Diurnal upwelling resonantly driven by sea breezes around an Adriatic island

Diurnal upwelling resonantly driven by sea breezes aroundan Adriatic island

Mirko Orlić,1 Gordana Beg Paklar,2 Vlado Dadić,2 Nenad Leder,3 Hrvoje Mihanović,3

Miroslava Pasarić,1 and Zoran Pasarić1

Received 11 January 2011; revised 20 May 2011; accepted 21 June 2011; published 21 September 2011.

[1] Diurnal coastal upwelling was previously observed when sea breezes wereexceptionally strong, or when the process occurred close to critical latitudes (30°N, 30°S)where local inertial oscillations may be resonantly excited. Our data collected in theAdriatic show that the pronounced diurnal upwelling is also possible under milderwind‐forcing and outside critical latitudes. It is found that the thermocline recorded inthe summer of 2006 at the south coast of the island of Lastovo was subject to diurnalvariability with a maximum range of about 30 m, and that the corresponding currentsmeasured off the west coast of the island pointed to internal waves propagating aroundthe island in a clockwise direction. We suggest that the summertime stratificationoccasionally promotes coastal waves that revolve daily around the island, creating theconditions needed for resonant excitation by sea breezes. Numerical modeling revealsthat the 24‐h waves are trapped around the island due to the influence of both theCoriolis force and bottom slope, and that the 12‐h waves radiate away from the island.The biogeochemical data show that the diurnal upwelling may stimulate primaryproduction in the area but may also adversely affect benthic organisms.

Citation: Orlić, M., G. Beg Paklar, V. Dadić, N. Leder, H. Mihanović, M. Pasarić, and Z. Pasarić (2011), Diurnal upwellingresonantly driven by sea breezes around an Adriatic island, J. Geophys. Res., 116, C09025, doi:10.1029/2011JC006955.

1. Introduction

[2] Upwelling occurs in oceans and seas when horizontaldivergence in the surface layer creates water deficit that isfilled by water brought up from a subsurface layer [Barber,2001]. The divergence is mainly caused by winds, eitheropen ocean or coastal. Winds blowing at a coast produceupwelling by forcing offshore transport in the surface layer,compensating onshore transport in a subsurface layer, andascending motion close to the coast. Because the subsurfacewater is rich in nutrients, coastal upwelling, which bringsthose nutrients to the sunlit surface layer, supports intenseorganic productivity and is of great economic importance:though the areas occupied by upwelling constitute less than1% of the world’s oceans, they account for about 20% ofthe global fish catch [Pauly and Christensen, 1995]. Thesubsurface water transported to the surface layer is rela-tively cold and therefore coastal upwelling is also recog-nized as an important link in the processes affected byincreased concentrations of greenhouse gases: due to globalwarming upwelling is presently intensifying [Bakun, 1990;Roemmich and McGowan, 1995]—although not every-where [McGregor et al., 2007]—and is expected to feed-

back on the atmosphere and to influence the drawdown ofatmospheric CO2.[3] Early studies of coastal upwelling were mainly based

on classical hydrographical surveys of four eastern bound-ary currents and one western boundary current found insubtropical ocean regions: the Canary Current off north-western Africa, the Benguela Current off southwesternAfrica, the California Current off western North America,the Peru Current off western South America, and the SomaliCurrent off the Arabian Peninsula [Smith, 1968]. Upwellingin the Arabian Peninsula region is related to strong summermonsoon winds and is therefore highly seasonal; in the otherfour regions, upwelling is rather stable in lower latitudes andmore seasonal in higher latitudes. Subsequent investigationscapitalized on the then‐new technology of continuous cur-rent and temperature measurements to reveal superseasonal,day‐to‐day variability of upwelling, and the term “upwell-ing event” came into use [Smith, 1981; Codispoti, 1981].Long‐term hydrographical measurements and analyses ofsediment cores taken beneath upwelling systems made itpossible to track subseasonal, i.e., interannual and climaticvariability [Roemmich and McGowan, 1995; Summerhayeset al., 1995; McGregor et al., 2007].[4] The collection of time series of currents and temper-

ature enabled the documentation of not only day‐to‐day butalso diurnal variability of coastal upwelling. The analyses ofthe latter were undertaken relatively slowly, probablybecause the diurnal wind‐driven signal can be masked bythe diurnal tidal signal. When empirical studies of diurnal

1Andrija Mohorovičić Geophysical Institute, Faculty of Science,University of Zagreb, Zagreb, Croatia.

2Institute of Oceanography and Fisheries, Split, Croatia.3Hydrographic Institute of the Republic of Croatia, Split, Croatia.

