Dynamics of Gap-leaping Western Boundary Currents with Throughflow Forcing

Abstract

The dynamics of gap-leaping western boundary currents (e.g. the Kuroshio intrusion, the Loop Current) are explored through rotating table experiments and a numerical model designed to replicate the experimental apparatus. Simplified experimental and numerical models of gap-leaping systems are known to exhibit two dominant states (leaping or penetrating into the gap) as the inertia of the current competes with vorticity constraints (in this case the β-effect). These systems are also known to admit multiple states with hysteresis. To advance towards more realistic oceanographic scenarios, recent studies have explored the effects of islands, mesoscale eddies, and variable baroclinic deformation radii on the dynamical system. Here, the effect of throughflow forcing is considered, with particle tracking velocimetry (PTV) used in the lab experiments. Mean transport in or out of the gap is found to significantly shift the hysteresis range as well as change its width. Because of these transformations, changes in throughflow can induce transitions in the gap-leaping system when near a critical state (leaping-to-penetrating/ penetrating-to-leaping). Results from the study are interpreted within a nonlinear dynamical framework and various properties of the system are explored.

Effect of the Coastline Geometry on the Boundary Currents Intruding through the Gap

“…For strong penetrating flows, Ekman dissipation can no longer balance the vorticity advection and vorticity must be dissipated from the current as westward-traveling eddies, which is consistent with the momentum imbalance paradox of Pichevin and Nof [17]. Most recently McMahon et al have extended both the experimental and numerical investigations to include the effects of mean flow through the gap [18,19] and applied a Newton iteration solution method to the numerical model which enables identification of unstable loop current states [20].…”

“…The numerical model utilized in this study is a barotropic version of the baroclinic model developed and validated in [16,26]. The numerical model was developed to support a series of rotating table laboratory experiments (referenced above).…”

Influence of multiple islands on the hysteresis and dynamics of a western boundary current perturbed by a mesoscale eddy at a gap

“…Sheremet (2001) studied the hysteresis of a WBC using a quasi-geostrophic model and summarized the gap-leaping problem in the balance between the beta effect (promoting the penetration) and the inertia (promoting the leap). Next, the presence of the multiple states and hysteresis of the WBC in a gap-leaping system was verified in laboratory experiments (Sheremet and Kuehl, 2007;Kuehl and Sheremet, 2009;Kuehl and Sheremet, 2014;McMahon et al, 2021). Moreover, the external factors, i.e., meridional wind (Wang et al, 2010), island in the gap (Mei et al, 2019), mesoscale eddy (Yuan and Li, 2008;Yuan and Wang, 2011;Mei et al, 2022), and largescale circulation in the SCS (Mei et al, 2023), are also proved to be crucial to affect the WBC path in a gap-leaping system.…”

Dynamical Characterization of the Loop Current Attractor

“…Recent theoretical advances (McMahon et al., 2021) found in an idealized 1‐layer model that the LC‐like system transitions between five steady states, three stable and two unstable. The analysis suggests that relatively few degrees of freedom may be sufficient to describe the LC system most of the time, and its potential predictability may be higher than previously realized and currently achieved by ocean forecast systems.…”

“…The mechanisms and processes that control the penetration of the LC into the Gulf of Mexico and trigger the Rings' separation are neither well simulated and forecasted by ocean models, nor fully understood (National Academies of Sciences and Medicine, NASEM, 2018). More skillful predictions of the LC dynamics would have consequential societal benefits, ranging from improved weather forecasts, especially relevant in the event of tropical storms and hurricanes, to better oil spill and emergency preparedness and response, and improved fishery management.Recent theoretical advances (McMahon et al, 2021) found in an idealized 1-layer model that the LC-like system transitions between five steady states, three stable and two unstable. The analysis suggests that relatively few degrees of freedom may be sufficient to describe the LC system most of the time, and its potential predictability…”