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1.
This paper examines the performances of various cumulus convective parameterization schemes in the tropical atmosphere using an aqua-planet atmospheric General Circulation Model forced by zonally symmetric but latitudinally varying sea surface temperature (SST) and solar angle. The intertropical convergence zone (ITCZ) is represented by intense precipitation. The assigned Control experiment with a specific SST distribution, as designated by the Aqua Planet Experiment, yields a single ITCZ when Zhang’s scheme or Manabe’s scheme is employed, whereas a double ITCZ occurs when Tiedtke’s scheme is used. The key to the occurrence of a double ITCZ is latitudinal variation in evaporation within the boundary layer. Such variation is induced mainly by latitudinal variation in the zonal wind speed, with the existence of a calm belt at the equator and a maximum wind speed located off the equator, arising from the evaporation–wind feedback (EWF) mechanism. The latitudinal distribution of evaporation results in a decrease in the height of the lifting condensation level in areas off the equator and an increase at the equator. The occurrence of a single ITCZ in Zhang’s scheme is attributed to the use of a Convective Available Potential Energy criterion by which convection occurs more readily at the equator. As a result, a precipitation maximum is maintained at the equator via a prevailing Conditional Instability of the Second Kind mechanism.  相似文献   

2.
The daily patterns and activity of Intertropical Convergence Zone(ITCZ) in the Western-Central Pacific Ocean are analyzed using NOAA interpolated Outgoing Longwave Radiation dataset during the period from 1979 to 2008, and the relationships between ITCZ patterns and Arctic Oscillation(AO) is investigated in this paper. In accordance with the central activity region the daily ITCZ can be divided into six patterns—north, south, equator, double, full and weak pattern, respectively. The statistic result shows that the north(accounting for 30.98% of the total observations), south(31.11%) and weak(24.05%) ITCZ patterns are the most active daily patterns within a 30-year period, while the other three ITCZ patterns occur infrequently. Results show that the February-April AO index has a significant positive(negative) correlation with the frequency of the north(weak) ITCZ pattern from March-May to August-October, with the strongest correlation in April-June(March-May). At the same time, the lower tropospheric atmosphere circulation(850-hPa wind field) and SST anomalies corresponding to the AO change significantly in the tropical Pacific. When AO is in the positive phase, there is an anomalous westerly from the equator to 15°N and warmer SST in the critical north ITCZ active region, while there is an anomalous easterly and insignificant change of SST from the equator to 15°S. The wind and SST anomalies share the same characteristics of the equatorial asymmetry and thus enlarge the gradient between the south and north of equator, which would help reinforce convection in the north of equator and result in more frequent occurrence of the northern type of ITCZ.  相似文献   

3.
SAMIL模式中Tiedtke 积云对流方案对热带降水模拟的影响   总被引：2，自引：1，他引：1  
刘琨  刘屹岷  吴国雄 《大气科学》2010,34(1):163-174
目前, 大多数全球耦合模式及大气环流模式在降水模拟中普遍存在不同程度的“热带偏差”问题, 中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室所发展的全球大气环流谱模式SAMIL-R42L26也存在这一现象, 主要表现在SPCZ (南太平洋辐合区) 降水过强且过分东伸、 赤道附近降水偏少等方面。本文通过修改SAMIL中的积云对流方案有效地削弱了这一偏差, 并进一步探讨其原因, 发现对流方案修改后, 改变了对流层低层至地面的温度分布状态, 进而影响了风速及散度场的模拟, 最终通过垂直速度的调整反作用于对流过程。比较修改前后对流过程云底质量通量, 发现修改后的方案主要通过削弱浅对流来提高热带降水的模拟性能。  相似文献   

