The Effect of Topographic Forcing on the Formation and Maintenance of Blocking

A barotropic channel model in β-plane is used to study the effect of topographic forcing on the formation and maintenance of blocking. The approximate analytical solution of potential vorticity equation can show the main property of the whole process of blocking. It is indicated that the topographic forcing is one of the main factors causing the blocking process. The results suggest that the nonlinear interaction plays a very important role in the stable 'Ω' situation of blocking. The atmospheric circulation with periodic and low-frequency oscillation, perhaps, is partly caused by topographic forcing.

Dynamical role of the Rocky Mountain controlled by East Asian topographies in modulating the tropospheric westerly jet in northern winter

Large-scale mountains like Asian topographies and the Rocky Mountains have important influences on subtropical jet streams(STJs)over downstream regions in winter.The dynamical role of the Rocky Mountains in modulating STJs with and without the existence of East Asian(EA)topographies in northern winter is investigated via numerical experiments.In agreement with previous studies,the Rocky Mountains(topographic forcing),with the existence of EA topographies,can only strengthen the STJ from the east coast of North America to the western Atlantic region.The independent role of the Rocky Mountains,however,strengthens the STJ over not only the east coast of North America but also over Pacific regions.It is found that the existence of EA topographies can dramatically strengthen the EA trough,as well as a downstream ridge which,in the upstream of the Rocky Mountains,acts to partly cancel out the strengthening of the anticyclone to the north of the Rocky Mountains and the northward warm air transport in the high latitudes of Pacific regions due to the Rocky Mountains’forcing alone.Such circulation changes effectively weaken the Rocky Mountains–forced strengthening of the meridional temperature gradient in the midlatitude North Pacific,and thus the STJ there.Therefore,EA topographies are of great importance in modulating the role of the Rocky Mountains as a dynamical forcing of STJ variability.

THE EFFECT OF ISOLATED TOPOGRAPHY ON WEST FLOW

With the Fourier-expanding expression of isolated topography,the forcing of isolated topography is studied in this paper.In the initial condition of steady uniform west flow in β-plan channel model,the approximate solution of quasi-geostrophic potential vorticity equation is obtained.The results indicate that a series of steady topographic standing and transient waves can be produced by the forcing of isolated topography.The wave can move eastward or westward according to the wave scales.The most slowly Rossby wave is the strongest wave.So is the standing wave corresponding to the wave number.The main action of topography is to produce these strong waves.It is suggested that action of topography is one of the main causes to pro- duce low frequency oscillation and long-range steady weather.