Jets and Topography: Jet Transitions and the Impact on Transport in the Antarctic Circumpolar Current

The Southern Ocean’s Antarctic Circumpolar Current (ACC) naturally lends itself to interpretations using a zonally averaged framework. Yet, navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models. In this study, both observational and numerical results indicate that zonal asymmetries, in the form of topography, impact global flow structure and transport properties. The conclusions are based on a suite of more than 1.5 million virtual drifter trajectories advected using a satellite altimetry–derived surface velocity field spanning 17 years. The focus is on sites of “cross front” transport as defined by movement across selected sea surface height contours that correspond to jets along most of the ACC. Cross-front exchange is localized in the lee of bathymetric features with more than 75% of crossing events occurring in regions corresponding to only 20% of the ACC’s zonal extent. These observations motivate a series of numerical experiments using a two-layer quasigeostrophic model with simple, zonally asymmetric topography, which often produces transitions in the front structure along the channel. Significantly, regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic. Jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets. Eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering. The combination of high eddy kinetic energy and recirculation features enhances particle exchange. These results stress the complications in developing consistent circumpolar definitions of the ACC fronts

Frontal circulation and submesoscale variability during the formation of a Southern Ocean mesoscale eddy

AbstractObservations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, O(1-10 km), variability along the periphery of a mesoscale O(10-100 km) meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~ 120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar fronts converge and modifications of Subantarctic mode water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly-formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from co-located drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current data set. Upwelling (downwelling) rates of O(100 m day-1) are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in T-S space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along �θ 27 - 27.2 kg m�3 have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously

Slowdown of Antarctic Bottom Water export driven by climatic wind and sea ice changes

Antarctic Bottom Water (AABW) is pivotal for oceanic heat and carbon sequestrations on multidecadal-to-millennial timescales. The Weddell Sea contributes nearly a half of global AABW through Weddell Sea Deep Water (WSDW) and denser underlying Weddell Sea Bottom Water (WSBW) that are form on the continental shelves via sea ice production. Here we report an observed 30% reduction of WSBW volume since 1992, with the largest decrease in the densest classes. This is likely driven by a multidecadal reduction in dense water production over southern continental shelf associated with a >40% decline in the sea ice formation rate. The ice production decrease is driven by northerly wind trend, related to a phase transition of the Interdecadal Pacific Oscillation since the early 1990s, superposed by Amundsen Sea Low intrinsic variability. These results reveal key influences on exported AABW to the Atlantic abyss and their sensitivity to large-scale, multidecadal climate variability

The Zero Emissions Commitment and climate stabilization

How do we halt global warming? Reaching net zero carbon dioxide (CO2) emissions is understood to be a key milestone on the path to a safer planet. But how confident are we that when we stop carbon emissions, we also stop global warming? The Zero Emissions Commitment (ZEC) quantifies how much warming or cooling we can expect following a complete cessation of anthropogenic CO2 emissions. To date, the best estimate by the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report is zero change, though with substantial uncertainty. In this article, we present an overview of the changes expected in major Earth system processes after net zero and their potential impact on global surface temperature, providing an outlook toward building a more confident assessment of ZEC in the decades to come. We propose a structure to guide research into ZEC and associated changes in the climate, separating the impacts expected over decades, centuries, and millennia. As we look ahead at the century billed to mark the end of net anthropogenic CO2 emissions, we ask: what is the prospect of a stable climate in a post-net zero world

First description of in situ chlorophyll fluorescence signal within East Antarctic coastal polynyas during fall and winter

