The dynamics of low-latitude sub-surface oceanic jets

Abstract

A complex system of alternating zonal jets has been observed in the low latitude Pacific ocean, both on the equator and extending away into the subtropics. The dynamics of the subtropical jets, which also seem to be associated with strong cross-jet gradients in chemical tracers, may be similar to those of much-studied beta-plane zonal jets, but the effect of longitudinal boundaries, which are present in oceanic cases, on such jets is still puzzling. The forcing mechanism behind this set of oceanic zonal jets and what determines its horizontal and vertical structure are also poorly understood. We consider a beta-plane two-dimensional model with rigid boundary conditions and apply stochastic forcing, which without rigid boundaries would generate zonal jets. The instantaneous flow field is strongly time-dependent, with a large component of stochastically forced basin modes. This seems to disrupt the formation of alternating zonal jets, which, in contrast to the case without rigid boundaries, are observable only in the time-mean field and much weaker than the instantaneous flow. There is some evidence that these apparent time-mean jets are primarily the signature of stochastically forced basin modes rather than genuinely persistent jet-like flows. Adding tracers to the model allows the investigation of the relation between jet structure and the transport and mixing of tracers, and act as an important diagnostic to verify the presence or absence of jets. The instantaneous tracer field in the case with rigid boundaries is highly unsteady and, unlike the doubly periodic case, the jet structure is not manifested in the tracer field. Two-dimensional simulations with simple rigid boundary geometry may overestimate the generation of basin modes relative to the real ocean. Non-uniform damping is applied as a model device to break the interaction between the flow and boundaries and test whether it can inhibit basin modes and hence allow generation of steady zonal jets. With this non-uniform damping, jets present in the zonal mean field become more persistent as our redefined zonostrophy parameter increases, but there exist caveats we need to examine further, such as small ratio of energy in the zonal flow to the total energy as compared to much-studied cases without boundaries.