Northern Hemisphere Teleconnection Patterns during Extreme Phases of the Zonal-Mean Circulation

Regional climate, anomalies associated with year-to-year changes in the tropospheric zonal-mean zonal wind (ū) are examined. This study focuses on the wintertime Northern Hemisphere extratropics and compares seasonal mean anomalies associated with ū to those associated with the El Niño–Southern Oscillation during the 1947–94 period. Dynamical model experiments indicate that a single zonal index, characterized by out-of-phase ū anomalies at 35° and 55°N, is of primary importance for zonal flow/stationary wave interactions in the Northern Hemisphere extratropies. Such fluctuations in the zonal-mean zonal flow are shown to occur independently of tropical SST variability, consistent with earlier studies. Dynamical model experiments and regression analyses of the historical data indicate that such a zonal index explains a significant fraction of the wintertime stationary wave variability in several regions. The principal centers of action reside within wave trains over the North Pacific–North American region and the North Atlantic–Eurdsian region where locally 30%–40% of the eddy height variability is explained by the zonal index. Only over the North Pacific does the stationary wave signal related to ENSO appreciably exceed that associated with the zonal index. The surface climate associated with the zonal index is described by a wavenumber 1 pattern, which has out-of-phase temperature anomalies between Eurasia and North America and amplitudes considerably larger than those experienced during ENSO. The analysis offers a physical basis for understanding extratropical seasonal climate anomalies as a simple linear combination of teleconnection patterns associated with ū states and ENSO states. The utility of such an approach is illustrated for several cases of hitherto unexplained observed extreme climate anomalies during northern winter. It is also shown that a significant fraction of the interannual variability in some regions cannot he explained by either a zonal index or ENSO. A further important feature of the zonal index in the 1947–94 period is the recurrence of anomalies over multiyear periods. Such behavior has important implications for decadal climate variations, examples of which are given for secular changes after 1976.