Basin that spreads moisture to the North African Sahel, with influence from the Pacific El Niño. Another facet of our study is the relationship between East African highlands rainfall and the Indian Ocean circulation. We find coupling between the Indian Ocean Rossby wave, a thermocline oscillation and Walker cell over the Indian Ocean that induces shifts in rainfall, particularly in the October to December season.  相似文献   

4.
Changes in growing seasons for 2041–2060 across Africa are projected using a regional climate model at 90-km resolution, and confidence in the predictions is evaluated. The response is highly regional over West Africa, with decreases in growing season days up to 20% in the western Guinean coast and some regions to the east experiencing 5–10% increases. A longer growing season up to 30% in the central and eastern Sahel is predicted, with shorter seasons in parts of the western Sahel. In East Africa, the short rains (boreal fall) growing season is extended as the Indian Ocean warms, but anomalous mid-tropospheric moisture divergence and a northward shift of Sahel rainfall severely curtails the long rains (boreal spring) season. Enhanced rainfall in January and February increases the growing season in the Congo basin by 5–15% in association with enhanced southwesterly moisture transport from the tropical Atlantic. In Angola and the southern Congo basin, 40–80% reductions in austral spring growing season days are associated with reduced precipitation and increased evapotranspiration. Large simulated reductions in growing season over southeastern Africa are judged to be inaccurate because they occur due to a reduction in rainfall in winter which is over-produced in the model. Only small decreases in the actual growing season are simulated when evapotranspiration increases in the warmer climate. The continent-wide changes in growing season are primarily the result of increased evapotranspiration over the warmed land, changes in the intensity and seasonal cycle of the thermal low, and warming of the Indian Ocean.  相似文献   

5.
Daily rainfall variability over southern Africa (SA) and the southwest Indian Ocean (SWIO) during the austral summer months has recently been described objectively for the first time, using newly derived satellite products. The principle mode of variability in all months is a dipole structure with bands of rainfall orientated northwest to southeast across the region. These represent the location of cloud bands associated with tropical temperate troughs (TTT). This study objectively identifies major TTT events during November to February, and on the basis of composites off NCEP reanalysis data describes the associated atmospheric structure. The two phases of the rainfall dipole are associated with markedly contrasting circulation patterns. There are also pronounced intra-seasonal variations. In early summer the position of the temperate trough and TTT cloud band alternates between the SWIO and southwest Atlantic. In late summer the major TTT axis lies preferentially over the SWIO, associated with an eastward displacement in the Indian Ocean high. In all months, positive events, in which the TTT cloud band lies primarily over the SWIO, are associated with large-scale moisture flux anomalies, in which convergent fluxes form a pronounced poleward flux along the cloud band. This suggests that TTT events are a major mechanism of poleward transfer of energy and momentum. Moisture transport occurs along three principle paths: (1) the northern or central Indian Ocean (where anomalous fluxes extend eastward to the Maritime Continent), (2) south equatorial Africa and the equatorial Atlantic, (3) from the south within a cyclonic flow around the tropical-temperate trough. The relative importance of (2) is greatest in late summer. Thus, synoptic scale TTT events over SA/SWIO often result from large-scale planetary circulation patterns. Hovmoeller plots show that TTT development coincides with enhanced tropical convection between 10°–30°E (itself exhibiting periodicity of around 5 days), and often with convergence of eastward and westward propagating convection around 40°E. Harmonic analysis of 200 hPa geopotential anomalies show that TTT features are forced by a specific zonally asymmetric wave pattern, with wave 5 dominant or significant in all months except February when quasi-stationary waves 1, 2 and 3 dominate. These findings illustrate the importance of tropical and extratropical dynamics in understanding TTT events. Finally, it is suggested that in November–Januar TTT rainfall over SA/SWIO may be in phase with similar rainfall dipole structures observed in the South Pacific and South Atlantic convergence zones. Received: 11 August 1998 / Accepted: 28 May 1999  相似文献   

