An analysis of a relatively rare case of continental blocking

Planetary- and synoptic-scale analyses of a relatively rare continental blo cking event that occurred over North America during the spring of 1980 are undertaken to determine whether or not this event was different from its co unterparts which occur over oceanic regions. The planetary-scale analysis d emonstrates that during the spring season a ridge was located further inlan d over the North American continent and amplified with respect to climatolo gy. The position of this ridge may have been linked to a broad region of co lder-than-normal sea surface temperatures found over the north central Paci fic during the spring season and much of the previous winter. Simple 'Sutcl iffe-type' and thermodynamic analyses of the accompanying lower-tropospheri c warm anomaly associated with the ridging show that lower-tropospheric tem perature advection and subsidence associated with anticyclonic-vorticity ad vection by the time-mean thermal wind produced much of the anomalous warmth . A simple synoptic-scale analysis was performed using both the Zwack-Okossi (ZO) equation and potential vorticity (PV) thinking approaches. These compl ementary analyses demonstrated that synoptic-scale cyclones were instrument al in the formation and maintenance (and/or intensification) of this blocki ng event. The PV analysis demonstrated that low-PV air was swept polewards and then was advected over the blocking region sustaining the broad region of low potential vorticity associated with the block over North America. Th e ZO analysis showed that the advection of anticyclonic vorticity was the m ost important mechanism forcing geopotential-height rises at 500 hPa over t he block centre. The region of low PV and ZO height rises could be associat ed with the anticyclonic-shear side of an upstream jet maximum typically fo und in association with developing and/or intensifying blocking events. Thu s, negative PV advection correlated significantly with calculated ZO height rises. Finally, it is suggested that a favourable phase relationship betwe en the upstream cyclones and the large-scale ridge is necessary for block d evelopment or intensification.