Multidecadal Regime Shifts in North Pacific Subtropical Mode Water Formation in a Coupled Atmosphere-Ocean-Sea Ice Model

A regime shift in the formation mechanisms of the North Pacific subtropical mode water (NPSTMW) and its causes were investigated using a 2,000-year-long pre-industrial control simulation of a fully coupled atmosphere-ocean-sea ice model. The volume budget analysis revealed that the air-sea flux and ocean dynamics (OD) were the two primary driving mechanisms for NPSTMW formation, but their relative importance has periodically alternated in multidecadal timescales of approximately 50-70 years. The regime shift of the NPSTMW formation was closely related to the meridional (50 years) and zonal (70 years) movements of the Aleutian Low (AL). When AL shifted to the south or east, it induces the sea surface height anomalies propagating westward from the central North Pacific and preconditions the NPSTMW formation, thus the OD become relatively more important.