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news & views OCEAN SCIENCE Arctic sea ice heated from below Beneath the fresh and cold surface water in the Arctic Ocean resides more saline and warmer water of Atlantic origin. Pan-Arctic measurements of turbulent mixing suggest that tidal mixing is bringing up substantial amounts of heat in some areas. Camille Lique T he Arctic Ocean has been undergoing rapid transformation over the past few decades. Among the features of this changing environment, the most striking is the fast shrinking of the sea-ice cover. Atmospheric warming in the Arctic region is probably the main driver of this sea-ice decline1, yet the ocean might also contribute significantly. Indeed, the layer of Atlantic water that is found at intermediate depths in the Arctic Ocean contains a large amount of heat that, if brought to the ocean surface, could melt all of the Arctic sea ice within a few years. Writing in Nature Geoscience, Rippeth and colleagues2 report that tidegenerated turbulent mixing can locally be large enough to bring significant amounts of heat upward to the surface layer of the Arctic Ocean. Salty water with relatively warm temperatures of 2 to 3 °C is transported from the North Atlantic Ocean to the Arctic Ocean through Fram Strait and the Barents Sea. This water mass brings a large amount of heat to the Arctic Ocean. As the Atlantic Water enters the Arctic Ocean, it descends beneath the fresh surface layer, the halocline. The surface water originates largely from river runoff. It is colder and contains so much less salt that its overall density is lower. The sea ice is thereby insulated from the heat contained in the Atlantic water by this surface layer. The Atlantic water circulates anti-clockwise within the intermediate layer of the Eurasian and Canadian Basins, following the slope of the seafloor. Eventually, it is exported back into the North Atlantic Ocean as cold water near the freezing point. Based on simple heat budget considerations, the Atlantic Water must undergo substantial cooling along its transit within the Arctic Basin. The loss of heat occurs mainly through upward transfer. Although crucial for understanding the role of the ocean in the stability of sea-ice cover, our knowledge of the upward heat flux from the Atlantic Water layer and its controlling mechanisms remains incomplete. Rippeth and colleagues2 address this question with the help of the most extensive Solar radiation Sea ice Cold fresh halocline Tide-generated mixing and associated upward heat flux Warm Atlantic water Figure 1 | Layers in Arctic waters. In the Arctic Ocean, a cold, fresh water layer insulates the sea ice from the more saline, warmer water of Atlantic origin. Vertical mixing and resulting upward heat flux could warm the fresh upper layer, and lead to melting sea ice. Rippeth and colleagues2 present measurements of turbulent mixing that suggest the tides generate locally substantial upward heat flux, whereas the presence or absence of sea ice seems to have little influence on turbulent mixing rates. 172 pan-Arctic survey of microstructure measurements of turbulent mixing to date. They infer large vertical heat fluxes, of up to 50 W m–2, in some locations. Their estimates of turbulent vertical mixing vary widely in space: vertical mixing tends to intensify over regions with steeper bathymetry, but it seems to be insensitive to presence or absence of sea ice. Based on these findings, Rippeth and colleagues suggest that the spatial variations of vertical mixing are mostly driven by variations in tidal dissipation, which can be locally important. These results improve our understanding of the mechanisms that can transfer heat from warm Atlantic water to the overlying surface layer. Tidal flow over topography, as well as wind blowing at the ocean surface, can act to generate internal waves at density interfaces within the ocean. As they propagate and eventually break, these internal waves are the main source for vertical mixing in the ocean. In the Arctic, however, sea ice forms a barrier between the atmosphere and the ocean, impeding the injection of wind energy to the ocean surface. Moreover, most of the Arctic basin lies poleward of the critical latitude beyond which the free propagation of tide-generated internal waves is inhibited. The amount of mixing at a distance from strong tide generation is therefore expected to be limited, in line with the findings by Rippeth and colleagues. As a result, the Arctic Basin is remarkably quiet compared with the rest of the world’s oceans, as shown by earlier direct observations of the turbulent mixing 3. However, as the sea-ice pack has been retreating further and for longer each year over the past few decades, the amount of energy input to the ocean from the wind forcing has increased. This effect may have led to a seasonal increase in internal wavemixing, with the possible consequence of destabilizing the stratification of the water column4. At the same time, competing effects may also have worked in favour of a more stable stratification: the Arctic hydrological cycle has intensified, and increases in river discharge, Greenland ice melting and precipitation over the Arctic Basin have all added more and more fresh water to the NATURE GEOSCIENCE | VOL 8 | MARCH 2015 | www.nature.com/naturegeoscience © 2015 Macmillan Publishers Limited. All rights reserved news & views very large fraction of the total heat lost by the Atlantic water mass during its entire transit through the Arctic Basin. The remaining heat is lost over a large area in the interior of the Arctic Basin through very small vertical heat fluxes6, with very limited effect for the sea-ice pack. The study by Rippeth and colleagues2 