SURFACE-TEMPERATURE RELATED VARIATIONS IN TROPICAL CIRRUS CLOUD AS MEASURED BY SAGE-II

Data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) so lar occultation satellite instrument have been used to study the prope rties of tropical cloud over the altitude range 10.5-18.5 km. By virtu e of its limb viewing measurement geometry, SAGE II has good vertical resolution and sensitivity to subvisual cloud not detectable by most o ther satellite instruments. The geographical distribution and temporal variation of the cloud occurrence have been examined over all longitu des on timescales from less than 1 day to that of the El Nino-Southern Oscillation (ENSO) cycle. Significant variations in cloud occurrence are found on each of these scales and have been compared with the unde rlying surface temperature changes. Maximum cloud occurs over the warm pool region of the Pacific Ocean, with secondary maxima over the Sout h American and Central African landmasses, where the percentage of clo ud occurrence in the upper troposphere can exceed 75%. Cloud occurrenc e at all altitudes within the Tropics, over both land and ocean, incre ases with the underlying surface temperature at a rate of approximatel y 13%degrees C-1. Extrapolated threshold temperatures for the formatio n of cloud are about 5 degrees C lower than those found from nadir vie wing observations. This difference is believed to be a consequence of the averaging process and the inclusion of outliers in the dataset. EN SO cycle changes in cloud occurrence are observed, not only over the T ropics but also over the subtropics, indicating a difference in the me ridional Hadley circulation between ENSO warm and cold years. Sunrise- sunset cloud differences indicate that large-scale variations, whose f orm resembles that of the Hadley and Walker circulations, are present, with a timescale of 1 day or less. The global distribution of upper-t ropospheric ice and its positive correlation with surface temperature