4.
Summary As revealed from the interannual variation of outgoing longwave radiation in the western Pacific, deep cumulus convection along the Meiyü-Baiu front and ITCZ is modulated by the anomalous summer circulation in the following manner: when the sea surface temperatures on the eastern tropical Pacific are anomalously warm (cold), cumulus convection is enhanced (suppressed) along the equator east of 150° E and along the Meiyü-Baiu front, but is suppressed (enhanced) along the equator west of 150° E and along a longitudinal zone (10° N–30° N) extending from the northern section of the South China Sea to the International Dateline. Since tropical deep cumulus convection exhibits a pronounced diurnal variation, the diurnal convection cycle in the western Pacific may undergo an interannual variation coherent with that of deep tropical cumulus convection. This inference is substantiated by our analysis of the diurnal convection cycle for 1980–1993 with 3-hour equivalent black-body temperature observed by the Japanese Geostationary Meteorological Satellite (GMS). As expected, the diurnal convection cycle in the western Pacific is subjected to an interannual variation in accordance with deep cumulus convection along the Meiyü-Baiu front and ITCZ. Except along the equator east of 150° E, the diurnal convection cycle does not exhibit a drastic interannual change in phase.  相似文献   

5.
Previous aqua-planet simulations reported in the literature have shown the existence of both single and double inter-tropical convergence zones (ITCZs). In this study, horizontal grid resolution strongly affects ITCZ morphology as well as the amount of tropical precipitation through its influence on resolved dynamics. The grid adaptation capability of our global model enables simulations that separate the influence of tropical and extra-tropical dynamics on both the ITCZ and tropical precipitation. The presence of single versus double ITCZs in our aqua-planet simulations depends on the resolution of convectively coupled equatorial waves. When the tropical resolution is sufficiently high to resolve prominent equatorial waves a double ITCZ occurs, otherwise a single ITCZ occurs. In contrast, tropical resolution does not affect the magnitude of tropical precipitation in our aqua-planet simulations. Instead the magnitude is sensitive to extra-tropical resolution, through its influence on the strength of baroclinic eddies and their forcing of the Hadley circulation.  相似文献   

6.
This paper investigates the processes behind the double ITCZ phenomenon, a common problem in Coupled ocean-atmosphere General Circulation Models (CGCMs), using a CGCM-FGCM-0 (Flexible General Circulation Model, version 0). The double ITCZ mode develops rapidly during the first two years of the integration and becomes a perennial phenomenon afterwards in the model. By way of Singular ValueDecomposition (SVD) for SST, sea surface pressure, and sea surface wind, some air-sea interactions are analyzed. These interactions prompt the anomalous signals that appear at the beginning of the coupling to develop rapidly. There are two possible reasons, proved by sensitivity experiments: (1) the overestimatedeast-west gradient of SST in the equatorial Pacific in the ocean spin-up process, and (2) the underestimatedamount of low-level stratus over the Peruvian coast in CCM3 (the Community Climate Model, VersionThree). The overestimated east-west gradient of SST brings the anomalous equatorial easterly. The anomalous easterly, affected by the Coriolis force in the Southern Hemisphere, turns into an anomalouswesterly in a broad area south of the equator and is enhanced by atmospheric anomalous circulationdue to the underestimated amount of low-level stratus over the Peruvian coast simulated by CCM3. Theanomalous westerly leads to anomalous warm advection that makes the SST warm in the southeast Pacific.The double ITCZ phenomenon in the CGCM is a result of a series of nonlocal and nonlinear adjustmentprocesses in the coupled system, which can be traced to the uncoupled models, oceanic component, andatmospheric component. The zonal gradient of the equatorial SST is too large in the ocean componentand the amount of low-level stratus over the Peruvian coast is too low in the atmosphere component.  相似文献   

7.
A statistically-based low-level cloud parameterization scheme is introduced, modified, and applied in the Flexible coupled General Circulation Model (FGCM-0). It is found that the low-level cloud scheme makes improved simulations of low-level cloud fractions and net surface shortwave radiation fluxes in the subtropical eastern oceans off western coasts in the model. Accompanying the improvement in the net surface shortwave radiation fluxes, the simulated distribution of SSTs is more reasonably asymmetrical about the equator in the tropical eastern Pacific, which suppresses, to some extent, the development of the double ITCZ in the model. Warm SST biases in the ITCZ north of the equator are more realistically reduced, too. But the equatorial cold tongue is strengthened and extends further westward, which reduces the precipitation rate in the western equatorial Pacific but increases it in the ITCZ north of the equator in the far eastern Pacific. It is demonstrated that the low-level cloud-radiation feedback would enhance the cooperative feedback between the equatorial cold tongue and the ITCZ. Based on surface layer heat budget analyses, it is demonstrated that the reduction of SSTs is attributed to both the thermodynamic cooling process modified by the increase of cloud fractions and the oceanic dynamical cooling processes associated with the strengthened surface wind in the eastern equatorial Pacific, but it is mainly attributed to oceanic dynamical cooling processes associated with the strengthening of surface wind in the central and western equatorial Pacific.  相似文献   

