A Theory for Midlatitude Forcing of Tropical Motions during Winter Monsoons

Abstract

In order to understand the northeasterly monsoon surges and associated tropical motions over Southeast Asia during northern winter, the dynamic response of the tropical atmosphere to midlatitude pressure surges is studied using the linearized shallow-water equations on an equatorial β plane. The forcing is specified to have a Gaussian spatial distribution with a zonal scale corresponding to approximately wavenumber 7 and a meridional scale of approximately 11°. It rises rapidly from zero to maximum within one day or less and then decays slowly over 2–4 days. The main results are as follows: 1) After an initial period of gravity-wave type motions with strong northerly winds, the main tropical response takes the form of a Rossby wave group. 2) A pronounced northeast-southwest tilt in this Rossby wave group develops due to the faster westward group velocity of the lower meridional modes relative to the higher meridional modes. 3) Several conspicuous features of the Rossby response closely resemble the observed flow pattern of the northeast monsoon region, notably the northeasterly wind streak over the South China Sea during cold surges, the mean winter condition of a cyclonic shear trough extending from Borneo to the Philippines, and the enhancement of cyclonic circulation near the northern Borneo coast after surges. 4) The pressure surge forcing also gives rise to eastward moving wave groups of the Kelvin, mixed Rossby-gravity, and inertia-gravity (mainly n=0) modes. The Kelvin wave response, as in the case of thermally forced Kelvin waves, has a preference for longer wavelengths. These wave groups offer a possible interpretation for the eastward moving cloud patterns observed during Winter MONEX by Williams (1981). Our results suggest that the gross features of the synoptic-scale tropical motions in the northeast monsoon region can be explained in terms of simple equatorial β-plane dynamics without taking into account other physical factors such as orography or boundary-layer friction.