Transient eddy forcing of blocking flows.

Abstract

The net forcing of blocking flows by transient eddies having synoptic time scales is examined within the framework of quasi-geostrophic theory. Temporal filtering is used to distinguish the effects of transients having synoptic time scales. The eddy flux convergences for individual blocking events are then composited according to the locations of the blocks. The quasi-geostrophic potential vorticity equation relating geopotential tendency and the eddy flux convergences of heat and vorticity is solved for composite blocking conditions. The diagnosis is performed for both observed blocks and blocks simulated by a general circulation model. The geopotential tendencies due to transient eddies exhibit a quadrature relationship with the blocking pattern throughout the troposphere, with anticyclonic eddy forcing being located about one-quater wavelength upstream of the blocking anticyclone. The effects of transient disturbances on blocking flows as depicted by our diagnostic technique are contrasted with those for earlier studies. Reasons for the inconsistencies in the results of earlier studies are discussed.-from Author