The Effect of Mistral Wind on the Ligurian Current Near

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.

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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. 25 temperature intervals constant speedof about 20 cm.seç-l when the wind in Figure 2, a to g, are rcliable. decreasedto about 5-6 m.sec-l, and acceleratedto 1 During this period, the nearshore current was about 30 cm.sec whenthewinddropped.Asarule,thespeed 20 km wide, being characterized by water about 1 "C to the West seemsto increase,as the wind from the West 1 warmer than that offshore. An interesting feature is the decreases;a surfacecurr-ent of about 30 cm. sec would existence of strong alongshore gradients, with 2 to 4 be expectedfrom a surfacewind of -10 m.sec-1, and isotherms perpendicular to the coast over a distance of this is not inconsistentwith the notion that strong local about 10 km. This frontal structure represents the winds could halt the Westward progress of the surface "head" of the Ligurian current, and gives.information outcrop of the Ligurian current. Detailed relations about its dynamics. between the current flow and the wind stress,need an extensiveknowledge of the meteorological and hydrolo- WIND-CURRENT INTERACTION S gical fields in the whole basin, and daily maps of the sea- surfacetemperature, do not allow more accurate studies The daily mean wind vectors of the Table are than those here presented. superimposedon the satellite-derivedtemperature charts (note the cyclonic circulation of the Mistral around the coast of Provence). On July17, 18 and 19, the Westerly wind blows strongly 1 (8-10 m. sec- at Le Levant) againstthe typical direction of the Ligurian current : during this time, the front is relatively stationary. On July 20, the wind dropped(4 m.sec-1 at Le Levant): no infrared sensingof the sea-surfacewas possible;but on July 21, the front had propagated Westward to the Western edge of the Island of Porquerolles. The front continued to progress Westward from July 2I to 24, a Figure 4 period during which the wind in its vicinity fell from - Although cloud-freeimages are in late Summerand early the oiew about 5 m. 1 1. few fall, sec to about 2 m. sec Strbngerwinds at taken on September27 , 1979,suggests a circulation which is supported the Bec de I'Aigle station occurred when the front was by in situ current measurements:during this time, the lowering of the N W located in the vicinity of Toulon: they did not affect its winds allows the Ligurian current to flow along the coûst onto the continental shelf of the Gulf of Lions. Westward motion. Bien que les images sans nuagessoient peu nombreusesvers la fin de The speed of propagation of the frontal stlucture is l'été et le début de l'automne, la vue prise le 27 septembre1979 suggère difficult to specify because of the irregularities of the une circulation qui est confirméepar desmesures in sita: à cetteépoque, l'atténuation des vents de NW permet au courant liguro-provençal de progresserle long de la côte sur le plateau continental du Golfe du Lion.

The later temperature charts show that the Ligurian current tends to follow the shore as far as the Gulf of Lions. But by July 24, the warm water has not reached the continental slope off the Gulf, so it is not possibleto say whether the flow would be limiTedby the continental slope or flow inshore along the coast. During July and August, North-Westerly winds are too frequent to allow the warm Ligurian current to penetrate onto the Gulf of Lions. In late summer, the NW component of the wind is small: in sl/z measurementsobtained near the surfaceat about 20 km off the coast of Camargue (LION 77 Figure 3 experiment), strongly suggestthat the Ligurian current Mean locations oJ thefront are indicated and claily speedsof propagqtion flows onto the shelf. The lowering of NW winds is hauebeen estimated along two lines parallel to the coast. Unfi[ July 79, the front is roughly stationary, thereafter it moues Westwards at speeds correlatedwith an increasingofthe cloud-cover,and then up to about 30 cn . sec I . when the current passesthe Provence coasts, it is not Les positions du front ont été reportéeset son déplacementa été estimé easily observed from space: fortunatly, this has been le long de deux lignesparallèles à la côte. Jusqu'au 19juillet le front est pratiquement immobile, puis sa vitesse de déplacement vers I'Ouest possible on September27, 1979, with the infrared - augmentejusqu'à des valeurs de i'ordre de 30 cm . s 1. scanner of Tiros N. Figure 4 represents the rough

401 C. MILLOT,L. WALD structure of the isotherms: it is clear that ( I ) the current not progress.When the wind stops,the Ligurian current follows the coastline and(2) it flows onto the shelf.Let us is found to flow along the coast, off Provence and then emphasizethat, in the frontal zone, the radiometer never onto the continental shelf of the Gulf of Lions. measuresa seasurface temperature of 18"C (mean value over a pixel): horizontal gradients in this area are Acknowledgements expectedto be very large; note that the outer edge ofthe current is well defined. The Centre de Météorologie Spatiale de Lannion generously made available the satellite data.

CONCLUSION REFERENCES

Albuisson M., Pontier L., Wald L., 1979. A comparison between sea- It is shown that somefeatures of the Ligurian current can surlace temperature measurements from satellite NOAA 4 and from be studied, using satellite-derived temperatures as a airborne radiometer Aries, Oceanol.Acta,2, l, 1-4. tracer. On the occasion studied, there'was a frontal Bethoux J. P., Prieur L., 1980. Évaluation des {lux d'eaux de la circulation du Nord-Est region between the warm water of the Ligurian du bassin occidental, Proc. XXVI Congrès current AssemblëePlénière C.I.E.S.M., Antalya, Turkey, 1978. and the cool water upwelled from the Gulf of Lions. In Lacombe H., Tchernia P., 1972. in: The Meditenanean Sea, edited by light winds periods, this frontal region propagated D. J. Stanley. Westward (the normal direction of the Ligurian current), 1. Millot C., 1979.Wind-induced upwellingsin the Gulf of Lions,Oceanol. at up to 30 cm. sec- But, with strong Westerlies,it does Acta,2,3,261-274.

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