Mesoscale eddies play an increasingly recognized role on modulating turbulence levels and associated diapycnal fluxes in the ocean, in particular with increased dissipation rates found in anticyclones. In September 2017, the last cruise of the ProVoLo project in the Nordic Seas (https://www.uib.no/en/rg/fysos/97330/provolo) intensively surveyed an energetic mesoscale anticyclone (the permanent Lofoten Basin Eddy) to characterize turbulence of the upper layer and eventually quantify the resulting vertical fluxes nutrients caused by turbulence.

The sampling strategy combined ship-borne measurements and autonomous platforms. The vessel carried out a radial transect with stations spaced by 5 km near the center and 10-20 km outside the eddy with measurements of temperature and salinity (CTD), currents (lowered ADCP) and turbulence (Vertical Microstructure Profiler, VMP2000). Water samples were analyzed to estimate the concentration of the main nutrients (nitrate, phosphate and silicate). In addition, two autonomous oceanic gliders were used. A first glider profiling 0-1000 m deep was completing a 6-month mission. A second glider was specifically deployed during the cruise (5 days). This glider was equipped with a dissolved oxygen Aanderaa optode, a WET Labs FLNTU fluorescence and turbidity sensor and a Rockland Scientific Microrider sampling turbulence. It sampled the surface layer (0-300 m) at high temporal (~30 min) and spatial (~500 m) resolution from about 60 km to 5 km of the eddy center.

By combining those measurements, we characterized the turbulence dissipation rates, vertical diffusion and its associated fluxes across the different nutriclines from the center to the outside region area of the eddy, revealing significant contrasts. Below the thermocline, turbulent patches were observed within the core with dissipation rates elevated by one order of magnitude relative to the values outside. The higher levels of dissipation rates supported 10-fold stronger vertical diffusion coefficients, substantially increasing vertical turbulent fluxes through the nutriclines. The transition between the eddy tangential velocity maximum and the zero vorticity was characterized by a frontal region exhibiting important oscillations of the thermocline, manifesting important vertical exchanges.

This study is not only relevant in a local context, but also has global implications for the ocean energy budget and highlights the need for more high-resolution observations resolving scales from the mesoscale to the dissipation.

How to cite: Bosse, A. and Fer, I.: Contrasts in turbulent vertical fluxes of nutrients across the permanent Lofoten Basin Eddy in the Nordic Seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5808, https://doi.org/10.5194/egusphere-egu22-5808, 2022.

In this study, we assess the ability of the ocean-sea ice general circulation models that participated in the CMIP6 Ocean Model Intercomparison Project (OMIP) to simulate the seasonal cycle of the ocean mixed layer depth in the area of the Arctic Ocean covered by multiyear sea ice. During summertime, all models understimate the mixed layer depth by about 20 m compared to the MIMOC (Monthly Isopycnal/Mixed layer Ocean Climatology) observational data. The origin of this systematic bias is unclear. In fall and winter, differences of several tens of meters are noticed between the models themselves and between the models and the observational data. Some models generate too deep mixed layers, while others produce too shallow mixed layers. Since the mixed layer deepening in ice-covered regions during these seasons is largely controlled by the brine rejection associated with ice growth, the discrepancies between models might be related to differences in the modelled sea ice mass balance. However, a detailed model comparison reveals that this is not the case, all models simulating more or less the same sea ice mass balance and thus salt flux into the ocean during sea ice freezing. By applying to model outputs the analytical model developed by Martinson (1990), that allows in particular to determine the main processes responsable for maintaining stablility in polar oceans, it is finally found that most of the disagreement between models can be explained by the accuracy with which the Arctic halocline is reproduced by those models. This feature is simulated generally poorly and quite differently from one model to another, and models with less stratified halocline generally lead to deeper mixed layers. It now remains to identify the model deficiencies responsible for this situation.

How to cite: Allende, S., Fichefet, T., and Goosse, H.: On the ability of CMIP6 OMIP models to simulate the seasonalcycle of the ocean mixed layer depth in the central Arctic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2231, https://doi.org/10.5194/egusphere-egu22-2231, 2022.