Copyright 2011 by the American Geophysical Union.0148‐0227/11/2011JC006955

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C09025, doi:10.1029/2011JC006955, 2011

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http://dx.doi.org/10.1029/2011JC006955

upwelling started, they concentrated more on current vari-ability [Rosenfeld, 1988;DiMarco et al., 2000; Lerczak et al.,2001; Rippeth et al., 2002; Woodson et al., 2007; Hunteret al., 2007; Zhang et al., 2009] than on temperature var-iability [Kaplan et al., 2003; Woodson et al., 2007; Zhanget al., 2009]. The studies were carried out not only in tra-ditional regions where coastal upwelling occurs but also insome new regions (Texas‐Louisiana shelf, Catalonian shelf,New York Bight). Analytical and numerical modelingdeveloped at the same time as this empirical activity.Diurnal winds were supposed to blow over the sea boundedby a straight coast, and the sea was assumed to be homo-geneous [Rosenfeld, 1988], two‐layered [Rippeth et al.,2002] or continuously stratified [Lerczak et al., 2001].The picture that emerged from these studies was of pro-nounced diurnal upwelling related either to strong seabreezes that develop due to temperature differences betweenthe sea and land or to inertial oscillations resonantly excitedclose to critical latitudes (30°N, 30°S).[5] A recent experiment in the Adriatic Sea revealed that

the pronounced diurnal upwelling is also possible undermilder wind‐forcing and outside critical latitudes. We aim in

this paper to present empirical findings based on theAdriatic data set and to interpret them using analytical andnumerical modeling.

2. Data

[6] A portion of the Adriatic experiment relevant for thestudy of diurnal upwelling was carried out at stations shownin Figures 1a and 1b between February and September 2006.During the experiment (1) acoustic Doppler current profiler(ADCP) measurements were performed at the station shownusing a trawl‐resistant bottommount, (2) sea temperature wasrecorded with a series of sensors deployed along a steep cliffon the south side of the island of Lastovo, (3) sea level wasmonitored at the permanent Dubrovnik tide‐gauge station,and (4) meteorological conditions were documented by rou-tine stations in the area (Dubrovnik and Vis).[7] We used an RDI Workhorse Sentinel ADCP operating

at 300 kHz with a 15 min sampling interval for currentmeasurements. The bin size was 3 m and the bottom depth95 m, implying 28 measurement layers that extended fromthe 8.4 to 89.4 m depths. While the current measurements

Figure 1. Study area and spectral characteristics of currents and sea temperature measured at two sta-tions. (a) The Adriatic Sea; the solid rectangle marks the inset shown in Figure 1b. (b) Stations at whichsea‐temperature, current, sea level, and meteorological data were collected between February and Septem-ber 2006. (c) Depth‐frequency plots of power density obtained from (top) along‐basin currents and (bot-tom) cross‐basin currents recorded at station ADCP and detided by allowing for seven barotropic tidalconstituents. (d) Depth‐frequency plot of power density obtained from sea temperature data collectedat Lastovo station. All spectra were computed using the FFT method and a 30‐day long, half‐overlappingHamming window. Note that depth scales in the current and temperature spectra are different.

ORLIĆ ET AL.: RESONANTLY DRIVEN DIURNAL UPWELLING C09025C09025

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were performed following standard practice in oceanogra-phy, the temperature measurements were done in a some-what unusual way: thermistors were not deployed on amooring but instead on a steep island cliff. Because fishingactivities in the area would limit mooring use to a month ortwo, this approach was necessary to enable data collectionfor the entire study period. As it turned out, practicalnecessity proved to be scientifically beneficial, because cliffmeasurements provided information on temperature vari-ability at the coast where the diurnal upwelling is mostpronounced rather than offshore where the variability isattenuated. This measurement strategy has rarely been used,and if so, it has been utilized primarily to document theinternal tides [Wolanski et al., 2004]. We used TidbiT type

thermistors to record sea temperature with a 10 min tem-poral resolution and an accuracy of ±0.2°C at 21°C. Tensensors were distributed between depths of 4 and 40 m witha 4 m vertical resolution, along an almost vertical cliffreaching to the 80 m depth. All sensors were successfullyrecovered at the end of the stratified season.

3. Results

[8] The first step in the data analysis was to removebarotropic tides from the current data by calculating verti-cally averaged currents, performing harmonic analysis onthe averages [Pawlowicz et al., 2002] and subtracting thesynthesized tidal signal from the data measured at each level

Figure 2. Diurnal variability of sea temperature and its dependence on tidal and wind‐forcing over theinterval extending from mid‐June to mid‐August 2006. (a) Wavelets at the 24‐h period of the 20°C iso-therm displacement registered at Lastovo, sea level measured at Dubrovnik, and northwestward barotropiccurrents measured at station ADCP. (b) Wavelets at the 24‐h period of the 20°C isotherm displacementregistered at Lastovo (as in Figure 2a), northeastward wind measured at Dubrovnik, and northwestwardwind measured at Vis. Wavelets were normalized by variances of respective original time series and bycorresponding significance levels so that the 95% confidence levels equal one (marked by the dashedblack line). (c) Vertical sea temperature profiles between 4‐ and 40‐m depth at Lastovo; the thick yellowline indicates the 20°C isotherm.