8.
观测表明,高云的夏季块状分布和冬季带状分布,与低层赤道辐合带的夏季与冬季的形状十分相似;并且卷云和卷层云可以独立于深对流单独存在。作者对这两个观测分析结果进行动力学分析,结论如下:1)由于印度洋北面是青藏高原与亚洲大陆,夏季不能在北面副热带地区形成反气旋,从而印度洋赤道北面为西南气流,导致了赤道辐合带在该地区断裂并且相应的深对流在亚洲季风区的块状分布。2)利用斜压超长波理论,将Rodwell等的亚洲季风单向模型(即非绝热加热导致季风形成)作了修改,扩展为双向闭合模型。印度洋跨赤道偏南风产生大范围水汽辐合,其与地形的共同作用,产生了降水云系的高层加热,由于Sverdrup涡度平衡关系,导致了低层的偏南风而形成了一个相互作用的闭合过程,从而表明了亚洲夏季风是准定常的。3)通过详细分析涡度方程,证明除了恰好在赤道上之外,赤道辐合带上的水平辐合均会产生涡,并且这些涡由点涡(涡度的奇异部分)与各种尺度的涡(涡度的正则部分)组成。正涡度对应于云区,负涡度对应于晴空区,与赤道辐合带(ITCZ)的观测结果一致。4)由于辐合和切变产生涡,得到赤道辐合带和深对流的带状准定常维持的动力机制,即:由于赤道辐合带的辐合,其南北风辐合与东西风切变将产生涡,其与水汽的共同作用产生了深对流的上升降水云系,而降水云系的潜热诱导上升,进一步加强了水平辐合,从而表明了赤道辐合带的带状准定常维持的中介是不同尺度的涡。5)卷云和卷层云可以独立于深对流的原因是热带卷云和卷层云与流场是可以互相激发的,深对流不是其唯一的源。  相似文献   

9.
 The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level. Received: 7 April 1997 / Accepted: 15 July 1998  相似文献   

10.
文中研究了耦合海-气环流模式中的双热带辐合带(Double ITCZ)现象,并对这一现象的成因从海洋热量收支的角度进行了初步分析。Double ITCZ现象是在热带太平洋赤道南北两侧各出现一条ITCZ的现象,这是耦合海-气环流模式中的较为普遍的一种异常现象,与实际气候中出现的Double ITCZ现象并非指同一问题。文中对比观测和模式结果,指出了Double ITCZ现象的主要特征,针对它的出现过程进行细致分析,再利用模式输出的热量收支各项进行统计,得到了从海洋热收支分析得到的海表温度变化原因。与观测到的正常模态相比,Double ITCZ是一个异常的模态,它的特征突出地表现为西太平洋暖池区的降温和东南太平洋10°S附近的升温。海洋热量收支分析表明,暖池区的降温主要是受到扩散的作用,而表层平流的异常增暖在决定异常辐合带区升温过程中占第一位的作用。  相似文献   

11.
Through study of observations and coupled climate simulations, it is argued that the mean position of the Inter-Tropical Convergence Zone (ITCZ) north of the equator is a consequence of a northwards heat transport across the equator by ocean circulation. Observations suggest that the hemispheric net radiative forcing of climate at the top of the atmosphere is almost perfectly symmetric about the equator, and so the total (atmosphere plus ocean) heat transport across the equator is small (order 0.2 PW northwards). Due to the Atlantic ocean’s meridional overturning circulation, however, the ocean carries significantly more heat northwards across the equator (order 0.4 PW) than does the coupled system. There are two primary consequences. First, atmospheric heat transport is southwards across the equator to compensate (0.2 PW southwards), resulting in the ITCZ being displaced north of the equator. Second, the atmosphere, and indeed the ocean, is slightly warmer (by perhaps 2 °C) in the northern hemisphere than in the southern hemisphere. This leads to the northern hemisphere emitting slightly more outgoing longwave radiation than the southern hemisphere by virtue of its relative warmth, supporting the small northward heat transport by the coupled system across the equator. To conclude, the coupled nature of the problem is illustrated through study of atmosphere–ocean–ice simulations in the idealized setting of an aquaplanet, resolving the key processes at work.  相似文献   

