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Geoffrey J. Stanley and David P. Marshall

Abstract

Downstream of Drake Passage, the Antarctic Circumpolar Current (ACC) veers abruptly northward along the continental slope of South America. This spins down the ACC, akin to the western boundary currents of ocean gyres. During this northward excursion, the mean potential vorticity (PV) increases dramatically (decreases in magnitude) by up to a factor of 2 along mean geostrophic streamlines on middepth buoyancy surfaces. This increase is driven by drag near the continental slope, or by breaking eddies further offshore, and is balanced by a remarkably steady, eddy-driven decrease of mean PV along these northern circumpolar streamlines in the open ocean. We show how two related eddy processes that are fundamental to ACC dynamics—poleward buoyancy fluxes and downward fluxes of eastward momentum—are also concomitant with materially forcing PV to increase on the northern flank of a jet at middepth, and decrease on the southern flank. For eddies to drive the required mean PV decrease along northern streamlines, the ACC merges with the subtropical gyres to the north, so these streamlines inhabit the southern flanks of the combined ACC–gyre jets. We support these ideas by analyzing the time-mean PV and its budget along time-mean geostrophic streamlines in the Southern Ocean State Estimate. Our averaging formalism is Eulerian, to match the model’s numerics. The thickness-weighted average is preferable, but its PV budget cannot be balanced using Eulerian 5-day averaged diagnostics, primarily because the z-level buoyancy and continuity equations’ delicate balances are destroyed upon transformation into the buoyancy-coordinate thickness equation.

Significance Statement

The Antarctic Circumpolar Current is the world’s largest ocean current and a key controller of Earth’s climate. As the westerly winds that drive this current shift poleward under global warming, it is vital to know whether the current will follow. To begin addressing this, we study the current’s fundamental dynamics, and constraints, under present-day conditions. By analyzing angular momentum and stratification together, we show that the current is weakened near boundaries and strengthened by eddies elsewhere. The strengthening effects of eddies are isolated to the current by merging the current with oceanic gyres to the north. This gives a new perspective on why the current travels so far northward alongside South America, and may provide dynamical constraints on future changes.

Open access
Zheen Zhang, Thomas Pohlmann, and Xueen Chen

Abstract

The characteristics and variability of intraseasonal internal coastal Kelvin waves (CKWs) along the Bay of Bengal (BoB) waveguide are investigated in the context of global warming by employing a regional ocean model. The analyzed period covers 120 years from 1980 to 2099, which includes the historical scenario and the RCP8.5 scenario. CKW information is successfully extracted from the temperature anomalies along the pycnocline by applying a newly developed methodology. The analysis reveals that intraseasonal CKWs in the BoB are highly in accordance with the intraseasonal zonal wind stress in the western equatorial Indian Ocean; the downwelling CKW lags the equatorial intraseasonal westerly winds, and the upwelling CKW lags the equatorial intraseasonal easterly winds. The CKWs significantly affect subsurface characteristics at the eastern BoB boundary, and the weakening of CKWs near the Irrawaddy Delta tip is a general feature occurring in the subsurface. With respect to the long-term scale, the occurrence of significant CKWs is predicted to be more frequent in the future under the high emissions pathway. Remarkably, the monthly climatology of CKWs varies over time; unlike the first two 30-yr analyzed periods, significant CKWs are predicted to mainly occur around August during the last two 30-yr periods due to the corresponding variabilities in the equatorial wind field, suggesting that the BoB characteristics may greatly deviate from the current climatological state.

Open access
Yidongfang Si, Andrew L. Stewart, and Ian Eisenman

Abstract

The Antarctic Slope Current (ASC) plays a central role in redistributing water masses, sea ice, and tracer properties around the Antarctic margins, and in mediating cross-slope exchanges. While the ASC has historically been understood as a wind-driven circulation, recent studies have highlighted important momentum transfers due to mesoscale eddies and tidal flows. Furthermore, momentum input due to wind stress is transferred through sea ice to the ASC during most of the year, yet previous studies have typically considered the circulations of the ocean and sea ice independently. Thus, it remains unclear how the momentum input from the winds is mediated by sea ice, tidal forcing, and transient eddies in the ocean, and how the resulting momentum transfers serve to structure the ASC. In this study the dynamics of the coupled ocean–sea ice–ASC circulation are investigated using high-resolution process-oriented simulations and interpreted with the aid of a reduced-order model. In almost all simulations considered here, sea ice redistributes almost 100% of the wind stress away from the continental slope, resulting in approximately identical sea ice and ocean surface flows in the core of the ASC in a fully spun-up equilibrium state. This ice–ocean coupling results from suppression of vertical momentum transfer by mesoscale eddies over the continental slope, which allows the sea ice to accelerate the ocean surface flow until the speeds coincide. Tidal acceleration of the along-slope flow exaggerates this effect and may even result in ocean-to-ice momentum transfer. The implications of these findings for along- and across-slope transport of water masses and sea ice around Antarctica are discussed.