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in order to obtain the residual current. The procedureappears to be suitable in our case of weak barotropic tidalcurrents (at most 3 cm/s and thus contributing not morethan 20% to the total current variance), in a rather deep sea(ca. 100 m), with the surface and bottom boundary layers(extending over some 8 and 5 m, respectively) being

excluded from the analysis; when applied on stronger cur-rents in a shallower sea the procedure may result in theboundary layer structure related to barotropic tides to bemistakenly interpreted as the internal tidal variability[Edwards and Seim, 2008]. From the detided current timeseries and the simultaneous temperature data we computedthe spectra [Jenkins and Watts, 1968] for the whole mea-surement interval and for all the depths (Figures 1c and 1d).There is considerable low‐frequency variability visible inthe spectra, probably related to the changing east‐coastinflow to the Adriatic and to the synoptic‐scale wind events.Higher frequencies are dominated by inertial oscillations ofa 17.6‐h period and by diurnal oscillations comprising 24‐,12‐ and 8‐h periods. The peaks are significant at the 95%level at the depths at which they attain maxima (not shown).The temperature variability is largest at a depth of 10–25 m,i.e., at the thermocline level, while the currents are surface‐and bottom‐intensified, which implies their baroclinicity.[9] There are two obvious causes of the diurnal oscilla-

tions: tidal forcing and wind‐forcing. We performed awavelet analysis [Torrence and Compo, 1998] to distinguishbetween the two. In this analysis tidal forcing was representedby sea level measured at Dubrovnik and by barotropic cur-rents recorded at station ADCP, whereas wind‐forcing wasrepresented by data collected at meteorological stationsDubrovnik and Vis. Temporal variability of the two forcingmechanisms was compared to fluctuations of the 20°C iso-therm, roughly corresponding with the thermocline at Las-tovo. Figure 2a shows that the beats of 24‐h tides are visible inboth the sea level and current time series and that they closelyfollowed each other. In June and late July 2006, when thetidal forcing was strongest, the 24‐h isotherm variability wasapparently related to it. Two‐input one‐output cross‐waveletanalysis, documenting relationship between barotropic tidalcurrents and wind stress on one hand and thermocline vari-ability on the other, confirmed the finding and made it pos-sible to consider internal tides in some detail [Mihanovićet al., 2009]. In the middle of July 2006, however, whenthe 24‐h isotherm oscillations were most pronounced, thetidal forcing was at a minimum. Figure 2b shows that the24‐h wind variability culminated at the time, stronglysuggesting that the wind‐forcing was responsible for themaximum diurnal oscillations of isotherm elevation.Figure 2c reveals that the typical range of thermoclinevariability is about 10 m, but it increased to 30 m in mid‐July 2006.

Figure 3. Variability of currents and sea temperature andits dependence on wind‐forcing between 14 and 19 July2006. (a) Hourly pseudo wind stress data collected at sta-tions Dubrovnik and Vis. (b) Hourly pseudo wind stressdata originating from stations Dubrovnik and Vis, afterbeing subjected to a high‐pass filter with the cut‐off periodplaced at 48 h. (c) Sea temperature data collected at Lastovo.Hourly mean values determined from original 10‐min data,without any additional processing. (d) Northwestward cur-rents originating from station ADCP, from which barotropictides and inertial oscillations were removed and to which ahigh‐pass filter with the cut‐off period placed at 48 h wasapplied. The thick black line indicates the 20°C isotherm.Note that depth scales in the Figures 3c and 3d differ.


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[10] A note on the wind field is in order here. Most usefulfor the present analysis would be wind data recorded on theisland of Lastovo. Since, however, such data were notavailable, we were forced to use time series originating fromthe nearby anemographic stations, Dubrovnik and Vis. Theobserved time series could be compared with the predictionsobtained by Aladin, a mesoscale meteorological model runby the Croatian Meteorological and Hydrological Service,not only for Dubrovnik and Vis but also for Lastovo. Themodel results suggested that the diurnal wind variability atLastovo, being pronounced not only in the NE‐SW directionbut also in the NW‐SE direction, resembles more closelythat at Vis than in Dubrovnik, reflecting the fact that Las-tovo and Vis are open‐sea locations whereas Dubrovnik is acoastal station. On the other hand, the model also indicatedthat the long‐term modulation of diurnal winds at Lastovo issimilar to the modulation in Dubrovnik while it differs fromthe modulation at Vis, probably due to a curvature of thecoastline to the north of Lastovo (Figure 1). We have

therefore decided to use the wind data originating from bothDubrovnik and Vis in the comparison of meteorological andoceanographic time series. The meteorological model resultswere not utilized in the comparison because the analysis ofmodel errors, which would be a necessary prerequisite forthe utilization, was beyond the scope of the present paper.[11] The wavelets shown in Figure 2 suggested that the

interval from 14 to 19 July 2006 was the most significant fordiurnal upwelling analysis. Temperature data collected overthat time interval at the south coast of the island of Lastovoare depicted in Figure 3c. The range of thermocline vari-ability increased over the interval, suggesting resonantexcitation. In the days with the largest variability, thethermocline was lowest at about 6 a.m. and highest at about4 p.m. with a secondary maximum occurring at about 10 p.m.As shown in Figure 3b, the diurnal thermocline oscillationswere phase‐locked to the sea breezes revolving clockwisethroughout the day; a relatively weak morning breeze,blowing in the open sea (station Vis) from the southeast,

Figure 4. Average daily course of (top) pseudo wind stress at station Vis, (middle) depth of the 20°Cisotherm at Lastovo, and (bottom) currents averaged in the layers above and below the average thermo-cline depth (20 m) at station ADCP for the 14–19 July 2006 interval. While determining the averages thefollowing data were used: pseudo wind stresses previously subjected to a high‐pass filter with the cut‐offperiod placed at 48 h, original 20°C isotherm depths, and currents from which barotropic tides and inertialoscillations were removed and to which a high‐pass filter with the cut‐off period placed at 48 h wasapplied.


