Atmospherically-Induced Water Oscillations Detected in the Port of Quequén, Buenos Aires, Argentina

Physics and Chemistry of the Earth 34 (2009) 998–1008

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Physics and Chemistry of the Earth

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Atmospherically-induced water oscillations detected in the port of Quequén, Buenos Aires, Argentina

Walter C. Dragani a,b,d,*, Enrique E. D’Onofrio a,b,c, Walter Grismeyer a, Monica M.E. Fiore a,b, María Inés Campos a a Servicio de Hidrografía Naval and ESCM-INUN, Av. Montes de Oca 2124 (C1270ABV) Ciudad Autónoma de Buenos Aires, Argentina b Departamento Ciencias de la Atmósfera y los Océanos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, (1428) Ciudad Universitaria, Pabellón II, 2do. piso. Ciudad Autónoma de Buenos Aires, Argentina c Instituto de Geodesia y Geofísica Aplicada, Facultad de Ingeniería, Universidad de Buenos Aires, Av. Las Heras 2214, (1127) Ciudad Autónoma de Buenos Aires, Argentina d CONICET, Consejo Nacional de Investigaciones Cientíﬁcas y Técnicas, Av. Rivadavia 1917, (C1033AAJ) Ciudad Autónoma de Buenos Aires, Argentina article info abstract

Article history: Sea level data gathered at Quequén, corresponding to the four most energetic events detected in 1982, are Received 22 December 2008 analyzed and compared with simultaneous sea level data recorded at Mar del Plata, Pinamar and Mar de Received in revised form 24 August 2009 Ajó. Large-amplitude sea-level oscillations at these locations are generally superposed to low-amplitude Accepted 24 August 2009 oscillations (‘‘background”) which are one or two order of magnitude lower than the ﬁrst ones. Back- Available online 29 August 2009 ground at Quequén is characterized by a broadband energy spectrum with maximum energy around 17–35 min. During energetic events at Quequén, the spectral peaks are situated between 0.8 and Keywords: 4.0 cph (15–75 min) and wavelet analysis shows intermittent activity of large-amplitude waves (they Atmospherically-induced waves show up irregularly during short lapses of 100–200 min long, approximately). The computed ratios Long waves Seiches between sea level variances of the active event and the preceding background at Quequén, Mar de Ajó, Tide gauge Pinamar and Mar del Plata apparently do not have a relationship among locations nor events. Such Spectral analysis noticeable variability in the spectral peak positions, variances and ratios could likely be related to the Port of Quequén celerity, amplitude, direction and period of atmospheric gravity waves in the region. Large-amplitude sea-level oscillations are always ﬁrstly observed at Quequén and, subsequently further north, at Mar del Plata, Pinamar and Mar de Ajó, respectively. Maximum amplitudes detected for each event at these different locations are very similar. These results support that atmospherically-induced large-amplitude sea-level oscillations (generated on the continental shelf) would enter the port of Quequén through its narrow mouth while they propagate towards Mar del Plata, Pinamar and Mar de Ajó, where they show up in tidal records some hours later. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Mar del Plata, 124 km apart, so it was then suggested that sea-level oscillations at the two locations were not related in a simple and Atmospherically-induced large-amplitude sea-level oscillations, direct way. which range from a few minutes to almost 2 h, have been fre- Vara et al. (1977) pointed out that these waves have regional quently observed at different tide stations in open sea and at loca- characteristics and they are present all along the Buenos Aires con- tions on the Buenos Aires coast between Mar de Ajó and Quequén tinental shelf. This observation is in agreement with the ﬁndings of (Fig. 1). Balay (1955) was the ﬁrst to report that these oscillations Lanfredi and Capurro (1971) and Lanfredi (1972), who detected frequently occur simultaneously with the passage of meteorologi- off-shore oscillations similar to the ones observed at the coast in cal fronts coming from central Patagonia. Subsequently, Inman a series of current measurements at the latitude of Mar del Plata. et al. (1962) obtained the spectra for a 10-day-interval of sea level Furthermore, Vara et al. (1978) pointed out that when sea-level records gathered at tide gauge stations located in Mar del Plata and oscillation activity increases, there is generally a greater concentra- Quequén. Very low coherence was obtained between Quequén and tion of spectral energy in the low frequency band at around 1 cph (cycle per hour). Between June 1981 and July 1982 the sea level at Pinamar was recorded by a digital instrument with a pressure * Corresponding author. Address: Departamento Oceanografía – 4 piso. Servicio sensor. Vara and Mazio (1982) obtained sea level spectra and de Hidrografía Naval, Av. Montes de Oca 2124, (C1270ABV) Ciudad Autónoma de Buenos Aires, Argentina. pointed out that, apparently, intense activity increases the spectral E-mail address: [email protected] (W.C. Dragani). amplitude by a constant factor. Dragani (1988) studied the possible

