The Feedback Of Latent Heating Over East Asian Continent On The Establishment And Maintenance Of The Asian Summer Monsoon Circulation

In this study, NCEP/NCAR reanalysis data for the period1948-2007and CMAP data for1979-2007are employed to delineate the seasonal variation of precipitation, and the3-dimensional structure of atmospheric circulation over the Asian monsoon region, thus to investigate the onset and seasonal march of the summer monsoon therein. And then we analyze the spatial and temporal features of the zonal land-sea thermal contrast on the mid-upper troposphere and the diabatic heating calculated by wind and temperature fields, in order to find their internal linkage with the Asian summer monsoon circulation. With a global climate CCM3, we reveal the contribution of the feedback of latent heating over East Asian continent to the establishment and maintenance of the subtropical East Asian summer monsoon circulation, as well as the summer mean of the tropical Asian summer monsoon circulation. The main conclusions are as follows:1. The general circulation over subtropical East Asia is clarified to transit to the summer phase on pentad22(from16th April to20th April), accompanied with the seasonal overturning of meridional winds both on the upper and lower troposphere, the establishment of meridional monsoon circulation and the transition of zonal geopotential height deviation from the winter quasi-barotropic structure to the summer baroclinic structure. At the same time, the positive diabatic heating penetrates through the whole troposphere.The seasonal march of the geopotential height on the upper troposphere shows that the center of subtropical high abruptly occurs over the Indo-China Peninsular around pentad22from the east of Philippines. These changes cause the northwesterly of the north part of the South Asian High (SAH) prevailing in the subtropical East Asia, and the transition of the meridional wind from winter southerly to summer northerly. In conjunction with the local vigorous ascending motion associated with the rainfall, the descent arm of Hadley circulation breaks up over the subtropical East Asia earliest, and a meridional monsoon circulation which is an anti-Hadley cell is found on pentad22. Furthermore, in winter, the zonal geopotential height deviation shows quasi-barotropic structure with the east higher than the west bordered along120°E; in summer, it performs baroclinic structure that the east is higher than the west on the mid-lower troposphere, according to the monsoon depression and the subtropical anticyclone, and the pattern is out of phase on the upper troposphere, according to the SAH and the mid-ocean trough. It is on the pentad22that the zonal geopotential height deviation shifts from the winter phase to summer. The aforementioned features are indicative that the earliest establishment of the summer monsoon circulation occurs over the subtropical East Asia on the pentad22. At the same time, we find the transition of the zonal thermal distribution over the subtropical East Asian on500-200hPa layer from winter to summer. Before pentad22, the East Asian continent is a cool zone, in contrast to the western North Pacific, but from pentad22on, the continent is under control of a relative warm zone. The zonal thermal contrast is the essential trigger of the subtropical East Asian summer monsoon circulation.It is found that the persistent rainfall occurs over Jiangnan zone (22.5°-30°N,110°-120°E) in south China from the14th pentad, and generates significant latent heating. The latent heating propagates upward accompanied with the convective activities, and warms up the air column on the mid-upper troposphere, leading to this area from heat sink to heat source. It appears an interesting story that it is just on the pentad22when the positive diabatic heating penetrates through the whole troposphere over the subtropical East Asia, and the period is consistent with the establishment of the summer phase of the atmospheric circulation and zonal thermal contrast.2. The feedback of latent heating is a driving force of the establishment of the subtropical East Asian summer monsoon circulation. Removing its affection will delay the seasonal transition of the zonal thermal contrast from winter to summer, and decrease the period of summer phase. And a close monsoon circulation fails to establish, accompanied with weaker ascending motion and southerly on the upper troposphere.The zonal thermal contrast over the subtropical East Asia has a close linkage with the latent heating. If the feedback of latent heating is switched off, it will significantly slow down and weaken the warming process in East Asia landmass (including the Tibetan Plateau) from spring to summer. Thus it not only delays the seasonal transition of the zonal temperature deviation on mid-upper layer from the winter mode of cold land and warm sea to the summer mode of warm land and cold sea, but also makes the overturning from summer to winter earlier. In other words, the summer period is shortened.The prevailing wind on the upper troposphere maintains southerly all year around, without seasonal transition from winter southerly to summer northerly. Without the feedback of latent heating, it shows extensive negative diabatic