OS1.4

The overturning of the ocean has been classically described by sinking at high latitudes and upwelling of deep water in the ocean interior. However, measurements showing bottom enhanced mixing have suggested that the ocean interior experiences downwelling, and it has been recently proposed that the upwelling of deep water should arise over sloping boundaries. The Bottom Boundary Layer Turbulence and Abyssal Recipes project was set up to test this paradigm in the Rockall Trough, a natural laboratory of the deep ocean overturning. We conducted a tracer experiment that began by the injection of 15 kg of long lived inert SF5CF3 on the deep part of a tidal canyon in July 2021. The injection was performed in the bottom boundary layer, ~7 meters above the bottom, along streaks between 1800 m and 2000 m depth, tagging water at potential temperature of 3.6°C within a temperature window of 0.1°C. Within 24 hours we started the tracer survey along the full canyon length for two weeks (totalling 81 stations) and we report here on the integrated diapycnal fluxes (upwellings and downwellings) at key locations between 900 m and 2600 m depth, at different time steps from neap to spring tides. The tracer dispersion along the canyon unprecedently documents a rapid diapycnal upwelling of the tracer ranging from 50 to 300 meters per day driven by tidal mixing implying an overturning circulation. As the tracer evolved in the canyon under tidal sloshing, its leading edge was detected reaching 8.5°C at the canyon head as we entered spring tides. We will also report  on the tracer chase outside of the canyon   to explore the contribution of sloping boundary mixing to ventilation at the scale of the Rockall Trough.
 

How to cite: Messias, M.-J., Mercier, H., Ledwell, J., Naveira Garabato, A., Ferrari, R., and Alford, M.: Diapycnal fluxes and overturning from a tracer release experiment in a tidal canyon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7924, https://doi.org/10.5194/egusphere-egu22-7924, 2022.

How to cite: Goris, N., Johannsen, K., and Tjiputra, J.: Gulf Stream and Deep Western Boundary Currents are key to constrain the future North Atlantic Carbon Uptake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6957, https://doi.org/10.5194/egusphere-egu22-6957, 2022.

The Labrador Sea plays a central role in the oceanic storage of carbon. In particular, several studies have shown that this region has amongst the highest integrated column inventories of anthropogenic carbon (C_ant) in the world’s ocean. The rate at which C_ant is stored in this region appears to be connected to changes in ocean circulation and can therefore vary over time. Nevertheless, it is still unclear whether the temporal variability of the total Dissolved Inorganic Carbon (DIC) inventory is solely due to the changes in C_ant concentrations or whether there is a contribution of the natural component of DIC to this signal.

The Bedford Institute of Oceanography has been maintaining the Atlantic Zone Off-Shore Monitoring Program (AZOMP) in the Labrador Sea since the early 1990s. The AZOMP involves annual occupations of the AR7W line that crosses the Labrador Sea and includes sampling of DIC, as well as multiple transient tracers such as CFC-12 and SF₆.  

By using observations of DIC along the AR7W line, as well as previous estimates of C_ant obtained with transient tracers (using a refined version of the Transit Time Distribution method; TTD) and new estimates of C_ant based on the extended Multiple Linear Regression (eMLR) method, we provide a first insight on the role that the natural component of DIC plays in the temporal variability of inorganic carbon in the central Labrador Sea between 1993 and 2016.

We show that different methods to estimate C_ant can lead to different conclusions on the role of the natural variability of DIC and that these discrepancies could be related to the assumptions implied in the C_ant estimation methods. In particular an analysis of C_ant estimates obtained with our refined version of the TTD method in different water masses, highlighted that further refinement of the tracers’ saturation assumption could be necessary in this region. This refinement could reconcile the C_ant estimates from the two methods and therefore lead to an unambiguous role of the natural DIC in this region.

How to cite: Raimondi, L., Tanhua, T., Azetsu-Scott, K., and Wallace, D.: Does the Natural DIC Affect the Storage of Total Inorganic Carbon in the Central Labrador Sea?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11982, https://doi.org/10.5194/egusphere-egu22-11982, 2022.

