Impact of cross-reef fluxes on the Ouano lagoon circulation.

Transcription

1 Impct of cross-reef fluxes on the Ouno lgoon circultion. Cristele Chevlier, Jen-Luc Devenon, Vincent Rey Mediterrnen Institute of Ocenogrphy (MIO), CNRS/INSU, UMR 7294 nd IRD, UR235, Mrseille Cedex, Frnce Corresponding uthor: Astrct. In meso-tidl lgoons the corl reef rrier cn e temporrily covered y wter t high tide nd emerged t low tide, inducing prticulr cross-reef dynmics. In order to understnd the importnce of crossreef fluxes on lgoon dynmics field survey hd een conducted on Ouno Lgoon (New Cledoni). The lgoon wter dynmics were nlyzed nd compred to numericl model output. The min current ws found to e northwrd during the flood, with inwrd flows through the ocen psses nd through the south pssge nd outwrd flows through the north pssge. During the e, currents re usully reversed. Concerning tidlly verged fluxes, they re minly inwrd t the ocen psses nd southwrd in pssges. Finlly, numericl tests were performed to evlute the impct of cross-reef fluxes on this circultion. The results of this study re, then, discussed. Key words: tidl reef-lgoon; corl reef rrier; cross-reef fluxes; New Cledoni Lgoon. Introduction Wter qulity in tropicl lgoons depends on wter renewl time, which is regulted essentilly y fluxes through pssges nd over the corl reef rriers (Pges nd Andrefouet 2001; Andrefouet et l. 2001). The slightly or strongly sumerged chrcter of the reef rriers cn reduce or enhnce these ocenlgoon hydrodynmic fluxes. In ddition, reef structures surrounding tropicl lgoons re prticulrly sensitive to the qulity of the mrine wters tht the them. It ppers therefore necessry to fully understnd the hydrodynmic regultions exerted y these reef rriers in order to forecst the influence of climte chnge or nthropogenic ctivities on the lgoon resilience. A numer of studies hve ddressed specific hydrodynmics in the vicinity of reef rriers. Some uthors hve studied enriched upwelling generted long them (Leichter nd Miller 1999, Henin nd Cresswel 2005) while others hve nlysed wve driven flows over corl reefs, s reviewed y Monismith (2007). Some studies hve lso tried to evlute the cross-reef fluxes induced y reking wves ove the reef flt (Kench 1998; Hern 1999; Bonneton et l. 2007, Tey et l., 2011) nd the influence of these fluxes versus through-pssge fluxes in micro-tidl lgoons (Atkinson, 1981, Lowe et l, 2009). Despite these studies, few hve tried to evlute the respective roles of through-pssge versus cross-reef fluxes in corl reef lgoons. Studies ddressing wter exchnges etween meso-tidl lgoons nd the open se often focus on fluxes through reef pssges only, s descried y Douillet (1998) nd Douillet et l. (2001) for the Noume lgoon in New Cledoni. Indeed, the rtio etween pssge size nd reef-rrier length in lrge lgoons such s Noume s induces primrily through-pssge tidl exchnges with the ocen. The cross-reef fluxes re minly ttriutle to swell reking over the reef (Bonneton et l., 2007) nd cn modify inside lgoon current dynmics in their vicinity only. For some smller meso-tidl lgoons with gret reef-rrier length compred to the length of pssges, verticl section cross pssge cn e similr to the one ove the reef flt when it is sumerged. Hence, t high tide, cross-reef fluxes cn e, t lest, s gret s throughpssge exchnges or even predominnt (Angwenyi nd Ryderg, 2005) in extreme cses such s when the lgoon is shllow with respect to the tide. Conversely, cross-reef fluxes ecome lmost negligile t low tide when the reef rrier is slightly sumerged or totlly exposed. Our im is to determine the possile influence of cross-reef fluxes, in quite deep meso-tidl lgoons. We focus our study on the Ouno reef Lgoon, locted in the southwest of New Cledoni Islnd. The current in pssges hs een cquired with ADCP t clssicl fixed moorings nd our results llow us to

