Tom P. Rippeth

Marine heatwaves are increasing in frequency and intensity globally against a background of long-term climate warming, yet their impacts on subsurface biogeochemistry in temperate shelf seas remain poorly constrained. Here, we present high-resolution in situ observations of chlorophyll during a Category IV marine heatwave in June 2023, collected using an autonomous ocean glider in a seasonally stratified shelf sea. Surface chlorophyll exhibited a rapid but short-lived enhancement, increasing from 5 mg m⁻³ and briefly exceeding the climatological ±2σ range by up to 40%. In contrast, subsurface chlorophyll showed a sustained enhancement, with concentrations within the subsurface chlorophyll maximum increasing from typical post-bloom values of 1–2 mg m⁻³ to >3 mg m⁻³ and episodic peaks exceeding 9 mg m⁻³. This intensification was accompanied by an increase in depth-integrated chlorophyll, reaching 140 mg m⁻². Accelerated bottom mixed-layer warming and strengthened horizontal temperature gradients are consistent with a possible influence of horizontal transport processes, while elevated surface irradiance may also have contributed. These findings highlight the importance of sustained, vertically resolved observations for detecting ecosystem responses to climate extremes.

The Arctic Ocean double estuary is a “three‐legged” overturning system in which inflowing waters are converted into both lighter and denser waters before being exported equatorwards. As the northern terminus of the Atlantic Meridional Overturning Circulation (MOC), it thus both affects, and is affected by, the Atlantic MOC. Here we quantify the magnitudes of the two overturning cells in density space, and then decompose the water mass transformation rates into net pan‐Arctic contributions from surface forcing and diapycnal mixing. We use a high‐resolution, quasi‐synoptic ice and ocean hydrographic data set spanning the four main Arctic Ocean gateways—Fram, Davis and Bering Straits, and the Barents Sea Opening. Two surface flux reanalyses and a hydrographic climatology are used to generate estimates of surface water mass transformation rates by density class. A box model then determines the profiles of turbulent mixing transformation rates, and associated turbulent diffusivities. We show that turbulent mixing and surface forcing drive transformations of similar magnitudes, while mixing dominates in the upper cell and surface fluxes in the lower cell. Consideration of uncertainties and timescales leads to the tentative suggestion that our results might be representative of recent decades. We discuss the possible significance of tides and sea ice brine rejection as energy sources driving turbulent mixing. Finally, we speculate as to whether water mass transformation rates may change in future as ocean heat transport into the Arctic increases. As sea ice declines and the efficiency of atmosphere‐to‐ocean momentum transfer increases, the Arctic Ocean is expected to “spin up,” causing more intense turbulent mixing, with uncertain consequences.

The Arctic Ocean differs from other oceans globally in several ways. Stratification is largely determined by changes in salinity, with cooler fresher water overlying warmer (intruding) saltier water. Until very recently the ocean was largely isolated from the atmosphere by sea ice restricting exchange of heat and momentum across the sea surface. As much of the Arctic Ocean lies poleward of the critical latitude for the dominant tidal forcing, preventing the formation of freely propagating internal tides, the major pathway of tidal energy to ocean mixing. As such mixing between layers in the Arctic Ocean is weak.An analogy is often drawn between the circulation in the Arctic Ocean and that in an estuary. Lateral gradients in density drive exchange through Arctic gateways with the exchange flow mediated by vertical mixing within the Arctic Ocean. Here we examine the potential impact of the recent decline sea ice extent on both the vertical mixing and the import of heat and export of freshwater through the Arctic gateways.

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer in response to deep water remineralisation of sunken organic matter. Model simulations suggest that a warming climate will accelerate this deficiency due to strengthening stratification and the increased duration of stratification. In the next decades the seasonally stratified seas will host the massive expansion of off-shore wind farms, and in particular the new generation of floating wind turbines. Here we integrate novel turbulence timeseries measurements with vertical profiles of water column properties from a seasonally stratified shelf sea to estimate oxygen and biogeochemical fluxes. These new results are used to investigate the role of vertical mixing in the development of the seasonal O2 deficit with particular reference to the potential impact of enhanced mixing associated with the wake of the floating wind turbines.

