AGCM Simulation of the North Pacific Climate Shift in the 1920s

The North Pacific climate has undergone interdecadal regime shifts during the twentieth century, such as in the mid-1920s, the mid-1970s, and the late-1990s. All these climate shifts are closely related to phase changes in the Pacific Decadal Oscillation. A strong linkage between changes in the tropical Pacific SST and the Aleutian low is evident in the 1970s and the 1990s climate shifts but has remained unclear in the 1920s due to limited in situ observations in the tropical Pacific. We revisit the tropical influence on the North Pacific climate shift in the 1920s by utilizing an atmospheric general circulation model (AGCM) and a series of new historical climate data sets that incorporate the updated data source of in situ observations not available in the conventional climate data sets. New SST data sets such as COBE-SST2 improve data coverage in the tropical Pacific during the early twentieth century compared with their previous versions. These new SST data sets exhibit a pronounced equatorial Pacific warming in the mid-1920s, consistent with observed weakening of the equatorial trade winds and the zonal SLP gradient between the tropical Pacific and Indian Ocean. This feature cannot be captured in conventional SST data sets such as HadISST1 and COBE-SST1. An AGCM forced by the new SST data sets successfully simulate not only the weakening of the Walker circulation but also a significant deepening of the Aleutian low observed in the mid-1920s. TOGA experiments suggest that the SST warming in the tropical Pacific is key to strengthen the Aleutian low as observed in the mid-1970s regime shift.