2 develop model of this lgoon. A detiled description of Ouno hydrodynmics functioning ws otined nd tests were mde on the impct of cross-reef fluxes on this circultion. Finlly, we discuss the role of cross-reef fluxes on the lgoon circultion. Mteril nd Methods Study site The field oservtions were performed t the Ouno lgoon locted t the north-west lgoon of New Cledoni. This lgoon, prtilly closed long the long reef, is pproximtely 30 km long, 10 km wide nd 10 m deep. It is opened to the Pcific Ocen through two psses of out 1 km wide (Isie nd Ouri). Ouri is the deeper with out 40m deep wheres Isie is only 10m deep. Furthermore, the lgoon is connected to northern nd southern lgoons y two pssges of 5 km wide. The north pssge is the shllower (out 5m), the south pssge eing 10 m deep. At high wter, the reef is sumerged (2 m), wheres it is wekly immerged t low wter. The incoming flow could e either of Pcific wter (entering through the two psses nd ove the reef) or lgoon wter from the lgoons situted t the south nd north ends (entering through the south nd north pssges). This lgoon is sumitted to the semi-diurnl wve M North pssge Ouri pssge Isie pssge TEREMBA OUANO 10 5 BOURAKE South pssge Figure 1: Ouno lgoon, its thymetry. Scttered ellipse locted on mooring position nd men current mesured during the Ouno field cmpign. Sensor deployment Our oservtions were performed from 2011, July 11th, to 2011, Octoer 3th. Four moorings equipped with ADCP were deployed to gin insight into temporl vriility of the velocity verticl profiles. The first is locted in the Ouri pss, the second in the Isie pss nd the two ltter in the north nd south pssges (Figure 1) Model The model employed is the Regionl Ocenic Modeling System (ROMS) (Shchepetkin nd McWillims, 2005), split-explicit, free-surfce ocenic model discretized in costline- nd terrinfollowing curviliner coordintes. The horizontl mesh ws 500 m nd the 9 s- coordinte verticl levels re stretched towrd the surfce (Hidvogel nd Beckmnn, 1999). Bottom topogrphy (h) is derived from SHOM dt. To prevent the genertion of pressure grdient errors, h is smoothed using selective Shpiro filter in order to keep the topogrphic prmeter (Hidvogel nd Beckmnn, 1999). Tide is forced on oundries y using the high resolution model developed y J. Lefevre. Surfce forcing is derived from the Comprehensive Ocen/Atmosphere Dt Set (COADS) (D Silv et l., 1994) monthly climtology. At the lterl oundries, n ctive rdition condition connects the model solution to the surroundings (Mrchesiello et l., 2001): in the cse of inflow conditions, the solution is nudged towrd dt. World Ocen Atls 2001 (WOA) (Conkright et l., 2002) men-monthly climtology provides temperture nd slinity. These dt nd COADS winds re comined to estimte geostrophic nd Ekmn velocity components for the oundries. Model initiliztion is done y using WOA temperture nd slinity nd no flow. The simultion ws run for 3 months. Integrted properties show tht fter spin-up of few dys, the model hs reched sttisticl equilirium (figure not shown). Results re then nlyzed fter this spin-up time. To nlyze the influence of the cross-reef fluxes on the lgoon circultion, three different reef prmeteriztions were developed. In the reference model, the reef is supposed to e n impervious wll. In the second one, the swell input due to the reking wves on the reef is modelize y wter input long the reef. In the lst model, the reef is immersed (1m) nd let go through the tidl current. Results ADCP dt Tidl mplitude t moorings ws pproximtely 0.8 m during the spring tide nd 0.25 m during nep tides. The tide ws minly semi-diurnl with diurnl inequlity. The se level power spectrum estimted using periodogrm nlysis (Bendt nd Piersol 2000) confirms the prevlence of the semi-diurnl nd diurnl tidl wves M2, S2 nd K1 in the studied re. Tidl mplitude vried slightly inside the lgoon, nd wek phse lg of few minutes ppered etween sttions (Tle 1).