Antarctic coastal polynyas are persistent and recurrent regions of open water located between the coast and the drifting pack-ice. In spring, they are the first polar areas to be exposed to light, leading to the development of phytoplankton blooms, making polynyas potential ecological hotspots in sea-ice regions. Knowledge on polynya oceanography and ecology during winter is limited due to their inaccessibility. This study describes i) the first in situ chlorophyll fluorescence signal (a proxy for chlorophyll-a concentration and thus presence of phytoplankton) in polynyas between the end of summer and winter, ii) assesses whether the signal persists through time and iii) identifies its main oceanographic drivers. The dataset comprises 698 profiles of fluorescence, temperature and salinity recorded by southern elephant seals in 2011, 2019-2021 in the Cape-Darnley (CDP;67˚S-69˚E) and Shackleton (SP;66˚S-95˚E) polynyas between February and September. A significant fluorescence signal was observed until April in both polynyas. An additional signal occurring at 130m depth in August within CDP may result from in situ growth of phytoplankton due to potential adaptation to low irradiance or remnant chlorophyll-a that was advected into the polynya. The decrease and deepening of the fluorescence signal from February to August was accompanied by the deepening of the mixed layer depth and a cooling and salinification of the water column in both polynyas. Using Principal Component Analysis as an exploratory tool, we highlighted previously unsuspected drivers of the fluorescence signal within polynyas. CDP shows clear differences in biological and environmental conditions depending on topographic features with higher fluorescence in warmer and saltier waters on the shelf compared with the continental slope. In SP, near the ice-shelf, a significant fluorescence signal in April below the mixed layer (around 130m depth), was associated with fresher and warmer waters. We hypothesize that this signal could result from potential ice-shelf melting from warm water intrusions onto the shelf leading to iron supply necessary to fuel phytoplankton growth. This study supports that Antarctic coastal polynyas may have a key role for polar ecosystems as biologically active areas throughout the season within the sea-ice region despite inter and intra-polynya differences in environmental conditions

The Zero Emissions Commitment and climate stabilization

How do we halt global warming? Reaching net zero carbon dioxide (CO2) emissions is understood to be a key milestone on the path to a safer planet. But how confident are we that when we stop carbon emissions, we also stop global warming? The Zero Emissions Commitment (ZEC) quantifies how much warming or cooling we can expect following a complete cessation of anthropogenic CO2 emissions. To date, the best estimate by the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report is zero change, though with substantial uncertainty. In this article, we present an overview of the changes expected in major Earth system processes after net zero and their potential impact on global surface temperature, providing an outlook toward building a more confident assessment of ZEC in the decades to come. We propose a structure to guide research into ZEC and associated changes in the climate, separating the impacts expected over decades, centuries, and millennia. As we look ahead at the century billed to mark the end of net anthropogenic CO2 emissions, we ask: what is the prospect of a stable climate in a post-net zero world?</jats:p

