A study on the distinct features of the Asian summer monsoon during the years of extreme monsoon activity over India

Summary In this paper, interseasonal characteristics of the Asian summer monsoon in the years of 1987 and 1988 are studied as 1987 is characterized by a large deficiency of monsoon rainfall (drought) and that of 1988 by a large excess monsoon rainfall (flood) over India. In order to compare the similarities and differences seen in the large scale dynamics and energetics of the Asian summer monsoon during the years of extreme monsoon activity, uninitialized analyses (12 Z) of the European Centre for Medium Range Weather Forecasts (ECMWF), U.K. are utilized in this study for the summer monsoon seasons of 1987 and 1988.It is found that the excess rainfall season (1988) is characterized by much stronger tropical easterly jet (TEJ) associated with the upper tropospheric easterlies and the East African low level jet (Somali Jet) associated with lower tropospheric westerlies. Such a feature mainly determines the strength of the reverse Hadley circulation which normally covers the South Asian continent during the northern summer. Further, the energetics of the TEJ show that the monsoon of 1988 has comparatively stronger zones of kinetic energy flux divergence (convergence) at its entrance (exit) regions. These zones of kinetic energy flux divergence are largely maintained by the adiabatic processes over the strong kinetic energy flux divergence zones over the Bay of Bengal and east central Arabian Sea as compared to that of 1987. Apart from this, both the zonal and meridional components of the ageostrophic flows are found to be stronger during 1988 monsoon season. Analysis of the vertically integrated thermodynamical features of the monsoon indicate that the monsoon of 1988 was characterized by an excess import of heat and moisture into the monsoon atmosphere as compared to that of 1987. Further, from the quantitative estimation of certain significant heat and moisture budget parameters during the contrasting monsoon seasons of 1987 and 1988, it becomes evident that considerable differences exist in the quantities of adiabatic production of heat energy, diabatic heating and the moisture source/sink.With 13 Figures     

EVOLUTION OF TROPOSPHERIC TEMPERATURE FIELDS AND CORRESPONDING THERMAL MECHANISMS BEFORE/AFTER ONSET PERIODS OF ASIAN SUMMER MONSOON

The evolution of the tropospheric temperature fields over Indian and South China Sea monsoon areas and their thermal mechanisms are compared and analyzed during the period from March to June, 1996. The results show that the onsets of the Indian and South China Sea summer monsoons are closely associated with the seasonal warming in the troposphere over the zonal belt of 10°N～30°N in these areas, which leads to the inversion of meridional temperature gradient. During the pre-onset period, the warming over the South China Sea monsoon region is mainly due to the warm horizontal advection and diabatic (latent) heating processes. Meanwhile, the warming is suppressed by the vertical adiabatic process (cooling). In spring over the Indian monsoon region, the significant adiabatic heating due to the subsidence motion, which compensates the cooling due to the strong cold advection and diabatic cooling processes, results in a larger warming rate than over the South China Sea monsoon region. However, the meridional temperature gradient over the Indian monsoon region is so large during the late winter and early spring that it takes longer time to warm the troposphere to have the reversion of meridional temperature gradient than it does over the South China Sea monsoon region. It results in the phenomenon that the South China Sea summer monsoon generally breaks out earlier than the Indian summer monsoon.     

Seasonal Variation of the Meridional Wind in the Temperate Jet Stream and Its Relationship to the Asian Monsoon

The features of the temperate jet stream including its location, intensity, structure, seasonal evolution and the relationship with the Asian monsoon are examined by using NCEP/NCAR reanalysis data. It is indicated that the temperate jet stream is prominent and active at 300 hPa in winter over the region from 45°-60°N and west of 120°E. The temperate jet stream is represented by a ridge area of high wind speed and dense stream lines in the monthly or seasonal mean wind field, but it .corresponds to an area frequented by a large number of jet cores in the daily wind field and exhibits a distinct boundary that separates itself with the subtropical jet. A comparison of the meridional wind component of the temperate jet stream with that of the subtropical jet shows that the northerly wind in the temperate jet stream is stronger than the southerly component of the subtropical jet, which plays an important role in the temperate jet stream formation and seasonal evolution, and thus the intensity change of the meridional wind component can be used to represent the temperate jet stream＇s seasonal variation. Analysis of the temperature gradient in the upper troposphere indicates that the temperate jet stream is accompanied by a maximum zonal temperature gradient and a large meridional temperature gradient, leading to a unique jet stream structure and particular seasonal evolution features, which are different from the subtropical jet. The zonal temperature gradient related to the land-sea thermal contrast along the East China coastal lines is responsible for the seasonal evolution of the temperate jet. In addition, there exists a coordinated synchronous change between the movement of the temperate jet and that of the subtropical jet. The seasonal evolution of the meridional wind intensity is closely related to the seasonal shift of the atmospheric circulation in East Asia, the onset of the Asian summer monsoon and the start of Meiyu in the Yangtze and Huaihe River Valleys, and it correlates well with summer and wint     

