Recent Regime Shift in the Kuroshio Extension and North Pacific Coupled Ocean-Atmosphere System

The Kuroshio is the western boundary current of the subtropical North Pacific Ocean that transports warm and salty equatorial waters northward. After separating from the coast of Japan, the Kuroshio becomes the Kuroshio Extension (KE), an eastward-flowing jet, that then carries these waters in the direction of the US. While flowing northward and eastward, respectively, the Kuroshio and the KE exchange heat with the atmosphere which influences weather patterns and long-term weather fluctuations in the region, but also over the North American continent.

This project seeks to investigate recent changes in the Kuroshio/KE path that have been occurring since 2018 and that are referred to as a “regime shift”. The newly detected path involves a large Kuroshio meander south of Japan and a KE system consisting of a highly stable jet that flows eastward without creating many additional meanders or pinched-off eddies.

The goal is to explore, with available data and models, what caused the regime shift and what the regime shift means for North Pacific oceanic and atmospheric conditions, with implications for North American weather patterns. This project will provide training for one graduate student who will learn about ocean-atmosphere interaction in the Kuroshio/KE system and other extratropical regions.

The emergence of a new dynamic Kuroshio/KE regime has added new complexity to the understanding of the variability in the North Pacific coupled ocean-atmosphere system. It is hypothesized that the new, on-going dynamic regime is a result of combined external wind forcing, of nonlinear interaction between the Kuroshio large meander and stabilized KE jet, and of coupled feedback by KE’s thermal forcing to the overlying atmosphere.

By conducting analyses of available satellite/in-situ data, ocean/atmosphere reanalysis products, and high-resolution coupled model output, the plan is to (1) elucidate the oceanic and atmospheric processes responsible for generating and sustaining the on-going Kuroshio/KE dynamic regime, (2) explore and contrast the basin-scale atmospheric responses to the Kuroshio/KE dynamic regime changes, and (3) quantify the impacts by the new Kuroshio/KE dynamic regime on changing upper ocean heat content and water mass properties in the northwestern Pacific basin. Since many of the scientific questions addressed in this proposal apply to other western boundary current systems, an improved understanding of the KE variability could serve as a useful step in elucidating ocean's role in extratropical ocean-atmosphere interactions in general.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.