<title>Tracking anticyclonic open-sea eddies in the Algerian basin by altimetry</title>

Abstract

ABSTRACT The evolution of an open-sea eddy, previously detected by infrared imagery, is studied using Sea Level Anomalies(SLA) maps. This evolution is compared with the information given by surface drifters deployed along eddy diam-eter, during ALGERS98 cruise in western Mediterranean. Preliminary results show a good agreement between thestructure observed from drifters trajectories and the velocity field obtained from SLA maps assuming a geostrophicbalance. The center of the eddy is taken as the grid point of maximum SLA. Differences between this center and thecenter given by the drifters seem to be due to the precision of the grid used by altimetric maps. From the positionof the centers the propagation velocity of th eddy is obtained. The mean velocity obtained from altimetry is 2.7km/day which is coherent with previous studies.Keywords: altimetry, AVHRR, in situ measurements, Algerian basin, mesoscale eddies 1. INTRODUCTION The Algerian basin, western Mediterranean sea, is characterized by the presence of a strong and narrow currentfollowing the Algerian coast. Meandering of this current can generate eddies that are anticyclonic for the biggest,long-lived and most energetic ones. These eddies tend to move eastwards until the Channel of Sardinia where theydetach from coast and become open-sea eddies that propagate westwards. These eddies are characterized by anhorizontal length scale of 100-200 km and surface velocities of 30-50 cm/s.' This information has been obtained,mainly, using infrared imagery2 because of the scarcity of hidrographic measurements in this area. Our objective isto complement the existing information using Sea Level Anomalies (SLA) maps.Typical surface elevations for the eddies (10-20 cm) and propagation velocities (2-5 km/day)'are adequate toSLA maps. In fact altimetry had already been used successfully in that area before. First work done by Vignudelli(1997) showed that altimetry was able to detect eddies even though it tends to underestimate surface velocities.Comparisons between altimetry and infrared imagery done by Tudicone et al. (1998) definitively proved the directrelation between SLA and the Mediterranean eddy field. Finally Bouzinac et al. (1998) showed the usefulness ofSLA maps to study mesoscale phenomena in the Algerian basin.During winter 1998 one of these eddies was tracked using infrared imagery. This information was used to definethe sampling strategy for the ALGERS98 cruise on board the Spanish R.V. Hesperides. The eddy 3D structure wassampled and its evolution tracked with surface drifting buoys. The confidence of SLA maps in tracking the eddy hasbeen tested with these in situ measurements.The paper is organized as follows: data and methodology used are presented in section 2, and results are givenin section 3. Finally section 4 presents preliminary conclusions.