Structure and evolution of the large-scale flow and an embedded upper-tropospheric cyclonic vortex over northeast Brazil

Abstract

Horizontal structure and evolution of large-scale flow and an embedded synoptic-scale cyclonic vortex over northeast Brazil as separate systems and dynamical interaction between them are studied at 200 hPa. A quasi-stationary cyclonic vortex with its average position at 10°S and 35°W that formed and remained active during 5-10 January 1993 is selected for the investigation. The evolution of large-scale flow in the prevortex period 1-4 January is also explored. An efficient and effective scale separation technique is developed and used to separate the large-scale flow and embedded synoptic-scale vortex. It is shown that a strong positive shear zone developed in the latitude domain 17.5°-7.5°S, within the South Atlantic trough region before the vortex formation. The shear zone has a characteristic meridional (zonal) scale of 1000 km (3000 km) and satisfies strongly the necessary condition for barotropic instability. It is identified that the development of a strong shear zone is associated with the intensification of a Bolivian anticyclone and associated ridge and their eastward shift, and intensification of the South Atlantic trough, east-west orientation of the Atlantic trough, and the presence of a transient trough over the equatorial Atlantic Ocean. The average structure of vortex including zonal and meridional characteristic scales is computed from the synoptic bandpass flow. The vortex is identified as a nonlinear wave packet with an average zonal wavelength of 2750 km and it is confined to a latitude belt of about 17.5°. The vortex shows a strong westward tilt with latitude; the convergence zone is located to its southwest and it is a weak cold cored system. Maximum cyclonic vorticity of the vortex is -3.24 × 10^-5 s^-1, which is comparable to the value for embedding flow. The momentum transports due to the vortex, large-scale eddy, and the vortex-large-scale eddy interaction are computed. It is found that the vortex and vortex-large-scale eddy westerly momentum transports are southward, down the gradient of embedding zonal flow, and their divergence (convergence) is located over the latitudes of large scale westerlies (easterlies). The sensible heat transports are weak. It is noted that the vortex-large-scale flow interaction leads to the weakening of the shear zone and restoration of the large circulation features to their January 1993 mean configuration, which have undergone significant deviation during the prevortex period. The signature of vortex-large-scale interaction is also seen in the evolution of dynamical parameters q_y and n² (square of refractive index parameter).