Diapycnal Diffusivity Research Articles

Revisiting Thorpe scale analysis and diapycnal diffusivities in Drake Passage

Ship heave effects in the swell-prevalent Southern Ocean induce frequent false overturns in density profiles, and conventional Thorpe scale analysis leads to unrealistically large diapycnal diffusivities. Another critical factor causing large positive biases in Thorpe scale analysis concerns the Ozmidov to Thorpe scales ratio α often considered constant and equal to 0.8. A revised Thorpe scale analysis is proposed with an innovative segment-by-segment approach to circumvent heave-induced false overturns together with best-fitting α functions to Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) data, which yields realistic diffusivities in the Drake Passage region. The revised Thorpe scale method applied to a finely-resolved CTD section across the Drake Passage yields typical diffusivities of O (10-5-10-3 m2 s-1), with the largest values being preferentially concentrated close to the bottom at major circumpolar fronts, consistent with nearby DIMES microstructure observations. In the southern Drake Passage the method highlights a middepth (500-1500 m) mixing maximum of 10-4-10-3 m2 s-1 in the Lower Circumpolar Deep Water (LCDW) layer due to large intrusions of Warm Deep Water (a cold/fresh variety of LCDW) of Weddell Gyre origin. The inferred middepth mixing maximum in intrusive regions, which is associated with isopycnal eddy stirring rather than caused by internal waves or double diffusive mixing, supports the short-circuiting paradigm of meridional overturning circulation in the southwestern Scotia Sea region and has important climatic implications in a warming climate.

Mixing and Water Mass Transformation Over Discovery Bank, in the Weddell‐Scotia Confluence of the Southern Ocean

AbstractThe South Scotia Ridge, in the Atlantic sector of the Southern Ocean, is a key region for water mass modification. It is the location of the Weddell‐Scotia Confluence, an area of reduced stratification which separates the Weddell Gyre to the south and the Antarctic Circumpolar Current to the north, and which receives input of shelf waters from the tip of the Antarctic Peninsula. To elucidate the transformations over the ridge, we focus on one of its largest seamounts, Discovery Bank, which has previously been observed as hosting a stratified Taylor column that retains water for months to years, during which time water masses are entrained from north and south of the Weddell Front and steadily mixed. Data from ship‐deployed sensors and autonomous platforms are analyzed to quantify and understand the diapycnal mixing, heat fluxes and water mass transformations over the bank. Ocean glider and free‐profiling drifting float data show that the mid‐depth temperature maximum of the Circumpolar Deep Water (CDW) is eroded between the northern and southern sides of the bank, while diapycnal diffusivity is enhanced by up to an order‐of‐magnitude over its steeply sloping portions. This is accompanied by heat fluxes from the CDW layer being increased by up to a factor of six, which may contribute to a reduction in mid‐depth stratification. Tidal model analysis shows that the southern side of the bank hosts strong barotropic to baroclinic energy conversion (>150 N m−2), emphasizing the role of internal tides in modulating water mass transformations in the Confluence.

A Framework for Constraining Ocean Mixing Rates and Overturning Circulation from Age Tracers

Abstract The age of seawater refers to the amount of time that has elapsed since that water encountered the surface. This age measures the ventilation rate of the ocean, and the spatial distribution of age can be influenced by multiple processes, such as overturning circulation, ocean mixing, and air–sea exchange. In this work, we aim to gain new quantitative insights about how the ocean’s age tracer distribution reflects the strength of the meridional overturning circulation and diapycnal diffusivity. We propose an integral constraint that relates the age tracer flow across an isopycnal surface to the geometry of the surface. With the integral constraint, a relationship between the globally averaged effective diapycnal diffusivity and the meridional overturning strength at an arbitrary density level can be inferred from the age tracer concentration near that level. The theory is tested in a set of idealized single-basin simulations. A key insight from this study is that the age difference between regions of upwelling and downwelling, rather than any single absolute age value, is the best indicator of overturning strength. The framework has also been adapted to estimate the strength of abyssal overturning circulation in the modern North Pacific, and we demonstrate that the age field provides an estimate of the circulation strength consistent with previous studies. This framework could potentially constrain ocean circulation and mixing rates from age-like realistic tracers (e.g., radiocarbon) in both past and present climates. Significance Statement The age of seawater—the local mean time since local water from different pathways was last at the surface—is a valuable indicator of ocean circulation and the transport time scale of heat and carbon. We introduce a novel constraint that relates total age flow across a density surface to its geometry, which provides new insights into constraining ocean circulation and mixing rates from age-like realistic tracers (e.g., radiocarbon).

