Footprint of Tropical Mesoscale Convective System Variability on Stratospheric Water Vapor

Mesoscale convective systems (MCSs) play a dominant role in tropical climate. However, the variabilities of their occurrences on a tropical-wide scale remain elusive, and the way they impact the tropical stratospheric moisture remains under debate. Based on a comprehensive global MCS data set during 1985-2008, we detect distinct transitions in tropical MCS activities, with the occurrence frequency of MCS during 1995-2002 being significantly lower than those before and after this period by over 10%. The stepwise transition of tropical MCSs has a significant impact on stratospheric water vapor: Active MCS occurrences result in an overall drier stratosphere, and vice versa. We demonstrate that these changes are closely related to the sea surface temperature pattern in the tropical central Pacific and the associated Walker circulation shift on convective systems. Our results suggest that the sea surface temperature variability in tropical central Pacific may exert an important forcing on the stratospheric water vapor. Plain Language Summary Mesoscale convective systems are essential ingredients of the tropical circulation and significantly regulate troposphere-stratosphere exchange of atmospheric constituents. But little attention has been paid to their variations on a global scale, and their role in troposphere-stratosphere water exchange is still a subject of ongoing debate. In this study, we show that there are prominent active and inactive phases of tropical mesoscale convective systems during 1985-2008. Specifically, their occurrence frequency during 1995-2002 is significantly lower than those before and after this period by over 10%. These stepwise transitions are closely related to the underlying sea surface temperature pattern in tropical central Pacific as well as the associated changes in Walker circulation. We further demonstrate that changes in these convective systems could strongly influence the stratospheric water vapor, which have significant implications for global climate variability.