Intraseasonal modulation of tropical cyclogenesis in the western North Pacific: a case study

The temporal clustering of the western North Pacific tropical cyclogenesis and its modulation by the Madden–Julian oscillation (MJO) during the 1991 summer were examined based on the tropical cyclone best track, outgoing longwave radiation, and NCEP/NCAR reanalysis datasets. The wavelet analysis shows that convective activities around the monsoon trough in the western North Pacific possessed a distinct MJO with a period of 20–60 days. Two or more tropical cyclones were observed to form successively during each active phase of the MJO, and tropical cyclones tended to generate around the southeastern part of the maximum vorticity of the low-frequency cyclonic circulation during the developing and peak stages of the active MJO phase. But tropical cyclogenesis scarcely occurred during inactive MJO phases. Thus the MJO was a major agent in modulating repeated development of tropical cyclones in the western North Pacific during the 1991 summer. The MJO in circulation was characterized by a huge anomalous cyclone (anticyclone) in the lower troposphere existing alternately over the western North Pacific, leading to an enhanced (weakened) monsoon trough. An examination of the meridional gradient of absolute vorticity associated with the zonal flow indicates that the zonal flow in the monsoon trough region satisfied the necessary conditions for barotropic instability, with both zonal flow and the meridional gradient of absolute vorticity varying on the similar MJO timescale. The intraseasonal oscillation of such an unstable zonal flow might thus be an important mechanism for temporal clustering of tropical cyclogenesis in the western North Pacific. The barotropic conversion could provide a major energy source for the formation and growth of tropical cyclones in the western North Pacific during active MJO phases, with the eddy kinetic energy generation being dominated by both terms of eddies interacting with zonal and meridional gradients of the basic zonal flow.     

Some dynamical properties of idealized thermally-forced meridional circulations in the tropics

Summary Through the use of a zonal balance model we investigate the properties of the tropical meridional circulation to a range of specified diabatic forcing fields for climatologically observed zonal winds. As in earlier studies, the solutions show that latent heat release away from the equator forces an asymmetric meridional circulation in response the anisotropy in the inertial stability parameter with respect to the meridional location of the forcing. The presence of strong zonal flows appears to play a relatively minor role in determining the magnitude and asymmetry of the meridional circulation, whereas the structure of the diabatic heating, particularly the meridional breadth, proves to be of much greater importance.A dynamic efficiency factor, which provides an analytic measure of the efficacy of diabatic heating at generating zonal kinetic energy, generally exhibits a meridionally symmetric structure except during Northern Hemisphere summer. This asymmetry gives rise to a pronounced sensitivity of zonal kinetic energy generation to the meridional location of ITCZ convection. Further examination of the flow pattern suggests that for zonal flows representative of those over the Indian Ocean during the Northern Hemisphere summer months, meridional displacements of the heating of less than 20° latitude can result in as much as an order of magnitude difference in the rate of kinetic energy generation. Solution of the balance system also implies the existence of a feedback mechanism, between zonally-organized convection and the energetics properties of the large-scale flow, that is highly sensitive to the meridional location of the convection.With 11 FiguresThe National Center for Atmospheric Research is sponsored by the National Science Foundation.     

