Description: In contrast with the atmosphere, which is heated from below by solar radiation, the ocean is both heated and cooled from above. To drive a deep-reaching overturning circulation in this context, it is generally assumed that either intense interior mixing by winds and internal tides, or wind-driven upwelling is required; in their absence, the circulation is thought to collapse to a shallow surface cell. We demonstrate, using a primitive equation model with an idealized domain and no wind forcing, that the surface temperature forcing can in fact drive an inter-hemisphere overturning provided that there is an open channel unblocked in the zonal direction, such as in the Southern Ocean. With this geometry, rotating horizontal convection, in combination with asymmetric surface cooling between the north and south, drives a deep-reaching two-cell overturning circulation. The resulting vertical stratification closely resembles that of the real ocean, suggesting that wind-driven pumping is not necessary to produce a deep-reaching overturning circulation contrary to common belief, and that buoyancy forcing plays a much more active role than is usually assumed.

Description: The data loggers attached to marine mammals represent an instrumentation type which provided a significant amount of temperature profiles for the moderate and polar regions since the 2000s. In the World Ocean Database WOD18 marine mammal data contribute ~60% of temperature profiles south of 50〈sup〉o〈/sup〉S for 2005 – 2018 and ~40% of profiles north of 40〈sup〉o〈/sup〉N being a significant data source for the estimation of the global ocean warming along with the data from ship-based CTDs and Argo floats. Use of the marine mammal data for the ocean heat content estimation requires the assessment of possible instrumental biases. For the first time we assess temperature biases in marine mammal data by comparing these data with temporarily and spatially collocated reference temperature profiles from Argo floats and ship-based CTDs. For the SRDL recorders implemented mostly in the Southern Ocean our estimates indicate a prevailing time- and sensor type dependent thermal negative temperature bias within the range -0.01 to -0.04〈sup〉o〈/sup〉C. The less accurate TRD recorders implemented earlier on mammals in the North Pacific are characterized by a different bias pattern, showing the predominantly positive bias of about 0.08 to 0.10〈sup〉o〈/sup〉C below 100 m. The derived total TRD bias can be decomposed into a small positive thermal bias (~0.02〈sup〉o〈/sup〉C) and the bias due to the systematic error in pressure (depth). Based on the results of the study we provide corrections for instrumental temperature biases both in SRDL and TRD recorders.

Description: The region of Maud Rise, a seamount in the Weddell Sea, is known for the occurrence of irregular polynya openings during the winter months. Hydrographic observations have shown the presence of a warmer water mass below the mixed layer along the seamount’s flanks, commonly termed the warm-water Halo, surrounding a colder region above the rise, the Taylor Cap. A combination of observational data sets, an eddy-permitting reanalysis product and a regional high-resolution configuration allows to investigate the properties of these features and shows a strong interannual variability. A warming of deep waters is observed in the Taylor Cap during the years preceding the opening of the Polynya in 2016 and 2017, starting in 2011. By analyzing regional simulations, we are able to demonstrate that most of the observed variability in the warm-water Halo is forced remotely by advection of Weddell Gyre deep waters in the region surrounding Maud Rise. Deep water anomalies found in the Halo are retroflected along the inner flank of the Halo and from there transported into the Taylor Cap by eddies within the next six months. In addition to the preconditioning of the ocean preceding the Polynya opening, we investigate the role of thermobaricity in the region of Maud Rise in the period before the Polynya opening by implementing the eddy-diffusivity mass-flux parameterization in 1D simulations and the regional configuration.

Description: The presentation will focus around the impact that major topographic features in the Southern Ocean have on the path of the Antarctic Circumpolar Current (ACC), and their relatively unknown role in the coupled ocean-atmosphere system. We investigate their influence by respectively flattening the bathymetry around the Kerguelen Plateau, Campbell Plateau, Mid-Atlantic Ridge, and Drake Passage in four simulations in a coupled climate model. The barriers lead to an increase in the ACC transport of between 3% and 14% in the four simulations. The removal of Kerguelen Plateau bathymetry increases convection south of the plateau and the removal of Drake Passage bathymetry reduces convection upstream in the Ross Sea, affecting the deep overturning cell. When the barriers are removed, zonal flattening of the currents leads to SST anomalies upstream and downstream of their locations. Interestingly, these SST anomalies strongly correlate to precipitation in the overlying atmosphere, with correlation coefficients ranging between r=0.92 and r=0.97 in the four experiments. Windspeed anomalies are also positively correlated to SST anomalies in some locations but other forcing factors obscure this correlation in general. Meridional variability in the wind stress curl contours over the Mid-Atlantic Ridge, the Kerguelen Plateau and the Campbell Plateau disappears when these barriers are removed, confirming the impact of bathymetry on overlying winds.

