Spatial and temporal variations of blocking and cyclogenesis in the 1978 / 79 winter

Complex empirical orthogonal function (CEOF) and Fourier analyses are applied to 500 hPa geopotential height anomaly for two selected latitude belts in the Northern Hemisphere from Dec 1978 through Feb 1979 based on the ECMWF FGGE III-b data. The positive anomalies in the three leading CEOFs for the high-latitude belt mainly show the preferred locations for blocking activity in the North Atlantic, the North Pacific and to the west of the Ural Mountains. The negative anomalies in the three leading CEOFs for the mid-latitude belt mainly show the preferred locations for cyclogenesis in the east coasts of Asia and North America, and the Mediterranean; weak cyclogenesis is also seen in the western United States and off the coasts of Spain and Morocco. The travelling components of the positive anomalies in the high-latitude belt mainly propagate westward, weakening as approaching the east side of some mountain chains while intensifying to the west side. On the contrary, the travelling components of the negative anomalies in the mid-latitude belt mainly propagate eastward, intensifying over the lee side of mountain and / or approaching the east coasts of the two continents. These preferred locations for blocking and cyclogenesis are basically consistent with the climatological results, and related to some teleconnection patterns found earlier. The temporal variation of blocking highs seems to relate with the vacillation of the potential vorticity (PV) index defined by Weng (1992). There are two build-up stages of the PV index during the winter. Each build-up stage corresponds to a westward propagation of a large-scale positive anomaly in the high-latitude belt, resulting in the occurrence of a series of blocking highs over the western Eurasia, Scandinavia, Greenland and the Pacific. In general, the temporal variation of cyclogenesis is less reflected by the PV index than blocking highs. The duration of a PV index cycle of build-up and break-down is about 30-50 days. Within this low-frequency envelope, there is a global quasi-two-week vacillation of the PV index, reflecting one of the preferred time scales of mid-latitude cyclone and anticyclone activity in some preferred locations during the 1978 / 79 winter.