Effects of changing baroclinicity on the southern hemisphere extratropical circulation

Abstract

AbstractExtratropical cyclones of the southern hemisphere provide the primary mechanism of poleward horizontal heat and moisture transport. Their behaviour is influenced by the strength of the meridional temperature gradient in the mid troposphere. As the semi‐annual oscillation (SAO) in the annual cycle of this gradient has become less dominant in recent years, we investigate the impact of modifying the gradient in two simulations with the Arpège general‐circulation model. Model temperatures at 50°S near the 500 hPa level are increased (strengthening the gradient) or decreased (weakening the gradient) in two different simulations T50+ and T50−, respectively.The temperature nudging had a greater impact on the temperature gradient in T50+, but both simulations showed significant surface warming on the Antarctic coast in winter. In T50+, 500 hPa temperature and surface pressure gradients had weaker SAOs, as the circumpolar trough contracted and intensified and cyclone numbers increased along the Antarctic coast. The SAOs strengthened in T50−, and the trough became more disperse, but cyclone numbers increased north of 50°S.Baroclinicity increased (decreased) with a stronger (weaker) meridional temperature gradient, but the static stability of the atmosphere also changed with the opposite impact on baroclinicity. Thus, in T50+ the increase in cyclone numbers was greatest near the Antarctic coast but this was counteracted somewhat by the static stability increase north of 60°S. In T50− cyclone numbers increased north of 50°S because baroclinicity was increased by a stronger meridional temperature gradient and reduced static stability. Hence, the effect on the Antarctic surface climate was nonlinear, with a warmer coast in both cases in winter, and a windier East Antarctica in T50+ but calmer katabatic flow in T50−. Copyright © 2002 Royal Meteorological Society