Satellite Altimetry and Current-Meter Velocities in the Malvinas Current at 41°S: Comparisons and Modes of Variations

Three year long current-meter arrays were deployed in the Malvinas Current at 418S below a satellite altimeter track at about 10 years intervals. Surface geostrophic velocities (SGV) derived from satel- lite altimetric data are compared with the in situ velocities at the upper current meter (􏰁300 m). Multisatel- lite gridded SGV compare better with in situ observations than along-track SGV. In spite of the proximity of the moorings to the complex Brazil-Malvinas Confluence (BMC) region, satellite SGV are significantly corre- lated with the 20 day low-passed in situ velocities (0.85 for along-isobaths velocities, 0.8 for cross-isobaths velocities). The recent in situ measurement period (2014?2015) stands out in the altimetry record with a long-lasting (4 months) high level of eddy kinetic energy at the mooring site and a southernmost location of the Subantarctic Front (SAF). The first two modes of variations of sea level anomaly (SLA) over the BMC remarkably match the first two modes of the low-passed in situ velocities. The first mode is associated with a latitudinal migration of the SAF, and the second with a longitudinal displacement of the Brazil Current overshoot. The two modes dominate the 24 year long record of SLA in the BMC, with energy peaks at the annual and semiannual periods for the first mode and at 3?5 months for the second mode. The SLA over the Southwest Atlantic was regressed onto the two confluence modes of SLA variations and showed remarkable standing wave train like structures in the Argentine Basin.Fil: Ferrari, Ramiro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; ArgentinaFil: Artana, Camila Indira. Universite de Paris VI; FranciaFil: Saraceno, Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; ArgentinaFil: Piola, Alberto Ricardo. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Provost, Christine. Universite de Paris VI; Franci

Atlantic Water Modification North of Svalbard in the Mercator Physical System From 2007 to 2020

The Atlantic Water (AW) inflow through Fram Strait, largest oceanic heat source to the Arctic Ocean, undergoes substantial modifications in the Western Nansen Basin (WNB). Evaluation of the Mercator system in the WNB, using 1,500 independent temperature‐salinity profiles and five years of mooring data, highlighted its performance in representing realistic AW inflow and hydrographic properties. In particular, favorable comparisons with mooring time‐series documenting deep winter mixed layers and changes in AW properties led us to examine winter conditions in the WNB over the 2007–2020 period. The model helped describe the interannual variations of winter mixed layers and documented several processes at stake in modifying AW beyond winter convection: trough outflows and lateral exchange through vigorous eddies. Recently modified AW, either via local convection or trough outflows, were identified as homogeneous layers of low buoyancy frequency. Over the 2007–2020 period, two winters stood out with extreme deep mixed layers in areas that used to be ice‐covered: 2017/18 over the northern Yermak Plateau‐Sofia Deep; 2012/13 on the continental slope northeast of Svalbard with the coldest and freshest modified AW of the 12‐year time series. The northern Yermak Plateau‐Sofia Deep and continental slope areas became “Marginal Convection Zones” in 2011 with, from then on, occasionally ice‐free conditions, 50‐m‐ocean temperatures always above 0 °C and highly variable mixed layer depths and ocean‐to‐atmosphere heat fluxes. In the WNB where observations require considerable efforts and resources, the Mercator system proved to be a good tool to assess Atlantic Water modifications in winter

Changes in Atlantic Water circulation patterns and volume transports North of Svalbard over the last 12 years (2008-2020)

Atlantic Water (AW) enters the Arctic through Fram Strait as the West Spitsbergen Current (WSC). When reaching the south of Yermak Plateau, the WSC splits into the Svalbard, Yermak Pass and Yermak Branches. Downstream of Yermak Plateau, AW pathways remain unclear and uncertainties persist on how AW branches eventually merge and contribute to the boundary current along the continental slope. We took advantage of the good performance of the 1/12° Mercator Ocean model in the Western Nansen Basin (WNB) to examine the AW circulation and volume transports in the area. The model showed that the circulation changed in 2008-2020. The Yermak Branch strengthened over the northern Yermak Plateau, feeding the Return Yermak Branch along the eastern flank of the Plateau. West of Yermak Plateau, the Transpolar Drift likely shifted westward while AW recirculations progressed further north. Downstream of the Yermak Plateau, an offshore current developed above the 3800 m isobath, fed by waters from the Yermak Plateau tip. East of 18°E, enhanced mesoscale activity from the boundary current injected additional AW basin-ward, further contributing to the offshore circulation. A recurrent anticyclonic circulation in Sofia Deep developed, which also occasionally fed the western part of the offshore flow. The intensification of the circulation coincided with an overall warming in the upper WNB (0-1000 m), consistent with the progression of AW. This regional description of the changing circulation provides a background for the interpretation of upcoming observations

Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Revisiter le courant des Malouines