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corresponded to the thermocline reaching its deepest point,and a relatively strong afternoon breeze, blowing in the openAdriatic from the northwest, coincided with the thermoclinebeing close to the sea surface. The sea breezes were super-imposed on a quasi‐steady airflow directed southwestward atthe coast (station Dubrovnik) and southeastward in the opensea (Figure 3a); the latter combined with the sea breeze in theafternoon to produce a pronounced wind known locally asmaestral. In Figure 3d, the elevations of the 20°C isothermare compared with the northwestward currents measured offthe west coast of the island of Lastovo. It is clear that when thethermocline was rising the currents tended to flow north-westward in the surface layer and to be of opposite directionin the bottom layer, and that the current directions werereversed when the thermocline was falling, which confirmsthe baroclinic nature of the process.[12] To illustrate the relationship between various para-

meters more vividly, their mean daily course was deter-

mined for the 14–19 July 2006 interval and is shown inFigure 4. It confirms the main characteristics of diurnalvariability noticed in the original time series, but also pointsto some complexities. Thus, for example, the currentsaveraged in the surface (bottom) layer are directed north-westward (southwards) when the thermocline is rising, andare therefore not exactly opposed to each other during thispart of diurnal cycle. This probably signals that the two‐layer interpretation should be regarded as approximate andthat eventually one should allow for vertical propagation aswell. Otherwise, Figure 4 clearly shows that the north-westward surface currents measured at the station locatedoff the west coast of the island of Lastovo coincide with therising thermocline recorded at the south coast of the island.This convincingly points to a thermocline trough being firstrecorded at the south and then at the west coast and thereforeto an internal wave propagating around the island in aclockwise sense. From the fact that about 6 h is needed forthe wave to propagate from the south to the west coast, itmay be concluded that the wave circumnavigates the islandin about a day and that therefore it may be resonantlyexcited by a forcing possessing daily periodicity.

4. Interpretation

[13] We first used a simple analytical model, whichaddressed the response of a reduced‐gravity f‐plane seabounded by a straight vertical wall to a periodic wind stress,to interpret the empirical findings. The model, originallyformulated by Cushman‐Roisin [1994], was extended byallowing not only for alongshore but also cross‐shore wind‐forcing and by considering both subinertial and superinertialfrequencies [Orlić and Pasarić, 2011]. Wind data collectedat Vis between 14 and 19 July 2006 had been utilized tocompute pseudo wind stress, which was then subjected toharmonic analysis. The results, shown in Figure 5, revealeda dominance of wind rotating clockwise at the 24‐h period,but also a presence of variability at other periods. Subse-quently, wind stress was determined by assuming the dragcoefficient equal to 1.5 × 10−3. The other parameters usedwere the proportional density defect (1.3 × 10−3) and thethermocline depth (20 m) estimated from our temperature timeseries averaged over the 14–19 July 2006 interval andassuming a uniform salinity equal to 38, as well as theacceleration due to gravity (9.81 m/s2) and the Coriolisparameter (10−4 s−1). The reduced‐gravity model then enabledthe amplitudes of the thermocline variability to be determinedfor a number of frequencies and to be compared with theamplitudes of the 20°C isotherm variability as observed on theisland of Lastovo between 14 and 19 July 2006. As is obviousfrom Figure 6a, the model reproduced well the 12‐h signal,but underestimated the 24‐ and 8‐h signals and the variabilityat largest periods. While the discrepancy at large periods isprobably due to phenomena that are not related to the diurnalwind‐forcing and are therefore not reproduced by the presentmodel, the difficulties encountered at the shorter periodsindicate that the model possibly failed to capture a wind‐related process relevant for the Adriatic. The fact that diurnalisotherm variability gradually increased between 14 and18 July 2006 (Figure 3) and that there were indicationsof an internal wave circumnavigating Lastovo in about aday (Figure 4) suggested that resonance was the missing

Figure 5. Result of harmonic analysis of pseudo windstress time series collected on the island of Vis between14 and 19 July 2006. Thick ellipses correspond to clockwiserotation, thin ellipses point to counter‐clockwise rotation,whereas gray lines indicate the phases.