​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system

State of the climate in 2018

In 2018, the dominant greenhouse gases released into Earth’s atmosphere—carbon dioxide, methane, and nitrous oxide—continued their increase. The annual global average carbon dioxide concentration at Earth’s surface was 407.4 ± 0.1 ppm, the highest in the modern instrumental record and in ice core records dating back 800 000 years. Combined, greenhouse gases and several halogenated gases contribute just over 3 W m−2 to radiative forcing and represent a nearly 43% increase since 1990. Carbon dioxide is responsible for about 65% of this radiative forcing. With a weak La Niña in early 2018 transitioning to a weak El Niño by the year’s end, the global surface (land and ocean) temperature was the fourth highest on record, with only 2015 through 2017 being warmer. Several European countries reported record high annual temperatures. There were also more high, and fewer low, temperature extremes than in nearly all of the 68-year extremes record. Madagascar recorded a record daily temperature of 40.5°C in Morondava in March, while South Korea set its record high of 41.0°C in August in Hongcheon. Nawabshah, Pakistan, recorded its highest temperature of 50.2°C, which may be a new daily world record for April. Globally, the annual lower troposphere temperature was third to seventh highest, depending on the dataset analyzed. The lower stratospheric temperature was approximately fifth lowest. The 2018 Arctic land surface temperature was 1.2°C above the 1981–2010 average, tying for third highest in the 118-year record, following 2016 and 2017. June’s Arctic snow cover extent was almost half of what it was 35 years ago. Across Greenland, however, regional summer temperatures were generally below or near average. Additionally, a satellite survey of 47 glaciers in Greenland indicated a net increase in area for the first time since records began in 1999. Increasing permafrost temperatures were reported at most observation sites in the Arctic, with the overall increase of 0.1°–0.2°C between 2017 and 2018 being comparable to the highest rate of warming ever observed in the region. On 17 March, Arctic sea ice extent marked the second smallest annual maximum in the 38-year record, larger than only 2017. The minimum extent in 2018 was reached on 19 September and again on 23 September, tying 2008 and 2010 for the sixth lowest extent on record. The 23 September date tied 1997 as the latest sea ice minimum date on record. First-year ice now dominates the ice cover, comprising 77% of the March 2018 ice pack compared to 55% during the 1980s. Because thinner, younger ice is more vulnerable to melting out in summer, this shift in sea ice age has contributed to the decreasing trend in minimum ice extent. Regionally, Bering Sea ice extent was at record lows for almost the entire 2017/18 ice season. For the Antarctic continent as a whole, 2018 was warmer than average. On the highest points of the Antarctic Plateau, the automatic weather station Relay (74°S) broke or tied six monthly temperature records throughout the year, with August breaking its record by nearly 8°C. However, cool conditions in the western Bellingshausen Sea and Amundsen Sea sector contributed to a low melt season overall for 2017/18. High SSTs contributed to low summer sea ice extent in the Ross and Weddell Seas in 2018, underpinning the second lowest Antarctic summer minimum sea ice extent on record. Despite conducive conditions for its formation, the ozone hole at its maximum extent in September was near the 2000–18 mean, likely due to an ongoing slow decline in stratospheric chlorine monoxide concentration. Across the oceans, globally averaged SST decreased slightly since the record El Niño year of 2016 but was still far above the climatological mean. On average, SST is increasing at a rate of 0.10° ± 0.01°C decade−1 since 1950. The warming appeared largest in the tropical Indian Ocean and smallest in the North Pacific. The deeper ocean continues to warm year after year. For the seventh consecutive year, global annual mean sea level became the highest in the 26-year record, rising to 81 mm above the 1993 average. As anticipated in a warming climate, the hydrological cycle over the ocean is accelerating: dry regions are becoming drier and wet regions rainier. Closer to the equator, 95 named tropical storms were observed during 2018, well above the 1981–2010 average of 82. Eleven tropical cyclones reached Saffir–Simpson scale Category 5 intensity. North Atlantic Major Hurricane Michael’s landfall intensity of 140 kt was the fourth strongest for any continental U.S. hurricane landfall in the 168-year record. Michael caused more than 30 fatalities and $25 billion (U.S. dollars) in damages. In the western North Pacific, Super Typhoon Mangkhut led to 160 fatalities and$6 billion (U.S. dollars) in damages across the Philippines, Hong Kong, Macau, mainland China, Guam, and the Northern Mariana Islands. Tropical Storm Son-Tinh was responsible for 170 fatalities in Vietnam and Laos. Nearly all the islands of Micronesia experienced at least moderate impacts from various tropical cyclones. Across land, many areas around the globe received copious precipitation, notable at different time scales. Rodrigues and Réunion Island near southern Africa each reported their third wettest year on record. In Hawaii, 1262 mm precipitation at Waipā Gardens (Kauai) on 14–15 April set a new U.S. record for 24-h precipitation. In Brazil, the city of Belo Horizonte received nearly 75 mm of rain in just 20 minutes, nearly half its monthly average. Globally, fire activity during 2018 was the lowest since the start of the record in 1997, with a combined burned area of about 500 million hectares. This reinforced the long-term downward trend in fire emissions driven by changes in land use in frequently burning savannas. However, wildfires burned 3.5 million hectares across the United States, well above the 2000–10 average of 2.7 million hectares. Combined, U.S. wildfire damages for the 2017 and 2018 wildfire seasons exceeded $40 billion (U.S. dollars)

