Continental Shelf Research ] (]]]]) ]]]–]]]

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Continental Shelf Research

0278-43

doi:10.1

n Corr

Concepc

E-m

PleasCont

journal homepage: www.elsevier.com/locate/csr

Research papers

Diurnal-period, wind-forced ocean variability on the inner shelf offConcepcion, Chile

Marcus Sobarzo a,c,n, Luis Bravo b,c, Carlos Moffat a

a Departamento de Oceanografıa, Universidad de Concepcion, Concepcion, Chileb Programa de Doctorado en Oceanografıa, Universidad de Concepcion, Concepcion, Chilec Programa de Investigacion Marina de Excelencia (PIMEX-Nueva Aldea), Facultad de Ciencias Naturales y Oceanograficas, Universidad de Concepcion, Chile

a r t i c l e i n f o

Article history:

Received 18 December 2009

Received in revised form

16 September 2010

Accepted 6 October 2010

Keywords:

Sea breeze

Diurnal currents

Surface enhanced currents

Inner shelf

Itata River

43/$ - see front matter & 2010 Elsevier Ltd. A

016/j.csr.2010.10.004

esponding author at. Departamento de Oc

ion, Casilla 160-C, Concepcion, Chile. Tel.: +5

ail address: msobarz@udec.cl (M. Sobarzo).

e cite this article as: Sobarzo, M., etinental Shelf Research (2010), doi:10

a b s t r a c t

We characterize the response of diurnal-period ocean current variability to the sea breeze using

measurements of current velocity taken off the mouth of the Itata River and wind stress collected at

Hualpen Point (central Chile) in spring of 2007 and summer of 2006 and 2008. During these three periods,

the winds are predominately towards the northeast, following the coastal topography, with the highest

variability found in the near-diurnal and synoptic frequency bands. The sea breeze amplitude is

intermittent in time and is associated with synoptic-scale variability on the order of three to 15 days,

so that the diurnal-period winds (and currents) are enhanced when the alongshore wind (i.e. upwelling-

favorable) is strong. The water current variability in the near-diurnal band is significant, explaining up to

40% (spring 2007) of the total current variance in the first 15 m depth.

The response of the near-diurnal band to the sea breeze is surface-intensified and elliptically polarized

with anticlockwise rotation. As is the case with the wind, the ellipses of the daily-period surface current

velocity are oriented alongshore, although the currents are significantly less rectilinear. In the along-shore

direction, the diurnal-period wind-stress is directed towards the northeast and the shore after mid-day

and reverse their direction at night. The currents also show counterclockwise rotation during the day,

with nearly in-phase along-shore motion in the surface layer and shoreward (oceanward) motion during

the morning (late afternoon). The deep layer also show significant counterclockwise motion but its

magnitude is roughly one third smaller than in the surface layer, and as much as 1801 out of phase. A two-

layer analytical model with no friction but that includes a cross-shelf pressure gradient and wind-stress

forcing is used to diagnose key properties of the observed response, suggesting that the observed diurnal-

period currents may be wind driven. However, data from different years shows that inertial motions and

probably other dynamics can be important when the winds are relatively weak or highly variable.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The coastal circulation and hydrographic structure of the con-tinental shelf off Concepcion (361490S, 731020W), central Chile, ischaracterized by a strong influence from alongshore winds that leadto enhanced coastal upwelling in spring and summer. This, in turn,sustains a highly productive ecosystem and large fishery (i.e. Moraleset al., 2007; Thiel et al., 2007). The system is also characterized bysignificant seasonal variability in water properties, which is partiallydue to the relaxation of the upwelling-favorable wind forcing in falland winter, variations in the discharge from the Itata and Biobıo rivers,and evolving radiation and freshwater balances (Sobarzo et al.,2007a).

ll rights reserved.

eanografıa, Universidad de

6 41 2207039.

al., Diurnal-period, wind-f.1016/j.csr.2010.10.004

Studies of the circulation over the Concepcion shelf haverevealed a subtidal circulation that is strongly influenced bywind and the complicated submarine topography of the area(Sobarzo and Djurfeldt, 2004), which includes a wide shelf andtwo large submarine canyons (Fig. 1). At higher frequencies, thecirculation is characterized by relatively weak tidal currents (18%or less than the total current variance) and strong near-inertialcurrents at the local inertial period (approx. 20 h), with decreasingintensity towards the coast (Sobarzo et al., 2007b). The strongdiurnal-period variability found in wind time-series from coastalstations, however, suggest that wind stress could be drivingsignificant current variability near the coast (Sobarzo et al., 2007b).

Diurnal-period winds are a persistent feature of many coastalareas, particularly in spring and summer. They are driven byevolving thermal gradients between ocean and land (Burt et al.,1973, 1974; Halpern, 1974, 1977; Price and Weller, 1986;Rosenfeld, 1988; Lerczak et al., 2001), as the smaller thermalheat capacity of land causes it to heat and cool faster than the sea.

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 1. Continental shelf off Concepcion showing the coastal area adjacent to the Itata River mouth, and the location of meteorological station at Hualpen Point (left panel). The

black rectangle indicates the study region, detailed in the right panel, showing the bathymetry of the study area and the location of the ADCPs during the three study periods.

