Structure and evolution of subtropical cyclone Anita 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).

Subtropical cyclones over the oceanic basins: a review

“…Dias Pinto et al . and Dutra et al . addressed these questions for the subtropical storm Anita, occurring in March 2010.…”

“…Dutra et al . discussed the relative importance of thermodynamic and dynamic processes during Anita's lifetime.…”

“…In the extratropical transition phase, diabatic and adiabatic cooling, and the warm air advection were the main mechanisms influencing the local temperature tendencies in surface cyclone environment. From the vorticity equation terms, most of the low‐level cyclonic tendency during Anita was due to mass convergence in the lower troposphere, which in turn was strongly associated with intensification of upward motions, evidencing the importance of convective activity for subtropical cyclones …”

“…29 The dynamics of subtropical cyclones and the environmental conditions associated with tropical or extratropical transitions are still largely unknown over the South Atlantic, since the study of such systems is recent and their frequency is low in the basin. Dias Pinto et al 64 and Dutra et al 65 addressed these questions for the subtropical storm Anita, occurring in March 2010. This was a long-lived (10-day) storm, which started as a pure subtropical cyclone, presented a potential phase for the tropical transition (which did not happen), underwent extratropical transition, and decayed as an extratropical cyclone.…”

“…Dutra et al 65 discussed the relative importance of thermodynamic and dynamic processes during Anita's lifetime. In the hybrid stage of the storm, low-level diabatic heating (due to surface turbulent latent and sensible fluxes) and horizontal temperature advection (warm during intensification and cold during its mature phase) were the main contributors to the local tendency of temperature.…”