Interannual Variation of Summertime Stationary Eddies

Abstract

An effort is made to explore the interannual variation of stationary eddies for the past 13 summers (1979–1991). Equatorial λ − t diagrams of sea surface temperature (SST), outgoing longwave radiation (OLR), the potential function of OLR (χ_OLR), and velocity potential (χ) derived from National Meteorological Center (NMC) analyses reveal the existence of a coherent and pronounced interannual variation, particularly over the Pacific. Performing regular empirical orthogonal function (EOF) analyses of eddy streamfunction and velocity potential departures from their multiple-summer averages at 200 mb [ψ_E(200 mb)] and χ_E(200 mb)] and correlating the distributions between these two variables, it is possible to identify two interannual variation modes of summertime stationary eddies. The horizontal structure of these two interannual variation modes, which differs from the Pacific-North American (PNA) teleconnection pattern, resembles the Matsuno-Gill type of tropically trapped modes. It is inferred from the coherent interannual variation of the tropical Pacific SST and ψ_E(200 mb) anomalies that a dynamic coupling exists between interannual variations of the Pacific SST and summertime stationary eddies. The coherent eigencoefficient time series of χ_OLR, the χ_E(200 mb) and ψ_E(200 mb) anomalies, the EOF analysis of the ψ_E(200 mb) budget, and numerical simulations of the response of summertime stationary eddies to the interannual variation of tropical forcing all reveal that the aforementioned dynamic coupling is accomplished through the interannual variation of the global-scale divergent circulation.