Les eaux modales de l'Océan Austral

Subantarctic mode water (SAMW) are formed in the Southern Ocean in the deep winter mixed layers north of the Subantarctic front. They influence the climate at interannual and decadal scales and play a fundamental role in the ventilation of Southern Hemisphere thermocline. We study the details of SAMW formation using the recent deployment of ARGO profiling floats and GDP surface drifters, which provide an excellent space-time coverage of the Southern Ocean upper ocean processes. Since the beginning of the ARGO international program, the number of vertical hydrographic profiles in the Southern Ocean have increased considerably so that nowadays we have a comparable number of profiles to decades of hydrographic ship data. Based on this dataset, we found that the dominant forcing for SAMW formation in winter in the Southern Indian Ocean was due to air-sea and Ekman fluxes. We found a rapid transition to thicker surface mixed layers in the central South Indian Ocean, at about 70°, associated with a reversal of the horizontal eddy heat diffusion in the surface layer and the meridional expansion of the ACC as it rounds the Kerguelen Plateau. These effects are ultimately related to the bathymetry of the region, leading to the seat of formation in the region southwest of Australia. SAMW formation is tightly linked to the Southern Ocean dynamics and position of the main polar fronts. A second study concerned the ACC circulation and frontal variability. In this study we mixed in-situ and altimeter data to monitor the position of the two main fronts of the ACC during the period 1993-2005. Then, we related their movements to the two main atmospheric climate modes of the Southern Hemisphere, the Southern Annular Mode (SAM) and the El-Nino Southern Oscillation (ENSO). We found that although the fronts are steered by the bathymetry, which sets their mean pathway at first order, in flat-bottom areas the fronts are subject to large meandering due to mesoscale activity and atmospheric forcing.In parallel, we developed a new estimate of the circumpolar distribution of eddy diffusion in the Southern Ocean. Diffusion has almost never been studied in the Southern Ocean with in-situ data. We analysed up to 10 years of surface drifter trajectories and applied a statistical analysis previously developed in other oceans. We mapped a climatological eddy diffusion coefficient and derived an altimetric parametrization of the coefficient for easy use by the modeling community and for future studies on the interannual evolution of eddy diffusion. This study shows that the Southern Ocean is highly diffusive north of the ACC core, with several spots of very strong diffusion: the Agulhas Retroflection, the Campbell Plateau region, and the Brazil Current region.These results lead us to a circumpolar analysis of the SAMW formation, and to a representaion of the link between Southern Ocean dynamics and SAMW formation. The constant increase in hydrological profiles in the Southern Ocean via the ARGO program allowed us to have a better spatial representation of the regions of SAMW formation. We found that eddy heat diffusion play a substantial role in the local heat balance. South of the western boundary currents and north of the SAF, the eddy heat diffusion is positive and counterbalances the cooling of the mixed layer by winter Ekman and air-sea fluxes. Specifically, it reduces the mixed layer destabilisation north of the SAF in the Western Indian Ocean downstream of the Agulhas Retroflection and in the Western Pacific downstream of Campbell Plateau.Les eaux modales Subantarctiques (SAMW) sont formées dans la profonde couche de mélange au nord du front Subantarctique (SAF) dans l'Océan Austral. Elles influencent le climat à des échelles décennales et inter-annuelles et jouent un rôle fondamental dans la ventilation de la thermocline de l'Océan Austral. Nous étudions la formation des SAMW en nous fondant sur les récents flotteurs profilants ARGO et sur les dériveurs de surface GDP. Ces jeux de données fournissent une très bonne couverture spatio-temporelle des processus à l'oeuvre dans les couches supérieures de l'Océan Austral. Depuis le lancement du programme international ARGO, le nombre de profils hydrographiques a augmenté de façon considérable dans l'Océan Austral. Une analyse de ces données a montré que les flux air-mer et les flux d'Ekman sont les forçages dominants dans la formation des SAMW. Nous avons trouvé une transition rapide, autour de 70°E, des couches de mélange peu profondes en amont vers des couches de mélange très profondes en aval. Cette transition est associée à un changement de signe de la diffusion tourbillonnaire horizontale dans les couches de surface, et à l'extension méridionale de l'ACC lorsqu'il passe autour du plateau de Kerguelen. Ces effets sont directement liés à la bathymétrie et laissent place à une région de formation des SAMW au Sud-Ouest de l'Australie.La formation des SAMW est intimement liée à la dynamique océanique Australe et à la position des principaux fronts polaires. Une deuxième étude concerne la circulation de l'ACC et la variabilité frontale. Dans cette étude, nous avons tiré parti de la complémentarité des données in situ et altimétriques afin de suivre l'évolution des deux principaux fronts de l'ACC pendant la période 1993-2005. Nous avons comparé leurs mouvements avec les deux principaux modes de variabilité atmosphérique de l'Hémisphère Sud, le mode annulaire Austral (SAM) et l'Oscillation Australe El-Niño (ENSO). La position moyenne des fronts est déterminée avant tout par les fonds océaniques. Cependant, nous avons trouvé que dans les régions à fond plat, les fronts forment de grands méandres dus à l'activité tourbillonnaire et aux forçages atmosphériques.En parallèle, nous avons développé une nouvelle estimation de la distribution circumpolaire de la diffusion dans l'Océan Austral. La diffusion n'a presque jamais été étudiée à partir de données in situ dans cet océan. Nous avons calculé une estimation du coefficient de diffusion tourbillonnaire à partir d'une analyse statistique de dix années de trajectoires de dériveurs de surface. Nous avons cartographié ce coefficient dans l'Océan Austral, puis nous l'avons paramétré à partir de données altimétriques pour pouvoir en étudier l'évolution inter-annuelle et en faciliter l'utilisation dans le futur. Cette étude montre que l'Océan Austral est fortement diffusif au nord de l'ACC, et particulièrement près des courants de bord Ouest, c'est à dire dans la Rétroflexion des Aiguilles, dans la région du plateau de Campbell, et dans le courant de Brésil-Malouines. Ces résultats nous ont menés à une analyse circumpolaire de la formation des SAMW, et à une meilleure conception du lien entre la dynamique océanique Australe et la formation des SAMW. La croissance constante des données hydrologiques du programme ARGO dans l'Océan Austral nous a également permis de mieux représenter la répartition des régions de formation des SAMW. Nous avons trouvé que la diffusion tourbillonnaire joue un rôle majeur dans les budgets de chaleur locaux. Au Sud des courants de bord Ouest, et au nord du SAF, la diffusion tourbillonnaire apporte de la chaleur, équilibrant et même dominant les refroidissements hivernaux dus aux flux d'Ekman et aux flux air-mer. Elle réduit en particulier la déstabilisation de la couche de mélange au nord du SAF dans l'Ouest du bassin Indien, en aval de la Rétroflexion des Aiguilles, et dans l'Ouest du bassin Pacifique, en aval du Plateau de Campbell

