Eddy-mean flow interactions in western boundary current jets: theory and observations

The Kuroshio Extension System Study (KESS) is a large-scale observational program of the Kuroshio Extension investigating the processes that govern the jet's variability and the relation between the jet and its recirculation gyres. Motivated by the KESS observations, we examine the nature and the importance of nonlinear eddy-mean flow interactions in the dynamics of Western Boundary Current (WBC) jet systems from both theoretical and observational perspectives.

From a theoretical perspective, the problem is approached via study of an idealized quasi-geostrophic numerical model which is used to examine the role of eddy-mean flow interactions in a simplified configuration that is appropriate to the Kuroshio Extension and Gulf Stream jet systems. The KESS observations are used to design and justify the set-up of this model, which consists of a baroclinic, mixed instability, boundary-forced, zonal jet in an open domain. It is found that, in a parameter regime relevant to the observed WBC jet systems, the unstable jet evolves through the shedding of eddies as it flows through the domain until it reaches stability, and the nonlinear eddy fluxes play one of two distinctive roles depending on the downstream location. Upstream of the jet's stabilization point, the eddies act to stabilize the jet through a diffusive-like, down-gradient potential vorticity flux. However downstream of this stabilization, the sense of this eddy flux reverses, and now acts to drive the recirculations through an anti-diffusive, up-gradient flux. This up-gradient flux is permitted by an eddy enstrophy advection from the upstream region where eddies are generated by the unstable jet, to the downstream region where eddies are dissipated. These two processes are related, and the properties of the eddy-driven time-mean circulation occurring downstream can be empirically predicted given the supercriticality of the upstream jet that is the source of the eddy variability. These results support the hypothesis that the observed recirculations in the Kuroshio Extension and Gulf Stream are, at least partially, eddy-driven, and provide the insight that the properties of the observed mean jet and its recirculations downstream may be related to the stability properties of the WBC at the coast.

From an observational perspective, the problem is approached using the KESS observations, satellite data and data from past observational programs in the region. The goals are to characterize the eddy-mean flow interactions in the Kuroshio Extension as well as to evaluate the relevance of the idealized theoretical results to the dynamics of the actual oceanic system. It is found that the observed system has several dynamically significant signatures in both mean and eddy properties consistent with the model's predictions. In particular, the downstream development of the time-mean jet-gyre structure is observed to exhibit a similar pattern to that in the model consisting of a strengthening and sharpening of the time-mean jet and the development of flanking westward recirculations up until the downstream location of maximum eddy kinetic energy (EKE), followed by a weakening and broadening of the jet and a weakening of the recirculation strength downstream of this location. Observed indicators of the eddy effect on the mean (such as the cross-jet distributions of Reynolds stresses) similarly show a model-consistent picture, transitioning from a pattern characteristic of an unstable jet to a pattern characteristic of a wave-radiating regime across the location of maximum EKE. These consistencies suggest that the simplified physics studied in the theoretical investigations is potentially useful in understanding the important role of nonlinear eddy-mean flow interactions in the Kuroshio Extension and potentially other WBC jet systems.

Overall, results contribute to an improved understanding of the processes that govern the strength and structure of the recirculation gyres that flank WBC jets, and in particular give insight into the changes that could be expected in jet structure, jet transport and mode water formation if oceanic conditions changed in such a way as to alter the stability properties of the upstream WBC. The results also have important implications for our numerical modeling efforts. They illuminate what aspects of the large-scale circulation we can expect to be missing if we fail to resolve the eddies responsible for these effects in our numerical simulations. They also are potentially useful in assisting in the design of effective parameterizations to include the eddies' dynamical impacts even when it is not possible to resolve the necessary scales. In particular, progress can potentially be made by combining knowledge of the eddy life cycle (i.e. eddy growth vs. eddy decay) and the jet's advection to define the downstream boundary between regions of down-gradient and up-gradient eddy fluxes.