Fig. 1. Buenos Aires coastal region (Argentina) and locations, in the southwestern Fig. 2. Sketch of atmospheric gravity wave activity area. The front (at the surface) Atlantic Ocean. The bathymetry is labeled in meters. and the jet streak, the inflexion axis and the ridge axis (at 300 hPa height) are pointed out. Source: Dragani (2007). connection between seismic activity and these sea-level oscilla- eral studies of mesoscale wave disturbance events, Uccellini and tions and found low correlation between them. Koch (1987) observed that a jet axis, a surface front, an inflexion Dragani (1997) and Dragani et al. (2002) analyzed continuous axis (between the trough and the ridge axis) and a ridge axis bound records for three different tide gauge stations located along the the area of wave activity (Fig. 2). Regarding the propagation direc- coast of Buenos Aires during 1982, the year of longest simulta- tion of atmospheric gravity waves, it tends to match best with neous records. The stations are located at Mar de Ajó (36°500S, wind direction of upper tropospheric westerlies (Gedzelman and 56°390W), Pinamar (37°080S, 56°500W) and Mar del Plata Rilling, 1978). (38°050S, 57°300W), as shown in Fig. 1. The stations are exposed A comparative study was carried out between sea-level oscilla- to the open sea; Pinamar is about 50 km south–southwest of Mar tions recorded at Mar del Plata station and atmospheric gravity de Ajó, and Mar del Plata is about 126 km southwest of Pinamar. waves at Punta Médanos Lighthouse (145 km north–northeast of Tides have a maximum range of almost 2 m at Mar del Plata and Mar del Plata) between 1984 and 1986 (Dragani, 1997; Dragani somewhat smaller to the north. Analog tidal records gathered et al., 2002). Fourteen highly active events of sea-level oscillations at the three stations have been digitized, filtered, processed and were detected in this period. All these events occurred during the spectrally analyzed and the total energy (spectral contents) corre- passage of meteorological fronts over the continental shelf of Bue- sponding to each event (1.42 day long) of atmospherically-induced nos Aires province, and atmospheric gravity waves were recorded sea-level oscillations has been computed. Dragani (1997) ranked in 12 cases. Filtered atmospheric pressure at Punta Médanos Light- these events taking into account the total energy of each one and house, for the highest energetic event of atmospheric gravity studied the spectral characteristics of the most energetic events. waves (October 12, 1985), is shown in Fig. 3. This event lasted During lapses of high activity of long waves, the spectral energy 26 h, large-amplitudes appear intermittently in the data record density detected was between 0.8 and 4.5 cph, but the most ener- and pressure fluctuations were higher than 3 hPa. Atmospheric getic peaks were located between 1.1 and 1.5 cph. The maximum pressure spectra at Punta Médanos (Dragani et al., 2002) showed spectral peak was located at 1.17 cph at Mar de Ajó and Pinamar the most energetic spectral contents located at low frequency stations, and at 1.49 cph at Mar del Plata. Maximum wave heights (close 0.8 cph) but it should be pointed out that due to the micro- were 0.41, 0.53 and 0.47 m, respectively. barographic record scale (7 days a page), high-frequency atmo- Dragani (1997), Nuñez et al. (1999), Dragani et al. (2002) and spheric disturbances (higher than 1.5 cph) were not well Dragani (2007) described the typical synoptic situation during resolved. Unfortunately, an effective net of micro-barometers, energetic sea-level oscillation events. Low-level atmospheric cy- developed to gather systematic measurements of high-frequency clonic circulation and the passage of atmospheric fronts were al- atmospheric pressure perturbations, has never been implemented ways present during those events. It was shown that energetic at the Buenos Aires coast; only a few, short and sporadic analog re- events of sea-level oscillations and atmospheric pressure distur- cords are available in this region. bances were simultaneously detected and they showed intense A numerical model (Caviglia and Dragani, 1996) – two horizon- energetic contributions in the same frequency band. During those tal dimensions, vertically integrated – forced by the passage of events, synoptic upper analysis showed jet streams at 250 hPa le- atmospheric cold fronts (Dragani, 1999) and atmospheric gravity vel, located approximately south of Bahía Blanca, behind — and waves, was used to investigate the generation of long ocean waves, parallel to — the surface front. Hourly meteorological data showed at Buenos Aires continental shelf (Dragani, 2007). It was concluded large pressure fluctuations and intense gusts at Mar del Plata. that atmospheric gravity waves are an effective mechanism to Upper-air soundings obtained at Comandante Espora Navy Base force long ocean waves. Significant differences between two simi- (6 km east of Bahía Blanca) showed a lower pronounced tropo- lar numerical experiments (phase speed and direction of propaga- spheric inversion which depicts an example of the state of the tion of atmospheric gravity waves were 30 m s1/330° and atmosphere when a frontal surface lies overhead. The propagation 40 m s1/310°, respectively) showed that long ocean wave genera- medium for atmospheric gravity waves appears to be a low-level tion (sea level amplitudes and spectral contents) is highly sensitive inversion (Bosart and Cussen, 1973; Gedzelman and Rilling, to the values of both parameters. After 36 h of simulation (Fig. 4) 1978; Stobie et al., 1983) represented, in this case, by warm air long ocean waves presented a well defined pattern characterized flowing from the north over cold air flowing from the south. In sev- by crest and troughs normally oriented with respect to the coastal 1000 W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008

Fig. 3. Passband ﬁltered microbarographic atmospheric pressure at Punta Médanos Lighthouse (Fig. 1). Hours since October 12, 17:00, 1985.

