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Da-Sheng Yang and T. N. Krishnamurti


A study of the potential vorticity budget for the low-level flows over the Arabian Sea and Indian Ocean is presented here. This study covers a 17-day period between 11 and 27 June 1979 during the GARP Monsoon Experiment (MONEX). Data sets for this study include the special observing systems for the Global Experiment, i.e., cloud winds from the geostationary satellites, surface and upper air data from the First GARP Global Experiment, FGGE and MONEX, data from low-level constant-level balloons, upper air data from dropwindsonde aircraft, and the World Weather Watch. The selection of the 17-day period is important logically for both phenomena and from the availability of the maximum data sets, over oceans during the global experiment. As many as 50 soundings per day were available from this observing system. The framework for the calculations are three equations that describe the changes in potential vorticity, absolute vorticity and the dry static stability. These equations include as relevant forcing the effects of shortwave and longwave radiative processes, shallow and deep cumulus convection, surface friction, and the air-sea transfers of momentum, heat and moisture.

The period of this study coincides with that of the onset and establishment of deep moist westerlies of the monsoons. The Somali jet established itself around 10°N during this period and the onset of monsoon rain had begun. A large increase of positive potential vorticity on the cyclonic shear side of the low-level jet is attributed to an increase of both the absolute vorticity and of the dry static stability. The mechanisms responsible for this increase are explored via detailed budget calculations. The salient mechanisms arise from the covariance of potential vorticity and differential radiative heating along the vertical, and vertical convergence of potential vorticity by the cumulus mass flux, which contributes to a generation of potential vorticity. On the other hand, the role of horizontal advection and friction seems to be opposite.

The meridional flow is countergradient, such that horizontal advection brings low values of dry static stability, absolute vorticity and potential vorticity into the region of their large positive values north of the equator. The wind-stress curl in this region is positive; it contributes to a diminution of the absolute as well as potential vorticity.

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