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

Abstract

The south Indian Ocean is characterized by enhanced midlatitude storm-track 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 storm-track 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 storm-track 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. Basinwide cold advection associated with the wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 158-258S, while seasonally enhanced storm-track activity augments SHF around 308S. The analysis highlights the significance of large-scale controls, particularly through SHF, on the seasonality of the climatological LCF distribution over the south Indian Ocean, which reflect the seasonality of the Mascarene high and storm-track activity.

Causes of Higher Climate Sensitivity in CMIP6 Models

“…Several recent studies have demonstrated the utility of cloud controlling factor analyses for understanding changes in extratropical low‐level cloud properties. These include Gordon and Klein () and Terai et al () for cloud optical depth, Ceppi et al () for liquid water path (LWP), Grise and Medeiros () and Kelleher and Grise () for SWCRE, and Zelinka et al () and Miyamoto et al () for low cloud cover (LCC). Encouraged by these studies, we apply CCF analysis to estimate how LCC and in‐cloud LWP for low clouds (LWP ) depend on five CCFs in the piControl simulations.…”

Spatial and seasonal variability of clouds over the southwest Indian Ocean based on the DARDAR mask product

“…The Mascarene High is a zone of high pressure located on average between 25°S and 35°S and between 40°E and 90°E in austral winter. It is strongest in austral winter but exists all the year round, and moves eastward in austral summer (Lee et al ., ; Miyamoto et al ., ).…”

“…a–b in Miyamoto et al . (). In region B2, the monthly averaged warm liquid cloud occurrence in July and August is 36% (relatively to all footprints) and 62% (of all clouds) (not shown); warm clouds predominate in winter.…”

“…It can be attributed to changes in the mean position of the Mascarene High, which is located further west of the basin during winter compared to summer (Fig. , Miyamoto et al ., ). It is also in agreement with Feng et al .…”

“…There are few studies on cloud variability focusing on the southwest Indian Ocean (Miyamoto et al ., ) and, up to now, none of them investigated the vertical distribution of clouds in the whole troposphere. Li et al .…”