Sea Level Oscillations in Coastal Waters of the Buenos Aires Province, Argentina

Continental Shelf Research 22 (2002) 779–790

Sea level oscillations in coastal waters of the Buenos Aires province, Argentina

W.C. Dragania,b,*, C.A. Mazioa, M.N. Nunez* b a Departamento Oceanograf!ıa-Armada Argentina, Servicio de Hidrograf!ıa Naval, Seccion Dinamica Costera, Montes de Oca 2124, 1271 Buenos Aires, Argentina b Centro de Investigaciones del Mar y la , Atmosfera,! CONICET - UBA, Ciudad Universitaria, Pabellon! 2, Buenos Aires, Argentina

Received 21 May 2001; accepted 7 September 2001

Abstract

Sea level oscillations, with periods ranging from a few minutes to almost 2 h, have been observed at various tide stations located on the coast of Buenos Aires.Simultaneous records of sea level elevation measured in Mar de Aj o,! Pinamar and Mar del Plata during 1982 have been spectrally analyzed.Signiﬁcant spectral energy has been detected between 0.85 and 4.69 cycles per hour (cph) and the most energetic peaks have frequencies between 1.17 and 1.49 cph. Spectra, coherence, and phase difference have been analyzed for the most energetic event of the year.During that event, the most intensive spectral peak is at 1.17 cph for Mar de Ajo! and Pinamar, and at 1.49 cph for Mar del Plata. Simultaneous total energy peaks at Mar de Ajo,! Pinamar and Mar del Plata, and the coherence function estimated between Mar de Ajo! and Pinamar suggests that sea level oscillations could be a regional phenomenon.The analyzed data suggest that sea level oscillations could be forced by atmospheric gravity waves associated with frontal passages. r 2002 Elsevier Science Ltd.All rights reserved.

Keywords: Sea level oscillations; Spectra; Coherence; Buenos Aires coast; Atmospheric gravity waves

1. Introduction detected in several coastal locations around the world (Munk, 1962).Over the Argentine shelf the Large-amplitude sea level oscillations, which occurrence of large-amplitude and long-period range from a few minutes to almost 2 h, have been oscillations is well known.These oscillations have frequently observed in different tide stations at the been observed frequently between Mar de Ajo! and open sea and at locations on the Buenos Aires Quequen! stations, on the Buenos Aires coast, but coast (Fig.1). Similar perturbations have been they have not been detected further south, over the broad Patagonian shelf.The most conspicuous event occurred on March 28, 1970, when a sea ! *Corresponding author.Departamento Oceanograf ıa-Arma- level oscillation of 162 cm height and 33 min da Argentina, Servicio de Hidrograf!ıa Naval, Seccion Dinamica Costera, Montes de Oca 2124, 1271 Buenos Aires, Argentina. period was recorded at Mar del Plata (Fig.2). Tel.: +54-11-4301-0061; fax: +54-11-4301-2918. Balay (1955) showed that these oscillations E-mail address: [email protected] (W.C. Dragani). frequently occur simultaneously with the passage

