Dynamic balances within tropical plumes in a global barotropic model

Abstract

Tropical plumes are a common synoptic scale feature over the Eastern Pacific associated with subtropical jet and ITCZ intensifications. Because of data sparseness, operational analysis has yielded little information of the details of the tropical plume formation process. Tropical plumes have been simulated in a global 200 mb shallow water model with a realistic basic state (Blackwell 1990). Diagnostic budgets were calculated based on model output for absolute vorticity, divergence, and kinetic energy. Also, the movement of the zero absolute vorticity isopleth during plume formation is examined. This information is compared for cases with plume formation, without plume formation, and the basic state. Budget results indicate the convergent forcing center, when located near a strong absolute vorticity gradient, creates an equatorial Rossby wave source. This source generates a large scale, quasi-stationary equatorial Rossby wave which, if located near the ambient eastern Pacific trough, strengthens this trough into the tropics. As the trough strengthens, it displaces the vorticity gradient equatorward into the cmvergence forcing region. This vorticity gradient encroaches upon the forcing region and creates a second Rossby wave source. This generates a second, smaller scale Rossby wave which propagates to the northeast along the axis of strong vorticity gradient. This Rossby wave is highly divergent in nature, due to the model's small Rossby radius of deformation. The accompanying train of convergence/divergence centers distorts the potential trough to create significant cross contour flow, which accelerates the subtropical jet to the east of the trough, resulting in a tropical plume. This study indicates the Rossby wave and accompanying cross contour and ageostrophic flow adequately explain the formation of all three key tropical plume features.