Time Scales of Variability of the Tropical Atmosphere Derived from Cloud-Defined Weather States

Abstract

The recent analysis of Rossow et al. used a clustering technique to derive six tropical weather states (WS) based on mesoscale cloud-type patterns and documented the spatial distribution of those WS and the modes of variability of the convective WS in the tropical western Pacific. In this study, the global tropics are separated into 30° ? 30° regions, and a clustering algorithm is applied to the regional WS frequency distributions to derive the dominant modes of weather state variability (or the climate state variability) in each region. The results show that the whole tropical atmosphere oscillates between a convectively active and a convectively suppressed regime with the exception of the eastern parts of the two ocean basins, where the oscillation is between a stratocumulus and a trade cumulus regime. The dominant mode of both those oscillations is the seasonal cycle with the exception of the eastern Indian and western?central Pacific region, where El Niño frequencies dominate. The transitions between the convectively active and suppressed regimes produce longwave (LW) and shortwave (SW) top-of-atmosphere (TOA) radiative differences that are of opposite sign and of similar magnitude, being of order 20?30 W m?2 over ocean and 10?20 W m?2 over land and thus producing an overall balance in the TOA radiative budget. The precipitation differences between the convectively active and suppressed regimes are found to be of order 2.5?3 mm day?1 over ocean and 1?2.4 mm day?1 over land. Finally, the transitions between the stratocumulus and shallow cumulus regimes produce noticeable TOA SW differences of order 10?20 W m?2 and very small TOA LW and precipitation differences. The potential climate feedback implications of the regime radiation and precipitation differences are discussed.