An analysis of a relatively rare case of continental blocking

Abstract

Planetary- and synoptic-scale analyses of a relatively rare continental blocking event that occurred over North America during the spring of 1980 are undertaken to determine whether or not this event was different from its counterparts which occur over oceanic regions. the planetary-scale analysis demonstrates that during the spring season a ridge was located further inland over the North American continent and amplified with respect to climatology. the position of this ridge may have been linked to a broad region of colder-than-normal sea surface temperatures found over the north central Pacific during the spring season and much of the previous winter. Simple ‘Sutcliffe-type’ and thermodynamic analyses of the accompanying lower-tropospheric warm anomaly associated with the ridging show that lower-tropospheric temperature advection and subsidence associated with anticyclonic-vorticity advection 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 complementary analyses demonstrated that synoptic-scale cyclones were instrumental in the formation and maintenance (and/or intensification) of this blocking 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. the ZO analysis showed that the advection of anticyclonic vorticity was the most important mechanism forcing geopotential-height rises at 500 hPa over the block centre. the region of low PV and ZO height rises could be associated with the anticyclonic-shear side of an upstream jet maximum typically found in association with developing and/or intensifying blocking events. Thus, negative PV advection correlated significantly with calculated ZO height rises. Finally, it is suggested that a favourable phase relationship between the upstream cyclones and the large-scale ridge is necessary for block development or intensification.