亚洲夏季风建立前后对流层温度场演变特征及其热力成因

对１９９６年亚洲夏季风爆发前后（３～６月）印度及南海季风区对流层温度演变特征及其热力成因作了比较分析。结果表明：印度和南海夏季风的爆发与各季风区对流层中上层南北温差逆转密切相关,而南北温差逆转是由１０～３０°Ｎ之间纬度带对流层的季节性增暖引起的。夏季风爆发前期,南海季风区的增温主要由暖平流及非绝热加热过程（主要为凝结潜热）共同作用所致。春季在印度季风区大陆上空存在显著的下沉绝热增温,使得对流层中上层的增温率比华南大陆及邻近地区上空的增温率显著得多。但印度季风区冬末春初的南北温差（南暖北冷）也非常明显,以至该地区对流层中上层增暖到引起南北温差发生逆转的时间较迟,而南海季风区对流层中层南北温差发生逆转的时间相对要早,因而印度夏季风比南海夏季风迟爆发。     

夏季南亚—西太平洋地区对流层环流的演变及其与低纬扰动的关系

本文着重讨论了对流层中、上层大气环流的演变对低纬度扰动的制约作用。应用两层模式中的扰动振幅方程,按实际的环流形势所确定的风场随纬度和高度的分布,计算了扰动特征相速及其振幅在区域内的分布,考察了南亚——西太平洋地区低纬度扰动的结构和性质。      

Characteristics of Zonal Propagation of Atmospheric Kinetic Energy at Equatorial Region in Asia

Based on the daily NCEP/NCAR reanalysis dataset from 1980 to 1997, the zonal propagations of 850 hPa kinetic energy (KE) and meridional wind (v) at equatorial region are examined respectively. Results show that the strongest center of KE in the tropical Asian monsoon region is located at 75°-90°E, with the secondary over the Somalia low-level jet channel, i.e., about 50°E. East to 90°E, disturbances of both KE and v observed are mainly coming from the western Pacific Ocean and propagating westward to the Bay of Bengal (BOB) passing through the South China Sea. But the propagation directions of both KE and v are rather disorderly between the BOB and the Somalia jet channel. Therefore, the East Asian summer monsoon and the Indian summer monsoon are different in the propagating features of the disturbances of KE and v. Above facts indicate that East Asian monsoon system exists undoubtedly even at the equatorial region, and quite distinct from the Indian monsoon system, it is mainly affected by the disturbances coming from the tropical western Pacific rather than from the Indian monsoon region. The boundary of the two monsoon systems is around 95°-100°E, which is more westward than the counterpart as proposed in earlier studies by 5-10 degrees in longitude.     

Barotropic Process Contributing to the Formation and Growth of Tropical Cyclone Nargis

This study reveals the barotropic dynamics associated with the formation and growth of tropical cyclone Nargis in 2008,during its formation stage.Strong equatorial westerlies occurred over the southern Bay of Bengal in association with the arrival of an intraseasonal westerly event during the period 22-24 April 2008. The westerlies,together with strong tropical-subtropical easterlies,constituted a large-scale horizontal shear flow,creating cyclonic vorticity and thereby promoting the incipient disturbance that eventually evolved into Nargis.This basic zonal flow in the lower troposphere was barotropically unstable,with the amplified disturbance gaining more kinetic energy from the easterly jet than from the westerly jet during 25-26 April. This finding suggests that more attention should be paid to the unstable easterly jet when monitoring and predicting the development of tropical cyclones.Energetics analyses reveal that barotropic energy conversion by the meridional gradient of the basic zonal flow was indeed an important energy source for the growth of Nargis.     

亚洲夏季风结构和变动与大气运动的斜压和正压特征:斜压模分析

通过定义流场的斜压性度量参量，利用观测资料，对大气运动的斜压特征量与夏季风变动的联系进行了探讨。结果表明：夏季风流场具有较强的斜压性；纬向气流的斜压强弱的时间变动与夏季风进退相吻合；强斜压区的范围可反映夏季风活动的基本范围。经向斜压运动强弱的水平分布及时间－经度剖面上斜压运动动能的纬向传播都与亚洲季风体系中季风子系统有关     

南海夏季风爆发前后深对流传播的多向性

根据1979～2004年NCEP/NCAR的逐日OLR资料, 对南海夏季风爆发前后南海地区热带深对流的纬向和经向传播特征进行分析, 结果表明, 在南海夏季风爆发前后, 热带深对流的传播方向无论是纬向还是经向在各年都不尽相同, 亦即具有多向性。在纬向上, 南海季风区深对流既可以由孟加拉湾向东传播到南海, 也可以由西太平洋向西传播到南海, 也可在南海地区直接生成。但在此时期, 印度季风区纬向传播相对单一, 主要为向西传播。在经向上, 南海地区的对流活动可以受到来自南半球热带地区和北半球中纬度的共同影响, 但印度季风区主要受热带地区对流的影响。上述结果表明, 南海夏季风爆发期季风区对流活动远比印度季风区复杂, 南海夏季风爆发的深对流云系既可来源于其东西侧, 也可来源于其南北侧, 或是局地发展。作者进一步分析了造成不同深对流来源的机制, 发现低空副高不同的移动路径是造成这一多样性的可能原因, 这可能也是目前对部分年份南海夏季风爆发日期确定存在争议的原因。     

