[ACG44-P05] On the maintenance mechanisms for the wintertime subtropical high over the South Indian Ocean
Keywords:subtropical high, storm track, South Indian Ocean, Asian summer monsoon
Over the South Indian Ocean, the subtropical Mascarene high exhibits a distinct seasonality from the other basins. While the high resides over the eastern portion of the basin in summer, the high resides over the western portion of the basin and its strength as a planetary-wave component is maximized in winter. The present study investigates the maintenance mechanisms for the wintertime subtropical high by using an atmospheric dynamical model and an atmospheric general circulation model.
We find that enhanced storm-track activity maintained by the Agulhas sea surface temperature (SST) front is important for the maintenance of the poleward portion of the subtropical high through the convergences of eddy heat and vorticity fluxes and the resultant acceleration of the climatological-mean westerlies on the poleward side of the high, as shown by the dynamical model experiments. The atmospheric general circulation model experiments further indicate that the Agulhas SST front acts to reinforce the high by energizing the storm-track activity.
The dynamical model experiments also reveal the remote influence from the tropics, especially the Asian summer monsoon region. The enhanced deep convection over the Asian summer monsoon region as well as the equatorial eastern Indian Ocean and western Pacific acts not only to shift the subtropical high westward but also to enhance mid-tropospheric subsidence and equatorward surface winds over the central and western portions of the subtropical South Indian Ocean. The induced subsidence acts to stabilize and dry the free troposphere, and the equatorward surface winds yield near-surface cold advection, both of which are favorable for the low-level cloud formation but unfavorable for the development of deep precipitating clouds. The resultant enhanced radiative cooling and reduced deep condensation heating can reinforce the equatorward portion of the subtropical high.
We find that enhanced storm-track activity maintained by the Agulhas sea surface temperature (SST) front is important for the maintenance of the poleward portion of the subtropical high through the convergences of eddy heat and vorticity fluxes and the resultant acceleration of the climatological-mean westerlies on the poleward side of the high, as shown by the dynamical model experiments. The atmospheric general circulation model experiments further indicate that the Agulhas SST front acts to reinforce the high by energizing the storm-track activity.
The dynamical model experiments also reveal the remote influence from the tropics, especially the Asian summer monsoon region. The enhanced deep convection over the Asian summer monsoon region as well as the equatorial eastern Indian Ocean and western Pacific acts not only to shift the subtropical high westward but also to enhance mid-tropospheric subsidence and equatorward surface winds over the central and western portions of the subtropical South Indian Ocean. The induced subsidence acts to stabilize and dry the free troposphere, and the equatorward surface winds yield near-surface cold advection, both of which are favorable for the low-level cloud formation but unfavorable for the development of deep precipitating clouds. The resultant enhanced radiative cooling and reduced deep condensation heating can reinforce the equatorward portion of the subtropical high.