The Port of Quequén (Fig. 5) is located at the mouth of the river Quequén, Necochea city is on the right bank and Quequén city is on the left bank. Trade consists mainly of exports of wheat to Brazil, with important quantities of maize, sunflower seed pellets and vegetable oil being exported elsewhere. The activities of the fishing fleet have diminished since most of them have been transferred to Puerto Deseado (Argentinean Patagonia). Imports are almost entirely confined to the reception of bulk fertilizers. There is an ‘‘anteport” immediately inside the breakwaters. The shortest width in the anteport is approximately 500 m, average depth 12.20 m. The anteport and all the berths in the port are sometimes affected by swell, especially during southerly gales (http://www.ssa-shipping.com/v2/index.php?option=com_con- tent&task=view&id=121&Itemid=96). Low amplitude sea-level oscillations, with heights lower than 0.15–0.20 m, are always present in sea level data series of Que- quén. They seem to be natural oscillations (‘‘background”) associ- Fig. 4. Instantaneous free-surface elevation (m) at Buenos Aires and the northern ated to the particular geometry of the harbor. In a previous study Patagonian continental shelf after 7 h of simulation. Source: Dragani et al. (2008). very low coherence was obtained between sea-level oscillations gathered at the port of Quequén and Mar del Plata (Inman et al., 1962), so these authors suggested that sea-level oscillations at the two locations were not related in a simple and direct way. This last statement has not been reviewed till the present work where this point is conveniently revised. The aim of this paper is to study if large-amplitude sea-level oscillations recorded at Quequén are related to atmospherically-induced sea-level oscillations observed at Mar de Ajó, Pinamar and Mar del Plata.

2. Data

Analog tidal records gathered at the standard tidal station of Quequén corresponding to the four most energetic events detected by Dragani (1997) at the Buenos Aires coast were selected and digitized at a rate of 60 samples per hour (Fig. 6). These events began in (i) September 9, 07:00, (ii) February 2, 05:00, (iii) March 23, 05:00 and (iv) August 27, 23:00, 1982 (time zone: +03:00). It can be noted that during the second event the tide gauge did not work after February 2, 20:00 (Fig. 6b). Sea level data at Quequén contain diurnal and semi-diurnal tides, storm surges and higher-frequency oscillations (ranging from a few minutes to almost 2 h). In general the best and simplest

Fig. 5. Port of Quequén, Quequén river and Quequén and Necochea cities. (Source: way to separate high frequency oscillations from measured sea Google Earth). level is by means of the subtraction of predicted astronomical tides from observed sea level records. However as high frequency sea- line. Inside the Río de la Plata estuary, long ocean waves are signif- level oscillation events are usually detected during negative and icantly attenuated probably due to bottom friction dissipation ef- positive storm surge episodes, the aforementioned subtraction is fects (in the middle section of the Río de la Plata the mean depth not the most convenient method to separate high frequency is less than 5 m and, in the inner part, the mean depth is less than oscillations. The event of February is presented in Fig. 7 and the 3 m). In general, greater oscillations occurred in the continental observed sea level, the predicted astronomical tide (generated by shelf near the coast. Sea-level disturbances decreased off-shore 60 tidal constituents) and the difference between them (residual) and vanished to the east of the continental slope (Dragani, 2007). are shown. It can be appreciated that storm surge heights can reach W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008 1001

Fig. 6. Sea level (m) and passband ﬁltered sea level (m) records at Quequén corresponding to the energetic events recorded in (a) September, (b) February, (c) March and (d) August, 1982.

Fig. 7. Observed sea level (solid line), predicted astronomical tide (dashed line), storm surge plus high frequency sea-level oscillations (heavy solid line) and passband ﬁltered sea level (heavy dashed line) corresponding to the event of February, 1982. 1002 W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008

Fig. 8. Passband ﬁltered sea level (m) data series at Mar de Ajó, Pinamar, Mar del Plata and Quequén corresponding to the energetic events recorded in (a) September, (b) February, (c) March and (d) August, 1982.

Table 1 The four most energetic sea-level oscillation events recorded at the Buenos Aires coast in 1982. Initial time of the activity at Quequén and Mar del Plata (time zone :+03:00), temporal lag (h) between initial times at both locations and maximum wave height at Quequén (QUE), Mar del Plata (MDP), Pinamar (PIN) and Mar de Ajó (AJO) are presented.

Event QUE inicial time MDP inicial time Temporal lag (h) QUE (m) MDP (m) PIN (m) AJO (m) i 09/09 07:00 09/09 10:00 3 0.62 0.47 0.42 0.41 ii 02/02 05:00 02/02 07:00 2 0.48 0.35 0.49 0.40 iii 23/03 05:00 23/03 05:00 0 0.33 0.37 0.36 0.23 iv 27/08 23:00 28/08 10:00 11 0.47 0.40 0.48 0.47

0.5 m and high frequency sea level perturbations are overlapped to ﬁlter, with cutoff frequency at 0.33 cph (180 min). Filtered sea level it. This same feature has been seen in the other three analyzed data series, initial time of the large-amplitude long wave activity events (not shown in the present paper). For this reason, it was and the highest wave height for the four selected periods are also decided to ﬁlter low frequency oscillations, related with tides shown in Fig. 6. It should be also noted that when the storm surge and storm surges, by means of a 101-point Kaiser–Bessel highpass heights are nearly zero (for example, between 5 and 8 h, 16 and W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008 1003

