A document by Bieito Fernández Castro. Click on the document to view its contents.

The Arctic Ocean has been covered by sea ice year-round for much of the past, inhibiting the transfer of momentum from atmosphere to ocean, with the consequence that Arctic Ocean currents are generally slow and turbulent mixing weak. However, recent decades have seen accelerated lower tropospheric warming accompanied by declines in sea ice concentration, thickness and extent, and more recently, changes in the ocean, termed ”atlantification”, are beginning to be observed. Against this background, here we explore the nature of the Arctic Ocean ”double estuary”, whereby (mainly) inflowing Atlantic-sourced waters are transformed into both lighter and denser components in a two-cell density-overturning circulation. The double estuary is quantified using measurements, and a box model is employed to determine the relative significance of surface forcing versus turbulent mixing to water mass transformation. We generate a net Arctic Ocean profile of turbulent diffusivity that is used to test the likely contribution of tides to mixing, and we find that the outcome is most sensitive to mixing efficiency. We note that Arctic Ocean dense water formation adds to the recognised sites of dense water formation in the Nordic Seas and northern North Atlantic. Finally, we discuss how mixing rates may change in future as sea ice declines and the efficiency of atmosphere-to-ocean momentum transfer increases, leading to ocean ”spin-up” and more intense turbulent mixing, and the possible consequences thereof.

Oceanic stratification plays a key role in climate by modulating the ocean’s resistance to be mixed vertically. It has been traditionally assumed that, beyond the deepest reach of winter mixing, stratification is relatively invariant on the time scales of contemporary climate evolution. Here, we test this view by performing a global investigation of the spatio-temporal variability of stratification from the surface to the main pycnocline, using 20 years (2003-2022) of data from the Argo float array. Both near-surface and main pycnocline stratifications are found to exhibit spatially-structured, vertically-coherent, global-scale variations on seasonal-to-decadal time scales, suggesting that both stratifications are a rapidly-evolving, readily-interactive component of the climate system. Variability in stratification is organized into well-defined patterns that replicate the spatial footprints and time scales of major climate modes, pointing to these modes of internal variability as important drivers of stratification changes. Our diagnosed patterns and forcings of stratification variability provide an important benchmark for advancing the climate models used to understand and predict the ongoing climate change

Sub-Antarctic Mode Waters (SAMWs) form to the north of the Antarctic Circumpolar Current (ACC) in the Indo-Pacific Ocean, whence they ventilate the ocean’s lower pycnocline and play an important role in the climate system. With a backward Lagrangian particle-tracking experiment in a data-assimilative model of the Southern Ocean (B-SOSE), we address the long-standing question of whether SAMWs originate from densification of southward-flowing subtropical waters, or lightening of northward-flowing Antarctic waters sourced by Circumpolar Deep Water (CDW) upwelling. Our analysis evidences the co-occurrence of both sources of SAMWs in all formation areas, and strong inter-basin contrasts in their relative contributions. Subtropical waters are the main precursor of Indian Ocean SAMWs (70-75% of particles) but contribute a smaller amount ($<$40%) to Pacific SAMWs, which are mainly sourced by CDW. By tracking property changes along particle trajectories, we show that SAMW formation from northern and southern sources involves contrasting drivers: subtropical source waters are cooled and densified by surface heat fluxes, and freshened by ocean mixing. Southern source waters are warmed and lightened by surface heat and freshwater fluxes, and they are made either saltier by mixing in the case of Indian SAMWs, or fresher by surface fluxes in the case of Pacific SAMWs. Our results underscore the distinct climatic impact of Indian and Pacific SAMWs, as net sources of atmospheric heat and net sinks of freshwater, respectively; a role that is conferred by the relative contributions of subtropical and Antarctic sources to their formation.

