A regime shifts in North Pacific subtropical mode water formation

A regime shift in the formation mechanisms of the North Pacific subtropical mode water (NPSTMW) was investigated using 50-year (1960-2009) ocean general circulation model (OGCM) and 2,000-year fully coupled atmosphere–ocean–sea ice model (Kiel Climate Model; KCM). We found that primary driving mechanism for NPSTMW formation is alternated between air–sea interaction (ASI) and ocean dynamics (OD) from two model simulations. In the OGCM simulation, we revealed that the local air-sea interaction process is a main driver of the NPSTMW formation prior to late-1980s, while ocean dynamics including the vertical entrainment become dominant since then. In the KCM simulation, the relative importance of two (ASI and OD) 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 induced the sea surface height anomalies propagating westward from the central North Pacific and preconditions for the NPSTMW formation, thus the ocean dynamics became relatively more important.