High-Frequency Transient Eddies and Blocking

In this paper we investigate the role of high-frequency transient eddies in the maintenance of the blocking high which occurred over Europe on 16-25 February 1979. Time-filtering is used to decompose the atmospheric large-scale flow into contributions from high-frequency and low-frequency fluctuations. the separation being roughly at the period of six days. The net effect on the ten-day mean flow of the high-frequency cyclone-scale eddies is studied using several methods. The vorticity flux convergence associated with the high-frequency eddies, although small in comparison with the dominating terms in the vertically averaged vorticity equation, appears to be important for counterbalancing the effects of the time-mean surface stress, especially in the vicinity of the storm tracks. The eddies are thus important for the distribution of the time-mean surface pressure. Also the “tendency method” of illustrating the eddy effects supports this view. The hemispheric average of the barotropic energy conversion from the high-frequency eddies to the time-mean flow is much stronger during the blocking episode than during the remaining part of the month. The response of a linear, hemispheric stationary wave model to the forcing caused by the high-frequency eddies is calculated by taking the eddy forcing and the zonally averaged mean flow as observed during 16–25 February 1979. Comparison of this calculated response with the observed time-mean waves further supports the view that during this blocking episode the high-frequency eddies had an important effect on the structure and maintenance of the time-mean flow in the lower troposphere.