Transient Cyclone-Scale Vorticity Forcing of Blocking Highs

Abstract

The forcing of Northern Hemisphere blocking highs by the transient vorticity transfer from the cyclone-scale eddies into the planetary flow is investigated. Thereby, a barotropic prediction model for the planetary modes is numerically integrated. The time-dependent cyclone-scale vorticity forcing is evaluated from the observations. The blocking activity in the forced model flow is examined by means of an objective analysis scheme. It is found that the model flow exhibits frequent blocking with some regional preference for the Atlantic and Pacific oceans. The mean structure of the model blocks compares well with the observations, particularly in the case of Atlantic blocking. However, the mean amplitude of the model blocks is only about two-thirds of those observed. Furthermore, it is found that the time-mean part of the forcing does not support the blocking pattern, so that the model blocking activity is due only to the transient forcing. The significance of the cyclone-scale vorticity forcing for the occurrence of observed blocks is established in terms of model flow pattern averaged over days of actually observed blocking. In the case of Atlantic blocks, the model reproduces the correct mean structure of the observed flow with about one-third of the observed intensity. On the other hand, this mechanism fails in the Pacific. It is believed that this shortcoming is partly due to poor data resolution in the Pacific.