Winter entrainment drives the mixed layer supply of manganese in the Southern Ocean

AbstractDespite the subnanomolar dissolved manganese concentrations that can co‐limit Southern Ocean primary production, their physical supply mechanisms during winter, for biological consumption in spring and summer have not yet been explored. During austral winter and spring 2019, two cruises were conducted in the Atlantic sector of the Southern Ocean, to determine the distribution and surface water supply mechanisms of dissolved manganese in the upper water column. The supply mechanisms were used to calculate the total flux of dissolved manganese to productive surface waters and were compared to biological consumption estimates. Mean dissolved manganese concentrations in the upper water column (< 500 m) during winter and spring were comparably low (≤ 0.34 nmol kg−1; p > 0.05), with seasonal mixed layer reservoir sizes averaging 65.21 ± 12.93 and 21.64 ± 19.32 μmol m−2, respectively. Winter entrainment contributed 89.33–99.99% (average 97.26% ± 5.28%) of the total dissolved manganese flux, while diapycnal diffusion contributed 0.52–10.58% (average 4.92% ± 5.14%), was identified as the dominant mechanisms for transporting dissolved manganese into the mixed layer in the subantarctic zone, polar frontal zone, and antarctic zone. Here, the winter physical supply rates were higher than the estimated consumption rates during spring, meeting phytoplankton biological demands. Whereas in the subtropical zone, the supply rates were lower than the consumption rates, indicating the presence of additional supply mechanisms such as coastal upwelling, which may help to meet the biological demands in this region.

Spatial and Temporal Variability of Turbulent Mixing in the Deep Northwestern Pacific

AbstractSmall‐scale turbulent mixing supplies potential energy for the upwelling of deep waters in the abyssal ocean, a key component of the global overturning circulation. This process is particularly significant in critical regions such as the Northwestern Pacific where the upwelling structure of deep waters remains poorly understood due to limited knowledge of deep ocean mixing. Here, we investigate the full‐depth spatiotemporal variability of turbulent mixing in the deep Northwestern Pacific based on hydrographic data collected over repeated surveys. Nineteen‐year‐average diapycnal diffusivity of 1.42 × 10−4 m2 s−1 is reveled in the deep Philippine Sea, indicating significantly stronger mixing compared to the stratified ocean interior. Spatially, turbulent mixing strengthens toward the bottom and intensifies westward from the open Pacific to the Philippine Sea due to rough topography. At certain mixing hotspots, enhanced mixing can penetrate up to 2,500 m above the bottom, suggesting a substantial potential for upwelling. Below 2,000 m, turbulent mixing exhibits pronounced seasonal variation that deep mixing is more intense in summer (winter) than in winter (summer) in the West Caroline Basin (the Parece Vela Basin). This spatially varying seasonality may be attributed to the inhomogeneous internal tidal energy dissipation in the Northwestern Pacific. Our study will serve to clarify the modulation of turbulent mixing to deep‐water mass transformation and circulation in the Northwestern Pacific.

Breaking Internal Waves and Ocean Diapycnal Diffusivity in a High‐Resolution Regional Ocean Model: Evidence of a Wave‐Turbulence Cascade