12.
Summary Based on ESSA-satellite imagery for the period July 1969–June 1970, this study investigates spatial and temporal variations of East African cloudiness. The major results of this work show that the mean annual cloud amounts over East Africa are lower than those in adjacent tropical areas. One of the main reasons for this is the quasi-meridional alignement of the ITCZ over East Africa during the winter months. Within the area itself, the highest mean annual cloud amount values can generally be found in a diagonally oriented zone extending from the eastern Congo Basin to the Ethiopian Highlands. In contrast to the cloudiness north of the equator, which is dominated by oscillation periods in the range of 30–60 days, the cloudiness fluctuations encountered south of the equator show periodicities around 2 days (in the western part) and 20 days (in the eastern part), respectively. The different oscillation patterns, which are roughly separated by the Rift Valley area (longitudinally) and the equator (latitudinally), resemble the signals of the adjacent (African and Asian) monsoon regimes. However, during the winter months oscillation periods around 40 days can be found north of the equator, whereas a quasi-biweekly oscillation appears over the coastal areas in summer. Further details of the seasonal variability of East African cloudiness are discussed.With 8 Figures  相似文献   

13.
The simulation of the mean seasonal cycle of sea surface temperature (SST) remains a challenge for coupled ocean–atmosphere general circulation models (OAGCMs). Here we investigate how the numerical representation of clouds and convection affects the simulation of the seasonal variations of tropical SST. For this purpose, we compare simulations performed with two versions of the same OAGCM differing only by their convection and cloud schemes. Most of the atmospheric temperature and precipitation differences between the two simulations reflect differences found in atmosphere-alone simulations. They affect the ocean interior down to 1,000 m. Substantial differences are found between the two coupled simulations in the seasonal march of the Intertropical Convergence Zone in the eastern part of the Pacific and Atlantic basins, where the equatorial upwelling develops. The results confirm that the distribution of atmospheric convection between ocean and land during the American and African boreal summer monsoons plays a key role in maintaining a cross equatorial flow and a strong windstress along the equator, and thereby the equatorial upwelling. Feedbacks between convection, large-scale circulation, SST and clouds are highlighted from the differences between the two simulations. In one case, these feedbacks maintain the ITCZ in a quite realistic position, whereas in the other case the ITCZ is located too far south close to the equator.  相似文献   

14.
Summary The global-scale intraseasonal and annual variations of divergent water-vapor transport and water vapor itself were examined by using outgoing longwave radiation (OLR) and data for 1979–1986 produced by the Global Data Assimilation System of the National Meteorological Center. An effort was also made to contrast results of this study with previous analyses of OLR and upper-level divergent circulation.As for intraseasonal oscillation, positive (negative) precipitable-water (W) anomalies and negative (positive) OLR couple with the convergent (divergent) center of the potential function of water vapor transport () anomalies and the divergent (convergent) center of upper-level divergent-circulation anomalies. It is inferred that the eastward-propagating divergent circulation of intraseasonal oscillation converges water vapor to maintain cumulus convection, which releases latent heat, possibly to support this low-frequency oscillation. Fluctuations of W and cumulus convection associated with this oscillation are large over the equatorial Indian Ocean and the equatorial western Pacific, but small over the tropical Americas and equatorial Africa. Moreover, during northern summer, W anomaly bands migrate regularly northward, following the low-level transient 30–50 day monsoon troughs and ridges over the northern Indian Ocean. To the south of the equator, a regular southward propagation of W anomaly bands is identified in both northern summer and winter. In contrast; over the northwestern Pacific, a signature depicting the north-south intraseasonal oscillation of the north Pacific Convergence Zone can be inferred by W anomalies.The annual cycle components of W and cumulus convection inferred from OLR anomalies exhibit three pairs of maximum-minimum centers over tropical continents. These centers correspond to those of and upper-level divergent circulation anomalies. It is shown that landmass cooling in the winter hemisphere and landmass warming in the summer hemisphere establish a pair of upper-level convergent-divergent centers over each tropical continent. Water vapor is converged (diverged) by divergent circulation, in order to maintain maximum (minimum) centers of W and cumulusconvection anomalies over each tropical continent.With 7 Figures  相似文献   

