Cloud Structure Over Tropical Pacific During 1985--1999: Preliminary Analysis

SAGE II observations from 1985 to 1999 during the month December--April are used to infer cloud vertical structure over tropical Pacific with the focus on two specific regions of climate sensitivity, namely, the Pacific warm pool (WP; 5^oS-10^oN, 100^oE-170^oE) and the Eastern Pacific (EP; 7.5^oS-7.5^oN, 200^oE-280^oE). Cloud distributions over these two geographic regions are closely linked through large scale circulation systems, particularly the Walker circulation. SAGE II cloud data have been used to study tropical cloud characteristics associated with the strong 1997/1998 El Niño. It was shown that, with respect to normal years, both opaque cloud (OC) and subvisual cloud (SVC) are enhanced over the EP and weakened over the WP as consistent with outgoing radiation observations from satellite instruments. The objectives of the present study are to explore the long-term behavior of the cloud structure over the WP and EP, and to study the relationship of cloud properties between these two areas. The preliminary results from the present analysis indicate that the opaque cloud occurrence over the WP declined during 1986/87, 1989/90, 1994/95, and 1997/98, with opposite changes over the EP, except in 1994/95. These interannual opaque cloud variations appear to be the manifestation of large scale circulation changes in the Tropics in response to El Niño events. In the case of subvisual cloud, the interannual variation over the EP closely resembles that of the opaque cloud in the same area. Over the WP, such a resemblance is much less evident, except from 1996 to 1999.