Table 1Features of each employed mooring arrays: ADCP type, geographical position, initial and final time of deployments.

ADCP Latitude Longitude Bottom Level Z (m) Start dd/mm/yy End dd/mm/yy

600 kHz �36121.50 �72153.10 �34 m 3.7–32.7 26/01/2006 26/02/2006

300 kHz �36121.70 �72153.50 �35 m 3.9–30.9 01/11/2007 31/12/2007

300 kHz �36121.70 �72153.20 �33 m 3.7–29.7 13/01/2008 24/02/2008

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]]2

The differential heating of these bodies results in a cross-shorepressure gradient that leads to daily cross-shore fluctuations of thelocal wind, with offshore scales around 200–400 km (Gille et al.,2003). This diurnal wind, frequently called ‘sea-breeze’, can forcecomplex baroclinic currents near the coast, particularly near thecritical latitude (i.e. 7301, see e.g. Lerczak et al., 2001). The responsecan include surface-intensified, rotating currents, which maydominate the high-frequency variability over the shelf (DiMarcoet al., 2000; Rippeth et al., 2002; Hyder et al., 2002; Simpson et al.,2002). Elsewhere, diurnal-period winds have been shown to advectriver plumes and other buoyant discharges (Pinones et al., 2005),produce significant sediment transport (Pattiaratchi et al., 1997),and generate upwelling events (Woodson et al., 2007). In thepresence of stratification, the response is typically surface-intensified (e.g. Davies and Xing, 2003). And if a well-definedpycnocline is found, the lower layer can show an out-of-phaseresponse with respect to the surface current (DiMarco et al., 2000;Hyder et al., 2002; Simpson et al., 2002).

In central Chile (32–371S), an estimation based on two dailypasses of satellite observations suggests the diurnal cycle of thesurface wind (estimated as the difference between 8 P.M. and 8A.M.) is of the order of 1 m s�1 (s¼3.5 m s�1) near the shore(Garreaud and Munoz, 2005). However, estimations of the diurnal-period wind cycle from in situ data and its effect on the inner-shelfcirculation are lacking for the Chilean coast. Here, we present thefirst analysis of diurnal-period wind variability and the currentresponse for the region. In particular, we focus on the dynamics ofthe inner shelf during three periods of upwelling-favorable windsoff the coast of Concepcion: November–December 2007 andJanuary–February of 2006 and of 2008. Section 2 presents thedata collected and basic processing. Section 3 presents the resultsand is followed by a brief discussion of the results in Section 4.Conclusions are presented in Section 5.

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

2. Available data and processing

The study area is located on the inner shelf off Concepcion,Chile (Fig. 1), approximately 40 km southeast of the Itata Riversubmarine canyon and 2 km northwest of the Itata River mouth.The coastline is oriented to the northeast (201 with respect toNorth), and the bathymetry is characterized by diverging isobathssouth of the Itata River, indicating a slightly narrower inner shelfnorth of the river mouth.

Current velocity measurements were collected using either a300 kHz or a 600 kHz RD Instruments, Acoustic Doppler CurrentProfiler (ADCP, see Table 1) deployed in spring of 2007 and summerof 2006 and 2008 (Fig. 1). The instrument was located between 33and 35 m depth, and it collected data every 15 min (192 pings perensemble) with a vertical bin resolution of 1 m. Table 1 shows thelocation of the deployments, bottom depths, and length of the time-series. Data within 3.8 m of the surface were discarded to avoidside-lobe and wave contamination. Overall, the data return was atleast 80%, although the two topmost bins (3.8 and 4.8 m) showedlower data quality. However, data from both of these bins werehighly correlated (r2

¼0.78) with data from farther down the watercolumn. Finally, wind magnitude and direction were recordedevery 10 min at the Hualpen Point meteorological station (�50 kmsouth of the ADCP deployment site) during each study period (seeFig. 1). Wind stress was calculated from the wind data followingSmith (1980).

Wind and current data were rotated into a coordinate systemapproximately aligned with the direction of the coast (201), withthe alongshore direction positive to the northeast and the cross-shore direction positive onshore. In order to focus on the diurnal-period current variability and its response to the correspondingwind variability, a diurnal-scale band between 0.73 and 1.60 cpd(1/15 to 1/33 cph) was defined and the corresponding dataset

orced ocean variability on the inner shelf off Concepcion, Chile.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]] 3

obtained from the original time series using a band-pass Cosine-Lanczos filter. The choice of filter explicitly leaves the inertialenergy in the time series due to the difficulty of being able tounambiguously separate it from daily-period variability. However,we conduct a complex demodulation analysis of the amplitude andthe phase of the diurnal-period current velocity to try to distinguishbetween these two important sources of near-diurnal variability(Simpson et al., 2002). Another potential source of diurnal-periodvariability is the K1 tidal component (1 cpd). However, studies overthe shelf indicate that K1 (the dominant diurnal constituent in theregion) is less than 2.5 cm s�1 (Sobarzo et al., 2007b). As we willsee, the diurnal-period variability of interest here is clearly surface-intensified, with amplitudes on the order of 10 cm s�1, andintermittency consistent with the wind variability, not with tidalforcing.

