Vorticity balances in the tropics during the 1982‐83 El Niñio‐Southern oscillation event
Journal Article
Overview
abstract
AbstractThe mean vorticity balances in the tropical atmosphere during December 1982 to February 1983, a season of extraordinary anomalies associated with the warming of the east equatorial Pacific Ocean, are determined using the 6‐hourly initialized and objectively analysed observations archived at the European Centre for Medium Range Weather Forecasts. the budgets are evaluated at one upper (150 mb) and one lower (850 mb) tropospheric level, with particular emphasis on the balance at 150 mb, near the level of maximum outflow associated with organized deep convection in the tropics.The balances at the upper and lower levels are found to differ qualitatively from one another. At 850 mb, the zonal flow is weak and the relative vorticity and its gradient small compared with that of the Coriolis parameter. the vorticity balance is therefore essentially a Sverdrup balance between the stretching and horizontal advection of planetary vorticity, and an assumption of linearity about a basic state at rest is not unreasonable. None of this applies, however, at 150 mb.At 150 mb, the balance is essentially nonlinear, and nearly inviscid, the primary balance obtaining between the stretching and advection of absolute vorticity by the time‐mean horizontal flow. Transients play a small though not insignificant role, but terms involving the vertical advection and twisting of vorticity are small enough to be neglected.The essentially nonlinear character of the upper tropospheric dynamics is highlighted by demonstrating the inability of a diagnostic barotropic model linearized about the observed steady zonal mean flow to simulate the anomalous flow in the central Pacific correctly. the idea of the expansion of material circuits in regions of strong convective outflow, leading to a rapid spin‐down of the local absolute vorticity, and the production of a region of almost zero absolute circulation, is of crucial importance. This is seen to be intimately related to the importance of nonlinearity.
