ALBERT

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: Understanding the drivers and physical processes influencing Antarctic sea ice and being able to predict Antarctic sea ice is crucial for improving our current climate projections. We investigate Antarctic sea ice predictability using a high resolution global coupled ocean-sea ice model and evaluate it against observations. We explored the physical processes in the upper ocean underlying sea ice predictability and found that memory in the upper ocean resides largely within the winter water layer. The intensity and the depth to which ocean memory forms in a region is controlled by upper ocean vertical structure. Ocean memory responds to seasonal changes in the upper ocean, especially to the changes in stratification strength, driven mainly by sea ice processes. Our results present sea ice predictability as a signature of local ice-ocean interaction. In our regional analysis, modelled and observed sea ice predictability diverge in most regions. Modelled sea ice produced higher predictability skills in summer and spring months, whereas in the observations, it is in winter. Assessing the spatial distribution of sea ice predictability skills, model and observation produced similar seasonal patterns, however, the predictability skill in the model was considerably higher than observation. Given, the sparsity of continuous ocean observation, especially under Antarctic sea ice, it is hard to pin point the differences in the representation of ocean conditions in these high latitudes between the model and observation. However, our findings indicate that better representation of upper ocean water column and its seasonal processes can improve Antarctic sea ice predictability skills.

Description: The ocean's global overturning circulation is a network of currents that redistributes vast amounts of heat, carbon and nutrients across the planet. It is traditionally conceptualized as a conveyor belt that carries and joins all major water masses of the global deep ocean. Here we show that a voluminous mid-depth layer, termed Upper Deep Water, is excluded from the global conveyor belt. This exclusion results from topographic constraints on abyssal overturning, dynamical constraints on upwelling in the Antarctic Circumpolar Current, and ocean-sea ice interactions around Antarctica. The findings establish a revised schematic of the global overturning circulation and a dominant role of diffusive processes in the ventilation of Upper Deep Water.

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.