Air-sea Interactions During Monsoon Onset Over The Bay Of Bengal

Air-sea interactions during monsoon onset over the Bay of Bengal (BOB) have been investigated by utilizing observational data and many analysis methods, such as statistical analysis, diagnostic analysis and numerical simulation. First, the seasonal cycle of sea surface temperature (SST) and wind show that this is an abrupt northward jump (ANJ) of the meridional warmest SST (WSSTA), and it can trigger the monsoon onset. Second, the analysis of seasonal cycle of heat budget of the ocean mixed layer (ML), general circulations and thermal condition of sea, land, and atmosphere indicated that ANJ of WSSTA is a response of ocean-land-atmosphere coupled system to seasonal cycle of solar radiation. Finally, the BOB has been selected as an example to investigate the climatology, case and interannual variability of how the ANJ of WSSTA triggers the monsoon onset. The major conclusions can be summarized as follows:(1) Influence of the annual cycle of SST on the monsoon onsetIt is found that SST is a major driver of tropical circulations in the atmospheric boundary layer. The warm SST zone is corresponds to the zone of convergence of southerly and northerly winds and precipitation maximum. In the trade wind regions, the annual cycle of SST involves a shift in the warmest SST axis (WSSTA) between two local maxima on either sides of the equator, or slight movement of WSSTA north of the equator. Consequently, WSSTA has little effect on the regional meridional SST gradient and wind direction. However, in the monsoon regions, the annual cycle of SST is characterized by an ANJ of WSSTA, resulting in a marked change in regional meridional SST gradient and consequent onset of winds and rain. The onset of the southwesterlies and the shift of rainfall maximum from the Southern Hemisphere (SH) to Northern Hemisphere (NH) lags behind the ANJ of WSSTA from the equator to north of 10°N. The ANJ of the WSSTA triggers convection by two ways. One is by destabilizing the atmosphere; the other is moisture convergence in the atmospheric boundary layer due to the SST gradient.(2) Why is there an ANJ of WSSTA? The ridgeline of subtropical high is located around 15°N, where the sky is clear due to subsidence, leading to formation of meridional maximum of surface short wave radiation flux in spring. As the winter monsoon weakens, the surface wind speed slows down, accompanied by weak latent heat flux and shallow ML. In spring, the land over south Asia warms rapidly. This leads to the anticyclonic circulations over the BOB and the Arabia Sea forced by sub-continent scale land-sea contrast, and southerlies in the northwestern Pacific forced by the thermal contrast between southern Asian and northwestern Pacific. In the center of the anticyclonic circulations, wind speed is weak and vorticity is positive, leading to weak sea surface evaporation, weak entrainment of cold water at the base of ML, and shallow ML. These processes contribute to the largest increase in the rate of SST around 15°N. Thus the SST around the 15°N is warmer than the equator before the SST south of 15°N greater than the equator, the result is the ANJ of the WSSTA before the monsoon onset.(3) Definition of the monsoon onset over the BOBDuring the monsoon onset, the winds in the BOB originate from different regions. Thus, the reversal of zonal wind or merdional wind is not stable which makes it difficult to define the monsoon onset by wind components. However, the monsoon onset can be defined objectively by using wind absolute angle and the westerly component in the southwest of the BOB. There is high correlation between the monsoon onset dates defined in these two ways and the dates determined by thermodynamic variables and convection. The variability of the westerlies in the southwest of the BOB can well represent the variability of convection in the BOB.(4) Characteristics of the monsoon onset over the BOB The climatological onset of the monsoon in the BOB shows that the ANJ of the WSSTA is caused by an increase SST in pentad 22 in the BOB, leading to the monsoon onset after about 2 pentads. Maximum SST values occur just before the monsoon onset. SST decrease after the monsoon onset, associated with an abrupt southward retreat to the equator. There are intensified rainfall and twin vortices in the equatorial central Indian Ocean (10). The meridional circulations show distinct differences after the monsoon onset, and intensified the interaction between the NH and SH. The monsoon onset leads to the formation of warm center in middle and upper troposphere over the BOB, resulting in the meridional warmest air axis shift from the SH to the BOB and the resultant reversal of merdional temperature gradient.(5) Interannual variability of the monsoon onset over the BOB and its relationship with ANJ of WSSTAThere is a significant positive correlation between the monsoon onset over the BOB and the ANJ of WSSTA. The dates of ANJ of WSSTA are prior to the dates of the monsoon onset for all years, although with different lead time. The interannual variability of ANJ of WSSTA affect the monsoon onset mainly by the meridional SST gradient, rather than the magnitude of SST. The earlier the monsoon onset, the quicker the monsoon onset is after ANJ of WSSTA. Compared with the late onset, the early onset show more rainfall in the Maritime Continent before the monsoon onset, resulting in westerly anomalies in the equatorial eastern 10 excited by the release of latent heat. The anomalous westerlies lead to the earlier ANJ of the WSSTA by cooling the SST in the equator, and the rapid monsoon onset.(6) Relative importance of ANJ of the WSSA and the Tibetan Plateau heating in the monsoon onset over the BOBThe experiment results of Community Atmosphere Model (CAM3) indicate that ANJ of WSSTA plays a more important role in the monsoon onset over the BOB than the Tibetan Plateau (TP) heating. After the ANJ of the WSSTA, convections occurred due to air convergence in lower troposphere, accompanied by cyclonic circulations. There is a positive feedback process between circulation and convection. The characteristics of circulation and rainfall induced by the ANJ of the WSSTA in the CAM3 are the same as observation. Affected by the westerlies in spring, the impacts of the TP heating are concentrated on its downstream regions. Therefore, TP heating has limited effects on the monsoon onset over the BOB.