The ocean around Iceland witnesses some of the most important transformations of water masses that drive the Global Ocean Circulation. Here, we analyze 28 years of continuous four-yearly hydrographic sections around Iceland from 1990 to 2018. The water-mass properties around Iceland show important spatial variability. From their temperature, salinity and stratification structure, we classified the Icelandic waters in three distinct regions with similar characteristics: the Southwest, the North and Northeast regions. The warm and salty Atlantic Waters that dominate the Southwest show the deepest winter mixed layer (~500m) while the North and Northeast have relatively shallow (< 100m) to moderate (~100m) winter mixed layer depth.  
Based on the decomposition of the total stratification into temperature and salt contributions, we find that the subsurface summer stratification is mainly dominated by temperature except for the North and Northwest regions where salinity dominates. 

The interannual variability of the mixed layer and its water properties is also large around Iceland. Mixed layer waters were generally colder in the 90's, then warmed until approximately 2015, and became colder again from 2015 to 2018.  Except for the southwestern region, the observed interannual variability seems unrelated with the North Atlantic Oscillation, and its main forcing remains an open question to address in future studies. Only in the northeastern region a multidecadal mixed layer warming trend clearly emerges from the interannual variability. This is associated with the Atlantification of the Arctic, which is also observed from the northward displacements of the isotherms derived from satellite SST. Elsewhere, rather than clear trends, we observe changes in the structure of the mixed layer temperature and salinity that compensate in density.  The present study provides an unprecedented and detailed regional description of the seasonal to decadal variability of the mixed layer depth and the stratification, and their link with the changing North Atlantic under global warming.

How to cite: Ruiz-Angulo, A., Portela, E., Perez-Hernandez, M. D., Ólafsdóttir, S. R., Macrander, A., Meunier, T., and Jonsson, S.: Impact of Ocean Warming and Natural Variability on the Stratification and Mixed Layer Depth around Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11925, https://doi.org/10.5194/egusphere-egu22-11925, 2022.

Water mass transformation in the Southern Ocean is vital for closing the large-scale overturning circulation, altering the thermohaline characteristics of upwelled Circumpolar Deep Water before returning to the ocean interior. Using profiling gliders, this study investigates how buoyancy forcing and wind-driven processes lead to intraseasonal (1-10 days) variability of the mixed layer temperature and salinity in three distinct locations associated with different Southern Ocean regions important for water mass transformation - the Subantarctic Zone (SAZ, 43°S), Polar Frontal Zone (PFZ, 54°S) and Marginal Ice Zone (MIZ, 60°S). Surface heat fluxes drive the summertime mixed layer buoyancy gain in all regions, particularly evident in the SAZ and MIZ, where shallow mixed layers and strong stratification further enhance mixed layer warming. In the SAZ and MIZ, the entrainment of denser water from below is the primary mechanism for reducing buoyancy gain. In the PFZ, turbulent mixing by mid-latitude storms result in consistently deep mixed layers and suppressed mixed layer thermohaline variability. Intraseasonal mixed layer salinity variability in the polar regions (PFZ and MIZ) is dominated by the lateral stirring of meltwater from seasonal sea ice melt. This is evident from early summer in the MIZ, while in the PFZ, meltwater fronts are proposed to be dominant during late summer, indicating the potential for seasonal sea ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of mixed layer thermohaline properties to small spatio-temporal processes, which suggests a similar regional dependence to surface water mass transformation in the Southern Ocean.

How to cite: du Plessis, M., Swart, S., Biddle, L. C., Giddy, I. S., Monteiro, P. M. S., Reason, C., Thompson, A. F., and Nicholson, S. A.: The daily-resolved Southern Ocean mixed layer: regional contrasts assessed using glider observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-917, https://doi.org/10.5194/egusphere-egu22-917, 2022.

