Planetary Waves Kinematically Forced by Himalayan Orography

Abstract

An analysis is made of the planetary-scale response of the atmosphere to the kinematic effects of orographic forcing by, in particular, the Tibetan Plateau-Himalayan Mountain complex. Theoretical scaling arguments are used to deduce a critical mountain height hc beyond which the component of flow around will dominate that over the orography. The hc is proportional to the meridional scale of the orography and depends on latitude, For north?south scales appropriate for the Himalayas hc ? 1.5 km which is much less than the actual height of 3706 m when resolved with tour zonal planetary waves, with the implication that the ?around? component will dominate. A steady-state planetary wave model which has a full kinematic nonlinear lower boundary condition is used to simulate the response to the eastern orography whose height has been multiplied by factors ranging from 0.1 to 2.0. Although the mountain configuration was fixed, the locations of the simulated perturbation highs and lows change substantially in such a way that the total flow increasingly adjusts to go around the high orography as the mountain heights are increased. This effect limits the total vertical motion induced by the orography and thus the amplitudes of the forced planetary waves increase at a rate much less than expected from linear theory. Neglected nonlinear terms in the model are shown to be relatively small in all cases. For shallow mountains the maximum response occurs at the latitude of the mountain (35°N) but both the maximum response and the maximum zonal mean poleward heat flux by the simulated waves are shifted poleward to ?55°N for orography >1500 m high, consistent with the observed location of the wintertime stationary waves in the Northern Hemisphere. Overall, results support the expectations from scaling considerations and show that linear theory may be reasonably applied for Himalayan orography up to ?1 km high when resolved on planetary scales, but the ?around? component dominates the ?over? component when the orography exceeds 1.5 km, as is the case in actuality. The around component should also dominate for the Greenland plateau and Antarctica but the effects are more equivocal for the Rockies which are only ?1 km high when resolved with planetary scales while hc ? 1.5 km.