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process and that the assumption of a straight coast shouldbe abandoned.[14] The next simplest model assumed that a circular

island is embedded in a two‐layer f‐plane sea. A series ofanalytical and numerical investigations [Longuet‐Higgins,1969; Wunsch, 1972; Hogg, 1980; Brink, 1999] indicatedthat in the Northern Hemisphere the model allows internalwaves to travel around the island in a clockwise direction atsubinertial frequencies and radiate away from the island atsuperinertial frequencies. If the island is large relative to theinternal Rossby radius of deformation [g"hh′/(h + h′)]1/2/f(where g is the acceleration due to gravity, " is the pro-portional density defect, h is the upper‐layer depth, h′ is thelower‐layer depth, and f is the Coriolis parameter), thewaves trapped around the island closely resemble internalKelvin waves. Although the mean radius of Lastovo(5200 m) is close to the Rossby radius of deformation, it isstill useful to make a first‐order estimate of the time thetrapped internal waves need to circumnavigate the island bytaking into account the speed of internal Kelvin waves[g"hh′/(h + h′)]1/2: with the parameters given above and thebottom depth equal to 80 m, the speed equals 0.44 m/s,implying that about 20 h would be needed for the internalwaves to travel around the island. The time is smaller thanthe diurnal period, but—as shown by Longuet‐Higgins[1969]—an underestimation may be expected since thespeed of internal Kelvin waves is larger than the speed ofinternal waves traveling around a relatively small island. Ittherefore appears that not only the data analysis but also thesimple calculation suggests that the pronounced diurnalupwelling observed at Lastovo between 14 and 19 July 2006could be interpreted as the internal waves resonantly drivenaround the island by clockwise‐rotating sea breezes. Let it bementioned that the speed of internal Kelvin waves consid-

erably varied over the 8‐month measurement interval. Thus,for example, our temperature time series indicate that thespeed was as small as 0.3 m/s in May 2006, and that it sur-passed 0.5 m/s in September 2006 due to a deepening of thethermocline and an increase of the proportional densitydefect. This implies that the stratification supported internalcoastal waves traveling around the island in about a day inmid‐summer and that the period was larger (smaller) andtherefore the conditions not so favorable for resonant exci-tation by diurnal winds earlier (later) in the year.[15] In order to check the possibility of resonant excitation

of internal waves by diurnal winds, we used a numericalmodel—the Princeton Ocean Model (POM). In the earliernumerical modeling studies, the generation of island‐trapped waves by tides [Brink, 1999; Wolanski et al., 2004]and by synoptic‐scale winds [Brink, 1999; Merrifield et al.,2002] had been considered. Our modeling aimed to dem-onstrate how a stratified sea in a wider area around Lastovoresponds to diurnal wind‐forcing. The POM is a three‐dimensional, nonlinear model based on the traditional,primitive equations for conservation of momentum, mass,heat and salt, combined with the equation of state [Blumbergand Mellor, 1987]. The model used for the Adriatic studyhas a 2 km resolution and either a constant depth of 300 mor realistic bathymetry. The bathymetry is based on the 7.5 sresolution depth field derived during the Dynamics of theAdriatic in Real‐Time (DART) project at the NATOUndersea Research Centre (NURC) using inverse distanceweighted interpolator, with the fine resolution NURC databeing bin averaged on the 2 km model grid and smoothed byShapiro filter. The model starts from the state of rest withthe initial vertical temperature profile deduced from ourtemperature time series averaged over the 14–19 July 2006interval in the upper part of the water column combined with

Figure 6. (top) Observed and modeled amplitudes of the 20°C isotherm variability at the island ofLastovo between 14 and 19 July 2006 and (bottom) their ratio. Prior to harmonic analysis the data weresubjected to a high‐pass filter with the cut‐off period placed at 48 h, but some energy leaked throughthe filter at the largest periods; inertial oscillations were not discernible in the data set. The theoreticalresults were obtained as described in the text, by (a) the reduced‐gravity, straight coast analyticalmodel, (b) the numerical model of the flat‐bottom Adriatic, and (c) the numerical model of the real‐bathymetry Adriatic.


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summertime middle Adriatic climatology [Artegiani et al.,1997] in the lower part and a uniform salinity equal to38. The model is forced by the wind stress observed at Visbetween 13 and 19 July 2006; the day before our intervalof interest is used to spin up the model, with the wind stresslinearly increasing from zero to the observed value over aninertial period. The elevations of the 20°C isotherm com-puted for the interval extending from 14 to 19 July 2006 weresubjected to harmonic analysis. In Figure 6 the modelingresults are compared to measurements carried out at Lastovo,whereas in Figure 7 the modeling results are depicted for awider middle‐Adriatic area.[16] The 24‐h isotherm oscillations are closer to obser-

vations in the flat‐bottom Adriatic (Figure 6b) than in thepreviously utilized analytical model (Figure 6a) and areeven better approximating the data in the real‐bathymetrycase (Figure 6c). This suggests that the 24‐h internal waveswere resonantly driven by sea breezes around the island ofLastovo in mid‐July 2006. The same improvement isobserved at the 8‐h period but not at the 12‐h period: sur-prisingly, the latter oscillation is better reproduced by theanalytical model than by either version of the numericalmodel. A possible explanation is inadequacy of wind‐forcing, since the winds observed on the island of Vis arenot fully representative of the winds in the area of Lastovo.The other possibility is that the numerical model errs at thisperiod, e.g., due to some numerical problems or an over-estimation of the nonlinear effects that accompany the res-onant generation of island‐trapped waves. The exact natureof the nonlinearity is not clear. It may be that advection andentrainment result in a reduced proportional density defect