AbstractIt is generally understood that the origin of ocean diapycnal diffusivity is primarily associated with the stratified turbulence produced by breaking internal (gravity) waves (IW). However, it requires significant effort to verify diffusivity values in ocean general circulation models in any particular geographical region of the ocean due to the scarcity of microstructure measurements. Recent analyses of downscaled IW fields from an internal‐wave‐admitting global ocean simulation into higher‐resolution regional configurations northwest of Hawaii have demonstrated a much‐improved fit of the simulated IW spectra to the in‐situ profiler measurements such as the Garrett‐Munk (GM) spectrum. Here, we employ this dynamically downscaled ocean simulation to directly analyze the nature of the IW‐breaking and the wave‐turbulence cascade in this region. We employ a modified version of the Kappa Profile Parameterization (KPP) to infer what the horizontally averaged vertical profile of diapycnal diffusivity should be, and compare this to the background profile that would be employed in the ocean component of a low‐resolution coupled climate model such as the Community Earth System Model (CESM) of the US National Center for Atmospheric Research (NCAR). In pursuing this goal, we also demonstrate that the wavefield in the high‐resolution regional domain is dominated by a well‐resolved spectrum of low‐mode IWs that are predictable by solving an appropriate eigenvalue problem for stratified flow. We finally suggest a new tentative approach to improve the KPP parameterization.

The Brazil Basin Tracer Release Experiment: Observations

Abstract Lightening of bottom water is required to close the abyssal overturning circulation, believed to play an important role in the climate system. A tracer release experiment and turbulence measurement programs have revealed how bottom water is lightened, and illuminated the associated circulation in the deep Brazil Basin, a representative region of the global ocean. Tracer was released on an isopycnal surface about 4000 m deep, over one of the fracture zones emanating from the Mid-Atlantic Ridge (MAR). Tracer that mixed toward the bottom moved toward the MAR across isopycnal surfaces that bend down to intersect the bottom at a rate implying a near-bottom buoyancy flux of 1.5 × 10−9 m2 s−3, somewhat larger than inferred from dissipation measurements. The diffusivity at the level of the tracer release is estimated at 4.4 ± 1 × 10−4 m2 s−1, again larger than inferred from dissipation rates. The main patch moved southwest at about 2 cm s−1 while sinking due to the divergence of buoyancy flux above the bottom layer. The isopycnal eddy diffusivity was about 100 m2 s−1. Westward flow away from the MAR is the return flow balancing the eastward near-bottom upslope flow. The southward component of the flow is roughly consistent with conservation of potential vorticity. The circulation as well as the pattern of diapycnal flux are qualitatively the same as in St. Laurent et al. (2001) but are more robust. The results indicate that diapycnal diffusivity is about twice that invoked by Morris et al. (2001) in calculating the basinwide buoyancy budget. Significance Statement Buoyancy flux into the abyssal waters is required to close the overturning circulation of those waters, an important part of the climate system. This contribution presents a robust view of the strength of that buoyancy flux and the associated circulation.

Influence of tidal mixing on bottom circulation in the Caroline Sea

Bottom circulation in the abyssal Caroline Sea is an important component of the global meridional overturning circulation. By use of a high-resolution regional ocean model, the influence of tidal mixing processes on bottom water and circulation in the Caroline Sea is investigated. Based on different configurations for diapycnal diffusivities of tidal mixing, three numerical experiments are performed: one completely without tidal mixing, one only with local tidal mixing due to the locally dissipated tidal energy, and one considering tidal mixing processes induced by the total dissipated tidal energy. The results show that tidal mixing processes in the abyssal Caroline Sea could sustain a relatively high horizontal density gradient and hence baroclinic pressure gradient not only across the two deep-water passages connecting to the open ocean, but also within the abyssal West Caroline Basin (WCB) and East Caroline Basin (ECB). Therefore, tidal mixing processes could maintain the large amounts of bottom water inflow, intensify the bottom basin/subbasin-scale horizontal circulation, and drive a more vigorous meridional overturning circulation in the abyssal WCB and ECB. Moreover, simulations of bottom water transport in the experiment with tidal mixing processes are more consistent with previous observations and estimates. These results suggest that tidal mixing processes play a crucial dynamic role in the bottom circulation, and is essential for ocean modelling.

Energetic Stratified Turbulence Generated by Kuroshio–Seamount Interactions in Tokara Strait

Abstract Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity, and microscale thermal-variance dissipation rate up- and downstream of the ∼10-km-wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates and diapycnal diffusivities above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence, which is expected to decay in less than 100 m based on turbulence decay time of N−1 ∼ 100 s and the 0.5 m s−1 Kuroshio flow speed. Thus, the turbulent wake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortex wake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy, and (iii) reduced-shear, are tested. Average ε vertical profiles are well reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.