identifies the important role of tides, compared with winds and sea ice, in controlling vertical mixing and associated heat fluxes in the Arctic basin. However, if we are to fully understand the influence of the ocean on current and future sea-ice loss, other processes such as the absorption of solar radiation in the surface layers of the Arctic Ocean will also have to be considered. ❐ Arctic Ocean. Combined with the warming of the surface ocean, these trends enhance the stratification within the top layers of the ocean, and are consequently expected to reduce the amount of vertical mixing 5. Regionally intensified levels of vertical mixing, as observed by Rippeth and colleagues, result in large upward heat fluxes from the Atlantic layer. The largest dissipation rates are found at the entrance of the Arctic Ocean, north of Fram Strait, where the Atlantic Water layer is at its warmest and can be in direct contact with the atmosphere during years with low sea-ice cover. Rippeth and colleagues2 infer vertical heat flux up to 50 W m–2 in this region. Although this heat loss occurs over a small region, it represents a Camille Lique is at the Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. e-mail: [email protected] References 1. Perovich, D. K. & Richter-Menge, J. A. Annu. Rev. Mar. Sci. 1, 417–441 (2009). 2. Rippeth, T. P. et al. Nature Geosci. 8, 191–194 (2015). 3. Rainville, L. & Winsor, P. Geophys. Res. Lett. 35, L08606 (2008). 4. Rainville, L., Lee, C. M. & Woodgate, R. Oceanography 24, 136–145 (2011). 5. Guthrie, J. D., Morison, J. H. & Fer, I. J. Geophys. Res. Oceans 118, 3966–3977 (2013). 6. Timmermans, M‑L., Toole, J., Krishfield, R. & Winsor, P. J. Geophys. Res. 113, C00A02 (2008). Published online: 16 February 2015 CARBON SEQUESTRATION The ocean is a remarkable sink for the increasing amount of carbon dioxide in the atmosphere. Carbon dioxide does not just diffuse into the ocean, it also reacts abiotically with seawater, producing bicarbonate and carbonate and thereby allowing more CO2 to diffuse into the ocean. All in all, as a result of these reactions the oceans take up roughly a quarter of anthropogenic CO2 emissions globally, and nearly all of this is converted to bicarbonate and carbonate. In the Southern Ocean, biological production and decomposition also play an important role in regulating the CO2 exchange between the ocean and the atmosphere. Although the Southern Ocean became a net CO2 sink following the industrial revolution, CO2 fluxes in this region are strongly seasonal. The Southern Ocean is a carbon dioxide sink in summer, when organisms use the dissolved inorganic carbon in the surrounding water — CO2, bicarbonate, and carbonate — to grow. In winter, when organic material decomposes and releases CO2 in the process, carbon dioxide is emitted back to the atmosphere. In the 1950s, Roger Revelle and Hans Suess noticed that the efficiency of ocean uptake of atmospheric CO2 can be quantified by a number that came to be known as the Revelle factor: a ratio relating changes in seawater CO2 concentrations to changes in seawater concentrations of total dissolved inorganic carbon. But the equilibrium of the reaction between CO2 concentration and dissolved inorganic carbon changes with pH. As oceans take up more CO2, they become more acidic © STEVE BLOOM IMAGES / ALAMY Biology’s growing role and the production of dissolved inorganic carbon becomes less favourable. Over time, the Revelle ratio increases, because a larger increase in the concentration of dissolved CO2 is required to create a given amount of dissolved inorganic carbon. As a result, the oceans become less efficient at taking up CO2. Yet, as the oceans’ chemical capacity to soak up CO2 diminishes, biological activity could play an increasingly important role in regulating CO2 uptake. Judith Hauck and Christoph Voelker simulated a scenario of large CO2 emissions and climate change in the twenty-first century with a coupled ocean–ecosystem model (Geophys. Res. Lett. http://doi.org/zzc; 2014). Their approach allowed them to distinguish how CO2 uptake NATURE GEOSCIENCE | VOL 8 | MARCH 2015 | www.nature.com/naturegeoscience © 2015 Macmillan Publishers Limited. All rights reserved is likely to change in the future in response to changes in the ocean’s chemical capacity to take up CO2 independent of the effects of changes in temperature, circulation, or resource availability. It is no surprise that ocean uptake of CO2 would increase alongside rising anthropogenic emissions of CO2 over the twenty-first century. But as the ocean’s ability to efficiently take up CO2 diminished, the strength of the seasonal cycle in CO2 uptake increased throughout the Southern Ocean. Late in the twenty-first century, the biological uptake of dissolved inorganic carbon caused a much larger decline in the amount of CO2 in the Southern Ocean surface waters than it did early in the century, seawater CO2 concentrations became more sensitive to changes in dissolved inorganic carbon concentrations: the change in the Revelle ratio means that a given decline in the levels of dissolved inorganic carbon will result in larger uptake of atmospheric CO2 by the oceans. In total, CO2 uptake from biological activity increased by roughly 2.5 times over the course of the twenty-first century, even though changes in biological activity were small. With some effort, we can avoid the large increases in atmospheric CO2 concentrations described in this highemissions scenario. But even if emissions grow at a slower pace, CO2 may eventually reach concentrations that shift the ocean system into a new chemical state, where marine organisms would play an increasingly important role in controlling CO2 uptake. JONATHAN E. HICKMAN 173