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A wavelet-based method is re-introduced in an oceanographic and spectral context to estimate wavenumber spectrum and spectral flux of kinetic energy and enstrophy. We apply this to a numerical simulation of idealized, doubly periodic quasi-geostrophic flows, that is, the flow is constrained by the Coriolis force and vertical stratification. The double periodicity allows for a straightforward Fourier analysis as the baseline method. Our wavelet spectra agree well with the canonical Fourier approach but with the additional strengths of negating the necessity for the data to be periodic and being able to extract local anisotropies in the flow. Caution is warranted, however, when computing higher-order quantities, such as spectral flux. Key Points A wavelet-based spectral method to estimate eddy variability is described Wavenumber spectra of eddies are estimated for a doubly periodic quasi-geostrophic flow The wavelet and Fourier approach agree well in their estimates of spectra and spectral flux Plain Language Summary Chaotic flows such as the ocean currents, atmospheric winds, and turbulence in general are fundamentally impossible to analytically predict, namely, to formulate a mathematical general solution. Nevertheless, the interest in describing such chaotic flows can be found in examples as old as Leonardo da Vinci's sketch of turbulence. While we cannot obtain an analytical description of turbulence, we can extract statistical information from turbulent flows and a common descriptor has been the wavenumber spectrum. The spectrum reveals at each spatial scale, the level of variability the flow carries. Here, we re-introduce an alternative method in estimating the spectrum based on wavelet functions.

With the increase in computational power, ocean models with kilometer-scale resolution have emerged over the last decade. These models have been used for quantifying the energetic exchanges between spatial scales, informing the design of eddy parametrizations, and preparing observing networks. The increase in resolution, however, has drastically increased the size of model outputs, making it difficult to transfer and analyze the data. It remains, nonetheless, of primary importance to assess more systematically the realism of these models. Here, we showcase a cloud-based analysis framework proposed by the Pangeo project that aims to tackle such distribution and analysis challenges. We analyze the output of eight submesoscale-permitting simulations, all on the cloud, for a crossover region of the upcoming Surface Water and Ocean Topography (SWOT) altimeter mission near the Gulf Stream separation. The cloud-based analysis framework (i) minimizes the cost of duplicating and storing ghost copies of data and (ii) allows for seamless sharing of analysis results amongst collaborators. We describe the framework and provide example analyses (e.g., sea-surface height variability, submesoscale vertical buoyancy fluxes, and comparison to predictions from the mixed-layer instability parametrization). Basin- to global-scale, submesoscale-permitting models are still at their early stage of development; their cost and carbon footprints are also rather large. It would, therefore, benefit the community to document the different model configurations for future best practices. We also argue that an emphasis on data analysis strategies would be crucial for improving the models themselves.

Understanding processes associated with eddy-mean flow interactions helps our interpretation of ocean energetics, and guides the development of parameterizations. Here, we focus on the non-local nature of Kinetic Energy (KE) transfers between mean and turbulent reservoirs. Transfers are interpreted as non-local when the energy extracted from the mean flow does not locally sustain an growth of energy in the turbulent flow, or vice versa. The novelty of our approach is to use ensemble statistics to define the mean and the turbulent flow. Based on KE budget considerations, we first rationalize the eddy-mean separation in the ensemble framework, and discuss the interpretation of a mean flow driven by the prescribed (surface and boundary) forcing and a turbulent flow u′ driven by non-linear dynamics sensitive to initial conditions. We then analyze 120-day long, 20-member ensemble simulations of the Western Mediterranean basin run at resolution. Our main contribution is to recognize the prominent contribution of the cross energy term to explain non-local energy transfers, which provides a strong constraint on the horizontal organization of eddy-mean flow KE transfers since the cross energy term vanishes identically for perturbations orthogonal to the mean flow . We also highlight the prominent contribution of vertical turbulent fluxes for energy transfers within the surface mixed layer. Analyzing the scale dependence of non-local energy transfers supports the local approximation usually made in the development of meso-scale, energy-aware parameterizations for non-eddying models, but points out to the necessity of accounting for non-local dynamics in the meso-to-submeso scale range. Key Points Ensemble-based eddy-mean decomposition of kinetic energy budget supports the view of an ocean turbulence driven by internal dynamics Turbulent fluxes of the cross energy term provide a potentially strong horizontal constraint on eddy-mean flow interactions Non-localities are leading

Circulation in the Southern Ocean is unique. The strong wind stress forcing and buoyancy fluxes, in concert with the lack of continental boundaries, conspire to drive the Antarctic Circumpolar Current replete with an intense eddy field. The effect of Southern Ocean eddies on the ocean circulation is significant – they modulate the momentum balance of the zonal flow, and the meridional transport of tracers and mass. The strength of the eddy field is controlled by a combination of forcing (primarily thought to be wind stress) and intrinsic, chaotic, variability associated with the turbulent flow field itself. Here, we present results from an eddy-permitting ensemble of ocean model simulations to investigate the relative contribution of forced and intrinsic processes in governing the variability of Southern Ocean eddy kinetic energy. We find that variations of the eddy field are mostly random, even on longer (interannual) timescales. Where correlations between the wind stress forcing and the eddy field exist, these interactions are dominated by two distinct timescales – a fast baroclinic instability response; and a multi-year process owing to feedback between bathymetry and the mean flow. These results suggest that understanding Southern Ocean eddy dynamics and its larger-scale impacts requires an ensemble approach to eliminate intrinsic variability, and therefore may not yield robust conclusions from observations alone. Plain Language Summary The Southern Ocean is the most turbulent part of the world’s oceans. This turbulence, often referred to as eddies, is critical to the evolution of the Southern Ocean under climate change. But it’s hard to get information about these eddies, because they occur on small scales in a large ocean basin that is poorly observed. In addition, the observational record is quite short, which makes it more difficult to use these observations to study what controls variations of these eddies. For this reason, we take an eddy-permitting ocean

We describe a form of AMOC variability that we believe has not previously appeared in observations or models. It is found in an ensemble of eddy-resolving North Atlantic simulations that the AMOC frequently reverses in sign at ∼35N with gyre-wide anomalies in size and that reach throughout the water column. The duration of each reversal is roughly one month. The reversals are part of the annual AMOC cycle occurring in boreal winter, although not all years feature an actual reversal in sign. The occurrence of the reversals appears in our ensemble mean, suggesting it is a forced feature of the circulation. A partial explanation is found in an Ekman response to wind stress anomalies. Model ensemble simulations run with different combinations of climatological and realistic forcings argue that it is the atmospheric forcing specifically that results in the reversals, despite the signals extending into the deep ocean. Key Points The AMOC can reverse in sign Reversals appear in the annual cycle The reversals are the result of atmospheric forcing Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is a climatically important component of the ocean circulation. It is routinely thought to flow northward at the surface and southward at depths of 2000-3000m. Here we show that a significant component of the annual AMOC cycles are intervals during which it actually reverses this sense of flow and argue further that this is a response of the AMOC to the atmosphere. The AMOC anomaly is basin scale in size and extends over the full depth. These results have implications for annual heat storage in the North Atlantic.