Turbulence Across the Antarctic Circumpolar Current in the Indian Southern Ocean: Micro‐Temperature Measurements and Finescale Parameterizations

AbstractTurbulence structures across the Antarctic circumpolar current (ACC) in the Indian Ocean at around 50°E were revealed using microstructure measurements. Depth‐averaged turbulent energy dissipation rates (ε) in 320 m segments estimated using a conductivity‐temperature‐depth (CTD)‐attached micro‐temperature fast‐response thermistor (FP07) were well reproduced using existing finescale parameterizations excluding near‐surface and near‐bottom 160 m. The spatial variations of ε are explained by density stratification N2, bottom topographic roughness, and depth‐averaged current speed U. Diapycnal diffusivity Kρ(=0.2εN−2) is found proportional to the squared internal wave energy () and increases toward the bottom, especially over rough bottom topography. The decay scale height is >1000m and the turbulence remains large even in the upper pycnocline. Existing shear‐based finescale parameterizations were generally consistent with the FP07 measurements but tended to overestimate in the areas with large shear‐strain ratio Rω and low ε in the Continental Zone (CZ) south of the ACC fronts (58–65°S). This overestimation is improved by using the strain‐based finescale parameterization with a constant Rω = 3.

Diapycnal mixing induced by salt finger and internal tides on the northwest coast of India

Microstructure measurements of velocity shear from the continental slope of the northwest coast of India (NWCI) in the eastern Arabian Sea are used to quantify the relative importance of double diffusion and internal tides induced diapycnal mixing in the different depth layers. It is found that the hydrographic conditions in the NWCI are conducive to the formation of moderately strong salt fingering (Turner angle between 55° and 72°). However, salt finger-induced vertical mixing dominates only in the upper 180 m of the water column, below which intense shear-driven turbulent mixing due to internal tide reduces its significance. As a result of this, the staircase structures, a measure of salt finger dominance in the water column, are frequent, and the mean temperature change across the interface (DTIH; 0.33 °C) is relatively larger in the upper 180 m compared to sporadic occurrence of steps with a small magnitude of DTIH (0.18 °C) below 180 m. It is also found that in the upper 180 m of the water column in the NWCI, mean diapycnal diffusivity (Kρ) is approximately a factor of eight larger (8.3 ± 1.3 × 10−5 m2s−1) than the estimation in the open ocean region of the eastern Arabian sea (5.4 ± 1.1 × 10−6 m2 s−1). However, due to internal tides, the magnitude of Kρ reaches as large as O (10−2) m2 s−1 below 180 m in the NWCI. The mean downward heat flux estimated in the salt finger-dominated (upper 180 m) layers is ∼ -6.1 Wm-2, and the shear-driven mixing-dominated layers (below 180 m) is ∼ -10.2 Wm-2.

Deep propagation of wind-generated near-inertial waves in the Northern South China Sea

Using a subsurface mooring deployed at a depth of ∼2290 m in the northern South China Sea, the characteristics of deep-propagating near-inertial waves (NIWs) (>1500 m) generated by a combination of a winter storm and a typhoon were examined. The NIWs had a large vertical wavelength and showed a high coherence in near-full water depth. Correspondingly, the strong vertical shear and energy of the NIWs were concentrated at vertical scales larger than ∼500 m. The average vertical group velocity was ∼160 m/day. The horizontal wavelength decreased from ∼600 km in the upper layer to a few tens of kilometers below a depth of 1500 m. Interestingly, the near-inertial energy did not decrease obviously at depths of 100–1500 m, where the average downward near-inertial energy flux (Fz) was (2.3 ± 0.9) × 10−3 W/m2 and nearly equaled that at 100 m depth. Combined with the slab model, the mean Fz at a depth of 100–1500 m can reach 19%–43% of that in the mixed layer. During the deep propagation of NIWs, the near-inertial shear was not strong in the upper layer. However, in the deep layer, the total shear was dominated by deep-propagating NIWs and was significantly enhanced. Correspondingly, the estimated diapycnal diffusivity increased, reaching a maximum value of ∼270% of the averaged value. This study contributes to our understanding of the deep propagation of wind-generated NIWs in the open ocean.