Some Aspects of the Characteristics of Monsoon Disturbances Using a Combined Barotropic－Baroclinic Model

ＳｏｍｅＡｓｐｅｃｔｓｏｆｔｈｅＣｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆＭｏｎｓｏｏｎＤｉｓｔｕｒｂａｎｃｅｓＵｓｉｎｇａＣｏｍｂｉｎｅｄＢａｒｏｔｒｏｐｉｃ－ＢａｒｏｃｌｉｎｉｃＭｏｄｅｌ￥Ｎ．Ｒ．ＰａｒｉｊａａｎｄＳ．Ｋ．Ｄａｓｈ（ＣｅｎｔｒｅｆｏｒＡ...     

Medium-range oscillations in the summer tropical easterlies at 200 hPa

By the use of space-time spectral analysis and band-pass filter, some of the features of the medium-range Oscillations in the summer tropical easterlies (10oS-20o) at 200 hPa are investigated based on a two-year (1980 and 1982) wind (u, v) data set for the period from May to September. Space-time power spectral analysis shows that the total energy of the westward moving waves was the largest and that of the standing waves and eastward moving waves was relatively small in the 200 hPa easterlies; the total energy of the eastward moving waves was at minimum at 10oN. Three kind of the medium-range oscillations with about 50 day, 25 day and quasi-biweekly periods were found in the easterlies, which all show a remarkable interannual variation and latitudinal differences in these two years. The wave energy of zonal wind is mainly associated with the planetary waves (1-3), which all may make important contributions to the 50 day and 25 day oscillations in different years or different latitudes. The quasi-biweekly oscillation is mainly related to the synoptic waves (4-6). In equatorial region, the 50 day oscillation was dominant with a eastward phase propagation in 1982 while the dominant oscillation in 1980 was of 25day period with a westward phase propagations in 1980. Both of them are of the mode of zonal wavenumber 1. Strong westward 50 day oscillation was found in 10oN-20oN in these two years. Regular propagations of the meridional wind 50 day oscillation were also found in the easterlies.The 50 day and 25 day oscillation of zonal wind all demonstrate southward phase propagation over the region of the South Asia monsoon and northward phase propagation near interational date line, where are the climatic mean position of the tropical upper-tropospheric easterly jet and the tropical upper tropospheric trough (TUTT), respectively.     

热带对流层低频振荡过程的能量学初步分析

本文利用线性化的能量方程和交叉谱方法分别研究了准两周振荡和准40天振荡的能量来源及其转换过程。发现:在热带对流层中,对于低频振荡过程来说,来自中高纬度的侧向强迫作用和水汽凝结的加热作用是非常重要的,为扰动的产生和维持提供了重要能源。而正、斜压不稳定作用对扰动发展的作用是极小的。并且指出,在东亚热带对流层上部,准两周振荡通过与平均气流的相互作用为基本气流提供许多能量,对于维持这里的高空东风急流有重要作用。准40天振荡向平流层输送能量,对平流层平均环流的演变可能有重要贡献。      

亚洲夏季风结构和变动与大气运动流场的斜压和正压特征:正压模分析

使用ＥＣＭＷＦ１９８０—１９８６年７ａ格点资料对大气运动的正压模进行了分析。指出：对流层中大气正压运动流场显示的副热带高压带仅能反映出行星风带的季节性移动；与亚洲夏季风有关且反映季风局地性的则是副热带高压带南侧的东风带上的波状扰动；东风带上的经向风分量存在着纬向传播且传播方向和扰动幅度与印度、东亚季风子系统有关；随着北半球夏季的到来，亚洲季风区大气运动的斜压模有较大的增长且斜压运动动能占气柱内整层平均总动能的绝大部分，而在赤道附近的其他经度上则是正压运动动能的成分明显增长。     

