Abstract
We continue our study of the role of curvature in modifying frontal stability. In Part I, we obtained an instability criterion valid for curved fronts and vortices in gradient wind balance ...(GWB): Φ′ =
L
′
q
′ < 0, where
L
′ and
q
′ are the nondimensional absolute angular momentum and Ertel potential vorticity (PV), respectively. In Part II, we investigate this criterion in a parameter space representative of low-Richardson-number fronts and vortices in GWB. An interesting outcome is that, for Richardson numbers near 1, anticyclonic flows increase in
q
′, while cyclonic flows decrease in
q
′, tending to stabilize anticyclonic and destabilize cyclonic flow. Although stability is marginal or weak for anticyclonic flow (owing to multiplication by
L
′), the destabilization of cyclonic flow is pronounced, and may help to explain an observed asymmetry in the distribution of small-scale, coherent vortices in the ocean interior. We are referring to midlatitude submesoscale and polar mesoscale vortices that are generated by friction and/or buoyancy forcing within boundary layers but that are often documented outside these layers. A comparison is made between several documented vortices and predicted stability maps, providing support for the proposed mechanism. A simple expression, which is a root of the stability discriminant Φ′, explains the observed asymmetry in the distribution of vorticity. In conclusion, the generalized criterion is consistent with theory, observations, and recent modeling studies and demonstrates that curvature in low-stratified environments can destabilize cyclonic and stabilize anticyclonic fronts and vortices to symmetric instability. The results may have implications for Earth system models.
The overturning circulation of the global ocean is critically shaped by deep-ocean mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of ...mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topography and the rate at which well-mixed boundary waters are exchanged with the stratified ocean interior. Here, we use innovative observations of a major branch of the overturning circulation—an abyssal boundary current in the Southern Ocean—to identify a previously undocumented mixing mechanism, by which deep-ocean waters are efficiently laundered through intensified near-boundary turbulence and boundary–interior exchange. The linchpin of the mechanism is the generation of submesoscale dynamical instabilities by the flow of deepocean waters along a steep topographic boundary. As the conditions conducive to this mode of mixing are common to many abyssal boundary currents, our findings highlight an imperative for its representation in models of oceanic overturning.
Abstract Numerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification and thus restratify the upper ocean via vertical flow. ...However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical midocean area of the northeast Atlantic. Vertical velocities inferred using the nondiffusive density equation are substantially stronger at submesoscales (horizontal scales of 1–10 km) than at mesoscales (horizontal scales of 10–100 km), with respective root-mean-square values of 38.0 ± 6.9 and 22.5 ± 3.3 m day −1 . The largest submesoscale vertical velocities and rates of restratification occur in events of a few days’ duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.
Abstract
Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this ...transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focusing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current’s denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unraveled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary layer stratification is generated by a downslope Ekman flow associated with the DWBC’s flow along sloping topography, and is further regulated by submesoscale instabilities acting to restratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling.
Clouds and other data artefacts frequently limit the retrieval of key variables from remotely sensed Earth observations. We train a natural language processing (NLP)-inspired algorithm with ...high-fidelity ocean simulations to accurately reconstruct masked or missing data in sea surface temperature (SST) fields—one of 54 essential climate variables identified by the Global Climate Observing System. We demonstrate that the resulting model, referred to as Enki, repeatedly outperforms previously adopted inpainting techniques by up to an order of magnitude in reconstruction error, while displaying exceptional performance even in circumstances where the majority of pixels are masked. Furthermore, experiments on real infrared sensor data with masked percentages of at least 40% show reconstruction errors of less than the known uncertainty of this sensor (root mean square error (RMSE) ≲0.1 K). We attribute Enki’s success to the attentive nature of NLP combined with realistic SST model outputs—an approach that could be extended to other remotely sensed variables. This study demonstrates that systems built upon Enki—or other advanced systems like it—may therefore yield the optimal solution to mitigating masked pixels in in climate-critical ocean datasets sampling a rapidly changing Earth.
For over 40 years, remote sensing observations of the Earth's oceans have yielded global measurements of sea surface temperature (SST). With a resolution of approximately 1 km, these data trace ...physical processes like western boundary currents, cool upwelling at eastern boundary currents, and the formation of mesoscale and submesoscale eddies. To discover the fundamental patterns of SST on scales smaller than 10 km, we developed an unsupervised, deep contrastive learning model named NENYA. We trained NENYA on a subset of 8 million cloud-free cutout images (<inline-formula> <tex-math notation="LaTeX">\sim \,\,80 \times 80 </tex-math></inline-formula> km2) from the MODerate-resolution Imaging Spectroradiometer (MODIS) sensor, with image augmentations to impose invariance to rotation, reflection, and translation. The 256-dimension latent space of NENYA defines a vocabulary to describe the complexity of SST and associates images with like patterns and features. We used a dimensionality reduction algorithm to explore cutouts with a temperature interval of <inline-formula> <tex-math notation="LaTeX">{\Delta T}= 0.5-1 </tex-math></inline-formula> K, identifying a diverse set of patterns with temperature variance on a wide range of scales. We then demonstrated that SST data with large-scale features arise preferentially in the Pacific and Atlantic equatorial cold tongues and exhibit a strong seasonal variation, while data with predominantly submesoscale structure preferentially manifest in western boundary currents, select regions with strong upwelling, and along the Antarctic circumpolar current. We provide a Web-based user interface to facilitate the geographical and temporal exploration of the full MODerate-resolution Imaging Spectroradiometer (MODIS) dataset. Future efforts will link specific SST patterns to select dynamics (e.g., frontogenesis) to examine their distribution in time and space on the globe.