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Yeqiang Shu, Jinghong Wang, Huijie Xue, Rui Xin Huang, Ju Chen, Dongxiao Wang, Qiang Wang, Qiang Xie, and Weiqiang Wang

Abstract

Strong subinertial variability near a seamount at the Xisha Islands in the South China Sea was revealed by mooring observations from January 2017 to January 2018. The intraseasonal deep flows presented two significant frequency bands, with periods of 9–20 and 30–120 days, corresponding to topographic Rossby waves (TRWs) and deep eddies, respectively. The TRW and deep eddy signals explained approximately 60% of the kinetic energy of the deep subinertial currents. The TRWs at the Ma, Mb, and Mc moorings had 297, 262, and 274 m vertical trapping lengths, and ∼43, 38, and 55 km wavelengths, respectively. Deep eddies were independent from the upper layer, with the largest temperature anomaly being >0.4°C. The generation of the TRWs was induced by mesoscale perturbations in the upper layer. The interaction between the cyclonic–anticyclonic eddy pair and the seamount topography contributed to the generation of deep eddies. Owing to the potential vorticity conservation, the westward-propagating tilted interface across the eddy pair squeezed the deep-water column, thereby giving rise to negative vorticity west of the seamount. The strong front between the eddy pair induced a northward deep flow, thereby generating a strong horizontal velocity shear because of lateral friction and enhanced negative vorticity. Approximately 4 years of observations further confirmed the high occurrence of TRWs and deep eddies. TRWs and deep eddies might be crucial for deep mixing near rough topographies by transferring mesoscale energy to small scales.

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Xiaohui Zhou, Tetsu Hara, Isaac Ginis, Eric D’Asaro, Je-Yuan Hsu, and Brandon G. Reichl

Abstract

The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper-ocean current observations [using electromagnetic autonomous profiling explorer (EM-APEX) floats deployed under five tropical cyclones] and a coupled ocean–wave (Modular Ocean Model 6–WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2–3 × 10−3 for wind speeds of 25–55 m s−1. While the drag coefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is significantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 45°, a feature that is underestimated by current models of sea state–dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left-front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state–dependent parameterizations of the drag coefficient in tropical cyclone conditions.

Open access
F. Sévellec, A. Colin de Verdière, and N. Kolodziejczyk

Abstract

We use an analog method, based on displacements of Argo floats at their parking depth (nominally located around 1000 dbar) from the ANDRO dataset, to compute continuous, likely trajectories and estimate the Lagrangian dispersion. From this, we find that the horizontal diffusivity coefficient has a median value around 500 m2 s−1 but is highly variable in space, reaching values from 100 m2 s−1 in the gyre interior to 40 000 m2 s−1 in a few specific locations (in the Zapiola Gyre and in the Agulhas Current retroflection). Our analysis suggests that the closure for diffusivity is proportional to eddy kinetic energy (or square of turbulent velocity) rather than (absolute) turbulent velocity. It is associated with a typical turbulent time scale of 4–5.5 days, which is noticeably quite constant over the entire globe, especially away from coherent intense currents. The diffusion is anisotropic in coherent intense currents and around the equator, with a primary direction of diffusion consistent with the primary direction of horizontal velocity variance. These observationally based horizontal diffusivity estimations, and the suggested eddy kinetic energy closure, can be used for constraining, testing, and validating eddy turbulence parameterization.

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Erica Rosenblum, Julienne Stroeve, Sarah T. Gille, Camille Lique, Robert Fajber, L. Bruno Tremblay, Ryan Galley, Thiago Loureiro, David G. Barber, and Jennifer V. Lukovich