Heating rates induced by optically significant water constituents (OACs), e.g. phytoplankton and coloured dissolved organic matter (CDOM), contribute to the seasonal modulation of thermal energy fluxes across the ocean-atmosphere interface in coastal and regional shelf seas. This is investigated in the Western Baltic Sea, a region characterised by considerable inputs of nutrients, CDOM and freshwater, and complex bio-optical and hydrodynamic processes. Using a coupled bio-optical-ocean model (ROMS-BioOptic), the underwater light field is spectrally-resolved in a dynamic ocean and the inherent optical properties of different water constituents are modelled under varying environmental conditions. We estimate the relative contribution of these water constituents to the divergence of the heat flux and heating rates and find that phytoplankton dominates absorption in spring, while CDOM dominates absorption in summer and autumn. In the Pomeranian Bight, water constituent-induced heating rates in surface waters are estimated to be up to 0.1^oC d^-1 in spring and summer, predominantly as a result of increased absorption by phytoplankton and CDOM, respectively during these periods. Warmer surface waters are balanced by cooler subsurface waters. Surface heat fluxes (latent, sensible and net longwave) all increase in response to warmer sea surface temperatures. We find good agreement between our modelled water constituent absorption, and in situ and satellite observations. More rigorous co-located heating rate calculations using an atmosphere-ocean radiative transfer model provide further evidence of the suitability of ROMS-BioOptic model for this purpose. The study shows that seasonal and spatial changes in optically significant water constituents in the Western Baltic Sea have a small but noticeable impact on radiative heating in surface waters and consequences for the exchange of energy fluxes across the air-sea interface and the distribution of heat within the water column. The importance of the light attenuation coefficient, K_d, in shelf seas as a bio-optical driver which provides a pathway to estimating heating rates and connects biological activity with energy fluxes is highlighted.

How to cite: Cahill, B., Graewe, U., Kritten, L., Wilkin, J., and Kowalczuk, P.: Seasonal impact of optically significant water constituents on radiative heat transfer in the Western Baltic Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12610, https://doi.org/10.5194/egusphere-egu22-12610, 2022.

The equation of state of seawater determines how density varies with temperature and salinity. Although it has long been known that the equation of state is nonlinear, there seems to be an overall feeling in the physical oceanography community that associated effects might be secondary in importance. This can be seen for example from the fact that most current theories of the large-scale circulation pre-assume a linear equation of state. Yet we contend here that these nonlinearities are responsible for the main transition in mixed layer properties observed in the World Ocean, the one separating so-called alpha regions (stratified by temperature) and beta regions (stratified by salinity). Beta regions are characterized by a halocline shielding surface cold waters from the influence of warmer deep waters, a condition for sea ice to form in polar region. Through numerical experiments where different equations of state are tested, we show that nonlinear effects of the equation of state: 1) strongly modulate surface buoyancy forcings, especially in mid- to high-latitudes, 2) generate the polar halocline by reducing there the influence of temperature on density, and consequently 3) enables sea ice formation in polar regions. The main nonlinear effect comes from the fact that the thermal expansion coefficient reduces to nearly zero at the freezing point, decreasing drastically the influence of surface cooling on the polar stratification. Other nonlinear effects, such as cabbeling or thermobaricity, are found of lesser importance although they have historically been the focus of intense research.

How to cite: Roquet, F., Ferreira, D., Caneill, R., and Madec, G.: How nonlinearities of the equation of state of seawater generate the polar halocline and promote sea ice formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1109, https://doi.org/10.5194/egusphere-egu22-1109, 2022.

Climatologies of the mixed layer depth have been provided using several definitions based on temperature/density thresholds or hybrid approaches. The upper ocean pycnocline (UOP) that sits below the mixed layer base, sometimes referred to as the transition layer or as the seasonal pycnocline, remains poorly characterised though it is an ubiquitous feature of the ocean surface layer. The UOP often consists in a rapid change in density with depth and enhanced vertical shear that connects the well-mixed surface layer to the stratified ocean interior. The UOP is important for the ventilation of the ocean as it represents a barrier to mixing between the upper ocean and the ocean interior.

Available hydrographic profiles (e.g., Argo, CTD on marine mammals) provide near-global coverage of the world's oceans and allow the characterisation of spatial and seasonal variations of the upper ocean vertical stratification, including the UOP. Based on these profiles, we estimate the depth, thickness and intensity of the UOP, and assess when and where the UOP can be considered as a layer with constant thickness. We provide monthly maps of the UOP complementing the available MLD climatologies and we compare the UOP characteristics with the depth and stratification of the mixed layer. We  aim at assessing the UOP intensity in winter and spring when the stratification is usually weak and submesoscale vertical motions can penetrate below the mixed layer base. During these seasons, the UOP intermittency must be taken into account because restratification may occur with intermittent events.

How to cite: Sérazin, G., Tréguier, A.-M., and de Boyer Montégut, C.: A seasonal climatology of the upper ocean pycnocline, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11443, https://doi.org/10.5194/egusphere-egu22-11443, 2022.

