Tropical Plumes in a Barotropic Model: A Product of Rossby Wave Generation in the Tropical Upper Troposphere

Abstract

A divergent barotropic model produces tropical cyclonic Rossby wave sources (RWSs) and tropical plumes (TPs) when forced by upper-tropospheric (UT) convergence and subsidence over the tropical central and eastern Pacific (TCEP). Experiments show that TPs form when equatorward amplification and zonal contraction of the TCEP trough cause rotationally balanced midlatitude flow to transition to buoyancy-driven, ageostrophic flow equatorward of 15°N. This transition results from the sharp reduction of the flow’s horizontal length scale below the Rossby radius of deformation in local near-equatorial regions. Sudden adjustment of mass and momentum fields in the absence of rotational balance produces concentrated convergence–divergence couplets and jet exit–entrance regions, which define actual TCEP TPs. Simulated TPs display remarkable feature agreement with observed TPs, even sharing a common length-scale evolution. Extensive UT cold advection, convergence, and subsidence exist west of the TCEP trough and equatorial westerly duct. These subsidence regions are collocated with large persistent areas of anomalous 6.7-μm water vapor brightness temperature (T_b) maxima. Plumes frequently form downstream of these very warm T_b regions. Simulations show that TPs result from convergence-induced cyclonic RWSs in near-equatorial gradients of absolute vorticity near the TCEP trough. Plumes do not form in simulations lacking tropical convergent forcing and cyclonic RWSs. Experiments display a wide variety of solutions because of the nonlinear nature of the RWS. The basic state is extremely sensitive to the location of tropical convergent forcing. Less than 50% of forced experiments produce TPs. The strongest TPs form in a vigorous Walker circulation. Forcing size may have a greater influence on TP development than forcing intensity.