A regime shift in the formation of the North Pacific subtropical mode water

Abstract

A regime shift in the formation mechanisms of the North Pacific subtropical mode water (NPSTMW) as well as its causes is investigated using 50-year (1960–2009) ocean general circulation model (OGCM) and 2,000-year preindustrial simulation of atmosphere-ocean coupled model (Kiel Climate Model). The volume budget analysis suggests that the formation of the NPSTMW is mainly controlled by the air‐sea interaction and ocean dynamics. In the OGCM simulation, it is found that there is a regime shift of the relative importance between the two around late-1980s. While the local air-sea interaction process is a main driver of the NPSTMW formation prior to late-1980s, ocean dynamics including the vertical entrainment become dominant since then. The NPSTMW formation is affected by the North Pacific Oscillation simultaneously in the early period, but with a few years lag in the later period. The interdecadal change of the driving mechanism of the interannual variability of the NPSTMW is probably due to stronger (weaker) remote signals in the ocean interior induced by the meridional shift of the Aleutian Low and weaker (stronger) influence of local atmospheric forcing in the western North Pacific during the former (later) period. Using an atmosphere-ocean coupled model (Kiel Climate Model), it is revealed that the transition of the driving mechanisms for the NPSTMW formation is periodically occurred in multi-decadal timescales. Once the epoch changes, each air-sea interaction and ocean dynamics forcing persists during about 10–30 years. The regime shift of the NPSTMW formation is related to the meridional shift of the Aleutian Low center.