An interesting climatological feature of upper troposphere during northern winter is the existance of regions of equatorial westerlies. Past research has focused on these westerly "ducts" as preferred locations for cross-equatorial wave propagation. However maps of potential vorticity from meteorological analyses and water vapor from satellites indicate that these westerly ducts may also be important regions for stratosphere to troposphere exchange. In particular, these maps show "Rossby wave breaking" events where tongues of dry, high PV (stratospheric) air intrude into regions of equatorial westerlies, thin, and eventually mix with the ambient tropospheric air.
We will present results from an examination of the dynamics of these stratospheric intrusions using a spherical shallow-water model, in which localized mass sources are used to produce non-zonal basic states (of varying structure). Preliminary calculations indicate that the occurrence of Rossby wave breaking within westerly ducts depends on the background flow structure and, in particular, on the strength of the diffluence in the subtropical jet. Further calculations are being performed to carefully examine this dependence, as well as the dependence on the wave focing.
The simple shallow water modelling will be supplemented by comparisons with with analyzed potential vorticity fields, and chemical tracers from 3-D chemical transport models. These latter fields will also be used to examine the vertical structure and the amount of transport during these events.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics