Formation And Pattern Variations Of The South Asian High And Their Impacts On The Asian Summer Monsoon Onset

Multiple reanalysis datasets and satellite observation were used to investigate the formation and pattern variation of South Asian High (SAH) from April to June and their influence on the onset processes of Asian Summer Monsoon (ASM). We started with the SAH evolution during the transition from spring to summer (April-June), and emphasized the vertical coupling of circulation in the upper and lower troposphere during the ASM onsetto reveal the related dynamical mechanisms. Thus the atmospheric external forcing and internal dynamics could be integrated together via the SAH pattern variation, which bridges the intrinsic associations among ASM subsystems finally. The primary conclusions are as follows.(1) The mechanism for SAH genesis from April to May is investigated to reveal the SAH effect on the Monsoon Onset Vortex (MOV) over the Bay of Bengal (BOB). Results show that the convection is developing rapidly over the southern Philippines on2weeks before the BOB summer monsoon onset. The released condensation heating could produce a negative vorticity source in the upper troposphere. Such negative vorticity source is steady before the BOB summer monsoon onset, and could force the SAH to form over the South China Sea (SCS), manifesting a Gill-type response. Subsequently, the local rainfall over the northern Indochina Peninsula leads the SAH to expand westwards to the northern BOB, inducing a trumpet-shaped flow field over the southeastern BOB. As a result, the evident divergence-pumping and strong atmospheric ascending is formed to favor the BOB MOV formation. Furthermore, the spring warm pool over the BOB provides energy to support the MOV formation. Therefore, both the upper pumping and lower warm pool trigger the BOB MOV together, and then the surface southwesterly is strengthened.(2) The ASM onset cannot propagate directly westwards to the India Peninsula and Arabian Sea in early May after it commences over the BOB. This phenomenon is named as the ASM "onset barrier", which is probably attributed to the atmospheric response and local land-air-sea interaction related to the latent heating realeased by BOB summer monsoon convection. After the BOB summer monsoon onset, the released latent heating brings on a cyclone in the lower but an anticyclone in the upper troposphere. Therefore, a vertical southerly shear appears near the western BOB and eastern coast of India Peninsula, accompanied by the local descending. When the downward flow arrives at the surface, the lower anticyclone is enhanced over the India to inhibit the ridgeline of subtropical anticyclone from titling southward. Thus the local vertical easterly shear is reduced to prevent the westward advancing of ASM onset. Moreover, the sinking flow over the India Peninsula could increase the land surface temperature to strengthen the land surface sensible heating. Then a warming center is formed over the west BOB under the influence of zonal advection of surface air temperature. Meanwhile the SST is decreased by monsoon convection to generate a cooling center over the BOB. The co-effect of warming air temperature and cooling SST makes the atmospheric column lose a lot of heat energy over the western BOB, suppressing the formation and development of monsoon convection to block the ASM onset to the west of BOB.(3) The forced inertial instability over the Arabian Sea plays an important role in the Indian summer monsoon onset. During the Indian summer monsoon onset, the strong crossing-equator gradient of surface pressure exists over the mid Arabian Sea, pushing the zero contour of absolute vorticity northwards to form an inertial instability region near the equator in the north hemisphere. As the air flow passing through this region, a lower convergence center to the north moves poleward gradually. Simultaneously the lower westerly is accellarating to form an evident zonal advection of momentum to the north of η=0. The analytic and diagnosing results point out that the zonal momentum advection could lead the lower convergence center to shift northeastward to the north of negative absolute vorticity area. It’s implied that the atmospheric response to the forced inertial instability over the Arabian Sea is determined by the zonal asymmetric distribution of lower flow except for the dynamical process in the boundary layer. Besides, the South Asian High is extending eastward with high potential vorticity (PV) vortex transported to the Arabian Sea in the upper troposphere prior to the Indian summer monsoon onset. Thus a "trumpt-type" stream field is generated to cause the strong upper pumping effect, providing a favorate background to the onset and advancing of summer monsoon. While the northward movement of lower convergence due to the forced inertial instability triggers the Indian summer monsoon onset.(4) The ENSO effect on the vertical coupling of circulation in the upper and lower troposphere from April to June is discussed to understand the interannual variability of ASM onset process. In April ensuing the El Nino events, the sea surface temperature anomaly (SSTA) is warm over the Indian Ocean and central-eastern Pacific, but cold over the western Pacific. On the one hand, the anomalous sinking of Walker circulation is located over the western Pacific, leading the Inter-tropical convergence zone (ITCZ) to stay on south of its climatological position. Therefore, the convection is weakened the convection over the south Philippines to result in the SAH formation over the northern Sumatra. At this moment the SAH is situated to south of the climate-mean state, weakening the upper divergence over the southern BOB. On the other hand, an anomalous lower easterly is established over the southern BOB due to the warm SSTA over the Indian Ocean and cold SSTA over the western Pacific. Thus the near-equatorial westerly is weakened to decrease the lower cyclonic vorticty, precluding the BOB MOV from generating and developing. Accordingly, the El Nino events can suppress the BOB monsoon convection and postpone the onset time of BOB summer monsoon by diminishing the upper divergence and lower cyclone circulation over the southern BOB.While following El Nino events in May, the suppressed BOB monsoon convection inhibits the SAH from expanding westward to the northern Arabian Sea, decreasing the upper divergence-pumping effect over the southern Arabian Sea. Meanwhile the weakened land surface sensible heating over the north Arabian Peninsula is adverse to the subtropical anticyclone development in the middle troposphere. Furthermore, the declined meridional gradient of SST over the western Indian Ocean decreases the cross-equatorial gradient of surface pressure, weakening the inertial instability over the Arabian Sea. Then both the Somali cross-equatorial flow and the lower convergence are abated in the Indian summer monsoon region. After the El Nino events, the cooperation of abnormal circulation on different levels contribute to the anomalous descending over the Arabian Sea, inhibiting the formation of monsoon convection and delaying the Indian summer monsoon onset. The situation in the La Nina category is opposite. Consequently, the ENSO events can modulate the onset time of Indian summer monsoon via influencing the SAH pattern in the upper, the feature of subtropical anticyclone in the middle and the strengthen of inertial instability in the lower troposphere.