The North Atlantic Oscillation in the NCEP–NCAR Reanalysis
Journal of Climate
The upper-air circulation characteristics of the North Atlantic oscillation (NAO) are studied from the 1958-97 National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, with regard to interannual variability and long-term trends. The low (high) phase of the NAO is defined by small (large) values of the Ponta Delgada, Azores, minus Akureyri, Iceland, surface pressure difference. For the fields of 200-mb topography, divergence, and divergent flow; 500-mb
... gent flow; 500-mb vertical motion; 1000-mb topography and total wind; and SST during January, differences are computed between ensembles of years of extremely low-minus high-NAO phase and are tested for statistical significance. In the low-NAO phase, topographies throughout the tropospheric column stand anomalously high (low) in the subpolar (subtropical) domains. Ascending motion and upper-tropospheric divergent outflow from the realm of the Icelandic low broadly southward, and convergent inflow and subsidence on the poleward side of the Azores high, are reduced. These vertical motion departures support the raised (lowered) 1000-mb topography in the subpolar (subtropical) domains. The weakened subtropical high entails slower trade winds that through reduced evaporation and wind stirring are conducive to warmer sea surface and slower midlatitude westerlies that through reduced Ekman transport lead to colder waters on the poleward side of the anticyclonic axis. Similarly, the weakened cyclonic circulation around the Icelandic low, through reduced Ekman transport, makes for a warmer sea surface. These SST departures are imparted to the overlying atmosphere. Long-term evolutions in the patterns of upper-tropospheric divergence and divergent flow, midtropospheric vertical motion, and SST accompany the trends in 1000-mb topography toward greater prevalence of the high-NAO phase.