18 h and 20 and 25 h) the filtered sea level and the residuals are Quequén, Mar del Plata, Pinamar and Mar de Ajó, respectively. practically coincident (Fig. 7). The long wave event lasted for approximately 10 h at the men- Filtered sea levels detected in September at Mar de Ajó, Pina- tioned locations. Sea-level oscillation variances were 0.066, mar, Mar del Plata and Quequén are presented in Fig. 8a (from 0.105, 0.076 and 0.092 (102 m2) for background (computed at a 09/07 00:00 to 09/13 00:00). It can be appreciated that large oscil- 18 h long lapse, between 24 and 42 h, Fig. 8c) and 0.219, 0.322, lations showed up firstly at Quequén and subsequently at Mar del 0.284 and 0.222 (102 m2) for the active event (between 51 and Plata, Pinamar and Mar de Ajó. Maximum observed amplitudes 69 h, Fig. 8c). The ratios between the energetic event and back- (Table 1) were 0.62, 0.47, 0.42 and 0.41 m at Quequén, Mar del Pla- ground variances were 3.3, 3.1, 3.7 and 2.4 at Quequén, Mar del ta, Pinamar and Mar de Ajó, respectively. The long wave event Plata, Pinamar and Mar de Ajó, respectively. lasted for approximately 18 h. The initial time of the activity at Sea levels for the fourth analyzed active event (August) are pre- Quequén was 09/09 07:00 (time zone: +03:00) and, at Mar del Pla- sented in Fig. 8d (from 27/08 00:00 to 01/09 00:00). It can also be ta, the large high frequency activity began 3 h later (Table 1, appreciated that large oscillations appeared firstly at Quequén and Fig. 8a). Sea-level oscillation variances were 0.034, 0.071, 0.030 subsequently at the other three locations. Maximum observed and 0.095 (102 m2) for background preceding to the activity amplitudes (Table 1) were 0.47, 0.40, 0.48 and 0.47 m at Quequén, (computed between 24 and 42 h, Fig. 8a) and 0.377, 0.446, 0.431 Mar del Plata, Pinamar and Mar de Ajó, respectively. The long wave and 0.847 (102 m2) for the active event (between 57 and 75 h, event lasted for approximately 30 h at the mentioned locations. Fig. 8a). The ratios between the energetic event and the back- The initial time of the activity at Quequén was 27/08 23:00 (time ground variances were 11.1, 6.3, 14.4 and 8.9 at Quequén, Mar zone: +03:00) and, at Mar del Plata, the large high frequency activ- del Plata, Pinamar and Mar de Ajó, respectively. ity began 11 h later (Table 1, Fig. 8d). Sea-level oscillation variances Large oscillations also showed up firstly at Quequén and subse- were 0.128, 0.081, 0.069 and 0.069 (102 m2) for background (com- quently at the other three locations during the active event de- puted for an 18 h long lapse, between 2 and 20 h, Fig. 8d) and tected in February (from 02/01 12:00 to 02/3 00:00, Fig. 8b). 0.436, 0.568, 0.470 and 0.524 (102 m2) for the active event (be- Maximum observed amplitudes (Table 1) were 0.48, 0.35, 0.49 tween 36 and 54 h, Fig. 8d). The ratios between the energetic event and 0.40 m at Quequén, Mar del Plata, Pinamar and Mar de Ajó, and background variances were 3.4, 7.0, 6.8 and 7.6 at Quequén, respectively. This large-amplitude wave event is a little shorter Mar del Plata, Pinamar and Mar de Ajó, respectively. than the detected in September (Fig. 8a).The initial time of the activity at Quequén was 02/02 05:00 (time zone: +03:00) and, at 3. Results Mar del Plata, it began 2 h later (Table 1, Fig. 8b). Sea-level oscillation variances were 0.031, 0.038, 0.038 and 0.063 (102 m2) for In order to examine the spectral features of the selected active previous background (computed at a 18 h long lapse, between 12 events, each data series was divided into two parts. Sea levels re- and 30 h, Fig. 8b) and 0.406, 0.632, 0.339 and 0.438 (102 m2) for corded before the beginning of the active event were selected for the active event (between 30 and 48 h, Fig. 8a). The computed ra- the analysis of ‘‘normal” activity and the subsequent lapses of long tios between the energetic event and the background variances wave activity were chosen for the analysis of large-amplitude sea were 13.1, 16.6, 11.2 and 7.0 at Quequén, Mar del Plata, Pinamar level perturbation. The period from September 6, 23:00, to Septem- and Mar de Ajó, respectively. ber 9, 02:00 (3060 min, 51 h) preceding the active event, was iden- Filtered sea levels for the third analyzed active event (March, tified as ‘‘normal” and selected for analysis of ‘‘background” 1982) are presented in Fig. 8c (from 03/21 00:00 to 03/23 22:00). signals; the ‘‘active lapse” from September 9, 09:00, to September Moderate sea-level oscillations (0.20–0.30 m height) can be appre- 10, 10:00 (1024 min, 17 h) was chosen for analysis of large-ampli- ciated during the first hours of these records, probably associated tude sea level perturbations. Sea level spectrum for the active lapse to a previous and weak atmospheric event. The beginning of this recorded in September (Fig. 9) was obtained by means of the fast event was approximately simultaneous at Quequén and Mar del Fourier transform procedure (Welch, 1967; Stoica and Moses, Plata (03/23 05:00) and the large high frequency activity began 1997). To improve the spectral estimates a Hamming spectral win- 3 h later at Pinamar and 6 h later at Mar de Ajó. Maximum ob- dow (Harris, 1978; Oppenheim and Schafer, 1989) with a constant served amplitudes (Table 1) were 0.33, 0.37, 0.36 and 0.23 m at time length of 201 min was performed. A confidence interval (95%)

