NOTES AND CORRESPONDENCE Regime Shifts in the Northern Hemisphere SST Field: Revisited in Relation to Tropical Variations

Abstract

The regime shifts previously identified by the authors over the Northern Hemisphere during the period from the 1910s to the 1990s is investigated in relationship with tropical sea surface temperature (SST) variation. It is found that SST of the Niño 3.4 region, which is known to be a good indicator of El Niño/Southern Oscillation, shows coherent changes with the regime shifts. The regression maps based on Niño 3.4 SST show typical El Niño condition. By subtracting the variations linearly correlated with Niño 3.4 SST from the raw SST field, the residual SST field is obtained. An empirical orthogonal function (EOF) analysis shows a dominant mode of variations in the residual field that is confined to the mid- and high-latitude North Pacific, which is known as the North Pacific mode. Another dominant mode of variation corresponds to the Arctic Oscillation. All the regime shifts are detectable in the residual SST field. Most of the regime shifts (the 1925/26, 1945/46, 1957/58, 1970/71, and 1976/77 shifts) took place concurrently with the two aforementioned EOF modes, and changes in the Niño 3.4 SST, while the 1988/89 shift was not associated with the Niño 3.4 SST changes. This indicates that the regime shifts can be divided into two groups: one is closely linked with the tropical Pacific and the Indian Ocean variations, and the other is independent of these tropical variations.