Tropical Cloud Plumes and the Interannual Variability of Precipitation Over the Southeastern United States and Mexico.

Tropical cloud plumes are easily identified on satellite imagery as elongated bands of upper-tropospheric clouds that extend from the Tropics into the midlatitudes. The first part of this dissertation presents the results of a 10-year (1974-1984) synoptic climatology of tropical cloud plumes in the Northern Hemisphere. The climatology indicates that tropical cloud plumes are primarily a cool season (October to May) phenomenon, and that tropical cloud plumes tend to form in the subtropical jet over the central and eastern Pacific and Atlantic Ocean, where the time -mean upper-tropospheric flow is westerly at the low latitudes. On interannual time scales, when there is below normal plume activity over the Pacific Ocean (e.g., during the 1982 -83 warm phase of ENSO), there is above normal plume activity observed over Mexico and the southeastern United States. This relationship is understood by considering the effects of the interannual variability of the planetary-scale basic state upon synoptic-scale eddy activity over the Pacific Ocean and adjoining North America through linear Rossby wave theory. The second part of the dissertation uses selected case studies during January 1992 to document the evolution and structure of the synoptic-scale features associated with tropical cloud plumes. The 1991-92 season features a warm phase of ENSO and above-normal precipitation over the southeastern United States and Mexico. It is found that plumes over the southeastern United States and Mexico are clearly associated with synoptic-scale eddies from the midlatitudes that originate within the Asian jet and reach the low latitudes of the eastern Pacific. The tropical cloud plumes are associated with ascent in the middle troposphere, moisture flux convergence, precipitation, and poleward moisture flux. It is suggested that the positive precipitation anomalies over Mexico and the southeastern United States during this warm ENSO event of 1991-92, and the warm ENSO event of 1982-83, are in part due to above-normal plume activity over the region.