PLANETARY AND SYNOPTIC-SCALE INTERACTIONS DURING THE LIFE-CYCLE OF A MIDLATITUDE BLOCKING ANTICYCLONE OVER THE NORTH-ATLANTIC

The formation of a blocking anticyclone over the North Atlantic has be en examined over its entire life-cycle using the Zwack-Okossi (Z-O) eq uation as the diagnostic tool. This blocking anticyclone occurred in l ate October and early November of 1985. The data used were provided by the NASA Goddard Laboratory for Atmospheres on a global 2.0 degrees l atitude by 2.5 degrees longitude grid. The horizontal distribution of the atmospheric forcing mechanisms that were important to 500 mb block formation, maintenance and decay were examined. A scale-partitioned f orm of the Z-O equation was then used to examine the relative importan ce of forcing on the planetary and synoptic scales, and their interact ions. As seen in previous studies, the results presented here show tha t upper tropospheric anticyclonic vorticity advection was the most imp ortant contributor to block formation and maintenance. However, adiaba tic warming, and vorticity tilting were also important at various time s during the block lifetime. In association with precursor surface cyc logenesis, the 300 mb jet streak in the downstream (upstream) from a l ong-wave trough (ridge) amplified significantly. This strengthening of the jet streak enhanced the anticyclonic vorticity advection field th at aided the amplification of a 500 mb short-wave ridge. The partition ed height tendency results demonstrate that the interactions between t he planetary and synoptic-scale through vorticity advection was the mo st important contributor to block formation. Planetary-scale, synoptic -scale, and their interactions contributed weakly to the maintenance o f the blocking anticyclone, with the advection of synoptic-scale vorti city by the planetary-scale flow playing a more important role. Planet ary-scale decay of the long-wave ridge contributed to the demise of th is blocking event.