Fig. 9. Sea level spectrum (heavy solid line) for the energetic event shown in Fig. 6a (from 09/09 09:00 to 10/09 00:00) and sea level spectrum of the background (heavy dash line) before the beginning of this energetic event, from 06/09 23:00 to 09/09 02:00. The periods of spectral peaks (min) and the fundamental period for Quequén harbor are pointed out. 1004 W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008 was computed following Oppenheim and Schafer (1975). The most events analyzed present similar characteristics to the ones shown energetic spectral band can be appreciated between 0.8 and in Fig. 9 but, for reasons of space, they were not included in this 4.0 cph (15–75 min). The uppermost spectral peaks are located at paper. 60.0, 33.3, 24 and 17.6 min and a gradual decrease of the spectral Wavelet analysis is a more appropriate spectral technique to energy can be seen from 4 cph to higher frequencies (Fig. 9). At fre- study the temporal variability of non-stationary sea level data ser- quencies higher than 4 cph the spectrum is ‘‘red”, with spectral en- ies. Filtered sea level wavelet transforms (Torrence and Compo, ergy rolling off at increasing frequency (as the inverse square of the 1998) for the four selected active events (Fig. 6a–d) are presented frequency) which is typical for long wave spectra (Rabinovich, in Figs. 10–13. The wavelet power spectrum for the most energetic 1997). Several weak spectral peaks can be appreciated at frequen- event (September) shows a lapse of intermittent sea level activity cies greater than 6 cph (periods lower than 10 min) having values between 400 and 1380 min (0 min corresponds to September 9, of two orders of magnitude lower than the spectral energy located 00:00, 1982, in Fig. 10). At the beginning, a short episode located at the most energetic band (frequencies lower than 4 cph). This last around 400 min, with associated periods of 60 min, can be ob- analysis suggests that sea level perturbations associated to the served in this figure. After that, a longer period of irregular activity high frequency band (frequencies greater than 6 cph) do not signif- (between 580 and 1250 min) is clearly manifested and, finally, icantly contribute to the large-amplitude sea-level oscillations re- maxima energy is appreciated around 1380 min. Between 580 corded at Quequén. and 1000 min, spectral energy is distributed between 0.8 cph Sea-level oscillations recorded previous to the energetic lapses (75 min) and 0.5 cph (30 min) and, subsequently, a noticeable at Quequén (‘‘background”) present a broad-band spectrum and shifting to higher frequencies (between 15 and 40 min, approxi- do not show any significant spectral peak (Fig. 9). At low frequen- mately) can be appreciated. cies (lower than 4 cph) the spectral energy is almost one order of The second energetic event (February, Fig. 6b) is slightly shorter magnitude lower than the spectra of the active lapse, while at fre- than the first one (750 min, approximately, but it should be noted quencies higher than 4 cph, the background spectra lies below but that the tide gauge did not work after February 2, 20:00) and it is fairly closer to the spectra of the active lapse. Background spectra also characterized by an irregular activity of large-amplitude sea- present a frequency band of maximum concentration of energy lo- level oscillations (0 min corresponds to February 2, 05:00, 1982, cated approximately around 17–35 min. It can be seen that three in Fig. 11). The most energetic lapse takes place between 950 significant spectral peaks corresponding to the energetic lapse and 1230 min; spectral energy is broadly distributed between al- (17.6, 24 and 33.3 min) are placed at this frequency band. Spectra most 0.5 cph (120 min) and 4 cph (15 min). Afterward, between obtained from filtered sea level data for the other three energetic 1300 and 1500 min, spectral energy appears in two separated

Fig. 10. (a) Quequén ﬁltered sea level record; 0 min corresponds to September 9, 00:00, 1982. (b) The wavelet power spectrum. Levels of color palette from white to dark gray have been chosen so that 75%, 50%, 25% and 5% of the wavelet power is above each level, respectively. The cross-hatched regions on the left and right sides are the ‘‘cone of inﬂuence’’—where edge effects become important (Torrence and Compo, 1998). W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008 1005

