Regime shift in the global sea-surface temperatures: Its relation to El Niño-southern oscillation events and dominant variation modes

Significant changes of mean state appearing widely in the global sea-surface temperature (SST) anomaly field have happened five times from the 1910s to the 1990s: 1925, 1942, 1957, 1970 and 1976. Since the regions of change spread over both hemispheres and/or multiple oceanic basins, they can be considered as 'global regime shifts'. The years of regime shifts are consistent with those of the Northern Hemisphere regime shifts reported by previous studies. It is also shown that the r egime shifts have happened concurrently with El Niño-southern oscillation (ENSO) events, which seems to suggest that the ENSO event acts as a trigger of the regime shift. At the regime shift, the tropical Pacific SSTs change from La Niña (El Niño) to El Niño (La Niña) conditions within 1 year. Further, the ENSO events occur just after the regime shifts begin in the July to September (JAS) season and reach the mature phase in the January to March (JFM) season as a typical evolution of the ENSO events. After that, they continue to at least the next year. The five regime shifts detected hav e similar features in their seasonal evolution and persistence of signals. First, the shifts start in the JAS season: an SST change occurs in the eastern and central tropical Pacific, and a change in the mid-latitudes of the North and South Pacific appears with the opposite sign. Then the shifts in the JFM season. The spatial patterns are similar to those of the JAS season, but signals in the North Pacific become remarkable. These features resemble those corresponding to a series of evolutions of ENSO events, but the signals in the North Pacific and the North Atlantic are much stronger than those of the typical ENSO events. After the shifts have happened, the changes in spatial patterns of SST that occurred at the regime shift persist until the next shift. The persistence of signals is more prominent in the JFM season than in the JAS season. From a review of th e dominant variation modes in global SSTs using empirical orthogonal function (EOF) analyses, four modes are identified: the ENSO mode, the Southern Hemisphere trend mode, the North Pacific (NP) mode, and the Arctic oscillation (AO) mode. In the years when regime shifts occur, the ENSO mode, the NP mode, and the AO mode show significant concurrent phase reversals on the global scale as previously shown in the Northern Hemisphere. These findings provide a possible reason why SST changes in the regime shift are similar but not exactly the same pattern as that of ENSO. Furthermore, it can be considered that a simultaneous phase reversal of the NP mode would suppress the growth of anticyclonic (or cyclonic) circulation in the atmosphere over the western tropical Pacific. This suggests that an ENSO event, which begins with a regime shift, would not reverse its condition and last for the following several seasons.