Signatures of the 1976-77 regime shift in the North Pacific revealed by statistical analysis

Regime shifts are abrupt changes in an ecosystem that may propagate through multiple trophic levels and have pronounced effects on the biotic and abiotic environment, potentially resulting in ecosystem reorganization. There are multiple mechanisms that could cause such abrupt events including natural and anthropogenic factors. In the North Pacific a major shift in the physics of the system, including a sudden increase in sea surface temperature, was reported in 1977 with a prominent biological response in the lower trophic levels and subsequent effects on the fisheries and economy of the region. Here we investigate the statistics of physical processes that could have triggered and maintained the late 1970’s shift. The hypothesis of an extreme sea level pressure event abruptly changing the oceanic conditions in winter 1976-1977, which was maintained by long-term changes in air-sea interaction processes, is tested. Using a novel statistical and dynamical approach, we show the occurrence of an extreme atmospheric event, specifically a persistent Aleutian Low during winter 1976-77, which constitutes a substantial part of the triggering mechanism of the regime shift. Subsequent sudden changes in the net heat flux occurred in the western North Pacific, particularly in the Kuroshio Extension region, which contributed to the maintenance of the new regime