Changes in the normal mode energetics of the general atmospheric circulation in a warmer climate

Changes in the normal mode energetics of the general atmospheric circulation are assessed for the northern winter season (DJF) in a warmer climate, using the outputs of four climate models from the Coupled Model Intercomparison Project, Phase 3. The energetics changes are characterized by significant increases in both the zonal mean and eddy components for the barotropic and the deeper baroclinic modes, whereas for the shallower baroclinic modes both the zonal mean and eddy components decrease. Significant increases are predominant in the large-scale eddies, both barotropic and baroclinic, while the opposite is found in eddies of smaller scales. While the generation rate of zonal mean available potential energy has globally increased in the barotropic component, leading to an overall strengthening in the barotropic energetics terms, it has decreased in the baroclinic component, leading to a general weakening in the baroclinic energetics counterpart. These global changes, which indicate a strengthening of the energetics in the upper troposphere and lower stratosphere (UTLS), sustained by enhanced baroclinic eddies of large horizontal scales, and a weakening below, mostly driven by weaker baroclinic eddies of intermediate to small scales, appear together with an increased transfer rate of kinetic energy from the eddies to the zonal mean flow and a significant increase in the barotropic zonal mean kinetic energy. The conversion rates between available potential energy and kinetic energy, C, were further decomposed into the contributions by the rotational (Rossby) and divergent (gravity) components of the circulation field. The eddy component of C is due to the conversion of potential energy of the rotational adjusted mass field into kinetic energy by the work realized in the eddy divergent motion. The zonal mean component of C is accomplished by two terms which nearly cancel each other out. One is related to the Hadley cell and involves the divergent component of both wind and geopotential, while the other is associated to the Ferrel cell and incorporates the divergent wind with the rotationally adjusted mass field. Global magnitude increases were found in the zonal mean components of these two terms for the warmer climate, which could be the result of a strengthening and/or widening of both meridional cells. On the other hand, the results suggest a strengthening of these conversion rates in the UTLS and a weakening below, that is consistent with the rising of the tropopause in response to global warming.     

Mid-Pliocene westerlies from PlioMIP simulations

The midlatitude westerlies are one of the major components of the global atmospheric circulation. They play an important role in midlatitude weather and climate, and are particularly significant in interpreting aeolian sediments. In this study, we analyzed the behavior and the possible mechanism involved in the change of the westerlies, mainly in terms of the jet stream position, in the mid-Pliocene warm period(3.3 to 3.0 million years ago) using simulations of 15 climate models from the Pliocene Model Intercomparison Project(Plio MIP). Compared to the reference period, the mid-Pliocene midlatitude westerlies generally shifted poleward(approximately 3.6 of latitude in the Northern Hemisphere and 1.9 of latitude in the Southern Hemisphere at 850 h Pa level) with a dipole pattern. The dipole pattern of the tropospheric zonal wind anomalies was closely related to the change of the tropospheric meridional temperature gradient as a result of thermal structure adjustment.The poleward shift of the midlatitude westerly jet corresponded to the poleward shift of the mean meridional circulation.The sea surface temperatures and sea ice may have affected the simulated temperature structure and zonal winds, causing the spread of the westerly anomalies in the mid-Pliocene between the atmosphere-only and coupled atmosphere–ocean general circulation model simulations.     

The East Asian Upper-Tropospheric Jet Streams and Associated Transient Eddy Activities Simulated by a Climate System Model BCC_CSM1.1

In this paper, major features of the upper-tropospheric jet streams simulated by a coupled Climate System Model BCC_CSM1.1 are evaluated through comparison with the NCEP/NCAR reanalysis. The jet streams consist of the East Asian subtropical jet (EASJ) and the East Asian polar-front jet (EAPJ). Associated stationary wave and synoptic-scale transient eddy activities (STEA) are also examined. The results show that the climatological positions of the westerly jet streams are well captured by BCC_CSM1.1, but with slight intensity biases. Statistics from the 6-h model outputs reveal that the jet core number (JCN) of ESPJ is significantly underestimated. Examination of the simulated seasonal evolution of the westerly jet stream indicates that the model has produced a westward movement of the EASJ core in May, one month earlier than that in the reanalysis. Analysis of stationary wave activities shows that the overestimated meridional wind component may have caused considerable enhancement of meridional momentum and heat transport. The stationary Rossby wave represented by the wave activity flux at the southern flank of the Tibetan Plateau is favorable to the growth of asymmetric zonal wind and the multiple-center pattern of JCN. Unlike the stationary wave heat flux transport, the model tends to systematically generate weaker transient heat flux over East Asia. Further analysis of STEA exhibits a general consistent pattern between the simulation and the reanalysis, while the intensity of the northern STEA branch associated with the EAPJ is greatly reduced. The deficiencies of eddy momentum and heat flux transport and accompanied eddy forcing may contribute to the biases of the simulated upper-tropospheric jet streams, suggesting the potential importance of midlatitude internal atmospheric dynamics in shaping the tropospheric general circulation, which is not yet fully and accurately resolved in the current BCC_CSM1.1.     