with the ﬁndings of Lanfredi and Capurro (1971) and Lanfredi (1972), who detected off-shore oscillations similar to the ones observed at the coast in a series of current measurements at the latitude of Mar del Plata.Furthermore, Vara et al. (1978) pointed out that when sea level oscillation activity increases, there is generally a greater concentration of spectral energy in the low frequency band.Between June 1981 and July 1982 sea level at Pinamar was recorded by a digital instrument with a pressure sensor.Vara and Mazio (1982) obtained sea level spectra and pointed out that, apparently, intense activity increases the Fig.1. Buenos Aires coastal region (Argentina) and its loca- spectral amplitude by a constant factor.Dragani tions, in the southwestern Atlantic Ocean.The bathymetry is (1988) studied the possible connection between labeled in meters. seismic activity and these sea level oscillations and found low correlation between them. In the present work, we have analyzed contin- uous records for three different tide gauge stations located along the coast of Buenos Aires during 1982, the year of longest simultaneous records. The stations are located at Mar de Ajo! (361500S, 561390W), Pinamar (371080S, 561500W) and Mar del Plata (381050S, 571300W), as shown in Fig.1. The stations are exposed to the open sea; Pinamar is about 50 km south-southwest of Mar de Ajo,! and Mar del Plata is about 126 km southwest of Pinamar.The continental shelf is E180 km wide Fig.2. Sea level for the most highly energetic event recorded at the station in Mar del Plata. off Pinamar and Mar del Plata, and somewhat wider off Mar de Ajo! (240 km).Tides in the region are mixed with both semi-diurnal and diurnal of meteorological fronts coming from central constituents present.Tides have a maximum range Patagonia.Subsequently, Inman et al.(1962) of almost 2 m at Mar del Plata and are somewhat obtained spectra for a 10-day-interval of sea level smaller to the north. records for three tide gauge stations along the The aim of this paper is to present evidence that Buenos Aires coast: two at Mar del Plata (port and sea level oscillations, ranging from a few minutes open sea) and one at Quequen! Port.The cross- to almost 2 h, are a regional phenomenon and to spectral analyses showed a peaked-shaped spec- consider their origin on the Buenos Aires coast.In trum which diminished monotonically with in- Section 2, the data analysis is brieﬂy described. creasing frequency.Very low coherence was Time changes of total spectral energy and the obtained between Quequen! Port and Mar del sequence of spectral density energy for each Plata (open sea), 124 km apart.It was suggested station are presented.Later, sea level data for that sea level oscillations at the two locations are the most highly energetic interval is analyzed by not related in a simple and direct way. means of spectra, coherence, and phase difference In a later study, Vara et al.(1977) pointed out functions (Section 3).In Section 4, a possible that these waves have regional characteristics and origin of such sea level oscillations on the they are present all over the Buenos Aires continental shelf is discussed.Results are summar- continental shelf.This observation is in agreement ized in Section 5. W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 781

2. Data analysis Sea level spectra were obtained by means of the fast Fourier transform procedure, and Parzen’s Analog tidal records gathered at standard tidal spectral window (Harris, 1978) of ﬁxed length stations at Mar de Ajo,! Pinamar and Mar del Plata 2048 min was performed.Spectra obtained were were digitized at a rate of 15 samples per hour.In smoothed by means of the sum of spectral consequence, a data base of 131400 values for each contributions over contiguous bands of 0.32 cycles station was obtained.Sea level data contain diurnal per hour (cph), resulting in 40 degrees of freedom. and semi-diurnal tides and higher-frequency oscilla- For the year 1982 and for each tidal station 254 tions ranging from a few minutes to almost 2 h. spectra were obtained from successive sets of 512 Upon digitalization, data were convoluted by observations (1.42 days). means of a 511-point Kaiser–Bessel bandpass ﬁlter, Total energy (or spectral contents) was com- with a passing-wave response function of E10– puted for each sea level spectrum by adding up all 180 min periods, and an attenuation factor of the spectral contributions placed at the range of 100 dB out of the last band.In this way, wave periods from 9.6 to 113.3 min (Fig. 3). When a sea periods shorter than 10 min are not transmitted. level oscillation period begins, the energy at the