Seasonal Variability of Near-Inertial/Semidiurnal Fluctuations and Turbulence in the Subarctic North Atlantic

Abstract Six profiling floats measured water-mass properties (T, S), horizontal velocities (u, υ), and microstructure thermal-variance dissipation rates χT in the upper ∼1 km of the Iceland and Irminger Basins in the eastern subpolar North Atlantic from June 2019 to April 2021. The floats drifted into slope boundary currents to travel counterclockwise around the basins. Pairs of velocity profiles half an inertial period apart were collected every 7–14 days. These half-inertial-period pairs are separated into subinertial eddy (sum) and inertial/semidiurnal (difference) motions. Eddy flow speeds are ∼O(0.1) m s−1 in the upper 400 m, diminishing to ∼O(0.01) m s−1 by ∼800-m depth. In late summer through early spring, near-inertial motions are energized in the surface layer and permanent pycnocline to at least 800-m depth almost simultaneously (within the 14-day temporal resolution), suggesting rapid transformation of large-horizontal-scale surface-layer inertial oscillations into near-inertial internal waves with high vertical group velocities through interactions with eddy vorticity gradients (effective β). During the same period, internal-wave vertical shear variance was 2–5 times canonical midlatitude magnitudes and dominantly clockwise-with-depth (downward energy propagation). In late spring and early summer, shear levels are comparable to canonical midlatitude values and dominantly counterclockwise-with-depth (upward energy propagation), particularly over major topographic ridges. Turbulent diapycnal diffusivities K ∼ O(10−4) m2 s−1 are an order of magnitude larger than canonical midlatitude values. Depth-averaged (10–1000 m) diffusivities exhibit factor-of-3 month-by-month variability with minima in early August.

Enhanced Mixing at the Edges of Mesoscale Eddies Observed From High‐Resolution Seismic Data in the Western Arctic Ocean

AbstractIn recent years, the Arctic Ocean has experienced a dramatic decrease in sea ice cover, leading to enhanced turbulent mixing and eddy activity. Due to the scarcity of high‐resolution observations, our understanding of turbulent mixing in the Arctic is incomplete. Using three seismic reflection transects with high spatial resolution (∼O (10) m) and concurrent ADCP‐derived current velocity, this study identified 11 mesoscale eddies on the ice‐free Chukchi Borderlands of the western Arctic Ocean. 82% of them are intra‐halocline anticyclones. The total length of horizontal scales of these 11 eddies occupies ∼40% of the total length of three seismic lines. Diapycnal diffusivities estimated from seismic data were enhanced and can be up to 10−4 m2 s−1 at the edges of these eddies, compared with the average values (∼10−6 m2 s−1) at the Chukchi Borderlands. Seismic‐estimated diffusivities were validated by fine‐scale parameterization using ADCP‐derived velocity data and historical hydrographic data. Enhanced diffusivities at the edges of eddies may be attributed to shear instabilities at the top and bottom edges and to submesoscale motions at the lateral edges of these eddies. We highlight the enhanced mixing at the edges of eddies in the halocline can increase the upward heat flux. This upward heat flux can transfer the heat of the warm Atlantic water below to the surface, which may further promote the melting of surface sea ice.

Multi‐Approach Analysis of Baroclinic Internal Tide Perturbation in the Ionian Sea Abyssal Layer (Mediterranean Sea)

AbstractDespite being widely recognized, the importance of deep layers thermohaline and mixing processes in the ocean circulation and variability is still poorly investigated, especially in the Mediterranean Sea. This limits understanding and parametrizing deep dynamics, which result in evident biases in the global circulation representation by observations and numerical ocean simulations. Having access to hydrological datasets, collected on a whole water column, we investigated the abyssal stratification and its variability of the Ionian Sea (Central Mediterranean). Applying multiple analyses, we found a tidal‐period oscillation and the resulting activation of mixing, pointing out that the combined effect of stratification, morphology, and tides has a key role in enhancing local diapycnal diffusivity in the deepest layers, being a mechanism that connects the whole water column with a compelling impact on the vertical transport of heat and tracers.

