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The Role of Forcing in the Local Instability and Regional Structure of Atmospheric Low Frequency Variability
Applied Math| Speaker: | Dan Hodyss, Naval Research Laboratory |
| Location: | 1147 MSB |
| Start time: | Fri, May 19 2006, 4:10PM |
Observations of atmospheric low frequency variability over the Northern Hemisphere show distinct structures in different geographical locations. These structures manifest as slowly modulated wave trains over the continents and zonally elongated features over the oceans. We hypothesize that these distinct regional structures can be attributed to the specific way in which the low-pass filtered flow is externally forced. To test this hypothesis we formulate a model of the troposphere using a quasigeostrophic homogeneous fluid on the β-plane, under the influence of potential vorticity (PV) sources and spatially varying bottom topography. Using numerical, Hamiltonian, and WKB methods we are able to relate these distinct regional structures to the local stability characteristics of the flow. In particular, we show that background flows forced by topography lead to oscillatory unstable structures, while background flows forced by PV sources may lead to smooth, envelope structures, both in qualitative agreement with observations