Fig.3. Evolution of total energy for Mar de Aj o,! Pinamar and Mar del Plata, and atmospheric pressure variations from the mean value at Mar del Plata, during 1982. 782 W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 three stations increases sharply, reaches a maximum simultaneously and then decreases, becom- ing very low after the event.Nevertheless, there are a few intervals where only one or two locations show a remarkable total energy peak.This point is discussed in section 4.In regard to the total energy at Mar de Ajo,! Pinamar and Mar del Plata stations, values o5cm2 were observed in 216, 213 and 225 cases (i.e. 85% of the total number of cases, approximately), between 5 and 25 cm2 in 34, 33 and 25 cases (12%), between 25 and 45 cm2 in 3, 5 and 3 cases (2%), and >45 cm2 in 1, 3 and 1 cases (o1%), respectively. A comparative study was carried out among the 254 spectra obtained from sea level records obtained at Mar de Ajo,! Pinamar and Mar del Plata.Time histories of spectra were prepared for the three stations and they showed very similar appearance.In this paper, only the time history of spectra for Pinamar is presented (Fig.4). Weak high-frequency disturbances are detected only at the time of highest energetic intensity while the longest-period waves are present before, during and after that time.The spectral contributions are distributed almost over the whole bandwidth during the events of high activity.Simultaneous quiet periods are also apparent, for example, between Julian days 53 and 64 (between February 22nd and March 5th).In these quiet periods, a weak contribution of spectral energy is only shown at frequencies o1 cph.According to all processed sea level records, the frequency of 4.7 cph is Fig.4. Contours of spectral density energy (from 0.25cm 2/cph, approximately the high frequency limit of the each 0.25 cm2/cph) during 1982 for Pinamar. phenomenon. A time interval was selected to study the temporal evolution of spectra and total energy at Pinamar.This interval must be long enough to confidence level of the spectral noise was plotted in totally include the period of strong activity.The each spectrum.This level is an overestimated limit period selected was the week of March 21–28, 1982 but it assures a realistic spectrum above it.The (Julian days from 80 to 87).The spectra sequence 95% confidence levels are also pointed out for corresponding to this interval is shown in Fig.5. each spectrum.Only peaks larger than the noise In this sequence it can be seen that the total energy level and the surrounding 95% confidence levels of increases from almost zero to a maximum the other nearby frequencies are labeled in Fig.5 (44.4 cm2), and later decreases again to almost (with the period in minutes).Highest peaks are zero.The sea level measurement error ( 72 cm) has placed between 1.1 and 1.3 cph (E50–40 min). a uniform statistical distribution with variance When the event is very energetic (Fig.5c and d) distributed throughout the frequency band like other lower peaks appear at higher frequencies white noise (spectral noise level).The 95% upper (>3 cph). W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 783

Fig.5. Sea level spectrum sequence for an energetic event from March 21–28, 1982, at Pinamar.Spectral peaks’ periods (minutes) and total energy (cm2) are pointed out; the noise level is indicated by dashed line.Sea level spectrum from data gathered from digital instrument with pressure sensor is also plotted in (d) (thin line).(a) March 21, 15:16, (b) March 23, 05:24, (c) March 24, 11:32, (d) March 25, 21:40, (e) March 27, 07:48, (f) March 28, 17:56.

From June 1981 to July 1982, a digital instru- 3. The highest energy event of 1982 ment with a pressure sensor also recorded sea level at Pinamar, at a rate of one sample per minute.In During September 8–10 (Julian days from 251 to Fig.5d, both sea level spectra are presented. The 253) the highest energy event on the Buenos Aires spectra are very alike; spectral peaks are placed at coast was recorded.Filtered sea level oscillations 40.3 and 19.4 min and they present similar total for this period in Mar de Ajo,! Pinamar and Mar energy values.Unfortunately, digital record pre- del Plata are shown in Fig.6. High sea level sented several long gaps and then few simulta- oscillations are observed ﬁrst at Mar del Plata and, neous spectra can be obtained. subsequently, further north at Pinamar and Mar 784 W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790