Oxygen isotope constraints on the ventilation of the modern and glacial Pacific

Changes in Pacific tracer reservoirs and transports are thought to be central to the regulation of atmospheric CO2 on glacial–interglacial timescales. However, there are currently two contrasting views of the circulation of the modern Pacific; the classical view sees southern sourced abyssal waters upwelling to about 1.5 km depth before flowing southward, whereas the bathymetrically constrained view sees the mid-depths (1–2.5 km) largely isolated from the global overturning circulation and predominantly ventilated by diffusion. Furthermore, changes in the circulation of the Pacific under differing climate states remain poorly understood. Through both a modern and a Last Glacial Maximum (LGM) analysis focusing on oxygen isotopes in seawater and benthic foraminifera as conservative tracers, we show that isopycnal diffusion strongly influences the mid-depths of the Pacific. Diapycnal diffusion is most prominent in the subarctic Pacific, where an important return path of abyssal tracers to the surface is identified in the modern state. At the LGM we infer an expansion of North Pacific Intermediate Water, as well as increased layering of the deeper North Pacific which would weaken the return path of abyssal tracers. These proposed changes imply a likely increase in ocean carbon storage within the deep Pacific during the LGM relative to the Holocene.

Radiation and energetics of M2 internal tides in the Okhotsk Sea

Tidal mixing around the Kuril Island Chain (KIC) significantly modulates the transport of Okhotsk Sea Mode Water into the Pacific Ocean and, hence, the formation of North Pacific Intermediate Water. Although previous studies have revealed the dominant role of trapped K1 internal tides in driving mixing, the energetic M2 internal tides that have been observed by altimetry are not sufficiently clarified. In this study, we investigated the generation, radiation, and dissipation of semidiurnal M2 internal tides in the Okhotsk Sea using a high-resolution model. The results showed that KIC is the main generation site for M2 internal tides, with an integrated energy conversion of 2.07 GW. The generated M2 internal tides propagated over a long distance into the Okhotsk Sea and Northern Pacific Ocean and exhibited asymmetry on the two sides of the KIC. In addition, the generation of M2 internal tides on the Kashevarov Bank in the northwestern Okhotsk Sea made a minor contribution. Although the integrated dissipation of the M2 internal tides at KIC was lower than that of the K1 internal tides, the M2 internal tidal energy dissipation rates exceeded 10−8 W/kg near the topography. More importantly, energy dissipation in the far field was dominated by radiating M2 internal tides, which induced higher diapycnal diffusivity than K1 internal tides. These results revealed the important role of M2 internal tides in driving mixing at KIC and surrounding regions, which was previously overlooked.

Different flavours of oxygen help quantify seasonal variations of the biological carbon pump in the Celtic Sea

Shelf seas represent only 10% of the World’s Ocean by area but support up to 30% of its primary production. There are few measurements of biological production at high spatial and temporal resolution in these physically and biologically dynamic systems. Here, we use dissolved oxygen to-argon (O2/Ar) ratios and oxygen triple isotopes in O2 (16O, 17O, 18O) to estimate net community production, N(O2/Ar), and gross O2 production, G(17O), in summer and autumn 2014 and spring and summer 2015 in the Celtic Sea, as part of the UK Shelf-Sea Biogeochemistry Programme. Surface O2/Ar concentration ratios were measured continuously using a shipboard membrane inlet mass spectrometer. Additional depth profiles of O2/Ar concentration ratios, δ(17O) and δ(18O) were measured in discrete water samples from hydrocasts. The data were combined with wind-speed based gas exchange parameterisations to calculate biological air-sea oxygen fluxes. These fluxes were corrected for diapycnal diffusion, entrainment, production below the mixed layer, and changes over time to derive N(O2/Ar) and G(17O). The Celtic Sea showed the highest G(17O) in summer 2014 (825 mmol m–2 d–1) and lowest during autumn 2014 (153 mmol m–2 d–1). N(O2/Ar) was highest in spring 2015 (43 mmol m–2 d–1), followed by summer 2014 (42 mmol m–2 d–1), with a minimum in autumn 2014 (–24 mmol m–2 d–1). Dividing the survey region into three hydrographically distinct areas (Celtic Deep, Central Celtic Sea and Shelf Edge), we found that Celtic Deep and Shelf Edge had higher N(O2/Ar) in summer (71 and 63 mmol m–2 d–1, respectively) than in spring (49 and 22 mmol m–2 d–1). This study shows regional differences in the metabolic balance within the same season, as well as higher net community production in summer than in spring in some areas and years. The seasonal patterns in biological production rates and the export efficiency (f-ratio) identified the importance of biology for supporting the Celtic Sea’s ability to act as a net CO2 sink. Our measurements thus help improve our understanding of the biological carbon pump in temperate shelf seas.

