The effect of Mistral wind on the Ligurian current near Provence Claude Millot, Lucien Wald To cite this version: Claude Millot, Lucien Wald. The effect of Mistral wind on the Ligurian current near Provence. Oceanologica Acta, Elsevier, 1980, 3 (4), pp.399-402. <hal-00463850> HAL Id: hal-00463850 https://hal-mines-paristech.archives-ouvertes.fr/hal-00463850 Submitted on 16 Mar 2010 HAL is a multi-disciplinary open access L'archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destin´eeau d´ep^otet `ala diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publi´esou non, lished or not. The documents may come from ´emanant des ´etablissements d'enseignement et de teaching and research institutions in France or recherche fran¸caisou ´etrangers,des laboratoires abroad, or from public or private research centers. publics ou priv´es. OCEANOLOGICA Ligurian current General circulalrr,rn Wind-induced circulatron The effect of Mistral wind Remote sensing Courant liguro-provençal Circulation générale Circulation induite par le vent on the Ligurian current Té1édétection near Provence C. Millot", L. Waldb uAntenne du Laboratoire d'Océanographie Physique du Muséum National d'Histoire Naturelle,B.P. 2, 83501La Seyne-sur-Mer,France. bCentre de Télédétectionet d'Analyse desMilieux Naturels de l'École des Mines. Soohia- Antipolis, 06560 Valbonne, France. Received1611180, in revised form22l4l80, accepted j l5lg0. The ABSTRACT effect of the Mistral wind on the Ligurian current has been studied using the NOAA 5-VHRRimagery. A frontalzone separates the Ligurian current and colder water upwelledfrom the Gulf of Lions. It is found that the surfaceflow associatedwith the c_urrent,is haltedby strongWesterly winds. When the wind drops,the frontal zone moves Westwardat speedsup to 0.3 m.sec-1.During a periodof siratification,the Ligurian currentin the surfacelayer tendsto flow along the coastsof the Gulf of Lions. Oceanol.Acta, 1980.3.4.399-402. RESUME L'action du Mistral sur le courant liguro-provençal le long des côtes de provence L'effet du Mistral sur le courant liguro-provençala été étudié à partir des images transmisespar le VHRR du satelliteNOAA 5. Une zone frontalesépure les eaux du courant liguro-provençalde celles,plus froides,qui sont associéesaux upwellingsdu Golfedu Lion. On montreque le courantde surface est stoppé par deforts vênts d'Ouest. Quand le vent cesse,la zonefrontale se déplace vers I'Ouest avec des vitesses pouvant i atteindre30 cm. sec- - Lorsquele milieuest stratifié, le courantliguro-provençal dans la couchede surfacea tendanceà progresserle long de la côte,dansle Golfe du Lion. Oceqnol.Acta, 1980.3.4" 399-402. INTRODUCTION Figure 1 The mean oceanic circulation is resisted.by the N I( winds near the coasts It is well known that the meancirculation in the North- of Prouence. Western MediterraneanSea (Fig. 1) is cyclonicin the La circulation océanique moyenne est contrariée par les vents de NW le surfaceand intermediatelayers (Lacombe,Tchernia, long des côtes de Provence. 1972). In the Ligurian Basin, the surface currenrs circulatein the same(cyclonic) sense as the prevailinu winds.In the Gulf of Lions, however,thc pr.ciailing windsare from NW (- 320"at Sète),and they divergè overthe Ligurian Sea( - 290"at Toulon),and overthe BalearicBasin ( -340" at Cape Béar). So, the mean surfacecurrent systemis reinforcedby the wind in the South-Westernpart of the Gulf, but resistedby the wind near the coastof Provence,in the vicinity of Toulon. Off Nice,Bethoux and Prieur(1980) have lbund that the transportof the Ligurian current,between the surface and 200m in depth, and from the coast to g0 km offshore, W is of the order of the flows in the Straitsof 0399-178411 980/399/$ 5.00/ @ Gauthier-Villars 399 C. MILLOT,L. WALD 1). Gibraltar (about 106 m3. sec* Depending on its the reported values are affectedby the varying exposure vertical extent,which is probably affectedby the seasonal of the anemometers,so the time variation and relative thermocline, the inshore edge of the Ligurian current valuesare more indicative. The major variation consisted may be defined either by the coast or by the 100-200m of two periods of strong to fresh wind separatedby a isobath, which is as far as 80 km offshore in the Gulf of relative lull, especially at the Eastern stations. The Lions. slackeningof the wind on July 20, was associatedwith an overcast sky, which prevented satellite infra-red In the Gulf of Lions, there are insufficientobservations to observations of the sea surface. define the mean current pattern, but it is known (Millot, 1979) that there is a coastal upwelling which brings cold water to the surface. There is sometimes a SATELLITE IMAGES rapid change in temperature, or a frontal zone, separating the Ligurian wai.mer current from the The NOAA 5-VHRR has a spatial resolution at the upwelled colder water from the Gulf of Lions; we have nadir of - 1 km2, when its radiometric resolution is "K "K. used satelliteimages of the Ligurian current and of this 0 . 