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 ((u) over bar) are examined. This study focuses on the wintertime Northern Hemisphere extratropics and c ompares seasonal mean anomalies associated with (u) over bar to those associated with the El Nino-Southern Oscillation during the 1947-94 pe riod. Dynamical model experiments indicate that a single zonal index, characterized by out-of-phase (u) over bar anomalies at 35 degrees and 55 degrees N, is of primary importance for zonal flow/stationary wave interactions in the Northern Hemisphere extratropics. Such fluctuatio ns in the zonal-mean zonal flow are shown to occur independently to tr opical SST variability, consistent with earlier studies. Dynamical mod el experiments and regression analyses of the historical data indicate that such a zonal index explains a significant fraction of the winter time stationary wave variability in several regions. The principal cen ters of action reside within wave trains over the North Pacific-North American region and the North Atlantic-Eurasian region where locally 3 0%-40% of the eddy height variability is explained by the zonal index. Only over the North Pacific does the stationary wave signal related t o ENSO appreciably exceed that associate with the zonal index. The sur face climate associated with the zonal index is described by a wavenum ber 1 pattern, which has out-of-phase temperature anomalies between Eu rasia and North America and amplitudes considerably larger than those experienced during ENSO. The analysis offers a physical basis for unde rstanding extratropical seasonal climate anomalies as a simple linear combination of teleconnection patterns associated with (u) over bar st ates and ENSO states. The utility of such an approach is illustrated f or several cases of hitherto unexplained observed extreme climate anom alies during northern winter. It is also shown that a significant frac tion of the interannual variability in some regions cannot be explaine d 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 decada l climate variations, examples of which are given for secular changes after 1976.