WINTERTIME LOW-FREQUENCY WEATHER VARIABILITY IN THE NORTH PACIFIC AMERICAN SECTOR 1949-93

A study of the impact of ENSO in the Hadley Centre's atmospheric clima te model HADAM1 is presented, with emphasis on the North Pacific-Ameri can (NPA) sector. The study is based both on observational data and an ensemble of six integrations for the period 1949-93, forced with obse rved global sea-ice and sea surface temperature data. The model is sho wn to reproduce most of the known features of the worldwide atmospheri c response to ENSO in boreal winter (January-March). Focusing on the N PA sector, the leading modes of low-frequency weather variability in t he winter season are identified on their natural timescales for both t he modeled and observed atmospheres. These modes are analyzed via rota ted EOF analysis of daily 500-hPa height data, filtered to remove syno ptic timescale variations. The model gives a reasonably skillful simul ation of the main features of the four leading modes in the NPA region : the Pacific-North American (PNA), the west Pacific (WP), the east Pa cific (EP), and the North Pacific (NP) modes. The sensitivity of these modes to SSTs is investigated. In particular, sensitivity to SSTs ass ociated with ENSO is analyzed in terms of the shift in frequency of oc currence of the opposing phases of a mode between warm event (El Nino) and cold event (La Nina) years. Three of the observed modes show such a sensitivity: the PNA, WP, and NP modes. Of the corresponding model modes, only the PNA responds significantly to ENSO (but too strongly i n warm event years), which is clearly illustrated by changes in both t he frequency and duration of PNA episodes between warm and cold event years. The EP mode shows no sensitivity to ENSO, in either model or ob served atmospheres. Finally, although the model is able to reproduce t he pattern of decadal anomalies seen in the North Pacific in the years 1977-87, which is related to the prevalence of the positive phase of the PNA in this period, it does so with a much reduced amplitude; poss ible reasons for this discrepancy are discussed.