Interpretation of Interdecadal Trends in Northern Hemisphere Surface Air Temperature

Monthly Mean time series of 1) surface air temperature anomalies averaged over the Northern Hemisphere based on data from land stations, 2) sea surface temperature anomalies averaged over the Northern Hemisphere oceans, and 3) 1000–500-mb thickness anomalies averaged over the region poleward of 40°N are examined. The data are stratified in terms of warm (May–October) and cold (November–April) seasons. Time series 1) and 3) based on cold-season data exhibit much larger month to month variability and a richer spectrum of interannual variability than those based on warm-season data: the interdecadal to century scale variability stands out more clearly in the warm-season time series. Cold- and warm-season time series for 1) differ substantially, even when heavily smoothed. It is hypothesized that the richer spectrum of variability in the cold-season time series is dynamically induced. To test this hypothesis, departures of cold-season data for 1) and 3) from their respective smoothed warm- season time series are regressed upon the hemispheric 1000–500-mb thickness field. Anomalously warm cold- season months are shown to be characterized by circulation regimes conducive to positive thickness anomalies over the high-latitude continents. A simple index of this distinctive “cold ocean–warm land” (COWL) pattern accounts for 65% of the variance of 1) during the cold-season months and for most of the discrepancies between its 5-yr running mean cold- and warm-season time series. The anomalous warmth of the winters of the 1980s is consistent with the strong positive bias of the COWL pattern index during the cold seasons of that decade.