A direct derivation of the Gent–McWilliams/Redi diffusion tensor from quasi-geostrophic dynamics

The transport induced by ocean mesoscale eddies remains unresolved in most state-of-the-art climate models and needs to be parametrized instead. The natural scale separation between the forcing and the emergent turbulent flow calls for a diffusive parametrization, where the eddy-induced fluxes are related to the large-scale gradients by a diffusion tensor. The standard parametrization scheme in climate modelling consists in adopting the Gent–McWilliams/Redi (GM/R) form for the diffusion tensor, initially put forward based on physical intuition and educated guesses before being put on firm analytical footing using a thickness-weighted average (TWA). In the present contribution, we provide a direct derivation of this diffusion tensor from the quasi-geostrophic (QG) dynamics of a horizontally homogeneous three-dimensional patch of ocean hosting a large-scale vertically sheared zonal flow on the $\beta$ plane. The derivation hinges on the identification of a useful cross-invariant defined as the product of the buoyancy and QG potential vorticity fluctuations. While less general than the TWA approach, the present QG framework leads to rigorous constraints on the diffusion tensor. First, there is no diapycnal diffusivity arising in the QG GM/R tensor for low viscosity and small-scale diffusivities. The diffusion tensor then involves only two vertically dependent coefficients, namely the GM transport coefficient $K_{GM}(z)$ and the Redi diffusivity $K_R(z)$ . Second, as identified already by previous authors, the vertical structures of the two coefficients are related by the so-called Taylor–Bretherton relation. Finally, while the two coefficients differ generically in the interior of the water column, we show that they are equal to one another near the surface and near the bottom of the domain for low-enough dissipative coefficients. We illustrate these findings by simulating numerically the QG dynamics of a horizontally homogeneous patch of ocean hosting a vertically sheared zonal current resembling the Antarctic Circumpolar Current.

Sensitivity of Seawater Temperature and Salinity to their Restoring Times, Appending in the Boundary Conditions for these Variables at the Free Surface of the Laptev Sea in the No-Ice Period

In order to study sensitivity of seawater temperature and salinity in the no-ice Laptev Sea to their restoring times, appending in the boundary conditions for these variables at the free surface, the 3D finite-element hydrostatic model QUODDY-4 and the indirect means of describing internal tidal waves have been applied. The latter is based on the use of the corrected eddy diffusivity representing the sum of the same uncorrected diffusivity and the diapycnal diffusivity. The first of them characterizes the influence of non-tidal factors, computed using 2.5-level eddy closure scheme, the second, determined by the ratio of internal tidal waves induced baroclinic tidal energy dissipation to the buoyancy frequency in square, the influence of purely tidal factor. The estimate of this dissipation obtained from a solution of auxiliary problem on the internal tidal waves dynamics and energetics is attached. Seawater temperature and salinity in the subsurface and near-bottom layers of the Sea and also their vertical distributions along the 120E meridional transection obtained for the strong, moderate, and mixed restoring times are discussed. As a result, it is clarified that seawater temperature and salinity are weakly sensitive to changes in the restoring time. The saying follows from a comparison of model averaged (over a chosen period and by the Sea area) values of seawater temperature and salinity, and from their local vertical profiles, computed with regard to accepted estimates of the restoring time.