15.
Summary  The circulation mechanisms instrumental in the origin of the Pacific equatorial dry zone are studied from a combination of data sources. A triple structure of convergence zones enclosing a near-equatorial zone of surface divergence is well developed only in March-April. Over the eastern Pacific the divergence band is centered to the north of the equator. Downstream acceleration and meridional divergence in the cross-equatorial flow from the southern hemisphere result from the rightward directed and northward increasing Coriolis acceleration. Upper-tropospheric convergence and subsidence along the equator is compensated by divergence in the realm of the Equatorial Mid-Tropospheric Easterly Jet. While not feeding the divergence near the surface, the subsidence throughout the mid troposphere is unconducive to deep convection and may thus also contribute to the scarcity of cloudiness. Proceeding from the eastern towards the central Pacific, the mid-tropospheric jet vanishes, the cross-equatorial surface airflow fades out, and concordant with the axis of smallest upward motion the divergence maximum and cloudiness minimum shift to south of the equator. Received January 5, 1999/Revised July 29, 1999  相似文献   

16.
A 10-year simulation with a coupled ocean-atmosphere general circulation model (CGCM) is presented. The model consists of the climate version of the Météo-France global forecasting model, ARPEGE, coupled to the LODYC oceanic model, OPA, by the CERFACS coupling package OASIS. The oceanic component is dynamically active over the tropical Pacific, while climatological time-dependent sea surface temperatures (SSTs) are prescribed outside of the Pacific domain. The coupled model shows little drift and exhibits a very regular seasonal cycle. The climatological mean state and seasonal cycle are well simulated by the coupled model. In particular, the oceanic surface current pattern is accurately depicted and the location and intensity of the Equatorial Undercurrent (EUC) are in good agreement with available data. The seasonal cycle of equatorial SSTs captures quite realistically the annual harmonic. Some deficiencies remain including a weak zonal equatorial SST gradient, underestimated wind stress over the Pacific equatorial band and an additional inter-tropical convergence zone (ITCZ) south of the equator in northern winter and spring. Weak interannual variability is present in the equatorial SST signal with a maximum amplitude of 0.5°C.  相似文献   

17.
Summary The ITCZ (Intertropical Convergence Zone) is an important parameter for climatic studies in tropical areas, and meteorological satellite imagery provides an original way to follow its location. Using archive imagery covering the 1971–1987 period, we attempted to study further some of the relationships (suggested by former studies) between ITCZ locations (followed here over the Atlantic ocean at 28°W), and climate anomalies in the Sahel, an area affected by periodic drought for the last seventeen years. We also paid close attention to more frequently studied parameters, such as upper air data, wind at sea level, and sea surface temperature. As for relative drought estimates, we assumed that runoff from the Senegal River was representative of the sahelian area and we observed that its variations were consistent with the Lamb's rainfall index over the 1965–1987 period.Since the onset of the rainy season for West Africa responds to wind changes, we assessed the link between ITCZ and wind at sea level and found the timing of northward ITCZ migration to be highly correlated (r=0.84) with the date of zonal wind stress intensification.On a general point of view, the relationships we found between rainfall amount and ITCZ position anomalies (or SST anomalies) agree with known results of precedent works, though better fit is found with the seventies than the eighties. We think this discrepancy is due in part to the fact that the parameters studied were not identical and, perhaps to a possible change in climatic conditions (on a long term basis, the data show a continuous trend for less intense equatorial upwelling in the gulf of Guinea, and our time series covers a more recent period than referenced works).With a closer look on the first half of the year, it appears that typical (wet/dry) schemes of the ITCZ migration can be evidenced more clearly, than in reporting the northernmost ITCZ location, that we found to be a less significant index: in other words, a sooner (respectively later) northward ITCZ migration corresponds to dry (respectively wet) episodes during the rainy season in sahelian areas. Hence, we propose the speed of ITCZ northwards movement as a parameterization of this event.Moisture content of the lower troposphere revealed that steady anomalies of this parameter may last several years over sahelian areas. Taking into consideration the relative strength African tropical and easterly jets, some limited results were obtained, in regard of climatic anomalies.As first conclusions, moisture transportation over sahelian area (associated with larger negative SST anomalies) is more efficient for wetter rainy season, than the intensity of convective process linked to higher local SST in the equatorial Atlantic area. In joining moisture analysis and ITZ migration (1980–1987 period), wetter rainy seasons were observed each time that positive humidity anomalies coincided with a later northward ITCZ migration (or greater northward ITCZ speed).With 8 Figures  相似文献   