when the thermocline rises. As shown by Orlić and Pasarić[2011], an increase of the thermocline displacement couldthen be expected. This may lead to a further decrease of theproportional density defect, etc. However, a thoroughanalysis of the positive feedback loop is beyond the scopeof this work. As for the variability at the largest periods, thenumerical model parallels the analytical model in under-estimating it, thus confirming that the variability is notrelated to the diurnal wind‐forcing considered here.[17] As for the conditions in a wider middle‐Adriatic area,

the 24‐h isotherm oscillations in the flat‐bottom Adriatic arefound to be pronounced close to the islands of Vis andLastovo, with the phase propagating in a clockwise directionaround them (Figure 7a). Surprisingly, the amplitude is notlargest near Lastovo unless real bathymetry is included(Figure 7b), indicating that a topographic Rossby effect isimportant for resonant excitation of subinertial island‐trapped waves. Both the amplitude and phase obtained forthe 12‐h thermocline oscillations show that the oscillationsare related to waves radiating away from the topographyrather than being trapped by it (Figure 7c). These modelingresults agree with the thermistor measurements that werecarried out not only on the island of Lastovo but also on themuch smaller islands of Sušac and Biševo (Figure 1); theirspectral analysis revealed that, whereas the 24‐h variabilityat Lastovo was much larger than at the other two islands, the12‐ and 8‐h variability at all three islands was similar[Mihanović et al., 2009]. It should be stressed again that thepronounced variability at Lastovo depended not only onisland dimensions but also on stratification (which in mid‐July 2006 supported internal waves circumnavigating the

Figure 7. (top) Amplitudes and (bottom) phases of the 20°C isotherm variability obtained by numer-ical modeling of the middle Adriatic subjected to wind‐forcing as observed on the island of Visbetween 14 and 19 July 2006. (a) Results obtained for the 24‐h period assuming uniform bottomdepth (at 300 m). (b) As in Figure 7a, except for realistic bathymetry. (c) Results obtained for the12‐h period assuming realistic bathymetry.


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island in about a day) as well as on diurnal wind‐forcing(which was most pronounced in mid‐July 2006). Differentstratification conditions would reduce the variability atLastovo and probably strengthen it at some other islands,whereas weaker diurnal wind‐forcing would obviouslyreduce variability everywhere.

5. Conclusion

[18] The data collected in the Adriatic in mid‐July 2006show that the thermocline was subject to a 30‐m diurnalvariability at a steep cliff of the south coast of Lastovo,being lowest at about 6 a.m. and highest at about 4 p.m. witha secondary maximum occurring at about 10 p.m. Thediurnal currents measured off the island west coast flowednorthwestward (southeastward) in the surface layer when thethermocline was rising (falling), thus pointing to internalcoastal waves propagating around the island in a clockwisesense. Comparison of the data with the winds simulta-neously measured in the area suggests that the diurnal waveswere driven by the sea breezes turning clockwise throughoutthe day. A simple analytical model confirms that over a partof the summer the stratification promotes internal coastalwaves that revolve daily around the island in a clockwisedirection, creating the conditions needed for resonant exci-tation by the diurnal winds. Numerical modeling reveals thatthe 24‐h waves are trapped around the island due to theinfluence of both the Coriolis force and bottom slope, andthat the 12‐h waves radiate away from the island.[19] In the present analysis we have concentrated on the

mid‐July 2006 episode because it was clearly wind driven.In a previous paper the three cases with prevalent tidalforcing were studied [Mihanović et al., 2009]. It should bementioned that the diurnal thermocline variability waspresent over a greater part of our measurement interval, butit was usually less pronounced and the two forcingmechanisms could not be readily disentangled. It thereforeseemed appropriate to focus in these, first analyses of theAdriatic data set on simpler cases, and to put off a morecomplete statistical analysis to a latter occasion.[20] How important is this, apparently new type of diurnal

upwelling for climate‐related and biological issues? Theoverall effect of periodic upwelling is a cooling of coastalzones above which time‐variable winds blow. It is notknown, however, whether global warming would intensifythis process by strengthening sea breezes and alteringstratification in a way that would change the conditions atLastovo and other similar islands to more closely resembleresonant ones and whether the process is widespread enoughto be of global importance. As for the productivity of coastalwaters, the diurnal upwelling may influence the generationof phytoplankton characterized by a near‐daily scale [Walshet al., 1977] and therefore may also influence the generationof zooplankton and nekton at much larger temporal scales.The well‐known fact that the larger areas of the islands ofLastovo and Vis are relatively productive ones in theAdriatic [Zavatarelli et al., 1998] supports the proposedmechanism and suggests that these islands represent thenatural laboratories in which the generation times of variousmembers of the food web can be studied. The diurnalupwelling at Lastovo may also influence benthic organisms.There is less diversity of bryozoans and other organisms

populating the cliffs down to a 40 m depth at Lastovo than atother Adriatic locations [Novosel et al., 2004], and this maybe partially due to the fact that organisms at Lastovo areoften exposed to large temperature changes over short timeintervals.