Fig.6. Simultaneous passband filtered sea level records at (a) Mar de Ajo,! (b) Pinamar and (c) Mar del Plata during the highest energetic event observed in 1982. de Ajo! stations, respectively.Maximum wave height determined was 41.4 cm at Mar de Ajo! station, 53.1 cm at Pinamar and 47.0 cm at Mar del Plata. Fig.7. Sea level spectra for the highest energetic event of 1982 ! at (a) Mar de Ajo,! (b) Pinamar and (c) Mar del Plata.Spectral Mar de Ajo, Pinamar and Mar del Plata sea 2 level spectra (for period shown in Fig.6) are peak periods (minutes) and total energy (cm ) are pointed out; the noise level is indicated. presented in Fig.7. Some peaks with different spectral energy density levels are clearly present at all locations.Another peak (at higher frequencies) series of sand ridges, averaging 4.7 m in height and is apparent at Mar de Ajo! and not at the other about 2.7 km apart. The ridges are oriented in the stations due, probably, to the different topography north–south direction, forming a 20–351 angle of each place.In this regard, it is important to with the coast (Parker et al., 1982). point out that the inner continental shelf adjacent The most energetic band covers periods ranging to the Buenos Aires coast, between Punta M- from 30 to 60 min while at lower periods (ranging edanos! and Mar del Plata, is characterized by a from 12 to 30 min) some not significant peaks are W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 785 observed.Nevertheless, spectral energy is greater spectral energy placed close to 1.0 cph and then the than the noise level in the frequency band ranging phase difference could be doubtful. from 0.5 to 4.0 cph. Using h ¼ 44 m as a realistic value for the Coherence and phase difference between Mar de continental shelf mean depth between the coast Ajo! and Pinamar (the nearest stations) for the and the 100 m contour, the wave speed c for the highest energetic event were estimated (Fig.8). edge-waves can be calculated as (gh)1/2, where g is Coherence values were plotted for the most the acceleration due to the gravity (Leblond and energetic frequency band (between 0.75 and Mysak, 1978).Using the aforementioned values, a 2.5 cph) and phase differences were only plotted wave speed of 20.9 msÀ1 and a lag of 0.7 h between at frequencies where coherence values are above Pinamar and Mar de Ajo are obtained.Taking the 90% confidence limit.Note that in the into account these values, the waves in this region frequency range 0.75–1.45 cph, where the spectra of the continental shelf appear to travel more energy and coherency are high, the phase differ- slowly than predicted by the theory. ence increases with the frequency from À401 to Coherence values between Pinamar and Mar del 1651.The phase difference is approximately a Plata and Mar de Ajo! and Mar del Plata are lower linear function of frequency, which implies that the than coherence obtained between Mar de Ajo! and wave motion is essentially non-dispersive.From Pinamar.A possible reason for these low coher- the slope of the line that represents the best fit to ence values could be that the distances between the difference phase/frequency data (see Fig.8), a Pinamar and Mar del Plata (126 km) and Mar de lag of 0.8 h between Pinamar and Mar de Ajois! Ajo! and Mar del Plata (176 km) are significantly obtained.Furthermore, a positive slope reveals greater than the distance between Mar de Ajo! and that the propagation is northward which agrees Pinamar (50 km). with the observations.Considering the distance between Pinamar and Mar de Ajo and the estimated lag between both stations a wave phase 4. Possible mechanisms responsible for sea level speed of 17.4 msÀ1 is obtained.Phase difference oscillations in Buenos Aires coastal close to 2.0 cph slightly decreases with frequency. watersFdiscussion Although spectral energy at 2.0 cph is above the noise level, it is one order of magnitude lower than Sea level oscillations, ranging from a few minutes to almost 2 h, could be originated by seismic, oceanic or atmospheric causes. Large tsunamis can be generated by fast tectonic displacements of the ocean floor over a large horizontal scale (from hundreds to over a thou- sand square kilometers) during strong earthquakes, causing vertical displacements of ocean floor of tens of meters.Other generation mechanisms are underwater subsidence or land avalanche in the ocean and submarine volcanic eruption (Wu, 1981).Dragani (1988) studied a possible relationship between