Skills and limitations of the adiabatic omega equation: how effective is it to retrieve oceanic vertical circulation at meso and submesoscale ?

The quasi-geostrophic and the generalized omega equations are the most widely used methods to reconstruct vertical velocity (w) from in-situ data. As observational networks with much higher spatial and temporal resolutions are being designed, the question rises of identifying the approximations and scales at which an accurate estimation of w through the omega equation can be achieved and what are the critical scales and observables needed. In this paper we test different adiabatic omega reconstructions of w over several regions representative of main oceanic regimes of the global ocean in a fully eddy-resolving numerical simulation with a 1=60o horizontal resolution. We find that the best reconstructions are observed in conditions characterized by energetic turbulence and/or weak stratification where near-surface frontal processes are felt deep into the ocean interior. The quasi-geostrophic omega equation gives satisfactory results for scales larger than ~ 10 km horizontally while the improvements using a generalized formulation are substantial only in conditions where frontal turbulent processes are important (providing improvements with satisfactory reconstruction skill down to ~ 5 km in scale). The main sources of uncertainties that could be identified are related to processes responsible for ocean thermal wind imbalance (TWI), which is particularly difficult to account for (especially in observation-based studies) and to the deep flow which is generally improperly accounted for in omega reconstructions through the bottom boundary condition. Nevertheless, the reconstruction of mesoscale vertical velocities may be sufficient to estimate vertical fluxes of oceanic properties in many cases of practical interest.

Symmetric Instability and Ageostrophic Secondary Circulation in the East Korea Warm Current

<p>Submesoscale dynamics and ocean-atmosphere exchange process in frontal regions play an important role in regulating ocean overturning circulation and cycles of materials (including carbon) and energy, yet our understanding on the dynamics is limited primarily due to lack of relevant observation. To investigate frontal processes such as symmetric instability (SI) and ageostrophic secondary circulation (ASC), multiple comprehensive hydrographic and current observations were made with marine meteorological measurements across a sharp front of the East Korea Warm Current (EKWC) over spring 2017, summer 2017 and fall 2018. Submesoscale features were identified from the observations, estimating diagnostic variables that are the Ertel’s potential vorticity (f<sub>q</sub>), balanced Richardson number angle ( ), and Ekman buoyancy flux (EBF). The results with f<sub>q</sub> < 0 along the front,  corresponding to SI regime, and enhanced EBF along the surface of front support that submesoscale overturning circulation induced by down-front wind is due to the SI and ASC. The ASCs with ageostrophic current estimated using the Omega equations further provide vertical motions in the vicinity of the front. Our results suggest that the western boundary currents like EKWC within the North Pacific marginal sea strongly interact with local wind to impact submesoscale overturning circulation and (re-)distribution of materials via SI and ASC.</p>