Table 2Results of the fit to the wind time-series using the least squares of a 24-h harmonic.

CS: cross-shore, AS: along-shore.

Year Amplitude (m s�1) Phase (deg.) % of explained variance

CS AS CS AS CS AS

2006 1.06 2.06 245 293 28.7 7.8

2007 1.20 2.72 240 287 37.4 13.5

2008 0.94 1.89 243 296 34.8 5.9

3. Results

3.1. Variability of the diurnal-period wind in summer

As is typical of the spring and summer wind variability in thisregion, the average wind direction during the three study period isfrom the southwest, with the principal axes strongly aligned withthe local coastline orientation (Fig. 2, left panels). These principalaxes explain between 93.4% and 96.4% of the total variability ofeach time-series. Hodographs of the hourly averaged wind velocity(Fig. 2, middle panels) reveal its daily evolution in this region: as the

Fig. 2. Dispersal diagrams of the hourly wind for January–February 2006 and 2008 and No

variance (EMV) and white arrow indicates the mean flows. For each period, we indicate

hourly averaged winds (middle panels) and the ellipses of the 24-h least-squares fit (ri

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

wind rotates counterclockwise, it reaches a maximum north-eastward velocity of 4–7 m s�1 between 15:00 and 24:00. Thewind then progressively diminishes to a minimum value of 1–3 m s�1

at around 08:00. The amplitude of the daily wind cycle is clearlysmaller than that of the average equatorward wind for all three dataperiods, although the ellipse of the diurnal wind variability is roughlytwice as large in the along-shore rather than the cross-shore direction.A least squares analysis of the time-series (Fig. 2, right panels) showthat roughly one-third of the total cross-shore variability and as muchas 14% of the along-shelf variability are explained by diurnal-periodvariability (Table 2).

Spectra of the wind velocity time series reveal that the abovediurnal cycle is a significant component in the wind variability ofthe region (Fig. 3). As shown by the hodographs, the motion in near-diurnal time-scales tends to be counterclockwise-polarized (Fig. 3,bottom panels), and it also shows more energy in the along-shoredirection. There are significant peaks in synoptic time-scales

vember–December 2007 (left column). The grey line indicates the axis of maximum

the angle of the EMV and the percentage of variance explained. Also shown are the

ght panels).

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 3. Cross-shore and alongshore (top panels) and rotary (bottom panels) spectra the wind for the three study periods. The dashed vertical line indicates the diurnal

frequency, and the shaded region the 15–33 h band.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]]4

(3–15 days) in this direction, but the motion in this frequency bandis clearly more rectilinear than in diurnal-periods. Therefore,despite the fact that the diurnal-period winds are relativelyweaker in the cross-shore direction, they dominate the cross-shore variability because so little energy is found at lower frequen-cies in this direction.

The analysis of the wind stress data reveals that the variability atthese two distinct time scales – diurnal and synoptic – is correlated.Plots of the low-pass and diurnal-period wind stress time series forthe three study periods consistently shows that the diurnal-periodwind stress intensifies when southwesterly wind stress increase,and weakens or nearly disappears with low-frequency northerly orweak southwesterly winds (Fig. 4). This relationship is partly due tothe nonlinearity of the stress formulation, so that the stressdepends on the total wind as well as the speed. However ahistogram of the synoptic-scale wind direction and the averagedaily-cycle amplitude for each wind speed and direction bin (Fig. 5,left panel) confirms that the magnitude of the daily cycle (in m/s) issignificantly larger with north and northeastward synoptic winds,i.e., upwelling-favorable, which occurred during an overwhelmingproportion (83%) of the three study periods combined. Moreover,the amplitude of the daily-period cycle was positively correlatedwith the synoptic wind intensity (Fig. 5, right panel). As aconsequence of this coupling, the daily-period wind, which canreach between 4 and 6 m/s during the afternoon and up tomidnight, can contribute an average of nearly 50% of the totalwind magnitude observed in the region during the spring andsummer periods.

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

3.2. Current variability

3.2.1. Mean flow and variability of the total current

The time mean of the ocean currents during the study period(Fig. 6) is characterized by a two-layer structure. The data suggeststhe flow in the top 5–10 m is towards the northeast and the shore,in the direction of the predominant wind direction, although thevalues are not statistically significant. The lower layer, from 10 m orso downwards, has typical values of 2–8 cm/s, with maximumvalues between 5 and 20 m depth. The variance of the total surfacecurrents is an order of magnitude greater than the near-bottomcurrents and, for the three study periods, the average variance inthe along-shore direction was as high as three times greater than inthe cross-shore direction (Fig. 6dh). The motion of the lower layer,flowing in the opposite direction to the prevailing winds, has beenthought to be caused by the presence of an along-shelf pressuregradient. This general two-layer structure has been observedelsewhere in this region and in other upwelling regions (Lentz,1994; Noble et al., 2002; Sobarzo and Djurfeldt, 2004).