The ocean surface boundary layer is a critical interface across which momentum, heat, and trace gases are exchanged between the oceans and atmosphere. Surface processes (winds, waves, and buoyancy ...forcing) are known to contribute significantly to fluxes within this layer. Recently, studies have suggested that submesoscale processes, which occur at small scales (0.1–10 km, hours to days) and therefore are not yet represented in most ocean models, may play critical roles in these turbulent exchanges. While observational support for such phenomena has been demonstrated in the vicinity of strong current systems and littoral regions, relatively few observations exist in the open‐ocean environment to warrant representation in Earth system models. We use novel observations and simulations to quantify the contributions of surface and submesoscale processes to turbulent kinetic energy (TKE) dissipation in the open‐ocean surface boundary layer. Our observations are derived from moorings in the North Atlantic, December 2012 to April 2013, and are complemented by atmospheric reanalysis. We develop a conceptual framework for dissipation rates due to surface and submesoscale processes. Using this framework and comparing with observed dissipation rates, we find that surface processes dominate TKE dissipation. A parameterization for symmetric instability is consistent with this result. We next employ simulations from an ocean front‐resolving model to reestablish that dissipation due to surface processes exceeds that of submesoscale processes by 1–2 orders of magnitude. Together, these results suggest submesoscale processes do not dramatically modify vertical TKE budgets, though such dynamics may be climatically important owing to their ability to remove energy from the ocean.
Key Points
We present a multimonth record of OSBL turbulence in the open ocean
The contribution of surface and submesoscale processes is examined
Dissipation rates due to surface processes dominate those of submesoscale processes
A signature of submesoscale flows in the upper ocean is skewness in the distribution of relative vorticity. Expected to result for high Rossby number flows, such skewness has implications for mixing, ...dissipation, and stratification within the upper ocean. An array of moorings deployed in the Northeast Atlantic for 1 year as part of the experiment of the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) reveals that relative vorticity is positively skewed during winter even though the scale of the Rossby number is less than 0.5. Furthermore, this skewness is reduced to zero during spring and autumn. There is also evidence of modest seasonal variations in the gradient Rossby number. The proposed mechanism by which relative vorticity is skewed is that the ratio of lateral to vertical buoyancy gradients, as summarized by the inverse gradient Richardson number, restricts its range during winter but less so at other times of the year. These results support recent observations and model simulations suggesting that the upper ocean is host to a seasonal cycle in submesoscale turbulence.
Key Points
Submesoscale relative vorticity (RV) is estimated for approximately 1 year in the Northeast Atlantic
Variance and skewness of RV within the upper ocean are enhanced during winter
Observations are consistent with seasonally varying submesoscale turbulence
Abstract
Mesoscale eddies contain the bulk of the ocean’s kinetic energy (KE), but fundamental questions remain on the cross-scale KE transfers linking eddy generation and dissipation. The role of ...submesoscale flows represents the key point of discussion, with contrasting views of submesoscales as either a source or a sink of mesoscale KE. Here, the first observational assessment of the annual cycle of the KE transfer between mesoscale and submesoscale motions is performed in the upper layers of a typical open-ocean region. Although these diagnostics have marginal statistical significance and should be regarded cautiously, they are physically plausible and can provide a valuable benchmark for model evaluation. The cross-scale KE transfer exhibits two distinct stages, whereby submesoscales energize mesoscales in winter and drain mesoscales in spring. Despite this seasonal reversal, an inverse KE cascade operates throughout the year across much of the mesoscale range. Our results are not incompatible with recent modeling investigations that place the headwaters of the inverse KE cascade at the submesoscale, and that rationalize the seasonality of mesoscale KE as an inverse cascade-mediated response to the generation of submesoscales in winter. However, our findings may challenge those investigations by suggesting that, in spring, a downscale KE transfer could dampen the inverse KE cascade. An exploratory appraisal of the dynamics governing mesoscale–submesoscale KE exchanges suggests that the upscale KE transfer in winter is underpinned by mixed layer baroclinic instabilities, and that the downscale KE transfer in spring is associated with frontogenesis. Current submesoscale-permitting ocean models may substantially understate this downscale KE transfer, due to the models’ muted representation of frontogenesis.
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