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

Abstract

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 Niño, the trends are approximately three times larger than those predicted by a linear relationship to sea surface temperature in the Niño3.4 region.