Poleward deflection of storm tracks

Abstract

An analysis of 11 years of European Centre for Medium-Range Weather Forecasts data focuses primarily on the vertically averaged high-frequency transients. The conclusions are discussed in the context of (a) the winter storm track, (b) monthly variability, and (c) interannual variability. (a) Winter storm track: Results show that the pattern of the forcing by the high-frequency eddies along the storm track is highly correlated with the stationary circulation, and the forcing itself is primarily responsible for the location of the trough ridge system associated with the stationary flow. The results also clarify the role of wind component covariance terms u′v′̄ and (u′2̄ - u′2̄) in the column-averaged vorticity forcing. The simpler term u′v′̄ has the well-known effect of intensifying the anticyclonic (cyclonic) tendencies on the southern (northern) side of the jet. thereby producing an increase in the barotropic component of the zonal jet. The (v′2̄ - u′2̄) term displays a quadrupole pattern, which is also approximately in phase with the trough-ridge system associated with the stationary flow. (b) Monthly variability: Eddy activity has been shown to possess a seasonal life cycle, increasing during the early fall and reaching a maximum around the month of November, then decaying for most of the winter months. Month-to-month variations in eddy activity over the Pacific Ocean show that energy levels increase up through November, decreasing thereafter, at the same time the trough-ridge circulation pattern is intensifying. By December, baroclinicity in the western Pacific has increased substantially, and low-level eddies are found to break by the middle of the ocean. Upper-level eddies start to break well before reaching the west coast of North America, resulting in a displacement of the maximum in (v′2̄ - u′2̄) westward from its November position and increasing the trough-ridge forcing by the high-frequency eddies, (c) Interannual variability: Wintertime eddy kinetic energy is seen to extend further eastward through the Pacific Ocean during the warm phase but displays an abrupt termination during the cold phase. Anomalies in the eddy transient forcing tend to be quite similar to that of the Pacific-North American pattern itself. The extension of the storm track during the warm phase resembles that of fall conditions and is present in the winter season because the source of low-level baroclinicity is extended well into the eastern Pacific for this El Niño-Southern Oscillation phase.