Fig. 11. (a) Quequén ﬁltered sea level record; 0 min corresponds to February 2, 05:00, 1982. (b) The wavelet power spectrum. Levels of color palette from white to dark gray have been chosen so that 75%, 50%, 25% and 5% of the wavelet power is above each level, respectively. The cross-hatched regions on the left and right sides are the ‘‘cone of inﬂuence’’—where edge effects become important (Torrence and Compo, 1998). frequency bands: one located at low frequencies (around 60 min) event of February, they were 16.6, 13.1, 11.2 and 7.0 at Pinamar, and another at higher frequencies (around 20 min). Finally, a short Mar de Ajó, Mar del Plata and Quequén, respectively. On the other energetic lapse can be observed around 0.5 cph, between 1600 a hand, the ratios were quite similar (around 3) at the four locations 1700 min. The third selected event (March, Fig. 6c) is longer than studied in March. Finally, the ratios were fairly comparable the ones discussed previously (1500 min, approximately) and also (approximately 7) at Pinamar, Mar del Plata and Quequén and a lit- presents irregular activity of sea-level oscillations. Spectral energy tle lower (3.4) at Mar de Ajó during the event of August. Such is unevenly distributed between 0.8 and 4 cph (Fig. 12). Initial time noticeable variability in the computed variances and ratios could (0 min) corresponds to March 25, 09:00, 1982 in Fig. 12. Between be likely related to the characteristics of the propagation of the 600 and 1600 min, the spectral contents are irregularly distributed atmospheric gravity waves in the region (for example, celerity, at high frequencies, between 15 and 30 min. amplitude, direction and period). The wavelet power spectrum of the fourth energetic event (Au- Background preceding large-amplitude sea-level oscillations at gust, Fig. 6d) is presented in Fig. 13 (0 min corresponds to August Quequén is characterized by a broadband and smooth spectrum 29, 06:00, 1982). Sea level activity is clearly intermittent too. Ini- with maxima concentration of energy approximately located be- tially (between 200 and 400 min and between 600 and 750 min) tween 1.7 and 3.5 cph and total variances lower than 0.0015 m2. spectral energy is distributed between 1.3 and 0.33 cph (20 and A classic model to estimate the theoretical fundamental period

75 min, approximately). Afterward (between 850 and 1000 min) (To) of an undamped free oscillation (seiche) in a long, narrow, it is located at lower periods, around 40 and 20 min. Finally (be- open mouth-basin of uniform depth and width is the modified 1/2 tween 1100 and 1300 min) high concentration of energy is ob- Merian formula given by To =4L/(gh) , where L and h are the served around 60 and 20 min, approximately. length and the depth of the open mouth-basin (Wilson, 1972). The length of the basin (Fig. 2) is considered from the mouth to 4. Discussion the first meander of the river (3300 m) and the mean depth of the basin is assumed equal to 7 m. This depth is an averaged value The computed ratios between variances corresponding to the among deeper soundings located at the navigation channel (10– event and the preceding background reached values close to 9 at 12 m) and shallower depths located near the banks of the river Quequén and, over 10 at Mar de Ajó, Pinamar and Mar del Plata. (4–5 m). Using these values in the Merian formula, a period (To) It seems that (Table 2) the computed ratios do not show a relation- of 27 min (2.22 cph) is obtained, which lies at the frequency band ship among locations nor events. During the event of September, where background spectral energy is highest. Merian formula can computed ratios were 14.4, 11.1, 8.9 and 6.3 m2 at Mar del Plata, be ideally applied to long basins which are closed in one of its ends. Mar de Ajó, Quequén and Pinamar, respectively. However, in the Even though in the specific case of Quequén the closed end is not 1006 W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008

Fig. 12. (a) Quequén filtered sea level record; 0 min corresponds to March 25, 09:00, 1982. (b) The wavelet power spectrum. Levels of color palette from white to dark gray have been chosen so that 75%, 50%, 25% and 5% of the wavelet power is above each level, respectively. The cross-hatched regions on the left and right sides are the ‘‘cone of influence’’—where edge effects become important (Torrence and Compo, 1998). present (it was imposed at the first meander of the river) the result that perturbations were firstly observed at Quequén and, subse- obtained with the Merian formula gives an excellent approxima- quently further north, at Mar del Plata, Pinamar and Mar de Ajó, tion to the problem. respectively. Maximum observed amplitudes at Quequén (0.62, During energetic events spectral peaks are located between 0.8 0.48, 0.33 and 0.47 m for events i, ii, iii and iv, respectively) com- and 4.0 cph (15–75 min) at Quequén spectra (Fig. 9). These spectra pare very well with maximum amplitudes detected at Mar del Pla- are fairly similar to Mar del Plata, Pinamar and Mar de Ajó spectra ta, Pinamar and Mar de Ajó corresponding to the same events but, at these latter locations, the most noticeable spectral peaks are (Fig. 8 and Table 1). Both features constitute the first evidence that situated at lower frequencies, between 1.1 and 1.5 cph (40– large-amplitude sea-level oscillations simultaneously recorded at 55 min). Three significant spectral peaks (17.6, 24 and 33.3 min) Quequén, Mar del Plata, Pinamar and Mar de Ajó could also be observed in Fig. 9 are situated at the frequency band of maxima linked and would be part of a regional phenomenon. But due to concentration of energy associated to the background (17– the particular geometry of this port (a very little harbor), large- 35 min). Then, it seems that these spectral peaks could be associ- amplitude sea-level oscillations (ranging from a few minutes to ated to amplified normal oscillations which are excited during two hours) could not be directly generated inside the port of Que- large-amplitude sea-level oscillation events. This last statement quén. Then, it can be supposed that atmospherically-induced large- is only a hypothesis based on the analysis of short data series. A amplitude sea-level oscillations, generated on the continental particular study based on long sea level records at Quequén should shelf, would enter the port through its narrow mouth while prop- be developed to elucidate this suggestion. agating towards Mar del Plata, Pinamar and Mar de Ajó, where they Inman et al. (1962) reported very low coherence between sea- would show up in tidal records some hours later. In this way, large- level oscillations gathered at the port of Quequén and Mar del Plata amplitude sea-level oscillations at Quequén would be episodic and suggested that these perturbations were not related in a sim- events and would also be associated to the passage of atmospheric ple and direct way. This last statement could be probably correct cold fronts propagating from the south/south-west which generate only if atmospherically-induced large-amplitude sea-level oscilla- perturbations all along the coast of Buenos Aires, from Quequén to tions are not present in the region. In such a case, computed coher- Mar de Ajó. These atmospherically-induced sea-level oscillations ency between normal oscillations (background) measured at would appear superposed to the normal oscillations (background, Quequén and Mar del Plata (or Pinamar or Mar de Ajó) would pres- heights lower than 0.15–0.20 m) which seem to be present before, ent very low values because the spectral characteristic of the local during and after the occurrence of large-amplitude events. background is associated to the particular configuration (bathym- Even though each wavelet spectra (Figs. 10–13) at Quequén etry) of the inner continental shelf. Initial times of sea-level presented distinguishable and particular characteristics, a couple oscillations observed at Quequén and Mar del Plata during large- of common features could be noted. Firstly, the activity of large amplitude events are presented in Table 1.InFig. 8 it can be seen sea-level oscillation is not continuous; it showed up intermittently W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008 1007

