An Anthology of Tropical Convection: Dynamical and Thermodynamical Interactions and the Organization of Large-Scale Tropical Convection
Webster, P.J., Program in Atmospheric and Oceanic Sciences, University of Colorado
Ninth Atmospheric Radiation Measurement (ARM) Science Team Meeting
There appears to be no universal relationship between large-scale organized convection and the magnitude of sea surface temperature (SST). Convection and mean precipitation maxima are often found on the equatorward side of maximum SST or even in the winter hemisphere of the tropics. Thus, there must exist other rules besides thermodynamical forcing that provide necessary conditions for convection. A survey of large-scale organized convection has been conducted in order to find necessary conditions for the existence of convection. It is found that a warm SST (not a maximum necessarily) is a necessary condition but that dynamical processes are also required. It is found that the cross-equatorial pressure gradient (XEPG) determines the dynamic regime and the location of convection. If the SST is sufficiently warm for the atmospheric column to be conditionally unstable, the following four regimes are found. One, when the XEPG is small, organized convection is collocated with maximum SST. Such organization occurs in the Pacific warm pool near the Atmospheric Radiation Measurement (ARM) Program's Tropical Western Pacific (TWP) site. Two, if the XEPG is moderate, convection is found on the equatorward side of the maximum SST and is associated with a dynamical instability caused by the advection of potential vorticity across the equator. Such regions occur in the eastern Pacific and Atlantic Oceans during summer and in the South Indian Ocean during the austral summer. These areas are shown to be the source regions of the tropical easterly waves. Three, if the XEPG is exceptionally large (e.g., in the Indian Ocean during the northern summer) convection is not collocated with maximum SST but part of a complex dynamical interaction of the monsoons. Four, in regions of the weak XEPG maxima in convection are sometimes found in the winter hemisphere. In fact, the most intense mean precipitation in the tropics occurs in early winter north of the equator. It is shown that this convection is the dynamical response over warm water to instabilities induced by the equatorward advection of extratropical waves towards the equator. Through these four processes, and with sufficient thermodynamical effects, the location and intensity of mean organized large-scale convection can be explained throughout the tropics.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
