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Emily Shuckburgh, Francesco d’Ovidio, and Bernard Legras

Abstract

The Lyapunov diffusivity is used to investigate local isentropic mixing events in the upper troposphere–lower stratosphere (UTLS) region. The diagnostic highlights the seasonal cycle of the longitudinally varying mixing properties, in particular those associated with the monsoon circulations and the westerly ducts in the subtropics. The results are broadly consistent with studies of Rossby wave–breaking frequencies. The mixing structure is shown to be modulated by modes of atmospheric variability. El Niño–Southern Oscillation (ENSO) is found to strongly influence the mixing structure throughout the tropics and subtropics. The subtropical jet is associated with longitudinal bands of mixing minima (isentropic mixing barriers) separated by synoptic-scale regions of strong mixing activity. The greatest ENSO modulation in December–February is confined to the Pacific sector, where the barriers associated with the subtropical jets extend farther into the eastern Pacific, and in the western Pacific a barrier is found at the equator during the positive phase. During June–August, the influence is seen to extend beyond the Pacific region, with the barrier at the subtropical jet in the Southern Hemisphere increasing in strength at all longitudes and with an increase in strength (and isolation of) monsoon-related mixing over Asia and North America. The local influence of the North Atlantic Oscillation in wintertime is investigated. During the positive phase, a double-barrier structure is associated with the subtropical jet, the northern branch crosses the Atlantic toward Scandinavia, and the southern branch tends toward North Africa. The Antarctic Oscillation is shown to influence whether the subvortex region is isolated from midlatitudes.

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Francesco d’Ovidio, Emily Shuckburgh, and Bernard Legras

Abstract

A new diagnostic (the “Lyapunov diffusivity”) is presented that has the ability to quantify isentropic mixing in diffusion units and detects local mixing events by describing latitude–longitude variability. It is a hybrid diagnostic, combining the tracer-based effective diffusivity with the particle-based Lyapunov exponent calculation. Isentropic mixing on the 350-K surface shows that there is significant longitudinal variation to the strength of mixing at the northern subtropical jet, with a strong mixing barrier over Asia and the western Pacific, a weaker mixing barrier over the western Atlantic, and active mixing regions at the jet exits over the eastern Pacific and Atlantic.

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Alice Pietri, Xavier Capet, Francesco d’Ovidio, Marina Levy, Julien Le Sommer, Jean-Marc Molines, and Hervé Giordani

Abstract

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.

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