The role of synoptic/planetary-scale interactions during the development of a blocking anticyclone

The period 19–21 January 1979 marked the development of a blocking anticyclone over the North Atlantic Ocean preceded by explosive cyclogenesis about 500 km south of Nova Scotia. Using fields derived from GLA analyzes (4° lat × -5° long) of the FGGE SOP-I data set, the general behavior of this block is diagnosed using the extended height tendency equation. This equation preserves much of the simplicity of the quasi-geostrophic form, but replaces the geostrophic wind and relative vorticity by the observed value. Three-dimensionally varying static stability and strong diabatic heating are also allowed in the extended form. To further analyze the relative importance of planetary-scale, synoptic-scale, and scale-interaction forcing of this block, height tendencies were solved from a scale-partitioned form of height tendency equation. The scale partitioning is accomplished using the Barnes objective analysis scheme. Results indicate that vorticity advection was the primary forcing mechanism during the block development. Growth in this mechanism occurred during and extended beyond the period of explosive cyclogenesis and was located downstream from the cyclone event. In fact, much of the vorticity advection was attributed to the northward advection of negative relative vorticity east of a jet streak that formed between the cyclone and anticyclone. The scale interactions implied by this relationship between the cyclone and anticyclone were confirmed in the partitioned height tendencies. The scale interaction component was consistently larger than the other two and was particularly significant during the block development. This component was followed in importance by the synoptic-scale component, although the latter was significant only in the vorticity advection term. Interestingly, despite pronounced northward warm air advection, the direct forcing of the block by thermal advection was relatively small. Rather, the thermal forcing was strongest in the upstream cyclone, which in view of the subsequent role of scale interactions in the block development suggests an indirect role for thermal advection.