Fig. 13. (a) Quequén ﬁltered sea level record; 0 min corresponds to August 29, 06:00, 1982. (b) The wavelet power spectrum. Levels of color palette from white to dark gray have been chosen so that 75%, 50%, 25% and 5% of the wavelet power is above each level, respectively. The cross-hatched regions on the left and right sides are the ‘‘cone of inﬂuence’’—where edge effects become important (Torrence and Compo, 1998).

Table 2 Event and preceding background variances and the ratios between them for the four most energetic sea-level oscillation events recorded at the Buenos Aires coast in 1982 at Quequén (QUE), Mar del Plata (MDP), Pinamar (PIN) and Mar de Ajó (AJO).

Location Background variance, m2 (102) Event variance, m2 (102) Event/background variances September February March August September February March August September February March August AJO .034 .031 .066 .128 .377 .406 .219 .436 11.1 13.1 3.3 3.4 PIN .071 .038 .105 .081 .446 .632 .322 .568 6.3 16.6 3.1 7.0 MDP .030 .038 .076 .069 .431 .339 .284 .470 14.4 11.2 3.7 6.8 QUE .095 .063 .092 .069 .847 .438 .222 .524 8.9 7.0 2.4 7.6 in short lapses (100–200 min long). Secondly, the position of the Quequén (Figs. 10–13) can be fairly comparable among different spectral peaks present a clear temporal variability and they are energetic events. This result is in good agreement with the well irregularly located between 15 and 75 min (4–1.25 cph). Even known fact that periods of meteo-tsunamis are mainly related to though high-resolution atmospheric pressure records on the Bue- resonant properties of local topography rather than to characteris- nos Aires coast (gathered with micro-barometer) are very limited, tics of the source (Monserrat et al., 1998; Rabinovich and Monser- the scarce and sporadic data available (Fig. 3) seem to show that rat, 1996). the atmospheric pressure signal is highly non-stationary, which is consistent with the significant variability found in Quequén 5. Conclusions wavelet spectra. This spectral variability is also in agreement with the significant temporal variability in sea level spectra found at The aim of this paper is to investigate if large-amplitude sea-le- Mar del Plata, Pinamar and Mar de Ajó (Dragani et al., 2002). vel oscillations recorded at Quequén could be related to atmo- Although sea level perturbations seem to be highly dependent spherically-induced sea-level oscillations observed along the on the forcing, long waves generated in the open ocean are Buenos Aires coast (at Mar de Ajó, Pinamar and Mar del Plata). Ana- strongly affected by local topography and bathymetry, such as con- log tidal records gathered at these locations corresponding to the tinental shelves and associated bays and harbors. These waves may four most energetic events detected during 1982 (the year of lon- amplify significantly near the coast due to local resonant effects gest simultaneous record) were digitized, filtered, processed and (Rabinovich and Stephenson, 2004). Consequently, although the spectrally analyzed and the total variances (previous and during forcing could present significant spatial and temporal variability, the activity) were computed. By comparing the filtered sea level re- sea-level oscillations (Fig. 6) and associated wavelet spectra at cords (Fig. 8) it can be seen that (i) perturbations were always 1008 W.C. Dragani et al. / Physics and Chemistry of the Earth 34 (2009) 998–1008

