| Based on the6-hour NCEP Climate Forecast System Reanalysis (CFSR) dataset, the daily NOAA Outgoing Longwave Radiation (OLR), the pentad NCEP Climate Precipitation Center Merged Analysis of Precipitation (CMAP), the pentad and daily NOAA precipitation (GPCP), the daily RMMs of MJO from Australian Meteorological Administration and the digital-typhoon dataset from1979to2010, onset-relative and withdrawal-relative composite analysis and the space-time wave filtering with respect to the composite anomalies are performed to examine the features of onset and withdrawal of East Asian subtropical summer monsoon (EASSM)(over110°-120°E,20°-30°N) and its interannual variations. The main results are as follows:1) It is shown that the climatological onset of EASSM or the earliest seasonal transition from winter pattern to summer pattern over East Asia occurs on the24th pentad with the most significant feature of the reversal of meridional wind in the upper troposphere and the formation of Ferrel-like monsoonal meridional flow. The reversal of the focus domain-averaged meridional wind shear (MWS) between925and200hPa from negative to persistent positive is thus used as the indice to best define the EASSM onset. Further analysis indicates that the vertical meridional flow reversal is most likely attributed to the reestablishment of the South Asian High (SAH) over southern Indo-China Peninsula after an abrupt "westward shift" in the upper troposphere.2) The climatological withdrawal of EASSM occurs on the52nd pentad with the distinguished features as following:firstly, the large amount rainfall retreats from focus domain due to the decreasing moisture transport according to the southward withdrawal of cross-equatorial vapor transport; secondly, the southward propagation of strengthening continental high (CH) changes the distribution of the thermal contrast between the continent and the ocean from warm in the west and cold in the east to the reversal one. Meanwhile, the spilt of the high from CH reaches over Shandong Peninsula and then changes the meridional wind at lower troposphere; thirdly, the reversal of meridional wind circulation averaged over110°-120°E accompanies with the southward shift and abrupt weakening of vertical; finally, the northwestward activities of tropical cyclones also favors to the circulation structure modulation especially in the meridional flow reversal.3) Slightly early than the climatological average, the mean onset (withdrawal) date for EASSM is the22.3(51.3) pentad with a standard variation of2.8(3.0) pentads and significant interannual variability in both time series based on the CFSR analysis from1979to2010. The earliest onset (withdrawal) date is on15th and46th pentad, while the latest one is on26th and58th pentad, respectively. The largest frequency is on19th as well as on23rd and24th pentad in onset cases, and the counterpart of withdrawal cases is on51st and52nd.4) Composite analysis shows that the SAH located over southern tip of Indo-China Peninsula in the early-onset years while over western tip in those late-onset years. In early-onset years, the westerly jet is stronger with southward axis and the ascending extension is narrower with lower center location of meridional cell, vice verse. As for the withdrawal situation, the robust convection exists over tropical areas, deep trough locates over110°E which favors the stronger southwesterly over domain in the upper troposphere, and the intensity of westerly jet is weaker in the early-withdrawal years, while vice versa.5) It is indicated that the onset of EASSM is strongly favored during the wet phases (phase1to3) of the tropical MJO when it propagates from the western Indian Ocean (IO) to central eastern10. The possible mechanism of the tropical MJO triggering the onset of EASSM is examined based on the space-time MJO-filtering to OLR, horizontal wind, geopotential height, and air temperature. It is suggested that the latent heating releasing by the MJO deep convection over central eastern10acts as the "Matsuno-Gill" effect of warm SST through the eastward propogation of Kelvin waves which contributes to the reestablishment of SAH over the southern Indo-China Peninsula in the upper troposphere. Thus the onset of EASSM is objectively connected with the reestablishment of SAH, the meridional wind reversal in the upper troposphere, and the tropical MJO activities.6) The three-dimension atmospheric features between the selected cyclone-influence and noncyclone-influence withdrawal years have been documented. The convection over tropical area is extremely strong (relatively weak) with fast (slow) southward shift of vertical ascending extension from subtropical area, and the Hadley-like cell has a slight higher (lower) center over subtropical area in the former (the latter) years. The western ridge of western Pacific subtropical high (WPSH) shifts northward and the strong cross-equatorial flow transports to the western Pacific in cyclone-influence years, while the western ridge of WPSH shifts southward and the weaker cross-equatorial flow is stalled over South China Sea in noncyclone-influence years. The SAH retreats westward (shifts southward), the trough over110°E deepens (shallows) its southern branch, thus the southwesterly (westerly) prevails over subtropical area as well as with weak (strong) westerly jet during the cyclone-influence (noncyclone-influence) withdrawal. Case analysis of the withdrawal in1991indicates that the main factor is the tropical cyclone activities. |
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