Low-Level Monsoon Dynamics Derived from Satellite Winds

John E. Stout Department of Meteorology, University of Wisconsin, Madison, 53706

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John A. Young Department of Meteorology, University of Wisconsin, Madison, 53706

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Abstract

The dynamics of low-level summer monsoon flow near 900 mb is studied using daily MONEX (1979) satellite wind data to estimate mechanisms influencing the horizontal momentum. We present an improved estimate of the large-scale monsoon geopotential field near the level of maximum wind, and a more approximate field of friction as well. Average fields for the premonsoon and established monsoon periods of 1.5 months are shown. The evolution of forces and accelerations along different trajectories crossing the western Indian Ocean are compared.

The net horizontal force, equal to the pressure gradient plus friction force, is obtained for the two periods by directly estimating the mean Coriolis and relative acceleration vectors. The contribution to mean acceleration by synoptic-scale transient eddies is significant only south of 30°S. Inertial acceleration by the mean flow produces a Rossby number in excess of 0.25 in an equatorial belt which expands to 10°N in the Somali Jet entrance.

A method is devised to split the observed net force field into its pressure gradient and friction force components; the method corresponds to solving the vorticity and divergence equations, respectively, and uses the property that pressure gradient is exactly irrotational and the assumption that friction force is mostly non-divergent. It is found that the diagnosed friction force tends to oppose the wind and is distinctly weaker than the pressure gradient force. The calculated geopotential field shows the development of a distinctive “reversed S” contour connecting the hemispheres and supporting strong cross-equatorial flow. The corresponding trajectories show that the degree of imbalance is greater in the Northern Hemisphere as the air adjusts to the changing Coriolis influence and monsoonal pressure gradient forces which increase and rotate. Northward moving air slows by friction as it approaches the equator, but increases speed in the Northern Hemisphere by flowing toward lower pressure. The assumptions involving frictional estimates for boundary conditions are evaluated using theory and wind data.

Abstract

The dynamics of low-level summer monsoon flow near 900 mb is studied using daily MONEX (1979) satellite wind data to estimate mechanisms influencing the horizontal momentum. We present an improved estimate of the large-scale monsoon geopotential field near the level of maximum wind, and a more approximate field of friction as well. Average fields for the premonsoon and established monsoon periods of 1.5 months are shown. The evolution of forces and accelerations along different trajectories crossing the western Indian Ocean are compared.

The net horizontal force, equal to the pressure gradient plus friction force, is obtained for the two periods by directly estimating the mean Coriolis and relative acceleration vectors. The contribution to mean acceleration by synoptic-scale transient eddies is significant only south of 30°S. Inertial acceleration by the mean flow produces a Rossby number in excess of 0.25 in an equatorial belt which expands to 10°N in the Somali Jet entrance.

A method is devised to split the observed net force field into its pressure gradient and friction force components; the method corresponds to solving the vorticity and divergence equations, respectively, and uses the property that pressure gradient is exactly irrotational and the assumption that friction force is mostly non-divergent. It is found that the diagnosed friction force tends to oppose the wind and is distinctly weaker than the pressure gradient force. The calculated geopotential field shows the development of a distinctive “reversed S” contour connecting the hemispheres and supporting strong cross-equatorial flow. The corresponding trajectories show that the degree of imbalance is greater in the Northern Hemisphere as the air adjusts to the changing Coriolis influence and monsoonal pressure gradient forces which increase and rotate. Northward moving air slows by friction as it approaches the equator, but increases speed in the Northern Hemisphere by flowing toward lower pressure. The assumptions involving frictional estimates for boundary conditions are evaluated using theory and wind data.

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