Rotary spectra of the current velocity time series (period o40 h)show intense energy peaks at semi-diurnal and diurnal-inertial time-scales (Fig. 7). Particularly in near-diurnal time scales, the energy islarger in the counterclockwise (CCW) rather than the clockwise (CW)component throughout the water column. The barotropic structureof the semi-diurnal tide is very clear, as well as the surface-inten-sified nature of motions at diurnal frequencies, which reach the top10-15 m of the water column, particularly in 2007 and 2008. Althoughwe lack concurrent hydrographic observations, individual CTD

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 4. Alongshore (right panel) and cross-shore (left panel) wind stress in the near-diurnal band (15–33 h, thin dashed line) and low-frequency wind (synoptic scale, thick

line) for the three study periods. Note the change in the scale in either direction.

Fig. 5. Histogram of synoptic wind direction (grey bars) and average amplitude (vertical bar corresponds to standard error) of the diurnal-period wind for each corresponding

direction (left panel) and histogram of synoptic-scale wind magnitude (grey bars) and average amplitude (vertical bar corresponds to standard error) of the diurnal-period

wind for each corresponding synoptic-scale amplitude for all study periods (right panel).

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]] 5

(Conductivity-Temperature and pressure for Depth) profiles collectedduring the summer of 2008 illustrate the vertical structure of thewater column (Fig. 8). The density profile shows a surface layer and

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

pycnocline reaching below 10 m, which is consistent with a two-layerstructure in the flow as shown in the mean profiles and currentvelocity variability discussed above.

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 6. Mean (solid lines) and confidence interval (grey bar) vertical profiles of the cross-shore (a, b, c) and alongshore (e, f, g) components of the total current. The panels (d)

and (g) show the vertical profile of the total variance for the cross-shore and alongshore currents.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]]6

3.2.2. Mean daily cycles

In this section we characterize the mean daily cycles of theband-passed-filtered currents. The average properties of thesecurrents reveal a counterclockwise-polarized circulation through-out the water column for the three study periods (with the singleexception of a near-surface velocity bin in 2006), with moderatelyellipticity of the order of 0.5–0.75 (Fig. 9). The currents are mostCCW-polarized and have a stronger ellipticity during 2007, and thisis also the period where the 15–33 h band-passed currentsaccounted for the largest proportion of the total current velocityvariance (40%), followed by 2008 (25%) and 2006 (20%).

The relative prominence of diurnal-period variability in 2007 isalso evident in the average wind-stress and diurnal-period currents(Fig. 10). During that year, the diurnal cycle was particularly strong,with average surface currents of nearly 7.5 cm/s flowing north-wards in the late afternoon, when the winds are also in theirmaximum, and reversing in the early morning. During 2008, thediurnal-period wind stress was weaker on average, and so is theresponse of the currents is weaker in the same time scales, while in2006 had the weakest average wind-stress forcing in daily time-scales, which is also reflected in the average daily-period currents.

The vertical structure of the daily-period currents (Fig. 10)reveals a surface-intensified, two-layer current velocity structure.The bottom layer shows, during 2007 and 2008, out-of-phase

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

motion with respect to the surface layer motion described above,with southwestwards velocities peaking in the mid- to late after-noon and northeastwards velocities peaking around 00:00–04:00.In the cross-shore direction, the surface flow tends to be towardsthe shore in the early morning (04:00–08:00) to the mid-afternoon(14:00–18:00), and towards the open ocean starting in the earlyevening and through the night. The strong phase difference in thelower layer therefore implies an exchange flow in diurnal-timescales in the cross-shore direction. However, in 2006, where thedaily wind was weaker and the daily-period currents explained asignificant smaller proportion of the total variance, the data showsa more complex response, with a less clear separation betweenupper and lower layers and a more complicated vertical structure,particularly in the cross-shore direction. Daily cycles for tworepresentative depths (roughly 3.9 and 20.9 m) are shown inFig. 10 (bottom panels) for the three study periods. A diurnal fitof these average cycles (Table 3) results in an anticlockwise phasedifference between surface and bottom layers equal to 551 (2006),1811 (2007) and 791 (2008). A dynamical interpretation of some ofthese results is provided in Section 3.2.5.

The evolution of the surface daily-period currents over time(Fig. 11) shows that during 2007, and to a lesser degree during2008, there is a clear response of the currents to the wind stress (toppanels). Pulses of wind stress from the northeast (and an offshore

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 7. Vertical variation of the counterclockwise (left panels) and clockwise (right panels) spectral energy for the 2006 (top panels), 2007 (middle panels) and 2008 (bottom

panels) datasets. Also indicated are the diurnal (di), inertial (f) and semi-diurnal tidal frequency (M2).

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]] 7

component) lasting two to five days are evident from midnight to10:00, with a reverse pattern (wind stress from the southwest andtowards the shore) after mid-day. The inertial-diurnal surfacecurrents show a similar, in-phase pattern of circulation in thealong-shore direction, with southwards motion during the first halfof the day turning into northwards during the second. As thecurrent rotates CCW during the day, the surface flow is towards theshore in the afternoon and seaward in the morning hours. During2006, when the winds are weaker on average and in synoptic scales,there are periods of CCW variability in phase with the wind (e.g.February 4–8th) with others where the phase is clearly changing(e.g. February 10–15th). This suggests that directly forced wind-driven current variability is dominant during 2007, less so in 2008,and not important during most of the sampling period in 2006.

