JpGU-AGU Joint Meeting 2020

Presentation information

[E] Poster

A (Atmospheric and Hydrospheric Sciences ) » A-CG Complex & General

[A-CG44] Extratropical oceans and atmosphere

convener:Youichi Kamae(Faculty of Life and Environmental Sciences, University of Tsukuba), Toyama Katsuya(Meteorological Research Institute), Hyodae Seo(Woods Hole Oceanographic Institution), Yoshi N(Hokkaido University)

[ACG44-P05] On the maintenance mechanisms for the wintertime subtropical high over the South Indian Ocean

*Ayumu Miyamoto1, Hisashi Nakamura1, Takafumi Miyasaka1,2, Yu Kosaka1, Bunmei Taguchi3, Kazuaki Nishii4 (1.Research Center for Advanced Science and Technology, The University of Tokyo, 2.Japan Meteorological Business Support Center, 3.School of Sustainable Design, University of Toyama, 4.Graduate School of Bioresources, Mie University)

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.