Barotropic mechanisms associated with tropical plume formation

The tropical plume is an upper level phenomenon with greatest frequency of occurrence over the northeast Pacific. Primarily wintertime events, plumes transport energy poleward and may be transient systems comprising a Hadley circulation. Previous studies of tropical plumes have focused on plume development and on describing plume structure and statistical properties. Possible mechanisms involved in plume initiation are investigated. From a case study, barotropic mechanisms are hypothesized to be important in initiating tropical plumes. These mechanisms are investigated by means of a one-layer, barotropic, gridpoint model using the shallow water equations on an equatorial beta-plane. Particular attention is given to wave-wave interaction occurring between equatorially trapped normal modes and between these modes and perturbations from higher latitudes. The numerical results indicate that interaction among equatorially trapped Rossby modes shows promise as being a key element in plume initiation. The effects are particularly apparent when the Rossby wave state is perturbed by a disturbance resembling an upper level cyclonic circulation, such as frequently occurs when a midlatitude trough intrudes into the tropics. The effects of a Kelvin-wave-like basic state are limited: active responses are observed only in the westerly regime of the Kelvin wave but do not interact strongly with the Kelvin wave itself.