On the importance of sea surface temperature front for the formation of low-level clouds over the South Indian Ocean

Over the South Indian Ocean, a strong SST front forms in the confluence of the Agulhas Return Current and Antarctic Circumpolar Current, acting to maintain locally-enhanced storm-track activity. The present study investigates how the SST front and storm-track influence low-level clouds over the South Indian Ocean by utilizing up-to-date MODIS satellite observations. Since the MODIS cannot nevertheless detect low-level clouds overlapped by middle and high-level clouds, our investigation is based only on observations without those clouds. Through this sub-sampling, we reveal that low cloud fraction (LCF) tends to be enhanced under both cold and warm advections near the surface. The nearly same results can be obtained under the random overlap assumption. In case of cold advection, enhanced upward sensible heat flux acts to destabilize the boundary layer, which is favorable for stratocumulus formation. In case of warm advection, by contrast, downward sensible heat flux acts to stabilize the boundary layer, which favors fog and stratus formation. This type of non-linear LCF response to near-surface temperature advection cannot be predicted by lower-tropospheric stability (LTS), a parameter often used for studying seasonal and interannual variabilities of LCF. The non-linear response of LCF to the temperature advections is most prominent over the warmer side of the SST front in summer, suggesting that the storm-track acts to expand the area of large LCF equatorward of the front. Overall, our analysis suggests that such a midlatitude SST front as observed in the South Indian Ocean can play an essential role for the formation of low-level clouds.