[21] Acknowledgments. We thank the scientists and crews of RVsBios and Palagruža for their skilled contribution to the field phase of theexperiment. In particular, we are indebted to Maja Novosel and AnđelkoNovosel for leading the team of divers who deployed and recovered thethermistors. Meteorological data and Aladin modeling results were pro-vided by the Meteorological and Hydrological Service of the Republic ofCroatia. We thank Paola Nardini and Michel Rixen for preparing and shar-ing the NATO Undersea Research Centre bathymetry data. We also thanktwo anonymous reviewers for providing constructive comments on themanuscript. The work was supported by the U.S. Office of Naval Researchand Naval Research Laboratory (project ITHACA, grant N00014‐05‐1‐0698) and the Croatian Ministry of Science, Education, and Sports (grants119‐1193086‐3085, 001‐0013077‐1122, and 022‐0222882‐2823).

ReferencesArtegiani, A., D. Bregant, E. Paschini, N. Pinardi, F. Raicich, and A. Russo(1997), The Adriatic Sea general circulation, Part I, Air‐sea interactionsand water mass structure, J. Phys. Oceanogr., 27, 1492–1514,doi:10.1175/1520-0485(1997)027<1492:TASGCP>2.0.CO;2.

Bakun, A. (1990), Global climate change and intensification of coastalocean upwelling, Science, 247, 198–201, doi:10.1126/science.247.4939.198.

Barber, R. T. (2001), Upwelling ecosystems, in Encyclopedia ofOcean Sciences, vol. 6, edited by J. H. Steele, S. A. Thorpe, andK. K. Turekian, pp. 3128–3135, Academic, San Diego, Calif.,doi:10.1006/rwos.2001.0295.

Blumberg, A. F., and G. L. Mellor (1987), A description of a three‐dimensional coastal ocean circulation model, in Three‐DimensionalCoastal Ocean Models, Coastal Estuarine Stud., vol. 4, edited byN. S. Heaps, pp. 1–16, AGU, Washington, D. C.

Brink, K. H. (1999), Island‐trapped waves, with application to observationsoff Bermuda, Dyn. Atmos. Oceans, 29, 93–118, doi:10.1016/S0377-0265(99)00003-2.

Codispoti, L. A. (1981), Temporal nutrient variability in three differentupwelling regions, in Coastal Upwelling, Coastal Estuarine Sci.,vol. 1, edited by F. A. Richards, pp. 209–220, AGU, Washington, D. C.

Cushman‐Roisin, B. (1994), Introduction to Geophysical Fluid Dynamics,Prentice Hall, Englewood Cliffs, N. J.

DiMarco, S. F., M. K. Howard, and R. O. Reid (2000), Seasonal variationof wind‐driven diurnal current cycling on the Texas‐Louisiana Continen-tal Shelf, Geophys. Res. Lett., 27(7), 1017–1020, doi:10.1029/1999GL010491.

Edwards, C. R., and H. E. Seim (2008), Complex EOF analysis as amethod to separate barotropic and baroclinic velocity structure in shal-low water, J. Atmos. Oceanic Technol., 25(5), 808–821, doi:10.1175/2007JTECHO562.1.

Hogg, N. G. (1980), Observations of internal Kelvin waves trapped roundBermuda, J. Phys. Oceanogr., 10, 1353–1376, doi:10.1175/1520-0485(1980)010<1353:OOIKWT>2.0.CO;2.

Hunter, E., R. Chant, L. Bowers, S. Glenn, and J. Kohut (2007), Spatial andtemporal variability of diurnal wind forcing in the coastal ocean, Geo-phys. Res. Lett., 34, L03607, doi:10.1029/2006GL028945.

Jenkins, G. M., and D. G. Watts (1968), Spectral Analysis and Its Applica-tions, Holden‐Day, San Francisco, Calif.

Kaplan, D. M., J. L. Largier, S. Navarrete, R. Guiñez, and J. C. Castilla(2003), Large diurnal temperature fluctuations in the nearshore watercolumn, Estuarine Coastal Shelf Sci., 57, 385–398, doi:10.1016/S0272-7714(02)00363-3.

Lerczak, J. A., M. C. Hendershott, and C. D. Winant (2001), Observationsand modeling of coastal internal waves driven by a diurnal sea breeze,J. Geophys. Res., 106, 19,715–19,729, doi:10.1029/2001JC000811.

Longuet‐Higgins, M. S. (1969), On the trapping of long‐period waves roundislands, J. Fluid Mech., 37, 773–784, doi:10.1017/S0022112069000875.

McGregor, H. V., M. Dima, H. W. Fischer, and S. Mulitza (2007), Rapid20th‐century increase in coastal upwelling off northwest Africa, Science,315, 637–639, doi:10.1126/science.1134839.

Merrifield, M. A., L. Yang, and D. S. Luther (2002), Numerical simulationsof a storm‐generated island‐trapped wave event at the Hawaiian Islands,J. Geophys. Res., 107(C10), 3169, doi:10.1029/2001JC001134.


