An indirect radiative surface-cooling due to water vapor in the tropics and its implications for pre-monsoonal heat stress

Abstract Severe pre-monsoonal heat stress has become a routine occurrence in several tropical regions. Motivated by the surmounting evidence that high ambient humidity worsens the perception of heat during these events, we analyse the general, net radiative impacts of humidity on pre-monsoonal surface temperatures using reanalysis and satellite data. Surprisingly, we find that there exists an indirect, radiative surface-cooling effect of water vapor within extended regions in the tropics during periods of high ambient humidity relative to drier periods in the pre-monsoonal season. This observation is contrary to the strong greenhouse effect generally associated with water vapor. We show that this radiative surface-cooling is associated with a previously unreported large positive correlation between lower tropospheric humidity and cloud cover in these regions bordering the ITCZ. These clouds are predominantly low-level and altocumuli, known to have a negative surface-radiative forcing, possibly causing the surface-cooling observed during periods of high ambient humidity. Therefore, periods of lower ambient humidity—or equivalently lesser low-to-mid-level cloud cover—lead to strong surface warming. We find that these pan-tropical regions, where heat stress is negatively correlated with ambient humidity, border the ITCZ and are co-located with the horizontal branch of the Hadley cell, moving north-to-south with the seasonal cycle. Hence, this indirect negative radiative effect of water vapor is particularly relevant for heat stress during the pre-monsoons. We demonstrate this with case studies of some recent major heat waves over the Indian sub-continent when this indirect water vapor effect is found dominantly active throughout the season and in its warming phase during the period of the heat wave. The evolution of this indirect, radiative effect of water vapor, under future climate warming, needs further investigation, considering its crucial importance for heat stress in the tropics.