Three-Dimensional Structure, Scale Interactions, and Moisture Transport in Three Types of Tropical 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 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.

The numerical model used to examine the three cases is the PSU/NCAR MM5. Two nested domains having 90 and 30 km horizontal resolution are utilized in all cases, and an additional nested domain having 10 km resolution is utilized in one case. The simulated fields of each case are shown to agree closely with satellite observations. Simulations with and without the effects of latent heat are examined to assess the feedback of organized mesoscale convection on the synoptic scale system.

Several important differences between the cases are evident. Convective and quasigeostrophic forcing affect the life cycles of the plumes in varying degrees, with both factors playing some role in each case. Important mesoscale circulations are present in different portions of each system. The difference that is most crucial to poleward moisture transport is the location of the cloud plume relative to the persistent northeastern Pacific anticyclone and to low pressure in the upper troposphere.

Poleward moisture flux within an individual plume usually exceeds the climatological mean, but one plume contributes only a very small percentage to the net global poleward moisture transport. However, extrapolating the results of the three cases examined to the rather typical 1995-97 seasons of plumes reveals a more 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.