Influence of the Subtropical High and Stormtrack on Low-Cloud Fraction and Its Seasonality over the South Indian Ocean

The South Indian Ocean is characterized by enhanced midlatitude stormtrack activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene High. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the stormtrack activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced stormtrack activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although its zonally extended distribution is correspondent with that of LCF, EIS alone cannot explain the wintertime LCF enhancement, which precedes the EIS maximum under continuous lowering of SST and enhanced SHF in winter. Basin-wide cold advection associated with wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 15ºS-25ºS, while seasonally-enhanced stormtrack activity augments SHF around 30ºS. Our analysis highlights the significant role of SHF on the climatological LCF over the South Indian Ocean, to which the subtropical high and stormtrack activity anchored by the prominent SST front contribute.