3.2.3. A case of study of wind-driven diurnal-period currents

The ocean response to the sea-breeze is illustrated by windstress and ocean current velocity time-series from the 2007deployment. During this period, the total northward current(Fig. 12b) is strongest and surface-intensified when the daily-period wind-stress amplitude is largest (Fig. 12a). When thediurnal-period winds weaken, the daily-period currents alsotend to weaken or reverse direction. Progressive vector diagrams(PVD) for the winds and currents (3–4 November) of the original,

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

unfiltered, hourly data (Fig. 12c, top panels) show the largestdisplacement between 13:00 and 22:00, when the land/seatemperature gradient is expected to be maximum and when thedirection of the sea-breeze and synoptic-scale winds directions areroughly the same. During the rest of the night and until mid-morning (10:00), the winds slowed down and shifted slightlynorthwards but did not reverse, as shown in Section 3.1 for theaverage daily-period wind cycle. The overall displacement of thesurface currents during a one-day period is 20 km, about 3% ofthe wind displacement, and the largest displacements occurredduring the afternoon, with a small phase difference with respect tothe wind-forcing. As with the wind, the currents did not reverse butdid slow down during the night and mid-morning. The band-passed filtered PVDs (Fig. 12c, bottom panels) show that theexcursion of the wind was nearly 100 km during the course of aday, where the wind rotated counterclockwise with a northeast-wards component from mid-day to 01:00 and southwestwardsuntil 10:00. A similar pattern is observed in the band-passedcurrents, albeit the excursion was roughly 5 km.

The vertical structure of the response to a wind event can beclearly seen in the daily-period ocean currents for the period ofNovember 1–14, 2007. In both the along-shore and cross-shore(Fig. 12d), the near-diurnal currents become surface-intensified asthe daily wind stress increases starting on November 1st, with thetop layer steadily deepening as the wind-stress forcing persist and

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 8. Vertical profile of temperature (left panel), salinity (middle panel) and sigma-t (right panel) collected with a CTD on January 21st, 2008 at the mooring site.

Fig. 9. (a) Polarization (clockwise/counterclockwise) and (b) ellipticity (minor axis/major axis) of the diurnal-period current velocity for the three study periods.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]]8

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-forced ocean variability on the inner shelf off Concepcion, Chile.Continental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

Fig. 10. Average of the diurnal-period along- and cross-shore wind stress (top panels), along-shelf velocity (middle panels), cross-shore velocity (middle panels), and along-

shelf and cross-shelf velocity at surface and near bottom (bottom panels) for the 2006 (left panels), 2007 (center panels) and 2008 (right panels) study periods.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]] 9

then becoming shallower as the forcing weakens starting onNovember 5th. Apart from a weaker overall response, the lowerlayer also moves with a clear phase difference of nearly 1801 in thecross-shore direction, which results in strong vertical shear duringthe maximum wind stress intensity. In the along-shore direction aphase difference is also evident during the build up of wind event,although by November 4th the upper layer motion appears tooccupy most of the water column.

During this event (November 10–11, 2007), the near-diurnalcurrents show strong counterclockwise rotation throughout thewater column (Fig. 13), a feature common to all the daily-periodcurrent velocity time-series. The currents ellipses rotatecounterclockwise with depth from a direction roughly alignedwith the coast at the surface-most bin to a roughly cross-shoredirection at depth. Extrapolating this trend to the surface, the near-

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

diurnal ellipses would become oriented closer to the cross-shoredirection, with a value of nearly 681 relative to due north, if theorientation was assumed to change linearly from the depth range ofthe observed currents.

3.2.4. Evolution of the diurnal-period currents

To further investigate the relationship between the wind-stressforcing and the response of the ocean, we compute a rotary diurnalfit using a 48 h window displaced every 6 h for every time series(Emery and Thomson, 2001). We thus separate the wind-stress andcurrent velocity into clockwise (CW) and counterclockwise (CCW)components, although we focus in the latter. For the currents, wecarry this analysis at two depths representative of the observedtwo-layer structure: the surface-most bin roughly at 4 m depth

orced ocean variability on the inner shelf off Concepcion, Chile.

Table 3Results of the complex demodulation for diurnal cycle for the wind stress, near surface and near bottom currents. (A: amplitude, CCW: Counter clockwise, CW: Clockwise.)

Wind stress

Year A (CCW) (N m�2) A (CW) (N m�2) Phase (CCW) (deg.) Phase (CW) (deg.)

2006 0.0244 0.0185 160 353

2007 0.0443 0.0346 173 14

2008 0.0324 0.0258 162 359

Currents

Depth (m) cm s�1 cm s�1 Deg. Deg.