Abstract

The Arctic seasonal halocline impacts the exchange of heat, energy, and nutrients between the surface and the deeper ocean, and it is changing in response to Arctic sea ice melt over the past several decades. Here, we assess seasonal halocline formation in 1975 and 2006–12 by comparing daily, May–September, salinity profiles collected in the Canada Basin under sea ice. We evaluate differences between the two time periods using a one-dimensional (1D) bulk model to quantify differences in freshwater input and vertical mixing. The 1D metrics indicate that two separate factors contribute similarly to stronger stratification in 2006–12 relative to 1975: 1) larger surface freshwater input and 2) less vertical mixing of that freshwater. The larger freshwater input is mainly important in August–September, consistent with a longer melt season in recent years. The reduced vertical mixing is mainly important from June until mid-August, when similar levels of freshwater input in 1975 and 2006–12 are mixed over a different depth range, resulting in different stratification. These results imply that decadal changes to ice–ocean dynamics, in addition to freshwater input, significantly contribute to the stronger seasonal stratification in 2006–12 relative to 1975. These findings highlight the need for near-surface process studies to elucidate the impact of lateral processes and ice–ocean momentum exchange on vertical mixing. Moreover, the results may provide insight for improving the representation of decadal changes to Arctic upper-ocean stratification in climate models that do not capture decadal changes to vertical mixing.

Open access
Tong Bo and David K. Ralston

Abstract

Idealized numerical simulations were conducted to investigate the influence of channel curvature on estuarine stratification and mixing. Stratification is decreased and tidal energy dissipation is increased in sinuous estuaries compared to straight channel estuaries. We applied a vertical salinity variance budget to quantify the influence of straining and mixing on stratification. Secondary circulation due to the channel curvature is found to affect stratification in sinuous channels through both lateral straining and enhanced vertical mixing. Alternating negative and positive lateral straining occur in meanders upstream and downstream of the bend apex, respectively, corresponding to the normal and reversed secondary circulation with curvature. The vertical mixing is locally enhanced in curved channels with the maximum mixing located upstream of the bend apex. Bend-scale bottom salinity fronts are generated near the inner bank upstream of the bend apex as a result of interaction between the secondary flow and stratification. Shear mixing at bottom fronts, instead of overturning mixing by the secondary circulation, provides the dominant mechanism for destruction of stratification. Channel curvature can also lead to increased drag, and using a Simpson number with this increased drag coefficient can relate the decrease in stratification with curvature to the broader estuarine parameter space.

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Manita Chouksey, Carsten Eden, and Dirk Olbers

Abstract

The generation of internal gravity waves from an initially geostrophically balanced flow is diagnosed in nonhydrostatic numerical simulations of shear instabilities for varied dynamical regimes. A nonlinear decomposition method up to third order in the Rossby number (Ro) is used as the diagnostic tool for a consistent separation of the balanced and unbalanced motions in the presence of their nonlinear coupling. Wave emission is investigated in an Eady-like and a jet-like flow. For the jet-like case, geostrophic and ageostrophic unstable modes are used to initialize the flow in different simulations. Gravity wave emission is in general very weak over a range of values for Ro. At sufficiently high Ro, however, when the condition for symmetric instability is satisfied with negative values of local potential vorticity, significant wave emission is detected even at the lowest order. This is related to the occurrence of fast ageostrophic instability modes, generating a wide spectrum of waves. Thus, gravity waves are excited from the instability of the balanced mode to lowest order only if the condition of symmetric instability is satisfied and ageostrophic unstable modes obtain finite growth rates.

Significance Statement

We aim to understand the generation of internal gravity waves in the atmosphere and ocean from a flow field that is initially balanced, i.e., free from any internal gravity waves. To examine this process, we use simulations from idealized numerical models and nonlinear flow decomposition method to identify waves. Our results show that a prominent mechanism by which waves can be generated is related to symmetric or ageostrophic instabilities of the balanced flow possibly occurring during frontogenesis. This process can be a significant mechanism to dissipate the energy of the geostrophic flow in the ocean.

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Giulio Passerotti, Luke G. Bennetts, Franz von Bock und Polach, Alberto Alberello, Otto Puolakka, Azam Dolatshah, Jaak Monbaliu, and Alessandro Toffoli

Abstract

Irregular, unidirectional surface water waves incident on model ice in an ice tank are used as a physical model of ocean surface wave interactions with sea ice. Results are given for an experiment consisting of three tests, starting with a continuous ice cover and in which the incident wave steepness increases between tests. The incident waves range from causing no breakup of the ice cover to breakup of the full length of ice cover. Temporal evolution of the ice edge, breaking front, and mean floe sizes are reported. Floe size distributions in the different tests are analyzed. The evolution of the wave spectrum with distance into the ice-covered water is analyzed in terms of changes of energy content, mean wave period, and spectral bandwidth relative to their incident counterparts, and pronounced differences are found between the tests. Further, an empirical attenuation coefficient is derived from the measurements and shown to have a power-law dependence on frequency comparable to that found in field measurements. Links between wave properties and ice breakup are discussed.

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