seismic activity (detected at La Plata seismographic station, located 240 km north-northwest of Mar de Ajo,! Fig.1) and eleven high sea level oscillation events detected at Fig.8. Coherence and phase differences between sea level Pinamar.It was shown that four sea level records for Mar de Ajo! and Pinamar, for the highest energetic event of 1982.Degrees of freedom: 30; the 90% significance oscillation events were detected 1 or 2 days after level is indicated.Spectral density energy for Mar de Aj o! and the underwater earthquakes had occurred.Never- Pinamar are also shown. theless, seven intervals of sea level oscillations 786 W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 occurred without previous seismic activity in the ocean.Furthermore, five submarine earthquakes were detected without subsequent high sea level oscillation activity.These results suggest that a different mechanism should be required to gen- erate the sea level oscillations discussed here. Studies carried out in Puerto Rico and the Philippines (Giese et al., 1982, 1990; Chapman and Giese, 1990) supported the hypothesis that large- amplitude coastal seiches can be produced by internal waves which have, themselves, been generated by strong tidal flows.Chapman (1984) proved by means of a simple two-layer step-shelf model that barotropic (surface) edge waves of Fig.9. Fortnightly distribution of sea level variance for Pinamar during 1982. substantial amplitude could, in principle, be generated by deep-sea internal waves incident upon the coastal topography.To show the standing wave with an antinode at the shore line generation of barotropic edge waves, Chapman and a node at the edge of the continental shelf considered a stratified deep ocean composed of (Munk, 1962).For the fundamental mode the two immiscible fluids with slightly different den- period T0 can be calculated as sities.The upper water general circulation of the sffiffiffiffiffi Western Argentine Basin is characterized by A T ¼ 8 : ð1Þ the southward flow of the Brazil Current along 0 sg the continental margin of South America and the equatorward flow of the Malvinas Current along Using a shelf width A ¼ 180 km and a continental À3 the continental slope (Bianchi et al., 1993). The shelf slope s ¼ 10 ; T0 ¼ 9:5 h is obtained. convergence of both currents near 381S causes a Continental shelf water density profiles present a strong thermohaline front referred to as Brazil– seasonal variability during the year.A strong Malvinas Confluence (Gordon, 1982).Associated stratification is presented from October to April with this confluence, the variety of water masses and a homogeneous sea is observed from March to and the large eddy variability, a complex vertical September (Charo et al., 2001). Although specific thermohaline structure is found.When subtropical studies of internal gravity waves in the Buenos water (Brazil Current) is found adjacent to the Aires continental shelf have not been carried out, Buenos Aires continental shelf the upper ocean is some repeated profiles of conductivity-temperature- strongly stratified.In that case the propagation of depth (CTD) suggest their probable existence tidal generated internal gravity waves could be during strong stratification periods (Bianchi, pers. possible.Nevertheless, unlike Puerto Princesa comm.2000).Nevertheless, sea level oscillation (Giese and Hollander, 1987) and the Caribbean activity does not present a characteristic seasonal coast (Giese et al., 1990) sea level oscillations in variability.Fig.10 shows the monthly mean var- the Buenos Aires coast are not apparently iance for Mar de Ajo,! Pinamar and Mar del Plata. correlated with the tide.The fortnightly distribu- The maxima variance values are reached on August tion of sea level variance for Mar de Ajo,! Pinamar and September, when continental shelf water is and Mar del Plata at the studied bandwidth do not vertically homogeneous.It can be seen that the show a distinct maximum (Fig.9).Moreover, the mechanism that supports coastal seiches produced estimated coastal seiche period at the Buenos Aires by internal waves generated by strong tidal flows do continental shelf is larger than the period of the not explain the observed oscillations at the Buenos analyzed sea level oscillations.The most simple Aires coastal waters.Another different generating model to estimate the coastal seiche period is a mechanism should be analyzed. W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 787

