Interpretation of Interdecadal Trends in Northern Hemisphere Surface Air Temperature

Abstract 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 tim...