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Coastal mesoscale eddies are important oceanic structures partially responsible for regulating ocean-shelf exchanges. However, their description and characterization are challenging; observations are often too scarce for studying their physical properties and environmental impacts at the required spatio-temporal resolution. Therefore, models and data extrapolation methods are key tools for this purpose. Observations from high-frequency radar, one satellite and two gliders, are used here to better characterize the three-dimensional structure of a coastal mode-water eddy from a multiplatform approach in the southeastern Bay of Biscay in spring 2018. After the joint analysis of the observations, a three-dimensional data reconstruction method is applied to reconstruct the eddy current velocity field and estimate the associated water volume transport. The target eddy is detected by surface observations (high-frequency radar and satellite) for two weeks and presents similar dimensions and lifetimes as other eddies studied previously in the same location. However, this is the first time that the water column properties are also observed for this region, which depicts a mode-water eddy behavior, i.e., an uplift of the isopycnals in the near-surface and a downlift deeper in the water column. The reconstructed upper water column (1–100 m) eddy dynamics agree with the geostrophic dynamics observed by one of the gliders and result in cross-shelf inshore (offshore) volume transports between 0.04 (−0.01) and 0.15 (−0.11) Sv. The multiplatform data approach and the data reconstruction method are here highlighted as useful tools to characterize and three-dimensionally reconstruct coastal mesoscale processes in coastal areas. [ABSTRACT FROM AUTHOR]

Abstract: An intensive observational programme of the Balearic Front, in the Western Mediterranean Sea, was carried out using conventional (CTDs from a ship) and new technologies (gliders). The strategy consisted of repeated sampling of a section almost perpendicular to the Balearic Current situated to the north of Mallorca Island. This section is coincident with track 773 of the ENVISAT satellite. In this work we present results of missions undertaken in July and September 2007. In July the sampling was designed to collect in-situ data from a ship and a coastal glider, almost simultaneously with the passage of the radar altimeter on board the ENVISAT satellite. In September the sampling was done only by the glider platform, and also simultaneously with the passage of the satellite. The Balearic Front was clearly detected by salinity (not temperature), with lower values (37.4 PSU) near the coast that are likely to be related to the input of recent Atlantic Water through the Mallorca Channel. The observed width of the front was about 35–40 km, with a vertical extension of 200 m. It had a significantly different pattern between the July and September samplings, which confirms the high mesoscale variability of this frontal area. Using a reference depth level of 180 m we have obtained maximum geostrophic velocities of 40–45 cm/s, flowing northeastwards along the north Mallorca coast. These velocity estimates are not sensitive to the test reference level (600 m), indicating that the layer between 200 and 600 m does not play a key role in the dynamics of the upper layer (200 m). A dynamic height rise from July to September (in 70 days) of about 3–7 cm reflects the seasonal cycle of sea level due mainly to thermosteric expansion of the water column. This gives a rise rate in the range of 1.3–3 cm/month, which is in agreement with previous estimates in the Mediterranean Sea. Absolute dynamic topography from altimetry data reveals good coherence with the dynamic height from the glider data in the area where there are common observations. In September, ENVISAT captured the sharp gradient observed with in-situ data, with mean velocities of about 24 cm/s, but with a smaller variance than the glider due to the larger wavelength of the filter applied to the altimeter data. From the technological point of view the observational experiment has confirmed the feasibility of using a coastal glider to monitor the spatial and low-frequency variability of the coastal ocean. The coastal glider was appropriate in this particular case since the vertical extension of the front only reaches up to 200 m. The autonomous platform was shown to be strongly robust, and able to monitor the area even under adverse meteorological conditions. This represents an important achievement, since under the same conditions the CTD sampling from a traditional research vessel would normally have been cancelled. Moreover, this type of platform allows autonomous collection of CTD casts and biogeochemical measurements at high spatial resolutions and at very low costs compared to conventional methods. [Copyright &y& Elsevier]

The analysis of deep-water glider hydrographic and fluorescence data, together with satellite measurements provides a new insight into eddy-induced anomalies within the South-Eastern Bay of Biscay, during summer. Two cyclonic eddies and a SWODDY have been observed in different glider transects and by means of different sources of satellite data. Vertical profiles reveal complex structures (characteristic of the second baroclinic mode): upward/downward displacement of the seasonal/permanent thermocline in the case of X13 and the opposite thermocline displacements in the case of the cyclones. This is a typical behaviour of mode-water and “cyclonic thinny” eddies. A qualitative analysis of the vertical velocities in the anticyclone indicates that though geostrophy dominates the main water column, depressing the isopycnals, near the sea-surface the eddy-wind interaction affects the vertical currents, favouring Ekman pumping and upwelling. The analysis of the Θ-S properties corroborates that inside cyclones and between the 26 and 27 isopycnals, net downwelling occurs. These two types of intra-thermocline lenses appear to deeply impact the Chl-a fluorescence profiles, since the maximum Chl-a fluorescence is located just below the seasonal thermocline. The mean Chl-a fluorescence was higher in the anticyclone than within the cyclones and the mean for the entire study period; the highest values were observed in the centre of the anticyclone. These results are in agreement with previous findings concerning the SWODDY F90 and surrounding cyclones, located in the South-Western Bay of Biscay. Significant differences in the Θ-S properties of the two cyclonic mesoscale structures have been observed: higher temperatures and lower salinity in the easternmost cyclone. Finally, time variation of the salinity content of the shallowest water masses of the anticyclone (salinity decreasing over time), probably indicates advective mixing processes occurred during the mission. [ABSTRACT FROM AUTHOR]