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 0278-4343/02/$ - see front matter r 2002 Elsevier Science Ltd.All rights reserved. PII: S 0278-4343(01)00096-6 780 W.C. Dragani et al. / Continental Shelf Research 22 (2002) 779–790 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 instru- ment 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.

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