Response of a Steady-State Model for Quasi-Stationary Perturbations to Simulated Anomalies at the Earth's Surface

Abstract

A steady-state quasi-geostrophic two-layer model for the quasi-stationary perturbations of the Northern Hemisphere midwinter conditions is used to study the response of the model to stimulated anomalies at the earth's surface. Three anomalous surface conditions are simulated: 1) a cold pool of water in the Pacific Ocean, 2) a vegetation cover over the Sahara Desert and 3) removal of a climatologically observed snow cover over Tibet. The cold pool of water in the Pacific Ocean is simulated by inserting a ?3°C perturbation in temperature at a level below the seasonal variation of the thermocline. An anomalous vegetation cover over the Sahara Desert is simulated by using appropriate values for surface albede and water availability for evaporation. The climatologically observed snow cover over Tibet in the control experiment is removed in the anomaly experiment. These changes affect the heat fluxes at the earth's surface and hence heating in the atmosphere. The anomalously cold subsurface temperature cools the surface by about -2.4°C. This change in the surface temperature produces a cooling of 0.2°C day?1 in the atmosphere and decreases the 500 mb temperature by 0.4°C. The vegetation over the Sahara Desert cools the earth's surface by 0.5?1°C. Heating in the atmosphere is increased by 0.2?0.3°C day?1 due to the release of latent heat by excessive precipitation resulting from increased evaporation at the surface. The temperature at 500 mb increases by 0.4°C downstream from the anomaly. Removal of snow over Tibet results in an increase in surface temperature of the region by 1.5?2°C. Atmospheric heating increases by 0.3°C day?1. The temperature at 500 mb increases by 0.5°C. In all cases, the heating (cooling) in the atmosphere produces a low (high) in the lower troposphere and a high (low) in the upper troposphere downstream from the anomaly. In spite of the simplicity of the model, the results of the experiments appear to be physically meaningful.