Description: In this study, we examine the factors that influence the stratification of the upper ocean pycnocline (UOP), focusing on the Southern Ocean. The UOP is a critical layer just below the mixed layer whose stratification controls the exchange of properties between the ocean and the atmosphere. We classify the UOP regions according to the relative contributions of temperature and salinity in stabilizing the layer, resulting in three categories: alpha (temperature-stabilized), beta (salinity-stabilized), and transition (temperature- and salinity-stabilized) zones. Our analysis uses observational profiles from the EN4.2 database and calculates annual mean buoyancy fluxes by combining existing heat and freshwater flux products. Ekman transport is taken into account as an additional term in the buoyancy flux. In the Southern Ocean, the deep mixed layers are located on the southernmost flank of the alpha region, with the exception of the southeastern Pacific sector where they are located in the polar transition zone. Regions with negative buoyancy flux exhibit mixed layer deepening along the water path, but deep mixed layers only form when the buoyancy flux is negative along the entire path. Ekman transport contributes to the formation of deeper mixed layers throughout the Southern Ocean by bringing cold water northward. Overall, our results reveal that the boundaries between alpha, transition, and beta regions are generally consistent with more traditional definitions of fronts and provide an overview of upper ocean pycnocline stratification in the Southern Ocean.

Description: The ocean's internal pycnocline is a layer of elevated stratification that separates the well-ventilated upper ocean from the more slowly-renewed deep ocean. Despite its pivotal role in organizing ocean circulation, the processes governing the formation of the internal pycnocline remain little understood. Classical theories on pycnocline formation have been couched in terms of temperature and it is not clear how the theory applies in the high-latitude Southern Ocean, where stratification is dominated by salinity. Here we assess the mechanism generating the internal pycnocline in the subpolar Southern Ocean through the analysis of a high-resolution, realistic, global sea ice-ocean model. We show evidence suggesting that the internal pycnocline's formation is associated with seasonal sea ice-ocean interactions in two distinct ice-covered regions, fringing the Antarctic continental slope and the winter sea-ice edge. In both areas, persistent sea-ice melt creates strong, salinity-based stratification at the base of the surface mixed layer in winter. The resulting sheets of high stratification subsequently descend into the ocean interior at fronts of the Antarctic Circumpolar Current, and connect seamlessly to the internal pycnocline in areas further north in which pycnocline stratification is determined by temperature. Our findings thus suggest an important role of localized sea ice-ocean interactions in configuring the vertical structure of the Southern Ocean.

Description: Air-sea-ice fluxes of heat, freshwater, and carbon in the Southern Ocean are critical to global ocean circulation and climate. During the ice-free summer months, carbon-rich Warm Deep Water (WDW) around Maud Rise in the eastern Weddell Sea is insulated from the atmosphere by a surface mixed layer and residual Winter Water layer. Fluxes across the mixed layer base control exchanges between WDW and the atmosphere, as well as determining water column stability in early winter at the onset of sea ice formation. A weakly stratified winter mixed layer could be susceptible to overturning, leading to ventilation of heat from WDW and the possible formation of an open ocean polynya, as observed over Maud Rise in the 1970s and 2016-17.We use high-resolution observations from autonomous platforms deployed during the 2022 SO-CHIC campaign to analyze the summer to winter mixed layer evolution in the Maud Rise region. 6 free-drifting UpTempO buoys collected over 12,500 hourly temperature profiles of the upper 60 m of the water column between January and June 2022. In addition, a Seaglider and Sailbuoy were deployed around Maud Rise from January to April, and 4 Argo floats remained in the region through austral summer 2023. We use ERA5 atmospheric reanalysis data to model the effect of 1-D processes on upper ocean variability, in order to distinguish the influence of synoptic storm events from submesoscale dynamics and quantify the integrated impact of each of these processes on mixed layer stratification at the onset of sea ice freeze-up.