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. Plain Language Summary In the western North Pacific, a thick subsurface layer with low potential vorticity and relatively uniform potential density called the North Pacific subtropical mode water (NPSTMW) is found between depths of 100 and 400 m. Using a coupled atmosphere‐ocean‐sea ice model integrated over 2,000 years with constant CO2 concentration at the pre‐industrial level, we studied the multidecadal regime shift of the NPSTMW formation. It was found that the major driving mechanisms of NPSTMW formation periodically shifted from local atmospheric forcing to ocean dynamics (OD) (and vice versa) in approximately 50–70 years. It was also found that the multidecadal regime shifts of the NPSTMW formation are associated with the latitudinal (50 years) and longitudinal (70 years) shifts of the Aleutian Low (AL) position, that is, large (small) southward and eastward extent of the AL during the epoch when the OD (air‐sea flux [ASF]) dominates. The position shift of the AL affects the variability of the local ASF and remotely driven OD via the Ekman pumping/suction. Key Points Driving mechanisms for the North Pacific subtropical mode water formation exhibit a regime shift with a periodicity of about 50–70 years Multidecadal regime shifts are associated with meridional and zonal shifts in the Aleutian Low (AL) Position shift of the AL affects the variability of the local air‐sea flux and remotely driven oceanic dynamics