20CR再分析资料在东亚夏季风区的质量评估

本文利用NCEP1和ERA40再分析资料, 并结合观测资料, 对最新公布的一套再分析资料——20CR再分析资料在东亚夏季风区的质量进行了综合评估。本文主要是从气候态、年际变率、年代际变化三个方面, 来评估20CR再分析资料在东亚夏季风区的质量。结果表明, 在气候平均态上, 20CR再分析资料基本合理再现了东亚夏季风区的高低层环流场(包括南亚高压、副热带西风急流、近地层风场)以及经向环流圈特征。但相较于NCEP1和ERA40, 20CR所刻画的南亚高压偏强, 西风急流偏北, 对流层中上层温度系统性偏高。在年际变率方面, 除了NCEP1在1967年之前存在偏差, 使其结果和ERA40、20CR不同之外, 三套再分析资料刻画的东亚夏季风变率在其它时段高度一致。三套资料在以纬向风为基础的东亚夏季风指数上的一致性, 高于以经向风为基础的东亚夏季风指数, 其中以低层纬向风为基础的东亚夏季风指数的一致性最高。20CR再分析资料可以较好地再现与东亚夏季风相联系的地表气温和降水年际变化特征, 其刻画的地表气温正相关中心位置偏西、强度最强, 且在河套平原地区有一个弱的负相关中心, 而其描述的降水在孟加拉湾和长江流域较之另外两套再分析资料更接近观测结果, 在热带地区和海上却反之。在年代际变化方面, 20CR再分析资料未能合理再现东亚夏季风年代际减弱的现象, 这也体现在不能合理再现青藏高原下游年代际变冷和“南涝北旱”降水型上, 这主要是因为20CR再分析资料所刻画的东亚地区对流层中上层年代际变冷信号偏弱所致。而在百年时间尺度上, 20CR再分析资料所刻画的东亚夏季风变化与观测较为一致;20CR再分析资料可以合理再现出东亚夏季风区1920年代前的显著冷期和1990年代之后的迅速增暖期, 但对1920～1950年代相对暖期和1950～1980年代相对冷期的再现能力较差。     

东亚与北美东部降水和环流季节演变差异及其可能机理分析

利用NCEP/ NCAR 再分析资料和CMAP、GPCP卫星反演降水资料,对比分析了东亚与北美东部地区降水和大尺度大气环流季节演变特征的差异。结果显示,东亚和北美东部地区冬季环流形势较为相似,而夏季差异则较大,这正是东亚为季风区,北美为非季风区的表现。此外,基于季风的两大特征量“风”和“雨”,分析了两地降水和低空风场季节变化的显著差异：东亚副热带地区降水季节变率大,呈“夏湿冬干”的季风降水特征,低层盛行风向随季节逆转,冬季盛行偏北风,夏季盛行偏南风,具有显著的副热带季风区特征。北美东部副热带地区全年雨量分配均匀,低层常年盛行偏西风,呈现非季风区特征。进一步的分析发现,作为季风基本推动力的海、陆热力差异在东亚和北美东部地区有着显著的区别：东亚地区的经向和纬向温度梯度随季节反转的特征显著;而北美东部地区虽有纬向温度梯度的季节反转但幅度很小,且经向海、陆热力差异随季节反转不明显。此外,与青藏高原和落基山脉相伴的纬向环流也存在显著差异。鉴于此,提出经向和纬向海、陆热力梯度反转特征的不同以及青藏高原和落基山脉地形的不同作用很可能是造成东亚副热带季风气候而北美东部非季风气候的主要原因,上述结论还有待于数值试验的进一步证实。