5 at 300 Data processingincludes a smoothing frontal zone to study the effect of stlong adversewinds of the imageswith a bidimensional filter, conditioned by during the period July 17 to 25, 1977. the importance of local gradients with respect to noise, which increasesthe reliability of relative values(accuracy of 0.5"K for a 3 km resolution;Albuisson et al., 1979). WIND OBSERVATIONS The set of observationslasts for about one week and in siru conditions (atmospheric absorption and atmosphe- Mean wind vectors were calculated from eight observations SEA SURFACETEMPERATURE at a three-hour interval for each of the five AND DAILYWINDS stations (Bec de 1'Aigle, Toulon, Porquerolles, Le FROMJULY 17T024, Levant, Camarat). The averagingperiod was arrangedto end at 09.00 UT, the approximate time at which the satellite was overhead (infrared data transmitted during the night were not recorded). During the period concerned,the Mistral wind was relatively high (Table): Table Three-hour wind. measurements obtained between successiuepassages of the satelli,te,are uectorially summedto get daily mean speeds.The drop in wind'speed on July 20,is signiJicantat all the stations.In Toulonand other Eastern stations, the wind speediluring theJirst part of the peùod is higher than during the second part. Les mesures de vent obtenues toutes les 3 heures entre les passagessuccessifs du satellite sont sommées vectoriellement pour obtenir dei vitesies journalières moyennes.L'atténuation du 20juillet est significativeà toutes les stations. A Toulon et aux stations situéesplus à I'Est, la vitessedu vent est plus élevée pendant la première partie du coup de vent que pendant la deuxième. Satellite Bec de Day view l'Aigle Toulon Porquerolles Le Levant Camarat 4.5 2.1 3.1 2.1 1.5 285 251 246 286 222 t6 8.9 4.0 +. I J.4 l-J 306 283 260 280 287 t7 8.0 6.6 8.4 7.3 3.8 296 284 260 285 237 Figure 2 8.5 7.4 9.0 10.0 5.1 18 The winr) and sea-surJàcetemperot ure f;eldsare drawn;t'romJuIy l'7 to 24. 278 289 260 276 218 Note the cyclonic rotation oJ the Mistral around the Prouencecoasts. 7.8 s.3 8.0 8.8 4.9 With respect to offshore waters, the eooler ones obseruedin the Gulf of 19 306 283 242 277 225 Lions are due to upwelling.and I he warmerones alonq the Easterncoâst"s of Prouence are linked to the Liguriàn current. The frontal sttucture 7.0 4.6 5.6 4.0 7.4 (gradients 20 of IoC per somekilometres) is stationaryfrom J uly Lj to 1.9, 293 276 233 280 )L and then it moDesWestwards along the cooit.:iiis is thought to be 2l 11.9 5.5 6.6 7.0 z-o related.to the lowefinq of the local wind stress. 307 288 259 283 189 La distribution spatiale du vent et de la température de surface est 11.8 5.1 5.9 5.5 5.8 reportée du 17 au 24jui11et.On peut noter la rotation cyclonique du 336 300 256 292 212 Mistral le long de la côte provençale.Par rapport aux eaux du large, les eaux plus froides observéesdans le Golfe du Lion sont associéesà des 1.9 .5 1.1 1.5 4.0 23 upwellings,et leseaux plus chaudesle long descôtes Est de la provence 119 339 204 211 193 sont celles du courant liguro-provençal. La structure frontale (gradients de "C 24 r.9 .3 1.7 2.8 3.0 1 pour quelqueskilomètres) est stationnaire du 17 au 19, puis elle 234 255 232 281 195 sedéplace vers 1'Ouest le long de la côte.Ceci estprobablement ielié à la baissede la tension du vent local. 400 THELIGURIAN CURRENT NEAR PROVENCE ric contentsin HrO, CO2, O3, roughnessand emissivity coastline. The daily speedscan be estimated along two of the seasurface, stratification of the upper layers, ...)are lines roughly parallel to the coast of Provence too variable to compare absolutevalues from one day to (seeFig. 3), and it is found that the front was stationary the other. Nevertheless,the induced errors are known to from 09.00 July 17 to 09.00 July 19, moved at about be sufficientlyhomogeneous, and they cannot significant- 19 cm.sec-r between09.00 July 19 and 09.00 July 21, ly affectthe large horizontal thermal gradientswhich are and at 19,23 and 32 cm sec-1 in the succeeding24-hour observed; when seen, clouds are not located in the periods. So, during this specific event, the front was vicinity of thesegradients. This paper usesonly the time roughly stationary when the local wind speed was of and space distributions of the sea-surfacetemperature about 8-9 m.sec 1; it moved against the wind with a "K gradients,and then, the + 0.