A Simulation of the January Standing Wave Pattern Including the Effects of Transient Eddies

Abstract

A steady-state, linear, two-level primitive equation model is used to simulate the January standing wave pattern as a response to mountain, diabatic and transient eddy effects. The model equations are linearized around an observed zonal mean state which is a function of latitude and pressure. The mountain effect is the vertical velocity field resulting from zonal mean wind over the surface topography. The diabatic heating is calculated using parameterized forms of the heating processes. The transient-eddy effects, i.e., the flux convergence of momentum and heat by transient eddies, are computed from observations. Separate responses of the model are computed for each of the three forcing functions. The amplitude of the response to diabatic heating is small compared to observed values. The vertical structure is highly baroclinic. At the upper level, the phase of the waves is approximately in agreement with the observations. The amplitude of the response to mountain forcing is comparable with observations. The wavelength of the response in the Pacific sector is shorter than observed. The vertical structure is equivalent barotropic. The combined response to diabatic heating and mountain forcing is dominated by the contribution from the mountains. The phase shows some agreement with the observations, but the Aleutian low is located too far to the west and an unrealistic high appears to the west of the dateline. The amplitude of the response to transient eddy effects is comparable to the observations in middle and low latitudes. At high latitudes the amplitudes are much too large. The assumption of linearity is not valid for strong forcing at high latitudes where the zonal wind is very weak. The vertical structure of the response is almost equivalent barotropic. A comparison of the responses to mountain and transient eddy effects shows an interesting phase relationship. The troughs produced by the transient forcing are found in the lee of the troughs produced by the mountains (very close to the ridge) indicating that transient forcing is organized by the mountain effects. The combined model response to all three forcing functions shows good agreement with observations except at very high latitudes.