The role of diabatic heating in maintaining the upper‐tropospheric baroclinic zone in the South Pacific

Abstract

AbstractA modified set of ECMWF FGGE level III‐b analyses is used to examine the four‐dimensional structure of the region containing the South Pacific convergence zone (SPCZ) during a period when two cyclones form in the tropics and propagate south‐eastward along the SPCZ into middle latitudes. During this period. 10–18 January 1979, the atmosphere is characterized by a highly baroclinic state which, together with the existence of an upper tropospheric subtropical jet, supports the growth and development of the observed cyclone activity. In a previous study, it was found that convective activity was prominent during the life cycle of each cyclone.The goal of the present study is to examine the significance of kinematic and thermodynamic processes in maintaining the upper baroclinic region coincident with the subtropical jet. This baroclinic zone lies just poleward of the warm core associated with the convectively‐active SPCZ. A partitioned form of the fronto‐genetical function is used to diagnose the adiabatic and diabatic contributions to the maintenance of baroclinicity in the vicinity of the SPCZ. A major balance is found between the frontogenetical contribution by differential diabatic heating and the opposing diabatic tilting processes. In contrast to the results of most mid‐latitude cyclone studies, the adiabatic contributions from the deformation and tilting terms appear to be of lesser importance. Also, the contribution from the deformation associated with the horizontal branch of the diabatically forced circulation is found to be relatively unimportant. The latter finding is in disagreement with some previously reported numerical results concerned with middle latitude moist frontogenesis. Possible reasons for these differences are discussed.