Cloud-radiative impacts on the tropical Indian Ocean associated with the evolution of ‘monsoon breaks'

Abstract

A detailed diagnostic analysis of a suite of observed datasets was carried out with a view to understand the importance of cloud-radiative effects on the evolution of prolonged ‘monsoon breaks’ over the Indian region. The study particularly focuses on the role of clouds in affecting the sub-seasonal/intra-seasonal variability of sea surface temperature (SST) and atmospheric convection in the equatorial and south-eastern tropical Indian Ocean (SETIO) during monsoon-break transitions. A characteristic feature of the monsoon-break evolution is the appearance of suppressed convection over the SETIO region nearly 7–10 days prior to the commencement of a break spell over India. It is seen from the present analysis that the lack of cloud cover over the SETIO during the 'pre-break' phase leads to significant warming of the tropical Indian Ocean due to strong solar insolation at the surface. During the ‘pre-break’ phase, the net cloud-radiative forcing (NETCRF) at the surface is found to be typically around − 30 Wm⁻² and the mean SST in the SETIO is about 29.3 °C. Following the transition to a monsoon-break phase, the cloud amount increases by about 25% over the SETIO region in association with intensified convection. The NETCRF at the surface over the SETIO averaged during the 'break' phase is found to be about − 60 Wm⁻² (i.e. a change of about − 30 Wm⁻² from the ‘pre-break’ phase). Consistent with the above change in the NETCRF, the SST in the SETIO shows a cooling of about 0.7 °C, although the mean SSTs during the ‘break’ phase remain as high as 28.6 °C. On the basis of the findings from this study, it is suggested that the SST warming during the ‘pre-break’ phase plays a key role in maintaining high SST and allows sustained convection to occur over the SETIO during prolonged monsoon breaks.