Low Cloud Modulation by Synoptic Waves over the Eastern Tropical Pacific

The relation between the synoptic variability of low cloud amounts and the wave disturbances over the eastern tropical Pacific is studied using data from an atmospheric reanalysis (ERA-Interim) and the International Satellite Cloud Climatology Project (ISCCP) over 12 years (from 1990 to 2001). Here, low clouds are defined as those having the tops between 800 and 680 hPa. A significant correlation is found between the low cloud amount and the meridional wind over 3-7 day periods. Composite analyses show that mixed Rossby-gravity waves in the tropics and Rossby waves in mid-latitudes contribute to the synoptic variability of the low cloud amount. These waves induce the fluctuations of the low cloud amount independently, and the low cloud variability is explained as their superposition. Quantitative analyses suggest that these waves affect the low cloud amount through horizontal advection of clouds and the modulation of static stability.