The North Atlantic is one of the hot-spot for ocean oxygen ventilation due to cold surface water and strong winter convection. This region is subjected to large interannual to multidecadal variability, which is suspected to strongly impact the regional and temporal oxygen ventilation and inventory.
Here we investigate the oxygen variability over 1991-2018 and driving mechanisms of the two main water masses of the Irminger Sea: the Labrador Sea Water (LSW) and the Island Scotland Overflow Water (ISOW). For this, we combined the most recent Argo dataset with ship-based hydrographic data in the Irminger Sea. The dissolved oxygen concentration of the LSW oscillated between 300 mu mol/kg in the early 90's and between 2016 and 2018, and 280 mu mol/kg in the period 2002-2015. The temporal changes in oxygen concentration are less pronounced in the underlying Iceland Scotland Overflow Water (ISOW).
We show that, while solubility changes partly explain the variability of the dissolved oxygen concentration within the Labrador Sea Water (LSW), the main driver of oxygen variability is the Apparent Oxygen Utilisation (AOU). 
In the early 90's and between 2015 and 2018, the deep convection was more intense and led to less stratified, thicker, colder, and more oxygenated LSW than during the period 1995-2015. This was attributed to larger ocean heat loss, stronger wind stress, and colder subpolar gyre under positive NAO conditions.   
The observed oxygen variability in the Irminger Sea between 1991 and 2018 does not show any significant linear trend. This study provides the first observational evidence of the impact of the subpolar gyre decadal variability on the oxygen ventilation in the Irminger Sea and advocates for continuing the monitoring of oxygen concentration and content in the subpolar gyre to separate any possible warming-induced long-term changes from the large decadal natural variability.

How to cite: Feucher, C., Portela, E., Kolodziejczyk, N., and Thierry, V.: Subpolar gyre decadal variability explains the recent oxygenation in the Irminger Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3692, https://doi.org/10.5194/egusphere-egu22-3692, 2022.

Subantarctic Mode Waters (SAMW), forming in the deep winter mixed layers in the Subantarctic Zone (SAZ) to the north of the Antarctic Circumpolar Current (ACC), connect the ocean thermocline with the atmosphere, contributing to ocean carbon and heat uptake and transporting high-latitude nutrients northward, to fuel primary production at low latitudes. Many aspects of SAMW formation are poorly understood due to the data scarcity during Austral winter. Here, we use biogeochemical Argo float observations to investigate the seasonal development, origin and significance of a subsurface salinity maximum in the SAMW formation regions. This conspicuous feature develops every summer in the seasonal thermocline of the SAMW formation regions as a consequence of the advection along the ACC of warmer and saltier waters from the western boundaries of the subtropical gyres, in particular the Agulhas Return current. The salinity maximum acts as a gatekeeper for SAMW ventilation, since it controls the seasonal evolution of stratification at the base of the mixed layer, modulating its rate of deepening during autumn and winter and re-stratifying the SAMW pool when winter mixing ceases. We also show that the subtropical influx, often overlooked, is key to understand the variability of SAMW properties, since it represents a leading order term in the heat and salt budgets at the formation regions. Finally, the analysis of the nitrate seasonal cycle at the SAMW formation regions as recorded by the Argo floats, revealed that the seasonal salinity increase goes along with a decrease in the concentration of this nutrient, as a consequence of the advection of subtropical waters containing low preformed nitrate. These results suggest that nutrient concentration in SAMW is controlled not only by the rate of upwelling of high-nutrient waters south of the ACC and the degree of biological drawdown during their northward transit, as frequently assumed, but also by the influx of subtropical waters, pointing to previously overlooked feedbacks in the redistribution of nutrients between high and low latitudes.

How to cite: Fernández Castro, B., Mazloff, M., Williams, R. G., and Naveira Garabato, A.: Subtropical contribution to Subantarctic Mode Waters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-101, https://doi.org/10.5194/egusphere-egu22-101, 2022.

Ocean gliders enable us to collect the ocean microstructure observations necessary to calculate the dissipation rate of turbulent kinetic energy, ε, on timescales of weeks to months: far longer than is normally possible using traditional ship-based platforms. Slocum gliders have previously been used to this end;  here, we report the first detailed estimates of ε calculated using observations collected by a Seaglider. Seaglider 620 was deployed in the western tropical Atlantic in early 2020 and was equipped with a FP07 fast thermistor. We use these same fast thermistor observations to calculate ε following the Thorpe scale method. We find very good agreement between estimates of ε calculated following the two methods. The Thorpe scale method yields the larger values of ε, but the average difference, less than an order of magnitude, is smaller than reported elsewhere. The spatio-temporal distribution of ε is comparable for both methods. Maximum values of ε (10^-7 W kg^-1) are observed in the surface mixed layer; relatively high values (10^-9 W kg^-1) are also observed between approximately 200 and 500 m depth. These two layers are separated by a 100 m thick layer of low ε (10^-10 W kg^-1), which is co-located with a high-salinity layer of Subtropical Underwater and a peak in the strength of stratification (i.e. in N²). We calculate the turbulent heat and salt fluxes associated with the observed turbulence that act to ventilate deeper layer of the ocean. Between 200 and 500 m, ε induces downward (i.e. negative) fluxes of both properties that, if typical of the annual average, would have a very small influence on the heat and salt content of the salinity-maximum layer above. We compare these turbulent fluxes with estimates of fluxes due to double diffusion, having objectively identified those regions of the water column where double diffusion is likely to occur. While the downward heat flux due to double diffusive mixing is lower than that due to mechanical mixing, the downward salt flux due to double diffusive mixing is six times greater.