2006 3.7 1.7128 1.5175 145 129

2007 3.9 4.1964 1.3361 223 56

2008 3.8 2.3856 0.9132 164 66

2006 20.7 1.3192 0.7181 90 257

2007 20.9 0.9742 0.3553 42 240

2008 20.8 0.8888 0.4159 85 283

Fig. 11. Evolution of the diurnal-period wind-stress (top) and near surface, near diurnal-period currents (bottom) during the three study periods. Colors show the along-shore

wind-stress and current magnitude, and the arrows the corresponding stress and velocity vectors.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]]10

and the deep layer at 20 m. Because of the diurnal and inertialfrequencies are relatively close in the region, the velocitytime-series are likely to contain diurnal-period (directly wind-forced), inertial and tidal (K1) currents. We can use the phaseinformation to distinguish among the first two (Simpson et al.,2002) because the diurnal fit to an inertial-period current wouldresult in the phase drift of a lead of the order of 681 per day in thestudy region (that is, with the phase resulting from the complexdemodulation analysis increasing with time), while a strictlydiurnal-period motion is expected to have constant phase

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

through time in this analysis. The K1 (freq¼0.0418 h�1) anddiurnal frequencies are close enough so that a lead phase drift ofthe order of one degree per day are to be expected. However, as wasexplained in Section 2, the tides in these regions are dominated bysemi-diurnal components and we do not expect diurnal tides to beanywhere as large, or have the strong baroclinic structure, as theobserved wind-driven or inertial response.

During 2007, when the winds were consistently strong in diurnaltime scales, the analysis shows a well-correlated ocean response(Fig. 14, right panels). The wind stress shows a tendency for CCW

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 12. (a) Alongshore diurnal-period wind stress and (b) total along-shore current velocity for the 2007 deployment. The greyed area indicates the data used for the

progressive vector diagram of total (top) and diurnal-period (bottom) winds and current velocity shown in panel (c). The along- and cross-shore diurnal-period currents

between November 1 and 14 are shown in panel (d).

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]] 11

motion, and a relatively stronger polarization is observed in thesurface currents (Fig. 14). Gusts of wind stress are well correlated withCCW currents of the order of 5–10 cm/s and nearly constant phase.During the same periods, the lower layer is also CCW polarized, butshows a 1801 phase shift with respect to the surface. The event ofNovember 1st–14th presented in the previous section is thusrepresentative of the dynamics observed throughout this period.However, there are also clear examples of brief periods where asignificant phase drift can be observed, such as November 14–17,November 22–27, December 8–9 and December 15–19, which seemmore consistent with inertial oscillations produced as the result of arelative sudden drop or increase in the wind stress (or change indirection). In fact, all of these events show a phase drift around 681/day(thick grey line in Fig. 14), which is consistent (within 2 h) with thelocal inertial period of 20.18 h. At the end of the 2007 study periodthe wind intensity drops, the currents weaken significantly, and thecurrent phase shows, perhaps expectedly, large variations.

During 2008 (not shown), the wind-stress forcing was generallyweaker, but similar patterns as in 2007 can be observed, with clearperiods of intense, CCW motion in the surface layer and significantphase shifts in the lower layer. But with overall weaker daily-

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

period wind-stress, there are relatively extended periods where thecurrents appear to be dominated by inertial variability or otherprocesses, and the current becomes CW-polarized over severaldays. Finally, in 2006 (Fig. 14, left panel) the diurnal-period windstress is the weakest of the three periods, and only a few days in themiddle of the time-series appear consistent with the CCW-polarized,constant-phase response observed in either of the other two datasets. Instead, the majority of this summer deployment is dominatedby weak diurnal-period wind-stress forcing, leading to weak oceancurrents or a CW-polarized circulation seemingly uncorrelated tothe wind stress. The current phase drifts observed during thisdeployment are highly variable and do not seem consistent withsome of the clear inertial-dominated periods evident in the 2007 and2008 datasets.

Overall, the results indicate a clear pattern of diurnal-periodcurrent variability in the inner shelf off Concepcion. When thewinds are strong as they were in 2008 and particularly in 2007, theresponse is characterized by a two-layer velocity structure, whichrotates counterclockwise during the day, and has a tendency to bepreferentially in the along-shelf direction. As the wind stressrotates to a maximum northeastward magnitude in the mid- to

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 13. Diurnal-period ellipses for the period of November 10–11, 2007, as a

function of depth.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]]12

late afternoon, the surface current responds with an intensity of theorder of 4–8 cm/s in the top 10–15 m, with a shoreward componentin the afternoon. When the daily-period wind-stress continues toturn as midnight approaches, the surface currents moves offshoreand southeastwards. The bottom layer, occupying 20–25 m, alsomoves counterclockwise but roughly with a phase shift of 1801with respect to the surface layer. This results in southeastwardsmotion with an onshore component in the late afternoon. It isworth noting, however, that data from different years reveal thatperiods of weak, inconsistent daily-period wind-stress forcing suchas 2006 do not fit the above description and are instead character-ized by weaker, highly variable currents. The circulation patternduring these times requires further investigation.

