Trends in upper-level cloud cover and surface divergence over the tropical Indo-Pacific Ocean between 1952 and 1997

This study investigates the spatial pattern of linear trends in surface-observed upper-level ( combined midlevel and high-level) cloud cover, precipitation, and surface divergence over the tropical Indo-Pacific Ocean during 1952-1997. Cloud values were obtained from the Extended Edited Cloud Report Archive (EECRA), precipitation values were obtained from the Hulme/Climate Research Unit data set, and surface divergence was alternatively calculated from wind reported by Comprehensive Ocean-Atmosphere Data Set and from wind derived from Smith and Reynolds Extended Reconstructed sea level pressure data. Between 1952 and 1997, upper-level cloud cover increased by about 4%-sky-cover over the central equatorial South Pacific and decreased by about 4-6%-sky-cover over the adjacent subtropics, the western Pacific, and the equatorial Indian Ocean. Trends in precipitation and surface convergence are usually positive where upper-level cover trends are positive and negative where they are negative. Consistency between time series of upper-level cloud cover reported by EECRA and the International Satellite Cloud Climatology Project (ISCCP) during 1984-1997 for various subregions provides further confirmation that the surface-observed upper cloud trends are real. Estimated radiative effects of surface-observed cloud cover anomalies are also well correlated with all-sky radiation flux anomalies reported by the Earth Radiation Budget Satellite in most areas. Contrastingly, EECRA and ISCCP low-level cloud cover regional time series exhibit little correspondence, and EECRA low-level cloud cover trends are uniformly positive across the tropical Indo-Pacific with no apparent relationship to changes in surface divergence. Although the spatial pattern of upper-level cloud trends resembles that associated with El Nino, the trends are approximately three times larger than those predicted by a linear relationship to sea surface temperature in the Nino3.4 region.