Evolution of atmospheric extreme events and the associated oceanic response in the North Pacific

Abstract

Ocean – atmosphere interactions have significant effects on climate variability, with extremes in atmospheric conditions playing a key role. Atmospheric extreme events have been associated with major marine ecosystem regime shifts and unusual weather patterns. Regime shifts may be defined as abrupt changes in an ecosystem that propagate through multiple trophic levels and have pronounced effects on the biotic and abiotic environment, potentially resulting in ecosystem reorganization. Such major changes were reported in winter 1976-1977 in the physics of the North Pacific with a prominent biological response in the lower trophic levels and subsequent effects on the fisheries and economy of the region. There are multiple mechanisms that can cause such shifts including natural and anthropogenic factors. An extreme sea level pressure event has previously been linked to abrupt changes of oceanic conditions in the late 1970’s, which were maintained by long term alterations in air-sea interaction processes. The evolution of the oceanic response to driving forcings, such as atmospheric extremes, able to cause abrupt shifts in complex systems as the North Pacific, calls for further scrutiny. This thesis begins by investigating the relationship of extreme atmospheric events with sudden oceanic changes by calculating the dynamical proxies of reanalysis data. The results show the occurrence of an extreme atmospheric event, specifically a deepened Aleutian Low during winter 1976–1977, which constitutes a substantial part of the triggering mechanism of the North Pacific 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. In order to verify the relationship of Aleutian Low extreme events with changes in Kuroshio Extension net heat flux, statistical and dynamical approaches are applied to historical simulations of the large ensemble of the Community Earth System Model (CESM-LENS). The response of Kuroshio Extension net heat flux to Aleutian Low extremes is found to be intensified around the time of the majority of detected events. The results suggest the validity of the hypothesized mechanism, taking into account the internal variability of the system. The results further allow the calculation of the dynamical proxies for future model simulations under the RCP8.5 scenario. The comparison of the detected extreme events between historical and future RCP8.5 ensemble members demonstrates that the frequency and persistence of atmospheric extremes are expected to increase in the future. An intensification of the net heat flux response in the Kuroshio Extension is also observed as a result of more frequent and persistent Aleutian Low extremes. The results outline the signature of climate change in the North Pacific atmospheric and oceanic properties, with wider implications for future weather patterns and ecosystem dynamics in the region.

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