A GCM Study on the Maintenance of the June 1982 Blocking in the Southern Hemisphere

Abstract

General Circulation Model (GCM) experiments have been performed to determine mechanisms that maintained the blocking episode in the Australian-New Zealand region during the period 8?22 June 1982. A control forecast reproduces the persistent ridge. Several mechanistic experiments lead to the following conclusions. (i) The block was not due to orographic forcing, which has only a small local influence on the winter atmospheric circulation in the Southern Hemisphere. (ii) The block was not produced by the sea surface temperature anomalies (SST). By comparing the relative location of low-level atmospheric vorticity and SST anomalies, we are able to show that during June 1982 the atmospheric blocking was the cause of the SST anomalies in the Pacific. (iii) The block was not a response to tropical heating or the Asian Monsoon. There are only weak effects on the block when the tropical heating or heating in the Pacific region is suppressed. (iv) The most important boundary forcing maintaining this blocking ridge is heating associated with the land-sea contrast. The height fields are more zonally symmetric when the land-sea contrast is suppressed. The local land-sea contrast in the Australian region also contributed to maintain the stationary blocking ridge. The sensible heat release in the subantarctic region is an important mechanism that maintains the block. (v) Finally, the daily spectral energetics of the control experiment suggests that the baroclinic amplification of planetary-scale waves forced by synoptic-scale disturbances played an important role in the evolution of this blocking process.