6.
We utilize a variety of climate datasets to examine impacts of two mechanisms on precipitation in the Greater Horn of Africa (GHA) during northern-hemisphere summer. First, surface-pressure gradients draw moist air toward the GHA from the tropical Atlantic Ocean and Congo Basin. Variability of the strength of these gradients strongly influences GHA precipitation totals and accounts for important phenomena such as the 1960s–1980s rainfall decline and devastating 1984 drought. Following the 1980s, precipitation variability became increasingly influenced by the southern tropical Indian Ocean (STIO) region. Within this region, increases in sea-surface temperature, evaporation, and precipitation are linked with increased exports of dry mid-tropospheric air from the STIO region toward the GHA. Convergence of dry air above the GHA reduces local convection and precipitation. It also produces a clockwise circulation response near the ground that reduces moisture transports from the Congo Basin. Because precipitation originating in the Congo Basin has a unique isotopic signature, records of moisture transports from the Congo Basin may be preserved in the isotopic composition of annual tree rings in the Ethiopian Highlands. A negative trend in tree-ring oxygen-18 during the past half century suggests a decline in the proportion of precipitation originating from the Congo Basin. This trend may not be part of a natural cycle that will soon rebound because climate models characterize Indian Ocean warming as a principal signature of greenhouse-gas induced climate change. We therefore expect surface warming in the STIO region to continue to negatively impact GHA precipitation during northern-hemisphere summer.  相似文献   

7.
Local and remote impacts of a tropical Atlantic salinity anomaly   总被引：1，自引：1，他引：0  
The climatic impacts of an enhanced evaporation prescribed during 50 years in the tropical Atlantic are investigated in a coupled ocean–atmosphere general circulation model. Locally, the salinity increase leads to a rapid deepening and cooling of the surface mixed layer. This induces a deepening of the equatorial undercurrent and an intensification of the south equatorial current. A remote atmospheric response to the tropical Atlantic perturbation is detected in the North Atlantic sector after ten years. It has the form of a robust wave-like tropospheric perturbation seemingly excited by the weakening of atmospheric deep convection over the Amazonian basin. Meanwhile, the salt anomaly is carried northward by the mean oceanic circulation. It is traced up to the convection sites and then on its return path at depth towards lower latitudes. Consistent with the density increase, deep convection is enhanced after the arrival of the salt anomaly and the Atlantic meridional overturning circulation (AMOC) intensifies about 20 years after the beginning of the perturbation. The adjustment of the tropical Atlantic to the AMOC intensification then modifies its initial response to the freshwater forcing, leading to a weaker cooling in the northern tropical Atlantic than in the southern tropical Atlantic, a slight northward shift of the tropical Atlantic precipitation pattern and an intensification of the North Brazil current. On the other hand, no significant anomalous precipitations are found in the Pacific. The initial remote atmospheric response is also modulated, by an NAO-like response to the AMOC intensification.  相似文献   

8.
The water cycle over the Amazon basin is a regulatory mechanism for regional and global climate. The atmospheric moisture evaporated from this basin represents an important source of humidity for itself and for other remote regions. The deforestation rates that this basin has experienced in the past decades have implications for regional atmospheric circulation and water vapor transport. In this study, we analyzed the changes in atmospheric moisture transport towards tropical South America during the period 1961–2010, according to two deforestation scenarios of the Amazon defined by Alves et al. (Theor Appl Climatol 100(3-4):337–350, 2017). These scenarios consider deforested areas of approximately 28% and 38% of the Amazon basin, respectively. The Dynamic Recycling Model is used to track the transport of water vapor from different sources in tropical South America and the surrounding oceans. Our results indicate that under deforestation scenarios in the Amazon basin, continental sources reduce their contributions to northern South America at an annual scale by an average of between 40 and 43% with respect to the baseline state. Our analyses suggest that these changes may be related to alterations in the regional Hadley and Walker cells. Amazon deforestation also induces a strengthening of the cross-equatorial flow that transports atmospheric moisture from the Tropical North Atlantic and the Caribbean Sea to tropical South America during the austral summer. A weakening of the cross-equatorial flow is observed during the boreal summer, reducing moisture transport from the Amazon to latitudes further north. These changes alter the patterns of precipitable water contributions to tropical South America from both continental and oceanic sources. Finally, we observed that deforestation over the Amazon basin increases the frequency of occurrence of longer dry seasons in the central-southern Amazon (by between 29 and 57%), depending on the deforestation scenario considered, as previous studies suggest.  相似文献   

9.
In this paper, the evolution of intraseasonal oscillation over the South China Sea and tropical western Pacific area and its effect to the summer rainfall in the southern China are studied based on the ECMWF data and TBB data) analyses. A very low-frequency waves exist in the tropics and play an important role in dominating intraseasonal oscillation and lead to special seasonal variation of intraseasonal oscillation over the South China Sea / tropical western Pacific area. The intraseasonal oscillation (convection) over the South China Sea and tropical western Pacific area is closely related to the summer rainfall (convection) in the southern China. Their relationship seems to be a seesaw feature, and this relationship resulting from the different pattern of convection in those two re-gions is caused by the different type of local meridional circulation  相似文献   

10.
Summary ?This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ). During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced) rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes. High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events. Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002  相似文献   

