Low clouds over the subtropical Indian Ocean and sub-seasonal circulation associations with the Indian summer monsoon

This study investigates anomalous low cloud fractions (LCFs) in the Mascarene High (MH) environment of subtropical Indian Ocean (SIO) during June-September, and their sub-seasonal (10-90 day) circulation changes in the SIO and associated variations of the Indian summer monsoon (ISM) using observations and ERA5 circulation products based on 1999-2014 period. Periods of anomalous excess [deficit] LCFs in the SIO are chosen based on regression analyses with a criterion that 4 or more consecutive days in the region 55 degrees E-95 degrees E, 15 degrees S-25 degrees S exhibiting anomalies larger [smaller] than 5% of JJAS mean LCF. The study shows that periods of excess and deficit LCF clearly reveal different sub-seasonal circulation attributes across the equator with precursor signals to the strength of ISM. Anomalous circulation composites from the excess LCF periods shows mean sea level pressure (MSLP) enhancements of about 2 hPa in the MH region in correspondence with increasing areal extent and intensifications in LCFs, and a net increase in low-level southerly momentum between MH and monsoon trough (MT) environments. The MSLP reinforcements in the MH are clearly demonstrated to emerge from the strength of cloud-top radiative cooling and associated winds and mass adjustments. The 10-20 [30-50] day modes of the circulation in the SIO further elucidates zonally propagating [quasi-stationary] manifestations on MH reinforcements. There is an increase in meridional transport of moisture fluxes, by about 7 times relative to deficit LCF periods, channelled through a conduit region (15-30 degrees S, 60-90 degrees E) juxtaposing the cross-equatorial circulation (CEC) from both western and eastern sides of the Indian Ocean. This occurs in tandem with a zone of moisture flux convergence in the ISM region advancing poleward towards the climatological MT region-implying that excess LCF periods portend the likelihood of stronger ISM. Deficit LCF periods on the contrary show a net northerly low-level wind anomalies between MH and MT, pressure deficits in the MH region, and also portend the likelihood of weaker ISM. Low clouds in the SIO are not only instrumental for MH stability, but also essential for circulation and moisture support across the equator and the signals for the strength of ISM on sub-seasonal scale.