3.2.5. A dynamical interpretation of the wind-driven response

The main characteristics of response of the daily-period, wind-forced inner-shelf circulation in the study area can be understoodby using a wind-driven, two-layer model which includes theinfluence of the coast in the form of a cross-shore pressure gradient.The model equations (see Craig, 1989 and Simpson et al., 2002 fordetails) are

@w1

@tþ ifw1 ¼

tw

rh1�g

@Z@x

@w2

@tþ ifw2 ¼�g

@Z@x

where w and t are the complex horizontal velocity (u+ iv) andwind-stress (tx+ ity), respectively, where x is the cross-shore and ythe alongshore components. By using the vertically integratedsolution from Craig (1989) to obtain the cross-shore pressure

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

gradient

@Z@x¼

tx

rgH

where H¼h1+h2. This approximation is valid for a shelf width scalesmaller than the barotropic wavelength 2p(gH)0.5o. The shelfwidth in the study region is roughly 60 km, so this condition iseasily satisfied. We look for solutions of the form Weiot and,following Simpson et al. 2002, we obtain the CCW amplituderesponse for the model

W1 ¼gTxþ iðgþ f=oþ1ÞTy

irHðf þoÞ

W2 ¼�ðTx�if=oTyÞ

irHðf þoÞ

where g is the ratio of the lower to upper layer thickness h2/h1. Themodel shows that a resonant response is expected around 301S.Also, it predicts that the response to CCW forcing should be of theorder 9(f–o)9/9(f+o)9 stronger than for equal magnitude CWforcing. For the study area, the ratio is about 11, and given thatthe wind-stress forcing is actually CCW-polarized (Fig. 14), thisratio should be even larger.

Although the study region is several hundred kilometers south ofthe latitude of resonant response, the ratio f/o is approximately 1.2 offConcepcion, so we can expect significant enhancement of theresponse. During the summer of 2007, which provides the clearestexample of wind-driven diurnal-period currents in the region, theaverage cross-shore and along-shore wind-stress amplitudes wereapproximately 0.04 and 0.06 N/m2, respectively. The upper layerthickness has a scale of approximately 10 m, which results in a depthratio g¼2.5. Using this value and the observed wind-stress ampli-tudes, the model predicts a ratio of the upper to lower layer CCWamplitude of roughly 2, with a phase difference between the upper andlower layers of approximately 1751 for both cases. The fit to the 2007data shown in Table 3 results in a phase difference of 1811 for the CCWcomponent and an amplitude ratio of 4.3, a reasonable agreement tothe model. The predictions also compares favorably with an averageratio of upper-layer (above 10 m) and lower-layer CCW amplitude of2.9 obtained from the complex demodulation analysis for 2007.

Overall, the main properties of the wind-driven response in theinner-shelf off Concepcion can be explained to a reasonable degreeby the model described above. In the observed CCW-polarized,two-layer response, the top layer is forced directly by the wind,while the lower layer moves roughly 1801 out of phase as a result ofthe wind-generated cross-shore pressure gradient. The CCWvelocity structure is surface-intensified by a factor of approxi-mately three to four in our observations, which is consistent, ifsomewhat larger, with the ratio derived from the model using theobserved wind-stress forcing and upper and lower layer thick-nesses. However, the daily-period currents are only about 2 to 5times more intense in the CCW rather than the CW direction ofrotation, values too low compared to the expected values of order10 for the observed wind forcing, which is also CCW-polarized.

4. Discussion

Diurnal-period winds near the coast have been observed inseveral coastal systems where strong synoptic winds are present(e.g. Burt et al., 1973; Halpern, 1974; Beardsley et al., 1987). Asimilar relationship between the magnitude of the diurnal-periodand synoptic-scale winds that we describe here (e.g. Fig. 5) for thecentral Chile coast has also been examined for the coasts ofCalifornia (Rosenfeld, 1988) and Australia (Pattiaratchi et al.,1997). Both of these systems have strong synoptic-scale windswhich are in balance by the large-scale pressure gradients, and

orced ocean variability on the inner shelf off Concepcion, Chile.

Fig. 14. Counterclockwise amplitude (top panel), phase (middle panels) and polarization (bottom panels) for a diurnal-period fit of the wind-stress (WS) and current velocity

at approximately 4 and 21 meters for 2006 (left panels) and 2007 (right panels). The thick, diagonal grey line in the middle panels is a 681/day phase drift expected for purely

inertial motions fitted with a diurnal frequency.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]] 13

which combined with the presence of the local, diurnal-scalepressure gradient can produce the observed enhancement of thesea-breeze regime, with the orientation of the daily-period windsprimarily in the along-shore direction (Pattiaratchi et al., 1997). Inthe case of our study region, synoptic winds are typically from thesouthwest during the spring and summer, and its variability islinked to the South Pacific anticyclone, while the daily variability is,as elsewhere, primarily driven by the near-shore thermal gradients,which are likely enhanced during periods of upwelling-favorablewinds when the days are clear.

The variability of the diurnal-period wind along the Chilean coast(between 191 and 431S) has been previously examined by Garreaudand Munoz (2005), who subtracted satellite-derived (QuikScat) windvelocities at 08:00 and 20:00 local time to calculate the amplitude ofthe daily cycle of the wind. They found that these amplitudes did notexceed 1.0 m s�1 off the coast of Concepcion. The sampling times ofthe satellite measurement are reasonable for this purpose (see Fig. 2),but our data shows that a daily-period wind amplitude of around5 m/s when calculated by subtracting the average daily cycle at thesame times used by Garreaud and Munoz (2005). The reason for thisdiscrepancy is not clear, but might be due to the relative coarseresolution of the QuickScat sampling grid (25 km) and the difficultiesof getting good-quality data within 20 or 30 km from the coast due tobackscatter interference from land. If the offshore decay scale of thesea-breeze is relatively short compared to this sampling issue, thesatellite-derived cycle might be more representative of the mid- toouter-shelf conditions than of the near-shore region. Anotherpossibility is interannual variability, as for this calculation Garreaudand Munoz (2005) used data from November–January of 2000–2003,while ours is from 2006–2008. It is important to note that our data is apoint measurement and that the frequent occurrence of gulfs and

Please cite this article as: Sobarzo, M., et al., Diurnal-period, wind-fContinental Shelf Research (2010), doi:10.1016/j.csr.2010.10.004

peninsulas along the coast (including Hualpen Point, where our winddata was collected) can result in significant along-shelf variability ofthe wind. These and other issues with the satellite data or the in situdata should be explored further.

