The Impact of Orography on Blocking Frequency in a General Circulation Model

Sensitivity experiments with a perpetual January version of a low-resolution general circulation model are conducted to investigate the influence of mountains on blocking in the Northern Hemisphere. Two 1200-day integrations, one with and the other without mountains, are compared. The absence of orography severely reduces the total number of days on which blocking is present anywhere over the Northern Hemisphere. It decreases the number of blocking episodes and shortens the duration of an average event. Strong blocking events do occur in the no-mountains simulation, however, and their structure and local maintenance characteristics are similar to strong blocks in the control run. It is concluded that orographic forcing greatly aids the blocking process in the model but is not crucial for the existence of model blocks.