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Abstract. During the last 15 years, substantial progress has been achievedin altimetry data processing, now providing data with enough accuracy toillustrate the potential of these observations for coastal applications. Inparallel, new altimetry techniques improve data quality by reducingland contamination and enhancing the signal-to-noise ratio. Satellitealtimetry provides more robust and accurate measurements ever closer to thecoast and resolve shorter ocean signals. An important issue is now to learnhow to use altimetry data in conjunction with other coastal observingtechniques. Here, we cross-compare and combine the coastal currents provided by largedatasets of ship-mounted acoustic Doppler current profilers (ADCPs), gliders,high-frequency (HF) radars and altimetry. We analyze how the differentavailable observing techniques, with a particular focus on altimetry, capturethe Northern Current variability at different timescales. We also study thecoherence, divergence and complementarity of the information derived from thedifferent instruments considered. Two generations of altimetry missions andboth 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 spatialcontinuity and coherence is observed at regional scale, showing thecomplementarity among the types of current measurements. In terms ofcurrent variability, there is still a good spatial coherence but the NorthernCurrent amplitudes derived from altimetry, glider, ADCP and HF radar datadiffer, mainly because of differences in their respective spatial andtemporal resolutions. If we consider seasonal variations, 1 Hz altimetrycaptures ∼60  % and ∼55  % of the continental slope currentamplitude observed by the gliders and by the ADCPs, respectively. Forindividual dates this number varies a lot as a function of thecharacteristics of the Northern Current on the corresponding date, with noclear seasonal tendency observed. Compared to Jason-2, the SARAL altimeterdata tend to give estimations of the NC characteristics that are closer to insitu data in a number of cases. The much noisier high-rate altimetry dataappear to be more difficult to analyze but they provide current estimatesthat are generally closer to the other types of current measurements. Thus,satellite altimetry provides a synoptic view of the Northern Currentcirculation system and variability, which helps to interpret the otherobservations. Its regular sampling allows for the observation of many featuresthat may be missed by irregular in situ data.

Synergy between in situ and altimetry data to observe and study Northern Current variations (NW Mediterranean Sea)