9 of 10

Mihanović, H., M. Orlić, and Z. Pasarić (2009), Diurnal thermocline oscil-lations driven by tidal flow around an island in the middle Adriatic,J. Mar. Syst., 78, S157–S168, doi:10.1016/j.jmarsys.2009.01.021.

Novosel, M., A. Požar‐Domac, and M. Pasarić (2004), Diversity and distri-bution of the bryozoa along underwater cliffs in the Adriatic Sea withspecial reference to thermal regime, Mar. Ecol. Berlin, 25, 155–170,doi:10.1111/j.1439-0485.2004.00022.x.

Orlić, M., and Z. Pasarić (2011), A simple analytical model of periodiccoastal upwelling, J. Phys. Oceanogr., 41, 1271–1276.

Pauly, D., and V. Christensen (1995), Primary production required to sus-tain global fisheries, Nature, 374, 255–257, doi:10.1038/374255a0.

Pawlowicz, R., R. Beardsley, and S. Lentz (2002), Classical tidal harmonicanalysis including error estimates in MATLAB using T_TIDE, Comput.Geosci., 28, 929–937, doi:10.1016/S0098-3004(02)00013-4.

Rippeth, T. P., J. H. Simpson, R. J. Player, and M. Garcia (2002), Currentoscillations in the diurnal‐inertial band on the Catalonian shelf in spring,Cont. Shelf Res., 22, 247–265, doi:10.1016/S0278-4343(01)00056-5.

Roemmich, D., and J. McGowan (1995), Climate warming and the declineof zooplankton in the California Current, Science, 267, 1324–1326,doi:10.1126/science.267.5202.1324.

Rosenfeld, L. K. (1988), Diurnal period wind stress and current fluctuationsover the continental shelf off northern California, J. Geophys. Res., 93,2257–2276, doi:10.1029/JC093iC03p02257.

Smith, R. L. (1968), Upwelling, Oceanogr. Mar. Biol., 6, 11–46.Smith, R. L. (1981), A comparison of the structure and variability of theflow field in three coastal upwelling regions: Oregon, northwest Africa,and Peru, in Coastal Upwelling, Coastal Estuarine Sci., vol. 1, edited byF. A. Richards, pp. 107–118, AGU, Washington D. C.

Summerhayes, C. K., C. Emeis, M. V. Angel, R. L. Smith, and B. Zeitzschel(1995), Upwelling in the ocean: Modern processes and ancient records, inUpwelling in the Ocean: Modern Processes and Ancient Records, editedby C. P. Summerhayes et al., pp. 1–37, John Wiley, New York.

Torrence, C., and G. P. Compo (1998), A practical guide to wavelet anal-ysis, Bull. Am. Meteorol. Soc., 79, 61–78, doi:10.1175/1520-0477(1998)079<0061:APGTWA>2.0.CO;2.

Walsh, J. J., T. E. Whitledge, J. C. Kelley, S. A. Huntsman, and R. D.Pillsbury (1977), Further transition states of the Baja California upwell-ing ecosystem, Limnol. Oceanogr., 22, 264–280, doi:10.4319/lo.1977.22.2.0264.

Wolanski, E., P. Colin, J. Naithani, E. Deleersnijder, and Y. Golbuu (2004),Large amplitude, leaky, island‐generated, internal waves around Palau,Micronesia, Estuarine Coastal Shelf Sci., 60, 705–716, doi:10.1016/j.ecss.2004.03.009.

Woodson, C. B., et al. (2007), Local diurnal upwelling driven by seabreezes in northern Monterey Bay, Cont. Shelf Res., 27, 2289–2302,doi:10.1016/j.csr.2007.05.014.

Wunsch, C. (1972), The spectrum from two years to two minutes of tem-perature fluctuations in the main thermocline at Bermuda, Deep Sea Res.Oceanogr. Abstr., 19, 577–593.

Zavatarelli, M., F. Raicich, D. Bregant, A. Russo, and A. Artegiani (1998),Climatological biogeochemical characteristics of the Adriatic Sea,J. Mar. Syst., 18, 227–263, doi:10.1016/S0924-7963(98)00014-1.

Zhang, X., S. F. DiMarco, D. C. Smith IV, M. K. Howard, A. J. Jochens,and R. D. Hetland (2009), Near‐resonant ocean response to sea breeze ona stratified continental shelf, J. Phys. Oceanogr., 39, 2137–2155,doi:10.1175/2009JPO4054.1.

G. Beg Paklar and V. Dadić, Institute of Oceanography and Fisheries,Šetalište I. Meštrovića 63, 21000 Split, Croatia.N. Leder and H. Mihanović, Hydrographic Institute of the Republic of

Croatia, Zrinsko‐Frankopanska 161, 21000 Split, Croatia.M. Orlić, M. Pasarić, and Z. Pasarić, Faculty of Science, Andrija

Mohorovičić Geophysical Institute, Unversity of Zagreb, Horvatovac 95,10000 Zagreb, Croatia.


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Diurnal upwelling resonantly driven by sea breezes around an Adriatic island

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