et al., 1989). Several studies about atmospheric gravity waves forcing sea-level oscillations have been published.For example, Wilson (1954) studied the generation of long-period seiches in Table Bay, Cape Town, by barometric oscillation. Munk et al.(1956) analyzed records of sea level oscillations in Oceanside and Scripps, which were associated to internal waves on the atmospheric inversion layer.Others, Tintor e! et al.(1988), Monserrat et al.(1991a, b), Gomis et al.(1993), Rabinovich and Monserrat (1996), Garcies et al. (1996), Monserrat and Rabinovich (1997), and Rabinovich and Monserrat (1997) studied large Fig.10. Monthly sea-level variance from Mar de Aj o,! Pinamar and Mar del Plata records during 1982. sea level oscillations in several bays and harbors of the western Mediterranean, which were also The third potential mechanism is atmospheric associated with atmospheric pressure fluctuations. forcing.The possible forcings are due to squall A comparative study was carried out between lines, passage of meteorological fronts and atmo- sea level oscillations for the Mar del Plata station spheric gravity waves.Cartwright and Young and atmospheric gravity waves at the Punta (1974) stated that a meteorological front is more Medanos! Lighthouse (165 km apart) between obviously associated with a discontinuity in wind 1984 and 1986.14 highly active events of sea level speed and direction than with one in pressure. oscillations were detected in this period.All these They considered obvious that its effects on sea events occurred with the passage of meteorological level should be at least qualitatively similar to fronts over the Buenos Aires continental shelf, and those of a fast-moving step in pressure.Total atmospheric gravity waves were recorded in 12 spectral energy at Mar de Ajo,! Pinamar and Mar cases.Filtered atmospheric pressure from Punta del Plata, and atmospheric pressure fluctuations Medanos! Lighthouse and filtered sea level oscilla- (relative to the yearly mean value) at Mar del tions from Mar del Plata station, for the highest Plata, during 1982, are shown in Fig.3.This figure energetic event of atmospheric gravity waves shows that total energy peaks occurred at the same (October 12, 1985), are shown in Fig.11a–b.Here, time those atmospheric pressure minima.Those both an atmospheric pressure fluctuation of al- minima are generally associated to (i) passages of most 4 hPa and a maximum sea level oscillation of atmospheric fronts (observed on surface synoptic 56 cm were detected.It should be pointed out that charts) and (ii) atmospheric gravity waves events due to the microbarographic record scale (7 days a over the Buenos Aires continental shelf (Nunez* page) high frequency atmospheric disturbances et al., 1999). Balay (1955) was the first in observing (higher than 1.5 cph) are not well resolved. The that sea level oscillations are usually coincident Mar del Plata sea level spectrum (Fig.11c) shows with the passage of atmospheric fronts at the the highest energetic peak placed at 0.85 cph Buenos Aires coast.In contrast, squall lines have (70.7 min), and another peak at 33.0 min. In this never been observed during sea level oscillation case, the total spectral energy is 46.1 cm2.In the events.When sea level oscillations are present, Punta Medanos! atmospheric pressure spectrum hourly meteorological data showed pressure fluc- (Fig.11d) the highest (and broad) spectral peak is tuations of up to 2.8 hPa/h and gusts of up to 36 also placed at 0.85 cph. Here, a weak peak placed knots, the synoptic surface analysis showed a front at high frequencies (28.2 min) is scarcely greater and an upper level jet at 250 hPa level was located than the surrounding broad noise of the other behindFand parallel toFthe surface front, which nearby frequencies.Both spectra show a similar is consistent with the presence of atmospheric appearance in low frequency (close 0.8 cph) where gravity waves (Uccellini and Koch, 1987; DeMaria the most energetic spectral contents appear.Using 788 W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790