How to cite: Sheehan, P., Damerell, G., Leadbitter, P., Heywood, K., and Hall, R.: Turbulent kinetic energy dissipation rate and attendant fluxes in the western tropical Atlantic estimated from ocean glider observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2848, https://doi.org/10.5194/egusphere-egu22-2848, 2022.

Oxygen is an essential component of the ocean biogeochemistry.  Relatively small variations in its content may have a significant impact on ocean productivity, biodiversity and fisheries and thus affect ocean health and ecosystem services.  Over the last decade, several studies have shown that regions with low oxygen concentrations are expanding over the world's oceans, a phenomenon which has been termed ocean deoxygenation. These changes are driven by a combination of anthropogenic climate change and the natural variability of the ocean. As climate change warms the upper ocean it reduces oxygen solubility,  increases upper ocean stratification and thus reduces oxygen mixing as well as induces changes in respiration rates. Disentangling the natural and anthropogenically-induced oxygen variability requires the use of models as prognostic or diagnostic tools, as they can be forced with different conditions which may or may not include the effects of climate change and allow a detailed examination of specific processes. In this work,  we compare two basin-scale coupled physical-biogeochemical simulations of the Tropical Atlantic ocean at different horizontal resolutions and show that more robust zonal jets at intermediate depths in the higher resolution simulation have a major impact on the overall structure of the North and South Atlantic OMZs by limiting their westward extent and supplying oxygen to the OMZ core regions between 300 m and 500 m. 

How to cite: R. Calil, P. H.: The Impact of Zonal Jets on the Atlantic Oxygen Minimum Zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4856, https://doi.org/10.5194/egusphere-egu22-4856, 2022.

Understanding what sets the T--S relation within the thermocline, and
how long and what volume of ventilated waters in each T--S class stay in the sub-surface
thermocline is a key question for climate prediction. In particular the sparsity of
the T--S distribution has been a puzzle since the days of
Stommel. Here we use runs performed for the TICTOC project, in which water is labelled by its
year of ventilation and its source region, to understand how the
volumetric T--S relation is laid down year on year, and  evaluate the
importance of near-surface (mostly vertical) mixing in the first year of ventilation
against longer term mixing (much of which is isopycnal) in specifying the T--S distribution.

How to cite: Nurser, A. J. G. and Marzocchi, A.: Using dye tracers to understand the development of the T–-S structureof the ocean thermocline, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1535, https://doi.org/10.5194/egusphere-egu22-1535, 2022.

Ocean ventilation provides the primary control of how the ocean takes up  excess carbon and heat supplied to the earth system due to carbon emissions. Ventilation involves an atmospheric source supplying a tracer to the mixed layer, which is then physically transported into the thermocline and deep ocean by the ocean circulation. For this physical transfer of tracer, there are two characteristic timescales: (i) a fast adjustment controlled by the depth of the mixed layer and (ii) a slow adjustment controlled by the rate of mass transfer to the ocean interior. However, this physical transfer is modified for heat and carbon by climate feedbacks and carbonate chemistry respectively. Here, we use a conceptual 2-dimensional ocean model that is designed to address the ocean adjustment to carbon emissions on yearly to multi-centennial timescales. The model includes  a source, an ocean mixed-layer and interior adjustments, and a feedback mechanism that includes a surface temperature feedback  (such as from clouds) and the effects of carbonate chemistry; the model ignores any seasonality, biological processes and chemical weathering. Using this conceptual model, we reveal  the similarities and differences in how ventilation controls the uptake of heat and carbon involving changes in how the fast and slow adjustments are controlled.  In summary, despite the physical transfer of fluid being determined by ocean ventilation, the effects of climate feedbacks and carbonate chemistry lead to differences in the ocean thermal and carbon adjustments to an increase in atmospheric CO₂.

How to cite: Katavouta, A. and Williams, R.: Ventilation controls of ocean heat and carbon uptake: similarities and differences in the response to carbon emissions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1626, https://doi.org/10.5194/egusphere-egu22-1626, 2022.