ALBERT

Description: The temporal evolution of the physical and biogeochemical structure of an oxygen-depleted anticyclonic modewater eddy is investigated over a 2-month period using high-resolution glider and ship data. A weakly stratified eddy core (squared buoyancy frequency N2  ∼  0.1  ×  10−4 s−2) at shallow depth is identified with a horizontal extent of about 70 km and bounded by maxima in N2. The upper N2 maximum (3–5  ×  10−4 s−2) coincides with the mixed layer base and the lower N2 maximum (0.4  ×  10−4 s−2) is found at about 200 m depth in the eddy centre. The eddy core shows a constant slope in temperature/salinity (T∕S) characteristic over the 2 months, but an erosion of the core progressively narrows down the T∕S range. The eddy minimal oxygen concentrations decreased by about 5 µmol kg−1 in 2 months, confirming earlier estimates of oxygen consumption rates in these eddies. Separating the mesoscale and perturbation flow components reveals oscillating velocity finestructure ( ∼  0.1 m s−1) underneath the eddy and at its flanks. The velocity finestructure is organized in layers that align with layers in properties (salinity, temperature) but mostly cross through surfaces of constant density. The largest magnitude in velocity finestructure is seen between the surface and 140 m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450 m. For both regions a cyclonic rotation of the velocity finestructure with depth suggests the vertical propagation of near-inertial wave (NIW) energy. Modification of the planetary vorticity by anticyclonic (eddy core) and cyclonic (eddy periphery) relative vorticity is most likely impacting the NIW energy propagation. Below the low oxygen core salt-finger type double diffusive layers are found that align with the velocity finestructure. Apparent oxygen utilization (AOU) versus dissolved inorganic nitrate (NO3−) ratios are about twice as high (16) in the eddy core compared to surrounding waters (8.1). A large NO3− deficit of 4 to 6 µmol kg−1 is determined, rendering denitrification an unlikely explanation. Here it is hypothesized that the differences in local recycling of nitrogen and oxygen, as a result of the eddy dynamics, cause the shift in the AOU : NO3− ratio. High NO3− and low oxygen waters are eroded by mixing from the eddy core and entrain into the mixed layer. The nitrogen is reintroduced into the core by gravitational settling of particulate matter out of the euphotic zone. The low oxygen water equilibrates in the mixed layer by air–sea gas exchange and does not participate in the gravitational sinking. Finally we propose a mesoscale–submesoscale interaction concept where wind energy, mediated via NIWs, drives nutrient supply to the euphotic zone and drives extraordinary blooms in anticyclonic mode-water eddies.

Description: The physical (temperature, salinity, velocity) and biogeochemical (oxygen, nitrate) structure of an oxygen depleted coherent, baroclinic, anticyclonic mode-water eddy (ACME) is investigated using high-resolution autonomous glider and ship data. A distinct core with a diameter of about 70 km is found in the eddy, extending from about 60 to 200 m depth and. The core is occupied by fresh and cold water with low oxygen and high nitrate concentrations, and bordered by local maxima in buoyancy frequency. Velocity and property gradient sections show vertical layering at the flanks and underneath the eddy characteristic for vertical propagation (to several hundred-meters depth) of near inertial internal waves (NIW) and confirmed by direct current measurements. A narrow region exists at the outer edge of the eddy where NIW can propagate downward. NIW phase speed and mean flow are of similar magnitude and critical layer formation is expected to occur. An asymmetry in the NIW pattern is seen that possible relates to the large-scale Ekman transport interacting with ACME dynamics. NIW/mean flow induced mixing occurs close to the euphotic zone/mixed layer and upward nutrient flux is expected and supported by the observations. Combing high resolution nitrate (NO3−) data with the apparent oxygen utilization (AOU) reveals AOU:NO3− ratios of 16 which are much higher than in the surrounding waters (8.1). A maximum NO3− deficit of 4 to 6 µmol kg−1 is estimated for the low oxygen core. Denitrification would be a possible explanation. This study provides evidence that the recycling of NO3−, extracted from the eddy core and replenished into the core via the particle export, may quantitatively be more important. In this case, the particulate phase is of keys importance in decoupling the nitrogen from the oxygen cycling.

Description: During the last 15 years, substantial progress has been achieved in altimetry data processing, providing now data with enough accuracy to illustrate the potential of these observations for coastal applications. In parallel, new altimetry techniques improve the data quality by reducing the land contamination and by enhancing the signal-to-noise ratio. Satellite altimetry provides ever more robust and accurate measurements ever closer to the coast and resolve ever shorter ocean signals. An important issue is now to learn how to use altimetry data in conjunction with the other coastal observing techniques. Here, we demonstrate the ability of satellite altimetry to observe part of the Northern Current variability. We cross-compare and combine the currents provided by large data sets of ship-mounted ADCPs, gliders, HF radars and altimetry. We analyze how the different available observing techniques capture the current variability at different time-scales. We also study the coherence/divergence/complementarity of the informations derived from the different instruments considered. Two generation of altimetry missions are used: Jason 2 (nadir Ku-band radar) and SARAL/AltiKa (nadir Ka-band altimetry); their performances are compared. In terms of mean speed of the Northern Current, a very good spatial continuity and coherence is observed at regional scale, showing the complementarity between all the types of current measurements. In terms of current variability, there is still a good spatial coherence but the amplitude of the seasonal variations is underestimated by ~50% in altimetry, compared to both gliders and ADCPs, because of a too low spatial resolution. For individual dates this number varies a lot as a function of the distance to the coast and width of the Northern Current. Compared to Jason 2, the SARAL/AltiKa data tend to give estimations of the NC characteristics that are closer to in situ data in a number of cases. Satellite altimetry obviously provides a synoptic view of the Northern Current circulation system and variability which helps to interpret the other current observations. Its regular sampling allows the observation of many features that may be missed by in situ measurements.