firstly observed at Quequén and, subsequently further north, at Bosart, L.F., Cussen Jr., J.P., 1973. Gravity waves phenomena accompanying east Mar del Plata, Pinamar and Mar de Ajó, respectively and (ii) maxi- coast cyclogenesis. Monthly Weather Review 101 (5), 446–454. Caviglia, F.J., Dragani, W.C., 1996. An improved 2-D finite difference circulation mum observed amplitudes at Quequén compare very well with model for tide- and wind-induced flow. Computational Geosciences 22 (10), maximum amplitudes detected at Mar del Plata, Pinamar and 1083–1096. Mar de Ajó corresponding to the same events (Table 1). Dragani, W.C., 1988. Análisis del proceso físico generador de ondas largas en la costa bonaerense argentina. Thesis, Inst. Tec. Bs. As., Buenos Aires, 58pp. From the inspection of wavelet spectra at Quequén (Figs. 10–13) Dragani, W.C., 1997. Una explicación del proceso físico generador de ondas de largo it can be appreciated that the activity of large sea-level oscillation período en la costa bonaerense argentina. Doctoral Thesis, Facultad de Ciencias is not continuous (it showed up intermittently in short lapses of Exactas y Naturales, Universidad de Buenos Aires, 222pp. Dragani, W.C., 1999. A feature model of surface pressure and wind fields associated 100–200 min long) and that the positions of the spectral peaks with the passage of atmospheric cold fronts. Computational Geosciences 25, present a clear temporal variability (they are irregularly located 1149–1157. between 15 and 75 min). The computed ratios between variances Dragani, W.C., Mazio, C.A., Nuñez, M.N., 2002. Sea-level oscillations in coastal waters of the Buenos Aires Province, Argentina. Continental Shelf Research 22, corresponding to the active event and the preceding background 779–790. reached values close to 9 at Quequén and, over 10 at Mar de Ajó, Dragani, W.C., 2007. Numerical experiments of the generation of long ocean waves Pinamar and Mar del Plata. It can be appreciated (Table 2) that in coastal waters of the Buenos Aires province, Argentina. Continental Shelf computed ratios apparently do not show a relationship among Research. doi:10.1016/j.csr.2006.11.009. Gedzelman, S.D., Rilling, R.A., 1978. Short-period atmospheric gravity waves: a locations nor events. Such noticeable variability in variances and study of their dynamic and synoptic features. Monthly Weather Review 106 (2), ratios could be likely related to the celerity, amplitude, direction 196–210. and period of the forcing (atmospheric gravity waves). Even though Harris, F.J., 1978. Of the use of windows for harmonics analysis with the discrete Fourier Transform. Proceedings of the IEEE 66, 51–83. high-resolution atmospheric pressure records on the Buenos Aires Inman, D., Munk, W., Balay, M., 1962. Spectra of low frequency ocean waves along coast (gathered with micro-barometer) are very limited, the scarce the Argentine shelf. Deep-Sea Research 8, 155–164. and sporadic data available (Fig. 3) seem to show that the atmo- Lanfredi, N.W., 1972. Resultados de mediciones directas de corrientes en el Atlántico Sudoccidental. Rep.H-650/2, Dpto. Oceanog., Serv. Hidrog. Nav., spheric pressure signal is highly non-stationary, which is consis- Buenos Aires, 107pp. tent with the significant variability found in Quequén wavelet Lanfredi, N.W., Capurro, L., 1971. Resultados de mediciones directas de corrientes en spectra and in the computed variances among locations. el Atlántico Sudoccidental. Rep. H-650/1, Dpto. Oceanog., Serv. Hidrog. Nav., Buenos Aires, 109pp. Finally, even though our results seem to support that large- Monserrat, S., Rabinovich, A., Casas, B., 1998. On the reconstruction of the transfer amplitude sea-level oscillations recorded at Quequén would be function for atmospherically generated seiches. Geophysical Research Letters associated to atmospherically-induced sea-level oscillations ob- 25 (12), 2197–2200. Nuñez, M.N., Mazio, C.A., Dragani, W.C., 1999. Estudio espectral de un lapso de served at the Buenos Aires coast (at Mar de Ajó, Pinamar and intensa actividad de ondas de gravedad atmosf!ericas registradas en la costa Mar del Plata) two different tasks should be carried out in order bonaerense argentina. Revista Meteorologica 23 (1–2), 49–59. to elucidate this subject better. Firstly, analog sea level records at Oppenheim, A.V., Schafer, R.W., 1975. Digital Signal Processing. Prentice-Hall, 556. Quequén (for 1982) should be entirely digitized and a complete Oppenheim, A.V., Schafer, R.W., 1989. Discrete-Time Signal Processing. Prentice- Hall. pp. 447–448. spectral analysis (wavelet transform and coherence spectra) Rabinovich, A.B., Monserrat, S., 1996. Meteorological tsunamis near the Balearic and including sea level data from Mar del Plata, Pinamar and Mar de Kuril Islands: descriptive and statistical analysis. Natural Hazards 13 (1), 55–90. Ajó should be performed. Secondly, a regional program of simulta- Rabinovich, A.B., 1997. Spectral analysis of tsunami waves: separation of source and topography effects. Journal of Geophysical Research 102 (C6), 12663–12676. neous observations of sea level and atmospheric pressure should Rabinovich, A.B., Stephenson, F.E., 2004. Longwave measurements for the coast of be implemented in order to face an integral study of this complex British Columbia and improvements to the tsunami warning capability. Natural atmosphere–ocean interaction. Hazards 32, 313–343. Stobie, J.G., Einaudi, F., Uccellini, L.W., 1983. 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