Dissipation of the energy imparted by mid-latitude storms in the Southern Ocean

The aim of this study is to clarify the role of the Southern Ocean storms on interior mixing and meridional overturning circulation. A periodic and idealized numerical model has been designed to represent the key physical processes of a zonal portion of the Southern Ocean located between 70 and 40°?S. It incorporates physical ingredients deemed essential for Southern Ocean functioning: rough topography, seasonally varying air–sea fluxes, and high-latitude storms with analytical form. The forcing strategy ensures that the time mean wind stress is the same between the different simulations, so the effect of the storms on the mean wind stress and resulting impacts on the Southern Ocean dynamics are not considered in this study. Level and distribution of mixing attributable to high-frequency winds are quantified and compared to those generated by eddy–topography interactions and dissipation of the balanced flow. Results suggest that (1) the synoptic atmospheric variability alone can generate the levels of mid-depth dissipation frequently observed in the Southern Ocean (10?10–10?9?W?kg?1) and (2) the storms strengthen the overturning, primarily through enhanced mixing in the upper 300?m, whereas deeper mixing has a minor effect. The sensitivity of the results to horizontal resolution (20, 5, 2 and 1?km), vertical resolution and numerical choices is evaluated. Challenging issues concerning how numerical models are able to represent interior mixing forced by high-frequency winds are exposed and discussed, particularly in the context of the overturning circulation. Overall, submesoscale-permitting ocean modeling exhibits important delicacies owing to a lack of convergence of key components of its energetics even when reaching ?x?=??1?km

Joint observation-model mixed-layer heat and salt budgets in the eastern tropical Atlantic

In this study, we use a joint observation-model approach to investigate the mixed-layer heat and salt annual mean and seasonal budgets in the eastern tropical Atlantic. The regional PREFCLIM observational climatology provides the budget terms with a relatively low spatial and temporal resolution compared to the online NEMO model, and this later is then re-sampled as in PREFCLIM climatology. In addition, advection terms are recomputed offline from the model as PREFCLIM gridded advection computation. In Senegal, Angola and Benguela regions, the seasonal cycle of mixed-layer temperature is mainly governed by surface heat fluxes; however, it is essentially driven by vertical heat diffusion in Equatorial region. The seasonal cycle of mixed-layer salinity is largely controlled by freshwater flux in Senegal and Benguela regions; however, it follows the variability of zonal and meridional salt advection in Equatorial and Angola regions respectively. Our results show that the time-averaged spatial distribution of NEMO offline heat/salt advection terms compares much better to PREFCLIM horizontal advection terms than the online heat/salt advection terms. However, the seasonal cycle of horizontal advection in selected regions shows that NEMO offline terms do not always compare well with PREFCLIM, sometimes less than online terms. Despite this difference, these results suggest the important role of small scale variability in mixed-layer heat and salt budgets.</p

Connecting flow–topography interactions, vorticity balance, baroclinic instability and transport in the Southern Ocean: the case of an idealized storm track

International audienceThe dynamical balance of the Antarctic Circum-polar Current and its implications on the functioning of the world ocean are not fully understood and poorly represented in global circulation models. In this study, the sensitivities of an idealized Southern Ocean (SO) storm track are explored with a set of eddy-rich numerical simulations. The classical partition between barotropic and baroclinic modes is sensitive to current-topography interactions in the mesoscale range 10-100 km, as comparisons between simulations with rough or smooth bathymetry reveal. Configurations with a rough bottom have weak barotropic motions, ubiquitous bottom form stress/pressure torque, no wind-driven gyre in the lee of topographic ridges, less efficient baroclinic turbulence and, thus, larger circumpolar transport rates. The difference in circumpolar transport produced by topographic roughness depends on the strength with which (external) thermohaline forcings by the rest of the world ocean constrain the strat-ification at the northern edge of the SO. The study highlights the need for a more comprehensive treatment of the Antarctic Circumpolar Current (ACC) interactions with the ocean floor, including realistic fields of bottom form stress and pressure torque. It also sheds some light on the behavior of idealized storm tracks recently modeled: (i) the saturation mechanism, whereby the circumpolar transport does not depend on wind intensity, is a robust and generic attribute of ACC-like circumpolar flows; (ii) the adjustment toward saturation can take place over widely different timescales (from months to years) depending on the possibility (or not) for barotropic Rossby waves to propagate signals of wind change and accelerate/decelerate SO wind-driven gyres. The real SO having both gyres and ACC saturation timescales typical of our "no gyre" simulations may be in an intermediate regime in which mesoscale topography away from major ridges provides partial and localized support for bottom form stress/pressure torque

Loop Current Eddies as a Possible Cause of the Rapid Sea Level Rise in the Gulf of Mexico

