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Abstract
The linearized shallow water equations on a sphere are solved numerically to examine the sensitivity of the steady response to midlatitude mountain forcing to the zonal mean basic state. The zonal mean basic state consists of meridionally varying zonal winds ū(y) and meridional winds v̄(y). Cases are considered where ū is westerly everywhere, outside a tropical region where it is easterly. A zonal wavenumber three mountain confined to the Northern Hemisphere midlatitudes, where ū>0, provides the forcing.
When v̄≡0 the usual result of negligible Southern Hemisphere response to the mountain forcing is found. However, a modest mean meridional velocity [0(3 m s−1)] that is directed from north to south through the easterly layer leads to significant Southern Hemisphere response. An argument based on the local dispersion relation is offered to explain this effect. It is concluded that critical latitude effects on wave propagation are sensitive to the structure of the mean meridional circulation in the critical latitude region of the model. The result of the simplified model suggests that a more relevant model with a zonally symmetric basic state consisting of zonal winds and meridional circulation varying with height as well as latitude should be investigated.
Abstract
The linearized shallow water equations on a sphere are solved numerically to examine the sensitivity of the steady response to midlatitude mountain forcing to the zonal mean basic state. The zonal mean basic state consists of meridionally varying zonal winds ū(y) and meridional winds v̄(y). Cases are considered where ū is westerly everywhere, outside a tropical region where it is easterly. A zonal wavenumber three mountain confined to the Northern Hemisphere midlatitudes, where ū>0, provides the forcing.
When v̄≡0 the usual result of negligible Southern Hemisphere response to the mountain forcing is found. However, a modest mean meridional velocity [0(3 m s−1)] that is directed from north to south through the easterly layer leads to significant Southern Hemisphere response. An argument based on the local dispersion relation is offered to explain this effect. It is concluded that critical latitude effects on wave propagation are sensitive to the structure of the mean meridional circulation in the critical latitude region of the model. The result of the simplified model suggests that a more relevant model with a zonally symmetric basic state consisting of zonal winds and meridional circulation varying with height as well as latitude should be investigated.
Abstract
The CSIRO9 general circulation model shows a zonally symmetric mode of variability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal mean zonal wind between 30° and 68°S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40° and 60°S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period. The momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49°S between the maxima in the zonal wind. The changes in angular momentum content are around 30% smaller in the southern band. Eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction.
A simple model of the zonal wind index with stochastic forcing and linear damping reproduces its short period variations well but is less successful in simulating the observed continuity over 10- to 20-day lags.
Abstract
The CSIRO9 general circulation model shows a zonally symmetric mode of variability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal mean zonal wind between 30° and 68°S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40° and 60°S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period. The momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49°S between the maxima in the zonal wind. The changes in angular momentum content are around 30% smaller in the southern band. Eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction.
A simple model of the zonal wind index with stochastic forcing and linear damping reproduces its short period variations well but is less successful in simulating the observed continuity over 10- to 20-day lags.