heating (heat sink) over the subtropical region in the Northern Hemisphere, and weak ascending motion. Generally, a close monsoon circulation can not be found in that sector.3. In summer mean, removing the feedback of latent heating, the geopotential height deviation appears quasi-barotropic structure both over the ocean and continent, and the SAH will move southward and split into two centers with weaker intensity, which lead to the southerly prevailing in the upper troposphere over subtropical East Asia. Consequently, it can’t be found of a local monsoon circulation meridionally.The sensitive experiment without the latent heating feedback is indicative that the warm area in the East Asian continent is narrowed, with the cold zone in the western Pacific intruding to the eastern China, and the zonal temperature gradient is significantly declined, in summer mean. Although, the land-sea thermal contrast performs that the west is wanner while the east is colder in general, the geopotential height deviation performs quasi-barotropic structure, and negative deviation dominates on the mainland of East Asia, while positive value over the subtropical western North Pacific. The local baroclinic configuration fails to form over the subtropical East Asia.Without the feedback of latent heating, it will lead to the subtropical high on the low layer migrating westward and an anticyclonic anomaly over the subtropical East Asia, which imply weakened summer monsoon therein. On the upper layer, the SAH shifts southward with weaker intensity, and more importantly it breaks up to two centers, with the transition belt of the two centers over the subtropical East Asia. Therefore, the upper troposphere over the subtropical East Asia is always under control of southerly, in a sharp comparison with the northerly in the control experiment. In that case, the ascent of the air column over the tropic expels it outwards in the upper troposphere, and then descends over the mid-latitude region. In summer mean, the meridional circulation doesn’t show a close mosoon circulation over the subtropical East Asia. Thus the feedback of latent heating is a necessity for the maintenance of the subtropical East Asian summer monsoon circulation.4、The tropical Asian summer monsoon establishes by stages, while the seasonal overturning of zonal wind in the upper troposphere occurs all together in the four subsystems.The equatorial westerly (?)nces and migrates northward gradually, accompanying with the establishment of Somalia Jet. Since the Indo-China Peninsular and Bay of Bengal are located in front of the Indo-Burma trough, the convection is active and leads to the subtropical high breaking up in that sector earliest and the transition of meridional thermal contrast. The onset of Indo-China Peninsular summer monsoon occurs on pentad26, and propagates to the Bay of Bengal later. Once the subtropical high moves out from the South China Sea (SCS), the SCS is under influences of the southwesterly in the west flank of the subtropical high, in conjunction with the southwesterly in the front of the Indo-Burma trough and the cross-equatorial current emanating from Australia, which is indicative of the onset of the SCS summer monsoon. Since the Indian Peninsular is under control of the descent motion of the Indo-Burma trough, the summer monsoon outbursts only when the transition of the meridional thermal contrast occurs on pentad31.Based on the analysis of the seasonal variation of the zonal wind on the200hPa layer, the easterly is found on pe(?)d26-27in the tropical Asia, which is a consequence of the re-establishment of SAH. The subtropical high abruptly occurs over the Indo-China Peninsular around pentad22from the east of Philippines. In that case, the low latitudes south of15°N, east of50°E is under control of the easterly in the south flank of the subtropical high, and the seasonal transition of the zonal wind occurs all together in the four subsystems.5、Removing the feedback of latent heating, it will decrease the magnitude of the summer mean of South Asian monsoon, and generate a cyclonic circulation on the low layer over SCS, which is favor of convective activities.Without the affection of latent heating, the heating center over the Tibetan Plateau can not be found, and the warm area on the continent is narrowed with southward moving. In summer mean, the land-sea thermal contrast is significantly weakened in the South Asian region. An extensive easterly anomaly occurs on the low layer of the low latitudes in South Asia, which diminishes the prevailing westerly belt in summer. Even the Indo-China Peninsular is under influence of northeasterly. These changes are indicative of the weakening of the South Asian summer monsoon, and decreasing of the monsoon rainfall. But the story is different over SCS. A cyclonic is forced in the low layer of SCS, which leads to strong convection and more rainfall. The latent heating associated with the rainfall propagates upward, and warms up the air column on the mid-upper troposphere. As a result, a warm center over SCS is found in the field of meridional temperature deviation, averaged from500to200hPa, and thus intensifies the land-sea thermal contrast. In general, the SCS summer monsoon is stronger to some extent.