Forced Planetary Waves in the Northern Hemisphere Winter: Wave-Coupled Orographic and Thermal Forcings

Abstract

A more complete and new formulation of the orographic forcing and new thermal forcings are included in a steady state model of the Northern Hemisphere planetary waves. When both forcings are included, the simulation produces excellent results which are compared in detail with observations. In particular, the Siberian high, the tropospheric East Asian trough and subtropical tropospheric East Asian jet stream maxima are well reproduced even though the forcing is primarily extratropical in origin. The modes uses a lower boundary condition in which the orographic forcing is determined by the effects of the total flow, not just the zonal mean basic state. Consequently, the net orographic forcing changes when thermal forcing is added and the tow solution is not equal to the linear sum of the solutions with orographic and thermal forcings separately. The thermally induced orographic forcing is found to be very significant and, in the troposphere, there is strong interaction between the two forcings with both of roughly equal importance. However, the Iowa-latitude vertically propagating waves am deflected by the subtropical jet and absorbed in the low-latitude easterlies. Thus only the mid-high latitude planetary waves are important in the stratosphere which seems to be dominated by the thermally forced component. The model is forced with new estimates of diabetic heating from several FGGE analyses. The sensitivity of the results to different heatings and their assumed vertical profile is examined. The amplitude of the lower-troposphere response is very sensitive to the vertical profile but there are much smaller changes at upper levels which are dominated by the remote response. Large differences in the response to the different diabatic heatings are found at high latitudes and over the Pacific Ocean. However, when orographic forcing is also included, these differences diminish indicating a smaller sensitivity to uncertainties in heating, and thus the orographic forcing is acting to constrain the total response. This is in marked contrast to the model results when the traditional lower-boundary condition (in which the waves are decoupled) is used since then the total response is entirely linear. Wave 1 is too weak in the model and this is most likely mainly due to deficiencies in the thermal forcing. The model eddy fluxes of heat and momentum show excellent agreement with observations in location although the poleward heat flux is somewhat weak. These reveal noticeable improvements over the wave-decoupled model which produces too large a response with the lower-tropospheric heat flux too far south at 35°N in association with a degraded simulation of the Siberian high and East Asian trough, in particular.