The advent of under-ice profiling float and biologging techniques has enabled year-round observation of the Southern Ocean and its Antarctic margin. These under-ice data are often overlooked in widely used oceanographic datasets, despite their importance in understanding the seasonality and its role in sea ice changes, bottom water formation, and glacial melt. We develop a four-dimensional climatology of the Southern Ocean (south of 40°S and above 2,000 m) using Data Interpolating Variational Analysis, which excels in multi-dimensional interpolation and consistent handling of topography and advection. The climatology captures thermohaline variability under sea ice, previously hard to obtain, and outperforms other products in data fidelity with smaller root-mean-square errors and biases. Our dataset will be instrumental for investigating seasonality and for improving ocean models. This work further highlights the quantitative significance of under-ice data in reproducing ocean conditions, advocating for their increased use to achieve a better Southern Ocean observing system.

Eddies have two distinct properties known as eddy advection (K_GM ) and eddy diffusion (K_iso). These are parameterized by Gent-McWilliams [1990] (K_GM ) and Redi [1982] (K_iso), respectively, in low to moderate resolution global ocean models. Both processes play an active part in the uptake of tracers such as CO_2 and CFCs. The advective role of eddies is associated with slumping of isopycnals, whereas the diffusive role is associated with the down-gradient diffusion [Lee et al., 2007]. In this work we investigate, for the first-time, which eddy process plays a dominant role in controlling tracer distribution in models. The model (pyOM2) used in this work is a coarse-resolution (2°x 2°) global ocean model [Eden et al., 2014]. We use a suite of nine different experiments, each with different K_GM and K_iso values. An idealised passive tracer is introduced at the surface of the ocean after the model is well equilibrated and it evolves for 100 years. We found on decadal time scales eddy diffusion and eddy advection both play equally important roles in controlling tracer distribution. However, on the time scale of hundred years or more eddy advection dominates over eddy diffusion. Small K_GM values set the isopycnal slopes to be steep, which allows the tracer to easily penetrate the deep ocean along isopycnals. On the contrary, large K_GM values flatten the isopycnals and therefore tracer only weakly penetrates the deep ocean. Near Antarctica, large values of K_iso exert strong control over the tracer distribution and brings tracer rich water to depth.

Abstract: In climate-ocean models, eddy mixing coefficients are often constant in space and time. However, observations, advances in theory, and high-resolution-eddy-resolving ocean models have revealed significant variability in the strength of isopycnal mixing both spatially and temporally. A theory that includes the effect of mean flow suppression has been developed by [1] it has a strong impact on tracer uptake and ventilation. This theory is used in offline calculations by [1, 2, 4] but has not been implemented as a parameterization in an ocean model and in this study we implement it for the first time in an ocean model [3]. The parameterization is tested in a non-eddy-resolving ocean model, demonstrating passive ventilation of a tracer as a function of depth and latitude. The first main result is this parameterization improves the mean state of the ocean. The other result is this parameterization improves the sensitivity of tracer uptake to changing winds.

Long-term temperature changes drive coastal Marine Heat Waves (MHW) trends globally. Here, we provide a more comprehensive global analysis of cross-shore gradients of MHW and SST changes using an ensemble of three satellite SST products during recent decades. Our analysis reveals depressed onshore SST trends in more than 2/3 of coastal pixels, including both eastern and western boundary current systems. These were well correlated with depressed trends of MHW exposure and severity, ranging from a -2 to -10 decrease in MHW days per decade and a –2.5 to –15°C.days per decade decrease in cumulative intensity. Results were consistent across all satellite products, indicating that these cross-shore gradients are a robust feature of observations. ERA reanalysis data shows that neither air-sea heat fluxes nor wind driven upwelling were found to be consistent drivers. Global ocean circulation models (OFAM3 and ACCESS-OM2) have limited ability to simulate the depressed onshore trends. A heat budget analysis performed in the Chilean coast region, where models agree with observations, showed that the gradient of temperature change was controlled by an onshore increase of longwave radiative cooling, despite an increase in upwelling. This highlights the complexity of small-scale coastal ocean-atmosphere feedbacks, which coarser resolution climate models do not resolve. Here, we show that global coastal regions may act as thermal refugia for marine ecosystems from aspects of climate change and pulsative (MHW) changes. Contrary to the literature, our results suggest that driving mechanisms are region dependant, stressing the necessity to improve climate models resolution.