Fig.11. (a) Passband ﬁltered microbarographic atmospheric pressure at Punta M edanos! Lighthouse and (b) sea level oscillations at Mar del Plata station.Hours since October 12, 17:00, 1985; (c) sea level spectrum (from data shown in Fig.11b) and (d) atmospheric pressure spectrum (from data shown in Fig.11a).

44 m as a value for the continental shelf mean Finally, although sea level oscillation activity is depth and an observed pressure anomaly of 4 hPa, generally simultaneous at the three stations, there and supposing an atmospheric gravity wave are a few intervals where only one or two locations celerity of 20 msÀ1 (which is in the observation show a remarkable energy peak.In Fig.3 (Julian range, DeMaria et al., 1989; Powers and Reed, day 18) it is clearly seen that energy reaches a 1993) a sea level perturbation of almost 0.50 m can strong maximum at Pinamar (47 cm2), while be produced by dynamic inverse barometric effect energy at Mar de Ajo! and Mar del Plata is very (Proudman, 1953).This shows that sea level low (o7cm2).Spatial variability of the atmo- oscillations could be forced by atmospheric gravity spheric forcing could be a possible explanation. waves in Buenos Aires coastal waters. For example, Nunez* et al.(1999) showed that W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 789 atmospheric gravity waves over the Buenos Aires analyzed high energetic sea level oscillation events continental shelf presented different pressure frontal passages and associated atmospheric grav- amplitudes and spectra between El Rincon! and ity waves with predominant northeastward propa- Punta Medanos! (Fig.1). gation were detected.(ii) Sea level oscillations were generally observed first at Mar del Plata and, subsequently, further north at Pinamar and 5. Summary Mar de Ajo,! and it could be associated to the predominant propagation direction of atmo- Simultaneous measurements of sea level oscil- spheric perturbation.(iii) Total energy peaks were lations from Mar de Ajo,! Pinamar and Mar generally simultaneous at all three stations and del Plata stations (Argentina) during 1982, were significant coherence values were found between spectrally analyzed.Maximum wave heights were sea level from Mar de Ajo! and Pinamar stations. 41.4, 53.1 and 47.0 cm, respectively. Spectral (iv) Sea level (Mar del Plata) and atmospheric density energy was detected between 0.8 and pressure (Punta Medanos)! spectra show the most 4.5 cph, but the most energetic peaks were energetic spectral contents at low frequency (close located between 1.1 and 1.5 cph. Time histories 0.8 cph). (v) Using realistic values for the con- of wave energy spectra have shown that weak tinental shelf mean depth and atmospheric pres- high-frequency disturbances occurred only at sure anomaly it was shown that a sea level moments of highest energetic intensities.Never- perturbation of almost 0.5 m could be produced theless, the longest-period waves were present by dynamic inverse barometric effect in Buenos during a more extensive period, which, approxi- Aires coastal waters. mately, appears 2 days before and vanishes 2 days Finally, although there is strong evidence that after the event. sea level oscillations could be forced by atmo- Spectra, coherence and phase differences were spheric gravity waves associated to frontal pas- estimated for the most energetic event of the sages, more detailed observations are required to year.The maximum spectral peak was located at elucidate the possible coupling processes. 1.17 cph in Mar de Ajo! and Pinamar stations, and at 1.49 cph in Mar del Plata. The relatively low values of coherence estimated (o0.6) are probably due, to: (i) to the irregular spatial distribution of Acknowledgements the forcing over the continental shelf of Buenos We would like to acknowledge the meteorolo- Aires, (ii) to the boundary and the topographic gical data provided by the Servicio Meteorologico! effects and (iii) to weak non-linear effects in the Nacional (Fuerza Aerea! Argentina).We thank propagation of the waves in shallow water. very much Alberto Piola and Alejandro Bianchi Using a simple model the continental shelf for their active participation in the discussion of seiche period was estimated (9.5 h). This value is far from the analyzed bandwidth.Moreover, it is this work.We want to thank Silvia Romero, who helped with the English. shown that sea level oscillations are not apparently correlated with the tide and the maxima variance values of sea level oscillations were reached on August and September, when continental shelf References water is vertically homogeneous.This suggests that the observed sea level oscillations are not Balay, M., 1955. La determinacion! del nivel medio del Mar coastal seiches associated with oceanic internal Argentino, influencias de las oscilaciones del mar no waves generated by strong tidal flows. causadas por la marea.Dir.Gral.de Nav.Hidrog.,Min. de Marina, 46pp. The following results suggest that sea level Bianchi, A.A., Giulivi, C.F., Piola, A.R., 1993. Mixing in the oscillations could be forced by atmospheric gravity Brazil–Malvinas Confluence.Deep-Sea Research 40 (7), waves associated to frontal passages.(i) During 1345–1358. 790 W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790

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