Oceanic Fronts, Subduction and Spin-up: Nonlinear Ekman Dynamics

University Of Washington, Seattle WA

Investigators

Abstract

ABSTRACT OCE-0351191 This study involves theory, modeling and observational work related to the forcing of the upper ocean by atmospheric winds and buoyancy flux. The PI and his team have found a new mechanism for the generation of fronts in a wind-driven ocean, using nonlinear theory. Atmospheric forcing by wind-stress and surface buoyancy flux drives the ocean through direct cooling, cooling augmented by horizontal heat transport in the upper Ekman layer, and divergence of the Ekman-layer volume transport modified by vertical vorticity. The latter two forcing effects are a consequence of nonlinear Ekman dynamics. Using a convection parameterization, the two-dimensional sub-inertial response of the subsurface ocean is predicted: it involves spontaneous development of fronts, downwelling at these fronts, buoyant convection at special sites related to the fronts, and the spin-up of downwind currents in a pattern that is highly structured by frontal downwelling. The potential vorticity field, q, acts as a key control; changes in the sign of q enable spin-up and instability to take on distinctive forms. q changes sign when surfaces of constant density slope more steeply than absolute momentum surfaces. Non-hydrostatic numerical simulations verify these results. The present study will start from this basis and emphasize three aspects of the ocean circulation: (i) generation of density- and velocity- fronts by strong wind fields in a fully three dimensional ocean; (ii) spin-up of circulation within and beneath the oceanic upper mixed layer, and (iii), amplification and forced propagation of anticyclonic circulation and eddies in the upper ocean by large-scale winds. Both 3- dimensional modeling and observations will be used. Model studies involve process oriented simulations aimed at understanding in the context of nonlinear Ekman dynamics, the preferential generation of anticyclones by wind-jets, propagation of wind-forced eddies by Ekman-pumping-induced vortex stretching, and evolution of wind-forced boundary currents in a Labrador Sea-like basin. Another goal is to use the theory to guide hydrostatic ocean circulation models so that they are able to capture the dynamics of wind-driven frontogenesis and frontal downwelling. Particular observational data-sets to be used include high-resolution satellite scatterometer wind-fields from Labrador Sea and subpolar fronts of the Atlantic, high-resolution sea-soar and ADCP data from the subpolar front of the Japan/East Sea, and high-resolution ctd sections taken with the Eriksen Seaglider. Sea-soar/ADCP surveys are now a commonplace tool used in frontal/mesoscale studies. The proposed work develops an inverse method to use such surveys with the theory above to both estimate ageostrophic overturning circulation and partition its driving force into contributions from wind, cooling, and confluent/diffluent geostrophic flow. Broader Impacts. Beyond physical science, this work relates particularly closely to ecosystems living near ocean fronts, in upwelling regions at coasts or along the Equator: regions which are particularly stressed by global change. In this way it contributes to understanding of Earth's habitability. This research also contributes to the PI's undergraduate teaching in environmental studies, in which the methods and results of science are put before young students who are the beneficiaries and custodians of the environment. It also motivates public lecturing on the environment.