Mechanisms for the existence of diagonal Southern Hemisphere Convergence Zones

• Main Author: Van Der Wiel, Karin
• Published: University of East Anglia 2015
• Subjects: 577
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.656148

This thesis considers the northwest-southeast, diagonal, orientation of the South Pacific and South Atlantic Convergence Zones (SPCZ and SACZ, respectively) which provide vital precipitation locally and influence mean climate globally. Their basic formation mechanism is not fully understood. A conceptual framework is developed to explain the mechanism responsible for the SPCZ diagonal orientation. Wind shear and Rossby wave refraction cause vorticity centres in the subtropical jet to develop a diagonal orientation and propagate equatorward towards the eastern Pacific upper-tropospheric westerlies. Ascent ahead of cyclonic vorticity anomalies in the wave then triggers deep convection parallel to the vorticity centre. Latent heat from condensation forces additional ascent and upper-tropospheric divergence; through vortex stretching this leads to an anticyclonic vorticity tendency. The calculation of a vorticity budget shows this tendency is strong enough to dissipate the wave. A similar sequence of events triggers diagonal bands of convection in the SACZ, though the vortex stretching feedback is not strong enough to dissipate the Rossby wave. An atmospheric general circulation model is used to investigate this mechanism. In an experiment the parametrisation of convection is modified: dynamic Rossby wave forcing is decoupled from the usual thermodynamic response. Consequently, Rossby waves over the SPCZ region are not dissipated, confirming the feedback in the framework. Furthermore, it is shown that SPCZ convective events decrease the strength of the eastern Pacific upper-tropospheric westerlies. Further experiments show which surface boundary conditions support the SPCZ diagonal orientation. Continental configuration, orography and absolute Sea Surface Temperatures (SST) do not have a significant influence. The key boundary condition is the zonally asymmetric component of the SST distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that transports and supplies moisture to the SPCZ. Convection is triggered when the dynamical forcing from Rossby waves is present.