The energetics of an upper tropospheric cyclonic vortex over north-east Brazil

Abstract

The daily budget of zonal and eddy components of kinetic and available potential energy (APE) including boundary and generation terms are computed for a limited area from 52.5 to 22.5°W and from 20°S to 2.5°N and extending in the vertical from 1000 to 50 hPa, during the period 1-10 January 1993. During the vortex period, 5-10 January, a cyclonic vortex in the upper troposphere with its mean centre at 10°S, 35°W was present. The combined boundary pressure work and dissipation terms of the kinetic energy budget equations, as well as APE generation terms, are computed as residuals. The meridional transports of eddy momentum and sensible heat are also computed. The time-pressure distributions of the zonal and eddy components of kinetic energy (KZ and KE, respectively) and APE are examined. The upper layer 500-100 hPa is identified as the vertical layer which contains the vortex and its interaction with the ambient flow. A sharp increase of KZ in the upper layer is seen as an important feature of the pre-vortex period 1-4 January. The role of boundary pressure work in the sharp increase of KZ associated with the development of a strong shear zone before the formation of the vortex is discussed. The vortex formation in the upper layer is associated with a rapid decrease of KZ, a sharp increase of KE and barotropic energy conversion from KZ to KE, the commencement of eddy APE (AE) to KE conversion and the generation of AE. These are identified as the main characteristics of the vortex formation from the time variation of energy variables. The vortex formation is also associated with a large increase of down-the-gradient eddy momentum transport. It is found from the vertically integrated mean energy cycle in the upper layer that both the barotropic and AE to KE energy conversions maintain the vortex. However, the former is found to be 0.63 W m^-2 and dominates over the latter by a factor of 3.5. A balance between the baroclinic energy conversion and the generation of AE is noted. The system is dynamically (thermally) more active in the vertical layer 350-100 hPa (500-100 hPa) as revealed by the vertical variation of energy variables. The reasonableness of computed energy variables is discussed qualitatively and quantitatively wherever possible.