Intrinsic versus Forced Decadal Variability in the Extratropical North Pacific

Decadal variability of the Kuroshio Current results from both the intrinsic dynamics of oceanic jets and from atmosphere-ocean feedback processes. The Kuroshio's distinct path configurations south of Japan are associated with differing states of intrinsic stability within the jet, with variations in volume and heat transport by the Kuroshio, and with the N-S position of the Kuroshio Extension (KE) downstream from Japan. This position affects the character and intensity of air-sea interactions, which in turn feed-back upon Kuroshio variability. This study will examine the mechanisms controlling shifts between paths and stability states, and the interplay between intrinsic ocean dynamics and air-sea feedbacks affecting variability. The scientific goals of the proposal are to (1) clarify the processes by which the intrinsic Kuroshio path variability south of Japan alters the KE dynamic state and how it competes with the KE variability controlled by the basin-scale wind forcing, (2) elucidate the dynamics underlying a recent Kuroshio large meander (LM) event, and (3) quantify the relative contributions of sea surface temperature (SST), SST fronts and mesoscale eddies due to the recent KE dynamic state change in modifying the atmospheric circulation. The project will use process-oriented oceanic and atmospheric numerical modeling to address the system variability. The Kuroshio and KE are important components of climate variability, influencing North Pacific ecosystems and basin-scale weather patterns, both of important societal relevance. Student involvement and international collaboration are planned.

Accumulation of high-quality satellite/in-situ observations and improvement in state-of-the-art ocean/atmosphere general circulation models over the past decades have significantly improved understanding of the roles played by the KE in the decadal North Pacific climate variability. The specific feedback loop involves the wind-forced interior ocean thermocline adjustment, its subsequent alteration to the KE dynamic state, and the KE's influence on the stormtracks and the surface wind field. This wind- forced feedback loop, which had been at work in the past three decades, was disrupted in late 2017 when the Kuroshio south of Japan changed to a large meander path, a manifestation of intrinsic western boundary current variability. Three testable hypotheses have been identified: Hypothesis 1: The 2017 KE index reversal caused the LM occurrence; Hypothesis 2: The 2017 LM was triggered by enhanced Subtropical Counter Current eddy activity; and Hypothesis 3: Meander-induced KE dynamic state reversal impacts the atmosphere. The first two hypotheses will be examined with the Hallberg Isopycnal Model, modified to a three-layer configuration, which (a) accommodates the interception of layer interfaces by steep bottom topography, (b) allows sea-surface outcropping of isopycnic layers, and (c) has isopycnal coordinates to avoid unphysical diapycnal mixing. Hypothesis 3 will be addressed using the Japan Meteorological Agency's non-hydrostatic mesoscale model, through a collaboration with Prof. Shusaku Sugimoto of Tohoku University, Japan.

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.