Mechanisms of the Northward Movement of Submonthly Scale Vortices over the Bay of Bengal during the Boreal Summer

Abstract

Abstract Mechanisms of the northward movement of submonthly scale vortices over the Bay of Bengal during the boreal summer (May–September) are studied with the use of a vorticity budget analysis applied to the ECMWF 40-yr Re-Analysis (ERA-40) data. To quantitatively evaluate the contribution from each term that constitutes the vorticity anomaly equation to the movement of the vortices, a vector measure, termed the forcing vector (FV), is used in the present study. Because the axis of the submonthly scale relative vorticity anomaly does not tilt meridionally below the 200-hPa level, the mechanisms of the northward movement of a composite submonthly scale vortex integrated from the surface to the 100-hPa level [the barotropic component (BTC)] are studied. The barotropic vortex moves northwestward, with northward speeds of 0.9° day−1. The meridional component of the FV (MFV), which represents the contribution to the meridional component of the movement, reveals that the primary and secondary terms that contribute to the northward movement are the advection of the vortex by the environmental meridional wind, and the tilting effect of the environmental horizontal vorticity vector by the vertical pressure velocity anomaly associated with the vortex, respectively. The former term works mainly in the lower troposphere, while the latter operates in the middle and upper troposphere. The first baroclinic component (FBCC) of the vortex in the troposphere also moves northwestward with almost the same northward speed as the BTC. Mechanisms of the northward movement of the FBCC are also clarified in the present study through examination of the MFV. The primary contributing term is the same as that of the BTC, while the tilting term hinders the northward movement of the FBCC. For the FBCC, the secondary contributing term is the advection of the planetary vorticity by the meridional wind anomaly associated with the horizontal convergence and divergence anomalies in the lower and upper troposphere, respectively. The present study also discusses the phase relation between the BTC and the FBCC from the viewpoint of their northward movement in an environment of easterly vertical shear.