Decadal changes in 1870-2004 Northern Hemisphere winter sea level pressure variability and its relationship with surface temperature

Decadal changes in correlations between boreal winter averages of indices of the North Atlantic Oscillation (NAO) and average temperature north of 20°N (NH20N) have been reported in the literature. We show that such fluctuations are caused by changing midfrequency and high-frequency correlations (periods less than 30 years) and are not the result of changes in the station network or a few erratic years. By examining the NAO-temperature squared correlation on a spatial basis, we show that there have been strong decadal changes in the Pacific but a more constant relationship in the Atlantic. We explain the varying NAO-temperature relationship by showing that sea level pressure (SLP) variance has changed markedly through time, especially in the North Pacific. We performed a principal component analysis (PCA) of the running variance of SLP using a recently released historical global gridded data set. The first PC corresponded to a reduction in variance of the NAO during the mid-20th century, which occurred concurrently with a similar change in the variance of the El Niño–Southern Oscillation (ENSO). The loading patterns for this PC suggest that the decadal changes in the variability of ENSO could be responsible for the changes in NAO variance. The second North Pacific–dominated PC was indicative of increased variability of the strength of the Aleutian Low during the mid-20th century, during which time the variability in the Pacific and Atlantic were in phase. These two changes in variance of SLP, together with the phase relationship between the two ocean basins, explain the changing NAO-temperature correlation.