How to cite: Greig, L. and Ferreira, D.: Submesoscale eddies and sea ice interaction , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3968, https://doi.org/10.5194/egusphere-egu22-3968, 2022.

We reconstruct the 3-D meso- and submesoscale structure of selected oceanic eddies from ship-based field observations of current velocity, in the mixed layer and below, in order to explore two main questions: what information on upwelling/downwelling can be derived; and inside what eddy radius is water trapped and transported.

The selected eddies have been intensively surveyed during the collaborative project REEBUS (Role of Eddies in the Carbon Pump of Eastern Boundary Upwelling Systems) in the eastern tropical North Atlantic. Making use of vertical sections of current velocities we fit an optimum eddy-like structure to the data. The structure is assumed a slowly drifting, circular symmetric but not necessarily linear velocity field, separated in horizontal layers. The composition of the reconstructed layers provides a 3-D velocity structure which is used to calculate derived variables as vorticity and divergence. We find submesoscale divergence patterns which support vertical flux occurring in the eddies. We further use current velocities from a high-resolution regional model based on ROMS to validate the method and estimate uncertainties.

How to cite: Fischer, T., Karstensen, J., Dengler, M., Onken, R., and Holzapfel, M.: Reconstructing meso- and submesoscale dynamics in ocean eddies from current observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12181, https://doi.org/10.5194/egusphere-egu22-12181, 2022.

It has long been appreciated that Ekman transport and pumping velocities are modified through interactions with underlying geostrophic currents. Nonlinearity involving interaction of the Ekman flow with itself is, however, typically neglected. This nonlinearity occurs when the Rossby number based on the Ekman velocity and horizontal length scale approaches order one values. Such values are common, for example, in the ice-ocean stress field across sharp gradients such as leads in the sea ice cover. Recent work has shown strong asymmetry in the pumping velocities, with cyclonic forcing producing diffuse upwelling and anticyclonic forcing producing sharp downwelling fronts. To better understand this dynamics, we consider the steady response to a simple specified prescription of the stress. In the (x-z) plane perpendicular to the stress, dynamics are described by the 2-D Navier-Stokes equation, with a forcing term dependent on vertical shear of velocity in the y-hat direction, specified by a pressureless momentum equation. An expansion in an Ekman-velocity based Rossby number is used to solve the system and to better understand the asymmetry. Interactions with stratification and underlying geostrophic currents are also considered, and examples of where these effects might be important are given.

How to cite: Rahnamaei, N. and Straub, D.: Intense Downwelling and Diffuse Upwelling in a Nonlinear Ekman Layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10579, https://doi.org/10.5194/egusphere-egu22-10579, 2022.

In fall 2020 and 2021, two field surveys examined the water column dynamics and surface mixing in a shallow lagoon, Szczecin (Stettin) Lagoon, located at the border between Germany and Poland. This was part of a larger experiment, looking into water column and air-sea interactions, and momentum fluxes, but this study is focused on how the presence of proposed Langmuir circulation affects the carbon and oxygen dynamics, and primary production in this shallow lagoon.

Measurements were collected from a station in Szczecin Lagoon, located near the Polish border, with water depth of about 4 meters. Measurements at and around the station were made using mobile FerryBox systems, or Pocket FerryBoxes, which measured almost continuously water temperature, salinity, dissolved oxygen, chlorophyll fluorescence, pH, turbidity, colored dissolved organic matter (CDOM) and in 2021 partial pressure of CO2 (pCO2). In addition, water column measurements of currents (ADCP) and water level were available, as well as surface drifters, and drone aerial measurements.

We found that during low wind conditions, the water column was well-mixed to a depth controlled by expected Langmuir cells, and bottom waters below this depth were quite different in most of the biogeochemical parameters measured. Therefore Langmuir circulation most likely controlled water column structure in large regions of the Szczecin Lagoon, consequently influencing the community, carbon and dissolved gas distributions in this shallow lagoon, and most likely the air-sea gas exchange rate. Only during short storm events, these conditions changed, and the water column structure and concentrations of biogeochemical parameters were altered.

How to cite: Voynova, Y. G., Buckley, M. P., Stresser, M., Cysewski, M., Bödewadt, J., Gehrung, M., and Horstmann, J.: Effect of Langmuir circulation on mixing and carbon dynamics in a shallow lagoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8271, https://doi.org/10.5194/egusphere-egu22-8271, 2022.