11.
The regions where the divergence of vertically integrated water vapor flux, averaged over a season or a year, is positive (negative) are sources (sinks) of moisture for the atmosphere. An aerial river is defined as a stream of strong water vapor flux connecting a source and a sink. Moisture flux, its divergence, and sources and sinks over the tropics of South and Central America and the adjoining Atlantic Ocean are obtained for dry years and for wet years in the Amazon Basin. Results show that the Amazon Basin is a sink region for atmospheric moisture in all seasons and that there are two source regions for the moisture in the basin, one situated in the South Atlantic and the other in the North Atlantic, both located equator-ward of the respective subtropical high-pressure centers. The convergence of moisture increases over the Amazon Basin in austral summer, and at the same time it decreases in the Pacific and Atlantic ITCZs. Box model calculations reveal that the wet years, on the average, present about 55 % more moisture convergence than the dry years in the Amazon Basin. A reduction in the moisture inflow across the eastern and northern boundaries of the basin (at 45°W and at the Equator, respectively) and an increase in the outflow across the southern boundary (at 15°S) lead to dry conditions. The annual mean contribution of moisture convergence to the precipitation over the Amazon Basin is estimated to be 70 %. In the dry years, it lowers to around 50 %. The net convergence of water vapor flux over the basin is a good indicator of the wet or dry condition.  相似文献   

12.
Summary Climatic determinants of summer (Nov-Mar) rainfall over southern Africa are investigated through analysis of sea surface temperatures (SST), outgoing longwage radiation (OLR) and tropospheric wind with respect to the Southern Oscillation Index (SOI) and the stratospheric quasi-biennial oscillation (QBO). Index-to-field correlation maps are presented at various lags for the austral spring and summer seasons to establish the spatial dependence and evolution of coherent, statistically significant features. The SOI signal is reflected in upper-level zonal wind anomalies over the equatorial Atlantic Ocean during spring. SSTs in the central Indian Ocean are significantly negatively correlated with the SOI in summer. On the other hand, OLR correlations are weak over southern Africa in the summer, implying that the SOI signal may not dominate interannual convective variability.QBO correlations with SST are relatively weak, but with 200 hPa zonal winds over the western equatorial Ocean, positive correlations are noted. A standing wave pattern is described in the sub-tropics. The OLR correlation pattern represents a dipole with increased convection over eastern and southern Africa in contrast to reduced convection over Madagascar when the QBO is in west phase.Contingency analyses indicate that the global indices are unreliable predictors in isolation. However the characteristics and domain of influence of SOI and QBO signals are identified and may offer useful inputs to objective multivariate models for different modes of southern African rainfall variability.With 12 Figures  相似文献   

13.
The impact of the inter-El Niño (EN) variability on the moisture availability over Southeastern South America (SESA) is investigated. Also, an automatic tracking scheme was used to analyze the extratropical cyclones properties (system density - SD and central pressure - CP) in this region. During the austral summer period from 1977–2000, the differences for the upper-level wave train anomaly composites seem to determine the rainfall composite differences. In fact, the positive rainfall anomalies over most of the SESA domain during the strong EN events are explained by an upper-level cyclonic center over the tropics and an anticyclonic center over the eastern subtropical area. This pattern seems to contribute to upward vertical motion at 500 hPa and reinforcement of the meridional moisture transport from the equatorial Atlantic Ocean and western Amazon basin to the SESA region. These features may contribute to the positive SD and negative CP anomalies explaining part of the positive rainfall anomalies found there. On the other hand, negative rainfall anomalies are located in the northern part of SESA for the weak EN years when compared to those for the strong events. Also, positive anomalies are found in the southern part, albeit less intense. It was associated with the weakening of the meridional moisture transport from the tropics to the SESA that seems have to contributed with smaller SD and CP anomalies over the most part of subtropics, when compared to the strong EN years.  相似文献   

14.
利用站点观测资料和再分析资料,采用相关分析,Morlet小波功率谱分析和复合分析等方法,研究了 1961-2011年南半球夏季后期(1-3月)坦桑尼亚降水的年际变化特征,并探讨了相关的大气环流和海温异常情况,以及坦桑尼亚干,湿年发生的机制.研究结果表明:坦桑尼亚1-3月降水变化存在显著的2-8年的年际变化周期和8-12...  相似文献   

