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 lime scales (i.e., periods less than approximately seven days) from those having longer time scales. Convergences of transient eddy transports of heat and vorticity during blocking conditions are computed from highpass-filtered time series. 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 net quasi-geostrophic geopotential tendencies due to transports by the synoptic-scale transient eddies exhibit a quadrature relationship with the blocking pattern throughout the troposphere, with anticyclonic eddy forcing being located about one-quarter wavelength upstream of the blocking anticyclone. The net quasi-geo- strophic temperature tendencies due to transports by the synoptic-scale transient eddies tend to be out of phase with the temperature perturbations of the block. These relationships were reproducible in all of the observed and model blocking events examined. 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.