The lower cell of the meridional overturning circulation (MOC) is sourced by dense Antarctic Bottom Water (AABW), which forms and sinks around Antarctica and subsequently fills the abyssal ocean. For the MOC to ‘overturn’, these dense waters must upwell through mixing with lighter waters above. Here, we investigate the processes underpinning such mixing, and the resulting water mass transformation, using an observationally forced, high-resolution numerical model of the Drake Passage in the Southern Ocean. In the Drake Passage, the mixing of dense AABW formed in the Weddell Sea with lighter deep waters transported from the Pacific Ocean by the Antarctic Circumpolar Current is catalysed by energetic flows impinging on rough topography. We find that multiple topographic interaction processes act to facilitate mixing of the two water masses, ultimately resulting in upwelling of waters with neutral density greater 28.19 kg m-3, and downwelling of the lighter waters above. In particular, we identify the role of sharp density interfaces between AABW and overlying waters, and find that the dynamics of the interfaces’ interaction with topography can enhance mixing. Such sharp interfaces between water masses have been observed in several parts of the global ocean, but are unresolved and unrepresented in ocean and climate models. We suggest that they are likely to play an important role in abyssal dynamics and mixing, and therefore require further exploration.

Turbulent mixing at the sub-meter scale is an essential component of the ocean’s meridional overturning circulation and its associated global redistribution of heat, carbon, nutrients, pollutants and other tracers. Whereas direct turbulence observations in the ocean interior are limited to a modest collection of field programs, basic information such as temperature, salinity and depth is available globally. Here, we show that supervised machine learning algorithms can be trained on the existing turbulence data to develop skillful predictions of the key properties of turbulence from $T,S,Z$ and topographic data. This constitutes a promising first step toward a hybrid physics-artificial intelligence approach to parameterization of turbulent mixing in climate models.

Major gaps exist in our understanding of the pathways between internal wave generation and breaking in the Southern Ocean, with important implications for the distribution of internal wave-driven mixing, its sensitivity to change, and the necessary ingredients of mixing parameterizations. Here we assess the dominant processes in internal wave evolution by characterizing wave and mesoscale flow scales based on full-depth measurements in a Southern Ocean mixing hot spot and a ray tracing calculation. The exercise highlights the importance of Antarctic Circumpolar Current (ACC) jets as a dominant influence on internal wave life cycles through advection, the modification of wave characteristics via wave-mean flow interactions, and the set-up of critical layers for both upward- and downward-propagating waves. Our findings suggest that it is important to represent mesoscale flow impacts in parameterizations of internal wave-driven mixing in the Southern Ocean.

The propagation characteristics of near-inertial waves (NIWs) and how mesoscale and submesoscale processes affect the waves’ vertical penetration (i.e., the chimney effect) are investigated using observations from a mooring array located in the northeast Atlantic. The year-long observations show that near-inertial motions are mainly generated by local wind forcing and that they radiate predominantly downward following several strong wind events. Once below the mixed layer, NIWs preferentially propagate equatorward primarily in the form of low modes. High-mode NIWs, however, are most likely dissipated locally near the base of the mixed layer. Enhanced near-inertial kinetic energy and vertical shear are found only in mesoscale anticyclones with Rossby number of O(0.1). By contrast, submesoscale motions with order one Rossby number have little effect on the trapping and vertical penetration of NIWs, due to their smaller horizontal scales and confined vertical extent compared to mesoscale eddies.

Diapycnal mixing shapes the distribution of climatically-important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation. The rate of water mass transformation in the Atlantic Ocean’s interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW), with a diapycnal circulation of up to 4 Sv between 24N and 32S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to hundreds of metres in the vertical. This result is confirmed with a zonally-averaged numerical model of the AMOC and indicates that tracer mixing can lead to divergent global pathways and ventilation timescales following the upwelling of tracers in the Southern Ocean. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.

The ocean’s permanent 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 permanent pycnocline remain little understood. Two factors in particular are generally overlooked: the presence of a zonal channel in the Southern Ocean, and the nonlinear interplay between temperature and salinity distributions. Here we assess the mechanism generating the Southern Ocean’s permanent pycnocline through the analysis of a high-resolution, realistic, global sea ice-ocean model. We show that the permanent pycnocline is formed by 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 leads to the formation of 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 are projected equatorward into the Southern Hemisphere basins along density surfaces. Our findings thus highlight the crucial role of localized sea ice-ocean interactions in configuring the vertical structure of the Southern Ocean.