Southern Ocean Warming

International audienceThe Southern Ocean plays a fundamental role in global climate. With no continental barriers, it distributes climate signals among the Pacific, Atlantic, and Indian Oceans through its fast-flowing, energetic, and deep-reaching dominant current, the Antarctic Circumpolar Current. The unusual dynamics of this current, in conjunction with energetic atmospheric and ice conditions, make the Southern Ocean a key region for connecting the surface ocean with the world ocean's deep seas. Recent examinations of global ocean temperature show that the Southern Ocean plays a major role in global ocean heat uptake and storage. Since 2006, an estimated 60%-90% of global ocean heat content change associated with global warming is based in the Southern Ocean. But the warming of its water masses is inhomogeneous. While the upper 1,000 m of the Southern Ocean within and north of the Antarctic Circumpolar Current are warming rapidly, at a rate of 0.1 degrees-0.2 degrees C per decade, the surface subpolar seas south of this region are not warming or are slightly cooling. However, subpolar abyssal waters are warming at a substantial rate of similar to 0.05 degrees C per decade due to the formation of bottom waters on the Antarctic continental shelves. Although the processes at play in this warming and their regional distribution are beginning to become clear, the specific mechanisms associated with wind change, eddy activity, and ocean-ice interaction remain areas of active research, and substantial challenges persist to representing them accurately in climate models