Structure and evolution of subtropical cyclone <i>Anita</i> as evaluated by heat and vorticity budgets

Abstract

This article explores the evolution of subtropical cyclone Anita , which occurred near the east coast of Brazil (∼19°S–37°W) in March 2010. Thermodynamic and dynamic processes during Anita 's life cycle are investigated using the heat and vorticity budget equations. The cyclone developed with hybrid characteristics and moved anomalously to the southwest, where it coupled with an upper‐level cut‐off low during the mature phase. This coupling was the main dynamical mechanism for further cyclone deepening. Anita then remained quasi‐stationary at about 30°S–47°W for 2 days, due to an upper‐level dipole pattern, which prevented earlier displacement of the upper‐level low counterpart. When the dipole pattern dissipated, the cyclone moved southeast and underwent extratropical transition whilst merging with a midlatitude frontal cyclone. Diabatic heating and horizontal temperature advection are found to be essential for subtropical development. During extratropical transition, instead it is diabatic cooling together with adiabatic cooling and warm‐air advection that act as the main mechanisms to influence the local temperature tendencies at low levels. Low‐level cyclonic tendencies were mostly due to convergent flow and the residual vorticity partially destroyed the vorticity tendencies produced by the divergence term. Moreover, in regions and levels where convection could explain some of the vorticity tendencies, it is found that apparent sinks of cyclonic vorticity were related to negative vorticity due to divergence (i.e. convergent flow), whilst apparent sources were related to positive vorticity due to divergence (i.e. divergent flow).