15.
The capability of a current state-of-the-art regional climate model for simulating the diurnal and annual cycles of rainfall over a complex subtropical region is documented here. Hourly rainfall is simulated over Southern Africa for 1998–2006 by the non-hydrostatic model weather research and forecasting (WRF), and compared to a network of 103 stations covering South Africa. We used five simulations, four of which consist of different parameterizations for atmospheric convection at a 0.5 × 0.5° resolution, performed to test the physic-dependency of the results. The fifth experiment uses explicit convection over tropical South Africa at a 1/30° resolution. WRF simulates realistic mean rainfall fields, albeit wet biases over tropical Africa. The model mean biases are strongly modulated by the convective scheme used for the simulations. The annual cycle of rainfall is well simulated over South Africa, mostly influenced by tropical summer rainfall except in the Western Cape region experiencing winter rainfall. The diurnal cycle shows a timing bias, with atmospheric convection occurring too early in the afternoon, and causing too abundant rainfall. This result, particularly true in summer over the northeastern part of the country, is weakly physic-dependent. Cloud-resolving simulations do not clearly reduce the diurnal cycle biases. In the end, the rainfall overestimations appear to be mostly imputable to the afternoon hours of the austral summer rainy season, i.e., the periods during which convective activity is intense over the region.  相似文献   

16.
This article presents an overview of the land ITCZ (Intertropical Convergence Zone) over West Africa, based on analysis of NCAR–NCEP Reanalysis data. The picture that emerges is much different than the classic one. The most important feature is that the ITCZ is effectively independent of the system that produces most of the rainfall. Rainfall linked directly to this zone of surface convergence generally affects only the southern Sahara and the northern-most Sahel, and only in abnormally wet years in the region. A second feature is that the rainbelt normally assumed to represent the ITCZ is instead produced by a large core of ascent lying between the African Easterly Jet and the Tropical Easterly Jet. This region corresponds to the southern track of African Easterly Waves, which distribute the rainfall. This finding underscores the need to distinguish between the ITCZ and the feature better termed the “tropical rainbelt”. The latter is conventionally but improperly used in remote sensing studies to denote the surface ITCZ over West Africa. The new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift, which in turn is controlled by the characteristics of the African Easterly Jet and Tropical Easterly Jet, rather than by moisture convergence. This new picture also includes a circulation feature not generally considered in most analyses of the region. This feature, a low-level westerly jet termed the African Westerly Jet, plays a significant role in interannual and multidecadal variability in the Sahel region of West Africa. Included are discussions of the how this new view relates to other aspects of West Africa meteorology, such as moisture sources, rainfall production and forecasting, desertification, climate monitoring, hurricanes and interannual variability. The West African monsoon is also related to a new paradigm for examining the interannual variability of rainfall over West Africa, one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north–south displacements of this feature. The new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the African Easterly Waves, the disturbances that are linked to rainfall over West Africa and spawn hurricanes over the Atlantic, and with our knowledge of the prevailing synoptic and dynamic features. This article demonstrate a new aspect of the West Africa monsoon, a bimodal state, with one mode linked to dry conditions in the Sahel and the other linked to wet conditions. The switch between modes appears to be linked to an inertial instability mechanism, with the cross-equatorial pressure gradient being a critical factor. The biomodal state has been shown for the month of August only, but this month contributes most of the interannual variability. This new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability, but also to the multidecadal variability evidenced in the region between the 1950s and 1980s.  相似文献   

17.
Summary Annual and interannual variations of rainfall over Brazil are discussed. First, rainy and dry seasons for several stations of Brazil are determined using the data of 21 years (1958–1978). The progressive movement of the Intertropical Convergence Zone seems to be associated with the progresive variation of rainfall seasons in the equatorial eastern Brazil. The annual migration of deep tropical convection from Central and Southern Portion of the Amazon basin in austral summer to the northwestern sector of South America in austral winter seems to be responsible for the annual cycle of rainfall in the Amazon basin. The conncection between the interannual variation of rainfall over Brazil and the Southern Oscillation is also discussed. The correlation coefficient between the Southern Oscillation index and the rainfall is generally small over most of Brazil except over Rio Grande do Sul. The correlation between the spring rainfall of Rio Grande do Sul and the Southern Oscillation index of the same or of the previous season is significantly high and shows prospects for seasonal rainfall prediction.With 5 Figures  相似文献   