Description: During the last 15 years, substantial progress has been achieved in altimetry data processing, now providing data with enough accuracy to illustrate the potential of these observations for coastal applications. In parallel, new altimetry techniques improve data quality by reducing land contamination and enhancing the signal-to-noise ratio. Satellite altimetry provides more robust and accurate measurements ever closer to the coast and resolve shorter ocean signals. An important issue is now to learn how to use altimetry data in conjunction with other coastal observing techniques. Here, we cross-compare and combine the coastal currents provided by large datasets of ship-mounted acoustic Doppler current profilers (ADCPs), gliders, high-frequency (HF) radars and altimetry. We analyze how the different available observing techniques, with a particular focus on altimetry, capture the Northern Current variability at different timescales. We also study the coherence, divergence and complementarity of the information derived from the different instruments considered. Two generations of altimetry missions and both 1 Hz and high-rate measurements are used: Jason-2 (nadir Ku-band radar) and SARAL/AltiKa (nadir Ka-band altimetry); their performances are compared. In terms of mean speed of the Northern Current, a very good spatial continuity and coherence is observed at regional scale, showing the complementarity among the types of current measurements. In terms of current variability, there is still a good spatial coherence but the Northern Current amplitudes derived from altimetry, glider, ADCP and HF radar data differ, mainly because of differences in their respective spatial and temporal resolutions. If we consider seasonal variations, 1 Hz altimetry captures ∼60 % and ∼55 % of the continental slope current amplitude observed by the gliders and by the ADCPs, respectively. For individual dates this number varies a lot as a function of the characteristics of the Northern Current on the corresponding date, with no clear seasonal tendency observed. Compared to Jason-2, the SARAL altimeter data tend to give estimations of the NC characteristics that are closer to in situ data in a number of cases. The much noisier high-rate altimetry data appear to be more difficult to analyze but they provide current estimates that are generally closer to the other types of current measurements. Thus, satellite altimetry provides a synoptic view of the Northern Current circulation system and variability, which helps to interpret the other observations. Its regular sampling allows for the observation of many features that may be missed by irregular in situ data.

Description: Two large-scale free-drifting isobaric-floats experiments, “SOFARGOS”/Marine Science and Technology Programme, phase 2 (MAST2) and Mass Transfer and Ecosystem Response (MATER)/MAST3, undertaken in 1994–95 in the northwestern Mediterranean Sea and in 1997–98 in the Algerian Basin, respectively, have revealed for the first time that Western Mediterranean Deep Water, newly formed by deep convection in the Gulf of Lion (the so-called Medoc site), can be advected several hundreds of kilometers away from the formation area by anticyclonic submesoscale coherent vortices (SCVs). This behavior implies that SCVs participate actively in the large-scale thermohaline circulation and deep ventilation of the western Mediterranean Sea. These SCVs are characterized by small radius (5 km), very low potential vorticity, high aspect ratio (0.1), and extended lifetime (〉0.5 yr).

Description: Mesoscale anticyclonic eddies in the Irminger Sea are observed using a mooring and a glider. Between 2002 and 2009, the mooring observed 53 anticyclones. Using a kinematic model, objective estimates of eddy length scales and velocity structure are made for 16 eddies. Anticyclones had a mean core diameter of 12 km, and their mean peak observed azimuthal speed was 0.1 m s(-1). They had core salinities and potential temperatures of 34.91-34.98 and 4.488-5.34 degrees C, respectively, making them warm and salty features. These properties represent a typical salinity anomaly of 0.03 and a temperature anomaly of 0.28 degrees C from noneddy values. All eddies had small (〈〈 1) Rossby numbers. In 2006, the glider observed two anticyclones having diameters of about 20 km and peak azimuthal speeds of about 0.3 m s(-1). Similar salinity anomalies were detected throughout the Irminger Sea by floats profiling in anticyclones. Two formation regions for the eddies are identified: one to the west of the Reykjanes Ridge and the other off the East Greenland Irminger Current near Cape Farewell close to the mooring. Observations indicate that eddies formed in the former region are larger than eddies observed at the mooring. A clear increase in eddy salinity is observed between 2002 and 2009. The observed breakup of these eddies in winter implies that they are a source of salt for the central gyre. The anticyclones are similar to those found in both the Labrador Sea and Norwegian Sea, making them a ubiquitous feature of the subpolar North Atlantic basins.