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.

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O₂ fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O₂ deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O₂ deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in the development of the seasonal deep water O₂ deficit.

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer, with predictions that global warming will accelerate this deficiency. Here we integrate novel turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum (SCM). We show for the first time that the associated mid-water oxygen maximum supports both upward and downwards O2 fluxes. The upward flux will out-gas to the atmosphere whilst the downward flux will partially off-set the deep water O2 deficit. These new results show that the fluxes are sensitive to both the water column structure and mixing rates indicating that the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that whilst tidal mixing supports the downward flux, the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in determining the development of the deep water O2 deficit.

The drive to achieve net zero carbon has motivated the development of offshore wind into deeper waters further from shore. The relatively weak tidal currents and deep water of future development sites means that infrastructure will, for the first time, be deployed at scale in seasonally stratified waters. Current designs for floating turbines have sub-structures which penetrate this stratification. Flow past such substructures generates turbulent wakes which can regionally enhance the very low levels of internal mixing observed in the seasonal thermocline. These low natural mixing rates drive nutrient fluxes which sustain phytoplankton growth at the subsurface chlorophyl maximum through the summer months and are responsible for 50% of the primary production in shelf seas. Since this production supports the marine food web, changes to the physical drivers will fundamentally impact the marine food web. Therefore, an anthropogenic source of turbulent mixing at the seasonal thermocline, has the potential to cause fundamental biogeochemical changes, impacting ecosystems, and fisheries in shelf seas. We present new measurements of strongly elevated turbulence within wakes at a shallow water wind farm. Strongly enhanced mixing is observed in the wake, and across the wider wind farm area and is associated with reduced stratification. These observations and our estimates for deeper water wakes suggest that mixing from these structures can be significant, and further research is essential to quantify the impact of this new source of anthropogenic mixing.

Whilst accounting for only 7% of the global ocean surface area, shelf seas are important links between multiple environments (including terrestrial, deep marine, and atmospheric) and modulate the freshwater influx from rivers before it reaches the open ocean. This freshwater acts as a buoyancy forcing, and, together with solar heating and tidal mixing, affects the seasonal stratification of shelf seas. As stratification impacts numerous processes within the shelf seas, such as heat uptake, ocean currents and biogeochemistry which may further be of global importance, it is important that freshwater fluxes are accurately simulated within models. Despite their importance, due to coarse model resolution, shelf seas are generally poorly represented in intermediate-complexity global climate models. Here, we examine the accuracy of shelf sea representation in the intermediate-complexity UVic Earth System Climate Model, with a primary focus on the North Sea. Using observational data, we show that the river basin configuration and freshwater discharge in the control model set up has large errors. As a result, the North Sea receives almost double the expected freshwater discharge on an annual scale, impacting the flushing time, seasonal stratification and biogeochemistry of the region. Through a series of simulations rerouting freshwater through more realistic drainage basins, and removing excess freshwater, we improve simulation results, with variations in freshwater fluxes having a significant impact on shelf sea processes. Our results indicate that the over-freshening of shelf seas may not solely be restricted to the UVic model but may be an issue in other global Earth system models due to their low spatial resolution.

Historically, the Arctic Ocean has been considered an ocean of weak turbulent mixing. However, the decline in seasonal sea ice cover over the past couple of decades has led to increased coupling between the atmosphere and the ocean, with potential enhancement of turbulent mixing. Here, we review studies identifying energy sources and pathways that lead to turbulent mixing in an increasingly ice-free Arctic Ocean. We find the evolution of wind-generated, near-inertial oscillations is highly sensitive to the seasonal sea ice cycle, but that the response varies greatly between the continental shelves and the abyssal ocean. There is growing evidence of the key role of tides and continental shelf waves in driving turbulent mixing over sloping topography. Both dissipate through the development of unsteady lee waves. The importance of the dissipation of unsteady lee waves in driving mixing highlights the need for parameterization of this process in regional ocean models and climate simulations.