3 Model result M2 S2 Mooring Ampl Phse Ampl Phse North Ouri Isie South Tle 1: Min tidl wves of the wter level vriility: Amplitude (cm) nd Phse ( ). zet - 4 Jn 1 The min current direction ws minly cross-pssge in the north pssge nd followed isoths in the Ouri nd Isie psses. This current revels wter exchnges with the studied lgoon nd the ocen. Current mesurements, lso revels current minly getting out of the y through the south pssge while the verged current ws minly inwrd through the north pssge. In Isie nd Ouri psses, the current ws wek nd rther in-flow. Therefore, the wter of the Ouno lgoon seems to minly come from the lgoon situted t the north of the studied one. Dt show n in- nd out-flow oscilltions linked to the tidl regime. Velocity ws found to e reltively high t moorings in north nd south pssges, regulrly up to 0.5 m.s -1 nd reching 1 m.s -1 in July due to high residul current (round 0.5 m.s -1 ). In Isie nd Ouri psses, this residul current is weker nd its mesured velocity is regulrly under 0.5 m.s -1. Hrmonic nlysis (Foremn, 1978) of the velocity dt confirms the predominnce of the semi-diurnl tidl wves M2 nd S2, nd diurnl tidl wves O1 nd K1 on the velocity signl. This nlysis lso revels the presence of qurter-diurnl wves (M4, MS4) in the velocity signl. It is proly due to wter exchnges cross the reef s it occurs on Myotte nd Tuler Lgoons (2011). The phse lg etween the wter level nd the current rnges from 2 to 3 hours for the M2 wve in pssges nd in Isie pss. At the Ouri pss, velocity nd current re quite in phse (Tle 2). Therefore, for the tidl wve M2, the wter rrives during the flow, from the south pssge nd from the two psses ut gets out from the north pssge nd reversely during the flow. M2 O1 Mooring Ampl Phse Ampl Phse North Ouri Isie South Tle 2: Min tidl wves of the velocity in the min direction Amplitude (cm.s -1 ) nd Phse ( ). The velocity is positive when the current is getting out. 40' ' Figure 2: Circultion in the Ouno lgoon during the flow () nd the e (), on the 2012, July 16 th. Reference model: the reef hs een considered s n impervious wll. Red quivers correspond to mesured velocities. Despite its schemtic chrcteristic, model qulittively grees with the field mesurements. The generl circultion dynmics seem to e well reproduced (figure 2). Amplitude nd phse of the min tidl wves of the wter level nd the velocity re in the sme order tht those mesured (for instnce, see plots on figure 3). However, in this configurtion, the vriility of north pssge residul current is not reproduced, notly due to the monthlyverged climtology.

4 zet - 4 Jn 1 Wter level 1 (m) Velocity 0.5 (m/s) Velocity 0.5 (m/s) 0 0 c dte July Figure 3: Mesurement (red line) nd model results (lck line): wter level in the north pssge (), tidl wve component of velocity in the min direction in the Isie pss () nd in the north pssge (c). 40' zet - 5 Jn 1 Discussion The glol circultion otined with the three models is then nlyzed. In oth simultions, the min chrcteristic of circultion is mintined. However, if this reef is immersed, we cn notice tht the circultion my e enhnced in some re ner the reef wheres, the glol circultion seems to e dmpened. If wter input is prmeterized t the reef, the intensity of circultion is slightly reduced during the flow ut it is slightly enhnced during the e. In contrst, the velocity in psses is strongly modified in the two cses. If the prmeteriztion include wter cross-reef input, the out-flow in psses is enhnced. If the reef is immersed, the tidl vriility of current in psses is reduced. 40' Figure 4: Circultion in the Ouno lgoon, on the 2012, July 16 th, if cross-reef in-flow is imposed: during the flood () nd the e ().

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