THE ROLE OF TOPOGRAPHY IN THE LOW-FREQUENCY VARIABILITY OF THE LARGE-SCALE MIDLATITUDE CIRCULATION

The effect of the zonally asymmetric forcing due to topography on the low-frequency variability of the large-scale flow is investigated for Northern Hemisphere winter conditions. Extended general circulation model integrations are used in which the topographic heights are reduced. The effect of reduced topographic heights is to reduce the mean persistence of recurrent regimes identified from the amplitude of the planetary waves with spatial scales comparable to the topography. The number of episodes of large wave amplitude and their anomaly patterns are affected very little. The impact of topography on the total gridpoint height variance includes two components. Decreased topographic heights lead to increased high-frequency eastward-traveling variance and decreased low-frequency variance. In addition, the regionalization of the Pacific and Atlantic storm tracks found in the control simulation diminishes as the topographic heights are reduced. From the results the authors conclude that the occurrence of persistent regimes in the large-scale Bow is linked to the presence of topographic forcing of sufficient amplitude but that the amplification mechanism of the planetary waves is not directly linked to the topographic forcing. Therefore, it appears that the topography plays a catalytic role in permitting longer persistence of a large-scale, amplified planetary wave flow regime.