The effects of the diurnal-scale winds on the inner-shelf watersoff Concepcion are evident in a surface-intensified, anticlockwise,and elliptically polarized current velocity response with ampli-tudes in the order of 10 cm s�1 and intermittency consistent withthe wind variability. Because the study site is located just 6.51 southof the critical latitude (301 S) the strong response and most of itsproperties can be explained by a relatively simple two-layer model(Simpson et al., 2002). Although inertial oscillations also generate abaroclinic structure with two layers out of phase by 1801 offConcepcion these oscillations appear with similar mean velocitiesin the surface and bottom layers (Sobarzo et al., 2007b). Thus,surface intensified baroclinic currents in the inner shelf offConcepcion would be a characteristic of the diurnal currentsdriven by the sea breeze in this region.

Other aspects of the ocean response that are not fully exploredhere include the modulation of the upwelling response. Becausethe diurnal-period and synoptic-scale winds are correlated, i.e.,diurnal-period wind magnitude is larger when the wind is equa-torward (upwelling favorable) and weak when the wind is pole-ward (downwelling favorable) it can be expected that coastalupwelling should experience important daily-scale variations inthis region. Since the upwelling process has normally been studiedwith subinertial time-series (periods440 h), the daily interactionbetween the sea breeze and coastal upwelling has not beenincluded in earlier studies (e.g. Sobarzo and Djurfeldt, 2004).Nonetheless, the land–sea breeze could be an effective mechanismfor modulating the wind from the southwest, although whether or

orced ocean variability on the inner shelf off Concepcion, Chile.

M. Sobarzo et al. / Continental Shelf Research ] (]]]]) ]]]–]]]14

not the sea breeze alone is capable of producing local diurnalupwelling, as was observed recently in Monterey Bay (Woodsonet al., 2007), has yet to be elucidated.

Finally, it is important to note the potential role of thesedynamics in cross-shore exchange transports. As both the ellipseof the diurnal surface currents and the low frequency flow areoriented roughly along the coast substances (pollutants or larvae)released on the surface of inner-shelf in the study region will tendto drift primarily northward describing a daily ellipse. However,due to the proximity of the coast (2 km) and the magnitude of theminor axis of the ellipse, these substances could reach the coast.This is the case during summer time and similar results have beenreported for the Maipo River plume by Pinones et al. (2005).

5. Conclusions

1.

PC

The spring–summer wind velocity was predominantly towardthe northeast, approximately parallel to the coastal topography,and included a diurnal cycle with a similar orientation. Themean amplitude of the daily wind speed cycle increased withthe magnitude of the synoptic wind, and its magnitude can be aslarge as 6 m/s. During the summer the sea breeze can con-tribute, on average, around 50% of the observed alongshorewind-speed magnitude.

2.

The response of the inner-shelf to the diurnal-period windforcing was observed clearly in two of the three deployments,where the wind-stress was consistently strong. The currentsrotate counterclockwise throughout the water column, but atwo-layer structure is formed, with the surface layer flowingnortheastwards during the afternoon and reversing direction atnight. The lower layer shows a similar pattern of motion, but itsmagnitude is only one third as large as the surface layer. Thelower layer also moves in a pattern 1801 out of phase with thesurface layer, which results in an exchange flow in the cross-shore direction, with onshore (offshore) flow in the surface(bottom) layer during the afternoon and early evening, whichreverses near midnight.

3.

The analysis of the observed currents reveals that the propertiesof the wind-driven response are consistent with a two-layermodel of the inner-shelf, which includes the presence of a coast.The model correctly predicts the prevalence of the CCW currentsin this site, as well as the surface-intensified nature of theresponse. However, the response is more rectilinear thanpredicted. Also, an analysis of three different periods suggestthat the conditions for the dominance of diurnal-period cur-rents in the inertial-diurnal band – i.e., strong, consistentdiurnal-period winds – are not always met (e.g. summer of2006) and that other dynamics can become prevalent.

Acknowledgments

This work has been supported by FONDECYT Grant No. 1070501.Support for data collection as well as for M. Sobarzo and L. Bravowas provided by the Programa de Investigacion Marina de Exce-lencia (PIMEX-Nueva Aldea) of the Faculty of Natural and Oceano-graphic Sciences of the University of Concepcion, which is fundedby Celulosa Arauco y Constitucion S.A. Current meters (ADCP) weredeployed and recovered by Julio Moscoso, whose assistance wegratefully acknowledge.

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orced ocean variability on the inner shelf off Concepcion, Chile.