18.
1986—1987年冬季亚澳地区大气环流异常主要表现在北半球中纬盛行纬向气流,副热带西风急流、西太平洋副热带高压脊及ITCZ位置异常偏北;西北太平洋热带气旋活动频繁;赤道盛行异常西风和异常南风;澳大利亚海平面气压偏高等现象。在这种大气环流异常形势下,东亚地区冬季风偏弱,冷空气主要在偏北地区东移,温度异常偏高,中国北方降水偏多,南方降水偏少。与此同时,澳大利亚夏季风偏弱并推迟一个月建立,整个澳大利亚地区降水偏少。大气环流异常是从低纬开始的。ITCZ位置异常偏于北半球,比其他环流系统异常要早一个月以上发生。1986年夏季以来,西太平洋赤道附近及其偏北地区SST始终维持异常偏高,可能是造成ITCZ异常偏北的原因,并进而引起瓦克环流减弱和南方涛动指数(SOI)偏低,从而对1986—1987年的ENSO事件的发生起了促进的作用。1986年夏秋季节西北太平洋30°N950hPa上北风异常,冬季南风异常又可能是引起西太平洋SST异常分布的原因,因此,1986—1987年冬季亚澳地区的异常事件必须从海—气相互作用的观点来加以说明。   相似文献   

19.
Summary The response of Lake Victoria basin climate to changes in the lake surface temperatures (LST) has been examined using NCAR-Regional climate model (RegCM2). In the control run uniform lake surface temperature of 24°C was prescribed and the model integrated for four months, starting at the beginning of September, 1988. In the anomaly experiments the LST was perturbed by ±1.5°C, and kept constant during the entire period of the integrations.Simulation results show significant relationship between basin-wide spatial distribution of rainfall and changes in LST. In general during the short rains at warmer/cooler LSTs, significant increase/decrease in the simulated rainfall occurs over the lake surface and surrounding areas. Rainfall exceeding the amount in the control run by more than 50%, particularly over the western, south/southwestern and central parts of the lake is simulated in the run in which the LST is 1.5°C warmer than the control. It is also evident from our results that different parts of the lake basin respond differently to LST changes which is in contrast to the common characterization of the lake basin as a single homogeneous climate regime in many previous studies.In general the results show that regions with largest response to LST anomalies during the short rains are collocated with the ITCZ. In October when the ITCZ is directly located over the lake, the largest response (maximum rainfall) is also located over the same region. As the season progresses and the ITCZ shifts out of the lake into northern Tanzania, the regions of rainfall maxima also shift with it. This appears to explain the unexpected reduction in over-lake rainfall in December in spite of the LST being warmer than control by 1.5°C. We believe this is a direct consequence of the enhanced convection to the south of the lake (over ITCZ) and the tendency of the system to conserve local moisture budget over the lake.  相似文献   

20.
The atmospheric general circulation models ARPEGE-climate and LMDz are used in an aquaplanet configuration to study the response of a zonally symmetric atmosphere to a range of sea surface temperature (SST) forcing. We impose zonally-symmetric SST distributions that are also symmetric about the equator, with varying off-equatorial SST gradients. In both models, we obtain the characteristic inter-tropical convergence zone (ITCZ) splitting that separates two regimes of equilibrium (in terms of precipitations): one with one ITCZ over the equator for large SST gradients in the tropics, and one with a double ITCZ for small tropical SST gradients. Transition between these regimes is mainly driven by changes in the low-level convergence that are forced by the SST gradients. Model-dependent, dry and moist feedbacks intervene to reinforce or weaken the effect of the SST forcing. In ARPEGE, dry advective processes reinforce the SST forcing, while a competition between sensible heat flux and convective cooling provides a complex feedback on the SST forcing in the LMDz. It is suggested that these feedbacks influence the location of the transition in the parameter range.  相似文献   

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