International audienceThe Gulf of Mexico (GOM), with its densely populated coastline, is one of the world's most vulnerable regions to climate change and sea level (SL) rise. Over the last three decades, various works have been conducted to assess coastal SL trends around the basin using tide gauge stations, separately from studies dealing with regional dynamical processes. Using altimetry, Argo, and eddy atlas products over the period from January 1993 to December 2020, we have analyzed the regional SL variations in the area to define their characteristics and explore their possible dynamical causes. We observe a mean GIA‐corrected SL rise rate of 4.81 ± 0.85 mm yr −1 , which is 25% higher than that of the adjacent Caribbean Sea and 44% higher than that of the global ocean. This result highlights the singular SL evolution in the GOM over the 28‐year study period. Over 2010–2020, the SL trend in the GOM has even accelerated, along with a strong warming of the upper‐layer (0.58 ± 0.17°C), which explains ∼60% of the SL rise rate through the thermosteric effect. Finally, the heat input estimates emphasize the role of the Loop Current eddies as a major contributor to the recent acceleration of SL rise due to upper‐layer warming

Partial Control of the Gulf of Mexico Dynamics by the Current Feedback to the Atmosphere

International audienceThe surface oceanic current feedback (CFB) to the atmosphere has been shown to correct long-lasting biases in the representation of ocean dynamics by providing an unambiguous energy sink mechanism. However, its effects on the Gulf of Mexico (GoM) oceanic circulation are not known. Here, twin ocean-atmosphere eddy-rich coupled simulations, with and without CFB, are performed for the period 1993-2016 over the GoM to assess to which extent CFB modulates the GoM dynamics. CFB, through the eddy killing mechanism and the associated transfer of momentum from mesoscale currents to the atmosphere, damps the mesoscale activity by roughly 20% and alters eddy statistics. We furthermore show that the Loop Current (LC) extensions can be classified into three categories: a retracted LC, a canonical LC, and an elongated LC. CFB, by damping the mesoscale activity, enhance the occurrence of the elongated category (by about 7%). Finally, by increasing the LC extension, CFB plays a key role in determining LC eddy separations and statistics. Taking into account CFB improves the representation of the GoM dynamics, and it should be taken into account in ocean models

Equatorial Atlantic interannual variability and its relation to dynamic and thermodynamic processes

The contributions of the dynamic and thermodynamic forcing to the interannual variability of the equatorial Atlantic sea surface temperature (SST) are investigated using a set of interannual regional simulations of the tropical Atlantic Ocean. The ocean model is forced with an interactive atmospheric boundary layer, avoiding damping toward prescribed air temperature as is usually the case in forced ocean models. The model successfully reproduces a large fraction (R-2 = 0.55) of the observed interannual variability in the equatorial Atlantic. In agreement with leading theories, our results confirm that the interannual variations of the dynamical forcing largely contributes to this variability. We show that mean and seasonal upper ocean temperature biases, commonly found in fully coupled models, strongly favor an unrealistic thermodynamic control of the equatorial Atlantic interannual variability

Variability and dynamics of the Yucatan upwelling : high-resolution simulations

The Yucatan shelf in the southern Gulf of Mexico is under the influence of an upwelling that uplifts cool and nutrient rich waters over the continental shelf. The analysis of a set of high-resolution (x=y approximate to 2.8 km) simulations of the Gulf of Mexico shows two dominant modes of variability of the Yucatan upwelling system: (1) a low-frequency mode related to variations in position and intensity of the Loop Current along the shelf, with upwelling intensified when the Loop Current is strong and approaches to the Yucatan shelf break and (2) a high-frequency mode with peak frequency in the 6-10 days band related to wind-forced coastal waves that force vertical velocities along the eastern Yucatan shelf break. To first order, the strength and position of the Loop Current are found to control the intensity of the upwelling, but we show that high-frequency winds also contribute (approximate to 17%) to a net input of cool waters (<22.5 degrees C) on the Yucatan shelf. Finally, although more observational studies are needed to corroborate the topographic character of the Yucatan upwelling system, this study reveals the key role played by a notch along the Yucatan shelf break: a sensitivity simulation without the notch shows a 55% reduction of the upwelling

Effective resolution in ocean models

International audienceThe increase of model resolution naturally leads to the representation of a wider energy spectrum. As a result, in recent years, the understanding of oceanic submesoscale dynamics has significantly improved. Also, the ubiquity of upper ocean frontal dynamics driving a direct energy cascade is now acknowledged. In the forward cascade framework, numerical and physical closures are more consistent in principle, but dissipation in submesoscale models remains dominated by numerical constraints rather than physical ones. Therefore, effective resolution can be defined by its numerical dissipation range, which is a function of the model numerical filters (assuming that dispersive numerical modes are efficiently removed). The COMODO project gathers the whole French ocean modeling community in order to assess current nu- merical methods and guide the development of future models. Within this framework, we present an idealized ACC-type Jet case, which provides a controllable test of a model capacity at resolving submesoscale dynamics. We compare analyses performed on simulations from two models, ROMS and NEMO, at different mesh sizes (from 20 to 1 km). Through a spectral decomposition of kinetic energy and its budget terms, we identify the characteristics of turbulent cascade, numerical dissipation, and effective resolution. It shows that numerical dissipation appears in different parts of a model, especially in spatial advection-diffusion schemes for momentum equations (KE dissipation) and tracer equations (APE dissipation) and in the time stepping algorithms