Three-dimensional structure, scale interactions, and moisture transport in three types of tropical cloud plumes

Three eastern Pacific tropical cloud plumes that occur within differing synoptic environments are compared using mesoscale numerical simulations. A tropical cloud plume is a winter season, synoptic scale system that usually is associated with an upper tropospheric trough and/or the subtropical jet stream. It is defined as a continuous band of upper and/or middle level clouds, at least 2000 km in length, which crosses 15--20 degrees north latitude from the south. Despite a common appearance in satellite imagery, however, variability exists in the mechanisms of plume formation, maintenance, and dissipation. The mechanisms at work in a plume determine the amount of moisture that is transported from tropical to middle latitudes. This study compares the synoptic and mesoscale structures and causal dynamics of differing plumes, and it quantifies their moisture transport; including their role in global moisture transport.;Examination of infrared cloud and water vapor satellite imagery over the eastern Pacific for two winter seasons (1995--97) reveals three major modes of plume formation. In the most common mode, an upper level middle latitude trough penetrates deep into the tropics and directly impacts the ITCZ. The second most common mode is a plume associated with a cutoff middle latitude low and a strong subtropical jet. The least common mode is the merger of tropical and middle latitude cloud masses within subtropical latitudes. One representative case from each category is examined in detail to describe the three-dimensional structure and moisture transport characteristics of plumes in that category.;Extrapolating the moisture transport results of the three cases to the rather typical 1995--97 seasons of plumes reveals a significant collective effect. During a typical winter season, eastern Pacific tropical plumes produce approximately 3% of the global, annual net poleward moisture transport out of the ITCZ into subtropical latitudes, and approximately 10% of the global wintertime net poleward moisture transport from subtropical into middle latitudes. Since approximately one third of all tropical plumes occur over the eastern Pacific, these percentages can be assumed to increase nearly threefold when plumes at all longitudes are considered.