The Direct Response to Tropical Heating in a Baroclinic Atmosphere

Abstract

The global response to tropical heating is studied by performing a time integration of a 15-level primitive equation model, starting with a basic flow maintained by a constant forcing. The direct, quasi-steady response to the tropical heating is seen during the first 20 days before baroclinic instability dominates. This technique enables the investigation of a variety of basic flows, from a resting state to a December–February 3D time-mean flow; of the timescales for establishing remote responses; and of nonlinear effects. It also allows the determination of timescales for the establishment of the response. The Gill-type response is seen in the lower troposphere in all cases. In the upper troposphere, depending on the basic conditions, the simple tropical quadrupole response of the Gill model shows considerable modification. The anticyclonic pair can be centered over the heating and can vary substantially in magnitude and vertical extent. The Rossby wave source and the upper-tropospheric divergence above the beating region is always found, but the existence and relative magnitudes of local Hadley and Walker cells as measured by upper-tropospheric convergence are strong functions of the flow. Both the Rossby wave source and the Rossby wave propagation are also strongly influenced by the ambient flow. Wave patterns extend to the equator in the regions in which the basic westerlies extend to the equator. Significant tropical zonal flow variations, which are also very dependent on the basic flow and the position of the heating, are also produced. The tropical and midlatitude response are generally established within a week. In an additional week the high-latitude pattern is determined and the subtropical wave pattern propagates back into the Tropics in the westerly wind regions. Nonlinear effects are found to be minor in all cases before the middle-latitude transients develop. On the two-week timescale of interest here, the sensitivity of steady-state models to the dissipations employed and to the existence of low-frequency modes is not found.