Bien que le Courant des Malouines (CM) soit le principal soutien de la productivité élevée qui caractérise l’Atlantique Sud-Ouest il reste encore peu documenté. Cette thèse combine des données in situ et satellitaires et des sorties de modèles opérationnels, pour étudier le CM. Le CM est une émanation du courant Antarctique Circumpolaire (ACC). Il s’écoule vers le nord jusqu’à 38°S puis il retourne vers le sud formant le Courant de Retour des Malouines (CRM). Nous montrons que l’énergie turbulente de l’ACC est filtrée sur le plateau des Malouines (55°S-48°S). Nous avons découvert l'occurrence d’évènements de blocage à 48S qui coupent le CM de sa source l’ACC. Le CM devient alors le bord ouest d’une grande cellule de recirculation cyclonique. Nous avons montré que les positions des fronts du système du CM (Front Polaire, Front Subantarctique et Front du Brésil) peuvent être suivies en définissant des critères sur la densité potentielle et la hauteur de l’eau. Des eaux du sud du front polaire (FP) sont injectées dans le CM (évènements d'alimentation) et recirculent entre le CM et le CRM. Les caractéristiques des eaux de la recirculation varient dans le temps en fonction des évènements d’alimentation et de blocage. Une série temporelle de 24 ans du transport du CM à 41°S a été construite à partir de données de mouillages et d’altimétrie satellitale. Les maxima et minima du transport sont liés à des perturbations de la circulation en provenance du Bassin Argentin (et non de l’ACC): les maxima sont dus à des tourbillons détachés du FP et les minima à des anomalies anticycloniques provenant du courant de Brésil.Although the Malvinas Current (MC) plays a key role over the Patagonian shelf in sustaining an extremely rich ecosystem it remains rather undocumented. In this thesis, we combined in situ, satellite data, and operational model outputs to study the MC. The MC is an offshoot of the Antarctic Circumpolar Current (ACC). It flows northward up to 38°S where it turns southward forming the Malvinas Return Flow (MRF). We show that a substantial portion of the mesoscale activity from the ACC is dissipated over the Malvinas Plateau. We discovered the occurrence of recurrent blocking events cutting the MC from its source, the ACC. However, the MC does not collapse as a recirculation cell is established. Criteria in potential density and dynamic topography were defined to study the MC system fronts (Polar Front, Subantarctic Front and Brazil Current Front). Waters from the South of the Polar Front (PF) are recurrently injected into the MC as pulses or feeding events. Polar waters accumulate in the recirculation region between the MC and the MRF. Variations in the water characteristics of the recirculation region are consistent with changes in the occurrence of blocking and feeding events. Combining mooring and satellite altimetric data, a 24-year long time series of the MC transport at 41 ° S was constructed. Maxima and minima of the Malvinas current transport at 41 ° S are not associated with the ACC, rather with eddies coming from the Argentine Basin. Transport maxima appear to be related with cyclonic eddied detached from the Polar Front and transport minima with large anticyclonic anomalies from the Brazil Current

Revisiter le courant des Malouines

Although the Malvinas Current (MC) plays a key role over the Patagonian shelf in sustaining an extremely rich ecosystem it remains rather undocumented. In this thesis, we combined in situ, satellite data, and operational model outputs to study the MC. The MC is an offshoot of the Antarctic Circumpolar Current (ACC). It flows northward up to 38°S where it turns southward forming the Malvinas Return Flow (MRF). We show that a substantial portion of the mesoscale activity from the ACC is dissipated over the Malvinas Plateau. We discovered the occurrence of recurrent blocking events cutting the MC from its source, the ACC. However, the MC does not collapse as a recirculation cell is established. Criteria in potential density and dynamic topography were defined to study the MC system fronts (Polar Front, Subantarctic Front and Brazil Current Front). Waters from the South of the Polar Front (PF) are recurrently injected into the MC as pulses or feeding events. Polar waters accumulate in the recirculation region between the MC and the MRF. Variations in the water characteristics of the recirculation region are consistent with changes in the occurrence of blocking and feeding events. Combining mooring and satellite altimetric data, a 24-year long time series of the MC transport at 41 ° S was constructed. Maxima and minima of the Malvinas current transport at 41 ° S are not associated with the ACC, rather with eddies coming from the Argentine Basin. Transport maxima appear to be related with cyclonic eddied detached from the Polar Front and transport minima with large anticyclonic anomalies from the Brazil Current.Bien que le Courant des Malouines (CM) soit le principal soutien de la productivité élevée qui caractérise l’Atlantique Sud-Ouest il reste encore peu documenté. Cette thèse combine des données in situ et satellitaires et des sorties de modèles opérationnels, pour étudier le CM. Le CM est une émanation du courant Antarctique Circumpolaire (ACC). Il s’écoule vers le nord jusqu’à 38°S puis il retourne vers le sud formant le Courant de Retour des Malouines (CRM). Nous montrons que l’énergie turbulente de l’ACC est filtrée sur le plateau des Malouines (55°S-48°S). Nous avons découvert l'occurrence d’évènements de blocage à 48S qui coupent le CM de sa source l’ACC. Le CM devient alors le bord ouest d’une grande cellule de recirculation cyclonique. Nous avons montré que les positions des fronts du système du CM (Front Polaire, Front Subantarctique et Front du Brésil) peuvent être suivies en définissant des critères sur la densité potentielle et la hauteur de l’eau. Des eaux du sud du front polaire (FP) sont injectées dans le CM (évènements d'alimentation) et recirculent entre le CM et le CRM. Les caractéristiques des eaux de la recirculation varient dans le temps en fonction des évènements d’alimentation et de blocage. Une série temporelle de 24 ans du transport du CM à 41°S a été construite à partir de données de mouillages et d’altimétrie satellitale. Les maxima et minima du transport sont liés à des perturbations de la circulation en provenance du Bassin Argentin (et non de l’ACC): les maxima sont dus à des tourbillons détachés du FP et les minima à des anomalies anticycloniques provenant du courant de Brésil

