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15 JULY 2002 1979A L L A N E T A L . Influence of Dynamics on the Changes in Tropical Cloud Radiative Forcing

Summary: 15 JULY 2002 1979A L L A N E T A L .
Influence of Dynamics on the Changes in Tropical Cloud Radiative Forcing
during the 1998 El Nin~o
Hadley Centre for Climate Prediction and Research, Met Office, Bracknell, Berkshire, United Kingdom
(Manuscript received 3 July 2001, in final form 26 November 2001)
Satellite measurements of the radiation budget and data from the U.S. National Centers for Environmental
Prediction­National Center for Atmospheric Research reanalysis are used to investigate the links between anom-
alous cloud radiative forcing over the tropical west Pacific warm pool and the tropical dynamics and sea surface
temperature (SST) distribution during 1998. The ratio, N, of the shortwave cloud forcing (SWCF) to longwave
cloud forcing (LWCF) (N SWCF/LWCF) is used to infer information on cloud altitude. A higher than
average N during 1998 appears to be related to two separate phenomena. First, dynamic regime-dependent
changes explain high values of N (associated with low cloud altitude) for small magnitudes of SWCF and LWCF
(low cloud fraction), which reflect the unusual occurrence of mean subsiding motion over the tropical west
Pacific during 1998, associated with the anomalous SST distribution. Second, Tropics-wide long-term changes
in the spatial-mean cloud forcing, independent of dynamic regime, explain the higher values of N during both
1998 and in 1994/95. The changes in dynamic regime and their anomalous structure in 1998 are well simulated
by version HadAM3 of the Hadley Centre climate model, forced by the observed SSTs. However, the LWCF
and SWCF are poorly simulated, as are the interannual changes in N. It is argued that improved representation


Source: Allan, Richard P. - Department of Meteorology, University of Reading


Collections: Geosciences