MECHANISMS FOR THE EXISTENCE OF DIAGONAL SOUTHERN HEMISPHERE CONVERGENCE ZONES

Van Der Wiel, Karin (2015) MECHANISMS FOR THE EXISTENCE OF DIAGONAL SOUTHERN HEMISPHERE CONVERGENCE ZONES. Doctoral thesis, University of East Anglia.

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Abstract

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.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Mia Reeves
Date Deposited: 01 Jul 2015 09:04
Last Modified: 01 Jul 2015 09:04
URI: https://ueaeprints.uea.ac.uk/id/eprint/53448
DOI:

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