In the vast expanse of the Southern Ocean, where the interplay between atmosphere and ocean dictates global climate dynamics, new research has revealed the critical role of storm-driven mixing in modulating summer warming. This groundbreaking study, conducted through an extensive observational campaign, highlights how turbulent mixing initiated by storms intricately regulates the temperature of the upper ocean in one of Earth’s most climatically sensitive regions.
Between December 2018 and March 2019, researchers deployed a coordinated array of autonomous vehicles south of the Polar Front at 54°S, 0°E, a location notorious for intense air–sea heat fluxes and relentless wind speeds. Two primary instruments—a spectral Wave Glider fitted with an ultrasonic weather station and a Slocum profiling glider equipped with a high-resolution microstructure profiler—were piloted simultaneously in tandem. This innovative deployment provided a coupled, high-resolution lens through which to observe the atmosphere and upper ocean’s synchronized responses to storm events amid the Southern Ocean’s turbulent environment.
The Slocum glider, housing a Rockland Scientific Microstructure Profiler known as MicroRider, traversed a 14-kilometer north-south transect, strategically collecting microstructure data only during its ascent to maximize battery efficiency and attain near-surface turbulence dissipation estimates. Equipped with conductivity, temperature, and depth sensors, the glider’s systematic profiling allowed investigators to estimate sea surface temperature (SST) and mixed-layer depth (MLD) with refined vertical resolution. These measurements were subjected to rigorous processing techniques to exclude spurious data in the upper ocean layers, ensuring accurate SST and MLD determinations crucial for understanding storm-induced oceanic mixing.
Simultaneously, the Liquid Robotics SV3 Wave Glider floated atop the ocean’s surface, continuously documenting atmospheric dynamics with an Airmar WX-200 Ultrasonic Weather Station. Mounted on a mast 0.7 meters above sea level, this refined instrument captured high-frequency wind speed data, which were later harmonized with ERA5 reanalysis datasets to fill observational gaps following mid-February. Through a carefully designed figure-of-eight navigation pattern over the Slocum glider’s path, the Wave Glider enabled a unique, co-located monitoring of wind-driven atmospheric forcing alongside the ocean’s turbulent response during storm events.
The observational dataset was augmented and contextualized using advanced reanalysis products, primarily the ERA5 atmospheric reanalysis, which offers highly resolved hourly data on wind vectors and air–sea heat flux components, including sensible and latent heat, as well as net solar and thermal radiation. This comprehensive dataset was critical for identifying storm tracks and quantifying the magnitude of wind and heat flux forcing on ocean surface regimes. The rigorous selection criteria excluded data near ice-covered areas, ensuring the fidelity of atmospheric and oceanic parameterizations pertinent to storm activities over open waters of the Southern Ocean.
Complementing atmospheric data, ocean temperature and salinity profiles from the Met Office Hadley Centre’s EN4 dataset, corrected for known instrumental biases, provided interannual MLD estimates. These profiles, collected from 2004 onward during the Argo float epoch, were processed to robustly characterize seasonal mixed-layer variability while mitigating spatial sampling biases. Despite these advances, the study acknowledged ongoing limitations in capturing finescale processes such as submesoscale eddies, which may locally influence stratification and SST beyond the resolution of the datasets employed.
Central to linking atmospheric forcing to upper ocean responses was the classification of storms using a Lagrangian tracking approach on ERA5 mean sea-level pressure fields. This method identified cyclone centers and defined storm influence zones extending 1,000 kilometers radially, filtering for mid-latitude cyclones south of 40°S while excluding proximity events near coastlines. This comprehensive storm catalog encompassed over half a million hourly instances during austral summer months from 1981 to 2019, forming a robust statistical basis for elucidating the cumulative effects of storm-induced mixing on Southern Ocean thermal dynamics.
To dissect the physical mechanisms governing SST evolution, the researchers developed a mixed-layer temperature budget framework. The equation accounts for net surface heat fluxes penetrating below the mixed layer, the entrainment velocity associated with changes in mixed-layer depth, and the entrainment temperature contrasts at the base of the mixed layer. This formulation explicitly treats entrainment as an irreversible process contributing colder or warmer water into the mixed layer when deepening occurs, thereby modulating SST.
The entrainment velocity was mathematically defined to activate only when the mixed-layer depth increased, reflecting the physical reality that entrainment modifies the temperature tendency only during deepening phases. This nuanced approach allowed precise quantification of the relative contributions of surface heat flux and turbulent entrainment in driving upper ocean heat content changes during storm passages, revealing the critical interplay between atmospheric forcing and oceanic mixing processes.
Further insights into ocean surface temperature dynamics were gleaned by evaluating the role of Ekman transport—wind-driven ocean surface currents induced by atmospheric wind stress and modulated by the Coriolis effect. Utilizing components of the wind stress vector and SST spatial gradients, the study calculated the Ekman-induced heat flux divergences and their subsequent effect on mixed-layer temperature tendencies. The vertical extent of Ekman transport influence was parameterized through an eddy viscosity model, relying on von Karman constants and frictional velocities, placing the phenomenon firmly within established boundary-layer theory.
By integrating these daily-scale temperature tendencies spatially and temporally throughout each summer season, the investigators approximated the net SST change attributable to Ekman dynamics over nearly four decades. This long-term perspective emphasized the cumulative and seasonally evolving influence of wind-driven ocean transport on Southern Ocean surface warming patterns, particularly under storm-dominated conditions.
Ultimately, this comprehensive experimental and analytical endeavor delineated how frequent and intense storm events over the Southern Ocean induce turbulent mixing that critically regulates summer SSTs. These findings underscore the indispensable role of atmospheric forcing variability and its oceanic manifestations in shaping regional and potentially global climate feedbacks. Enhanced understanding of these processes not only advances fundamental ocean–atmosphere science but also improves climate model representations of Southern Ocean heat budgets, with implications for predicting future climate trajectories.
This research substantially deepens our knowledge of mid-latitude storm impacts on ocean surface conditions, demonstrating the power of autonomous observational platforms combined with reanalysis data to capture complex coupled system dynamics. As climate change alters storm characteristics and frequency, such insights become increasingly vital for assessing the resilience and response of polar and subpolar ocean systems, which exert disproportionate influence over Earth’s climate.
The study also points to the need for further high-resolution investigations into submesoscale turbulence and eddy interactions, which modulate the intensity and nature of storm-driven mixing. These finer-scale processes can locally counteract or amplify mixing effects, potentially modulating the spatial heterogeneity of warming and stratification patterns within the Southern Ocean.
In sum, this integration of innovative field experimentation, robust data analysis, and theoretical modeling articulates a pivotal mechanism regulating Southern Ocean summer warming—the turbulent feedback between storms and the mixed-layer ocean. It sets a compelling foundation for future research avenues aimed at unraveling the complex interplay of atmospheric and oceanographic forces that govern Earth’s climate extremes and variability in polar regions.
Subject of Research: The interaction between storms and the upper ocean in the Southern Ocean, focusing on how storm-driven mixing regulates summer sea surface temperature and upper ocean heat content.
Article Title: Southern Ocean summer warming is regulated by storm-driven mixing.
Article References:
du Plessis, M.D., Nicholson, S.A., Giddy, I. et al. Southern Ocean summer warming is regulated by storm-driven mixing. Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01857-3
Image Credits: AI Generated
