JOINT SPATIOTEMPORAL MODES OF SURFACE-TEMPERATURE AND SEA-LEVEL PRESSURE VARIABILITY IN THE NORTHERN-HEMISPHERE DURING THE LAST CENTURY

Coherent spatiotemporal modes of climatic variability are isolated bas ed on a multivariate frequency domain singular value decomposition (SV D) of nearly a century of monthly Northern Hemisphere sea level pressu re (SLP) and surface temperature data. Insight into the underlying phy sical processes associated with potential climatic signals is obtained by examining the relationship between surface temperature and inferre d atmospheric circulation patterns as they evolve over the a typical c ycle, taking potential seasonal distinctions into account. Our analysi s provides evidence for two significant independent secular variations describing a secular warming trend (and accompanying changes in circu lation patterns) and a century timescale ''oscillation'' marked by hig h-amplitude variations in temperature and SLP in the North Atlantic th at are similar to those observed in recent model simulations. Quasi-os cillatory interdecadal (16-18 yr timescale) variability also displays a pattern similar to those predicted in recent model experiments, with an apparent origin in the North Pacific. Weaker quasi-decadal (10-11- yr timescale), largely cold-season oscillatory behavior is more closel y tied to the North Atlantic and may involve analogous mechanisms. Int erannual variability is examined with an ''evolutive'' generalization of our procedure to captures the time-evolving frequency and amplitude characteristics of the associated climate signal. Variability exhibit ing the characteristic climatic patterns of the global El Nino-Souther n Oscillation (ENSO) phenomenon is described by two largely distinct f requency bands within the broader 3-7-yr ENSO band. The drifting centr al frequencies of these two dominant bands is suggestive of nonstation ary behavior in ENSO. A quasibiennial signal exhibits a gradual trend toward increasing frequency. Prospects for improved long-range climate forecasting are discussed.