Intense anticyclones at the global Argentine Basin array of the Ocean Observatory Initiative

International audienceWe analyzed physical oceanic parameters gathered by a mooring array at mesoscale spatial sampling deployed in the Argentine Basin within the Ocean Observatory Initiative, a National Science Foundation major research facility. The array was maintained at 42∘ S and 42∘ W, a historically sparsely sampled region with small ocean variability, over 34 months from March 2015 to January 2018. The data documented four anticyclonic extreme-structure events in 2016. The four anticyclonic structures had different characteristics (size, vertical extension, origin, lifetime and Rossby number). They all featured near-inertial waves (NIWs) trapped at depth and low Richardson values well below the mixed layer. Low Richardson values suggest favorable conditions for mixing. The anticyclonic features likely act as mixing structures at the pycnocline, bringing heat and salt from the South Atlantic Central Water to the Antarctic Intermediate Waters. The intense structures were unique in the 29-year-long satellite altimetry record at the mooring site. The Argentine Basin is populated with many anticyclones, and mixing associated with trapped NIWs probably plays an important role in setting up the upper-water-mass characteristics in the basin

Topographically Trapped Waves Around South America With Periods Between 40 and 130 Days in a Global Ocean Reanalysis

International audienceThe South American continental slope hosts a variety of topographic waves. We use a 27-year-long global ocean reanalysis (1/12° Spatial resolution) to examine trapped waves (TWs) around South America at periods ranging from 40 to 130 days. The waves propagate from the Equatorial Pacific to the Tropical Atlantic (22°S) with phase velocities between 1.8 and 7 m/s according to the local background characteristics, such as stratification, slope steepness, latitude, mean flow and shelf width. The Madden-Julian Oscillation (MJO) plays a key role in forcing the TWs in two ways (a) through an oceanic connection implying equatorial Kelvin waves reaching the western American Coast and (b) through an atmospheric teleconnection enhancing southerly winds in the south-east Pacific. Furthermore, local winds, not necessarily linked with the MJO, modulate and trigger waves in specific locations, such as the Brazil-Malvinas Confluence. Trapped waves impact the along-shore currents: during the positive phase of the waves the near-surface flow is enhanced by about 0.1 m/s

Atlantic Water Inflow Through the Yermak Pass Branch: Evolution Since 2007

International audienceThirty-four months (2017–2020) of mooring data were recently obtained at 80.6°N, 7.26°E in the main branch of Atlantic Water inflow to the Arctic, the Yermak Pass Branch. The Yermak Pass Branch was sampled at that same location during 14 months a decade ago (2007–2008) when sea ice was abundant (mean sea-ice concentration of 74% vs. 39% during the recent deployment). We focus on time scales larger than 50 hr. The new mooring data set shows an increase in the velocity variations of 40% compared to the 2007–2008 period. Year 2018 was exceptional with ice-free conditions over the entire year and an intensified mesoscale activity compared to other years. Temperature and salinity time series at 340 m showed significant trends over 3 years (freshening of −0.07 g/kg and cooling of about −0.9°C in 3 years). The performance of 1/12° Mercator-Ocean operational model at the mooring location was precisely assessed. The modeled Atlantic Water transport was on average larger during 2017–2020 (40% larger) than during 2007–2008. The synoptic transport time series ranged between −1 and 5 Sv over 2007–2020 and showed large seasonal and interannual variations. The transport was larger in winter than summer. However, occasionally negative transport (<−0.7 Sv) through the Yermak Pass Branch occurred during winters (“Blocking events”). These blocking events are associated with recirculations and eddy activity and were more common over the last years from 2016 onward. The model suggested that a Northern Branch crossing the Yermak Plateau further north (81.6°N) intermittently developed