An Assessment of the Impact of Transient Eddies on the Zonal Flow during a Blocking Episode Using Localized Eliassen-Palm Flux Diagnostics

The Eliassen-Palm (E-P) flux, applied to zonal men flows, is an indicator of both the flux of eddy activity and the eddy forcing of the zonal mean flow. For time mean flows, a localized E-P flux is derived and used diagnostically to assess the impact of transient eddies on a major blocking episode that occurred over the South Pacific during the Southern Hemisphere winter of 1979. In contrast to previous studies that have focused on the mean quasi-geostrophic potential vorticity equation, the focus here is on the mean momentum equations. Eddy transports and the associated induced meridional circulation and other internal adjustments necessary to maintain the thermal wind balance, are gathered together allowing the residual circulation and the effects of the eddies to be determined. The time-mean equations of motion are thus transformed to consist of mean terms, the residual circulation and the divergence of a localized E-P flux vector. The latter is a measure of the eddy forcing of the mean flow, and the east-west component is shown to be related to the flux of wave activity. For the zonal mean case it is identical to the E-P flux. The local E-P flux is closely related to, but differs from, the E-vector of Hoskins et al. and Plumb's radiative wave activity flux, but has several advantages over both. For the blocking episode, defined as 20 July-31 August 1979, transient eddies were steered around the location of the blocking anticyclones following the two branches of the split westerly jet. However, the transient eddies in each branch differed in character, both from each other and from those in the main Southern Hemisphere storm track that extends across the southern Indian Ocean near 50°S. In the latter, the high frequency synoptic-scale baroclinic eddies are barotropically damped. The eddies have similar character to the south of the block but consist mainly of zonal wavenumbers 3 and 4 with periods shorter than a week. In contrast, the transient eddies in the subtropical branch of the jet are higher wavenumber (mostly waves 5 and 6) with periods longer than a week and, although primarily baroclinic, they are also maintained by barotropic processes. Most transient wave energy propagates eastward and wave packets can be followed around the entire hemisphere, mostly following the split westerly jet, with a period of about six days. The local E-P flux divergence is divided into barotropic and baroclinic components. The former is coherent in the vertical but strongest at 300 mb near the tropopause. The transient eddies barotropically accelerate the westerlies in the main storm track and branch south of the block, and this is partially balanced by the baroclinic component. Thus a large part of the momentum balance is between transient eddy momentum convergence and the Coriolis torque arising from the poleward heat transport induced Ferrel cell, in combination with Surface friction. Where the main westerly jet splits as part of the blocking flow configuration, both the barotropic and baroclinic local E-P flux components are acting to decelerate the westerlies and thus the transient eddies are helping to maintain the blocking episode. The main differences between the storm track and blocking regions arise in the barotropic component of the local E-P flux. It appears that the configuration of the split westerly jet acts to systematically deform the transient eddies in such a way that they feed back to help maintain the split structure.