Planetary and synoptic-scale interactions during the life cycle of a mid-latitude blocking anticyclone over the North Atlantic

Abstract

The formation of a blocking anticyclone over the North Atlantic has been examined over its entire life-cycle using the Zwack-Okossi (Z-O) equation as the diagnostic tool. This blocking anticyclone occurred in late October and early November of 1985. The data used were provided by the NASA Goddard Laboratory for Atmospheres on a global 2.0° latitude by 2.5° 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 form of the Z-O equation was then used to examine the relative importance of forcing on the planetary and synoptic scales, and their interactions. As seen in previous studies, the results presented here show that upper tropospheric anticyclonic vorticity advection was the most important contributor to block formation and maintenance. However, adiabatic warming, and vorticity tilting were also important at various times during the block lifetime. In association with precursor surface cyclogenesis, the 300 mb jet streak in the downstream (upstream) from a long-wave trough (ridge) amplified significantly. This strengthening of the jet streak enhanced the anticyclonic vorticity advection field that aided the amplification of a 500 mb short- wave ridge. The partitioned height tendency results demonstrate that the interactions between the planetary and synoptic-scale through vorticity advection was the most important contributor to block formation. Planetary-scale, synoptic-scale, and their interactions contributed weakly to the maintenance of the blocking anticyclone, with the advection of synoptic-scale vorticity by the planetary-scale flow playing a more important role. Planetary- scale decay of the long-wave ridge contributed to the demise of this blocking event.