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 10:04
    Last Modified: 01 Jul 2015 10:04
    URI: https://ueaeprints.uea.ac.uk/id/eprint/53448
    DOI:

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