Clustering the cloud clusters: moving from weather to climate states

George Tselioudis and W. B. Rossow

A recent analysis of cloud properties (Rossow et al. 2005) derived the time and space 
distribution of the primary weather states (WSs) over the global tropics. In basin-wide 
regions at each point in time a multitude of such WS are present, and the particular 
combination of WSs determine the regional properties of the radiatiative and hydrologic 
budgets. In this study we explore whether there exist preferred WS combinations that 
dominate the regional scales and determine the scales of variability of the tropical 
atmosphere. This is done by applying a cluster analysis technique to the WSs of basin-
wide regions, in order to determine the dominant combinations of WSs (thereafter 
referred to as Climate States or CSs) in each region. The time and space scales of 
variability and the radiative and hydrologic properties of the CSs are subsequently 
examined. The results show that the tropical atmosphere oscillates between a 
convectively active and a convectively suppressed regime with the exception of the 
eastern ends 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 Western-Central Pacific region where ElNino frequencies 
dominate. The radiative differences between the convectively active and suppressed 
regimes show that in the Indian Ocean and the Western-Central Pacific basins both the 
LW and SW differences are of the order 20-30W/m2, while in the Eastern Pacific and the 
Atlantic basins the SW differences are of the order 10-20W/m2 and the LW differences 
are mostly of similar order but reach very low values in the stratocumulus-shallow 
cumulus regimes. The differences in precipitation between the convectively active and 
suppressed regimes reach peak values of 3-4mm/day and have very low values in the 
stratocumulus-shallow cumulus regimes.