Extended Abstract (144K)16.4
Atmospheric Response to mid-latitude thermal forcing: a mechanism for changes in climate regimes?
Peter G. Baines, CSIRO, Aspendale, VIC, Australia
In mid-latitudes the atmosphere is dominated by the zonal mean circulation forced by the latitudinal heating distribution, and large synoptic scale eddies that are attributable to its instability. It is also influenced by local topography and heating. On the quasi-geostrophic scale, the effects of the latter are manifested in forced barotropic and baroclinic Rossby waves. In this paper I describe the effects of periodic and isolated thermal forcing in realistic sheared mid-latitude wind profiles, and the associated wave and flow fields that are produced. Typically, this consists of lee-side Rossby waves, and columnar upstream motions that may be identified with observed quasi-stationary high pressure systems, such as those upstream of Australia and in the Great Australian Bight, particularly in summer. The analysis shows that when the atmosphere is highly sheared and the mean flow near the ground is reduced to small values (< 1 ms-1), as it is in the mid-latitude zonal high pressure belt, higher order vertical modes may become subcritical (that were formally supercritical). This means that they will form part of the stationary response to thermal forcing from the Australian continent, or parts of it. In these conditions, a reduction in the mean surface wind speed due to a movement in the latitude of the high pressure belt, may cause abrupt changes in the flow field. In particular, this may cause a change in the direction of the forced surface wind from, say, southerly to northerly. The systematic effects of such changes at particular times of the year may result in effective changes in climate and rainfall. Hence, a systematic change in the mean position of the high pressure belt may cause abrupt changes in regional wind fields, for given seasons.
Session 16, Interdecadal and Longer Term Climate Variability II
Friday, 28 March 2003, 8:30 AM-11:45 AM
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