18.
基于1979-2016年ERA-Interim再分析资料和CAM5.3模式,研究了2016年和1998年北大西洋海温异常对中国夏季降水以及大尺度环流的可能影响及其机制。结果表明,这两年前夏（6-7月）长江中下游及其以南地区降水均异常偏多,但1998年降水异常较2016年更为显著。后夏（8月）,2016年长江以南地区降水异常偏多,长江-黄河流域降水异常偏少,而1998年降水异常分布与之相反。2016年和1998年夏季中国东部降水异常的差异与西北太平洋对流层低层异常反气旋以及欧亚中高纬度环流变化的共同作用直接相关。敏感性数值试验的结果表明,北大西洋海温异常的显著差异是导致2016年和1998年夏季中国东部降水以及大尺度环流异常存在明显差异的重要原因之一。一方面,北大西洋海温异常可以通过改变欧亚中高纬度环流进而对中国夏季降水产生影响。1998年北大西洋海温异常自热带至副极地呈类似"+ - +"型分布,这种海温异常型能够在前夏欧亚中高纬度地区激发出双阻型的环流异常响应。2016年北大西洋海温异常自热带至副极地呈相对弱的"- + -"型分布,欧亚中高纬度环流异常响应总体偏弱。另一方面,北大西洋海温异常还可以通过影响热带纬向环流进而对西北太平洋对流层低层异常反气旋起调制作用。1998年北大西洋海温异常对夏季西北太平洋异常反气旋起增强作用,这与热带印度洋-太平洋海温的强迫作用相协调。然而,2016年北大西洋海温异常则有利于西北太平洋异常反气旋的减弱,这与热带印度洋-太平洋海温的强迫作用相反。因此,在这3个大洋的协同作用下,2016年和1998年前夏西北太平洋异常反气旋均偏强,但前者的振幅弱于后者。在后夏,1998年西北太平洋对流层低层仍受异常反气旋控制,2016年则为异常气旋控制。   相似文献   

19.
张永生  吴国雄 《气象学报》1998,56(5):513-528
该工作将亚洲季风区作为一个复杂的海-陆-气耦合系统,来深入考察季风区海-气、陆-气相互作用的基本事实和物理过程,探讨它们在决定亚洲季风爆发及北半球行星尺度大气环流的季节突变的物理机理。本文是系列文章的第一篇,着重研究亚洲夏季风爆发的区域性和阶段性特征,以及过渡季节热带、副热带地区海-气、陆-气相互作用的基本事实,初步分析了它们之间的联系。研究表明,热带季风对流于4月底到5月初越过赤道进入北半球,首先出现在孟加拉湾东部-中南半岛西南部地区,然后于5月中旬和6月上旬末分别出现在南海和印度半岛地区,呈阶段性爆发的特征。季风对流在孟加拉湾东部-中南半岛西南部地区爆发阶段,在大气环流变化和对流活动中心位置出现区别于南海季风和印度季风爆发的特征。通过对地表感热通量和海表潜热通量的分析,表明热带海洋上海表感热通量甚小于海表潜热通量,南海季风爆发时期印度洋上海表潜热通量显著增大,印度季风爆发后海表潜热通量的高值中心在孟加拉湾和阿拉伯海上建立起来。印度洋上低层增强的过赤道气流引起的强烈的海-气相互作用导致海表水汽的大量蒸发,并通过其输送作用,为季风对流的爆发提供了充足的水汽来源。过渡季节在副热带地区(沿27.5～37.5°N纬带上), 青藏高原和西太平洋上地(海)表感热通量和潜热通量均有迅速的季节变化性, 但趋势相反。当青藏高原上地表感热通量和潜热通量呈阶段性的显著加大, 西太平洋上海表感热通量和潜热通量迅速减小。这种大陆和海洋对大气加热的显著的季节化的差异, 影响着大气环流的季节转变。  相似文献   

20.
许乐心  张人禾  齐艳军 《大气科学》2017,41(6):1125-1140
利用1979~2013年中国站点逐日降水资料和NCEP/NCAR再分析资料,对长江中下游夏季降水的季节内振荡最显著周期进行了分析研究。结果表明长江中游最显著周期为10~30天,长江下游最显著周期为30~60天。为了揭示这种差异产生的物理原因,进一步利用位相合成的方法对这两个区域不同周期的季节内振荡降水、高低空风场和高度场以及垂直结构和水汽等循环过程的演变特征进行分析。在200 hPa环流场上,长江中游的降水主要受到高纬度自西向东传播的波列影响,而长江下游的降水与鄂霍次克海的高度场的变化相关。在风场的垂直涡度和散度的位相结构演变过程中,10~30天的垂直涡度和散度有自北向南的移动,30~60天的垂直涡度和散度在长江以南地区有自南向北的传播。水汽输送的位相发展过程表明,长江中游的水汽分别来自于南海的向北输送和长江以北地区向南的水汽输送;长江下游地区的水汽则主要来自于热带东印度洋经孟加拉湾的向东输送并在南海的北向输送,以及西太平洋水汽向西输送到南海再向长江下游的输送。从高层大尺度环流场和整层积分的水汽通量输送上解释了长江中游10~30天降水的自北向南移动,和长江下游30~60天降水自南向北传播的原因。  相似文献   

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