Meteorologically Induced Strong Seiches Observed at Arraial Do Cabo, RJ, Brazil

Physics and Chemistry of the Earth 34 (2009) 989–997

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Physics and Chemistry of the Earth

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Meteorologically induced strong seiches observed at Arraial do Cabo, RJ, Brazil

Rogério Neder Candella

Instituto de Estudos do Mar Almirante Paulo Moreira, Rua Kioto 253, 28930-000 Arraial do Cabo, RJ, Brazil article info abstract

Article history: Meteorologically seiche oscillations can sometimes reach signiﬁcant heights, causing severe damage. Received 28 January 2009 There is only one reference to an extreme event of this type in Brazil, but it is based on only eyewitness Received in revised form 25 June 2009 reports, without data conﬁrmation. However, this lack of observed events is due to the very small number Accepted 29 June 2009 of high-resolution sea level measurements along the coast. The main purposes of the present study were Available online 1 July 2009 to analyze a 4-year high-resolution series, collected at Arraial do Cabo, RJ, Brazil, to identify the main high-frequency sea level events and to relate them to possible forcing. In the series examined, high-fre- Keywords: quency oscillations (T < 1 h) were almost always present, with a dominant period close to the fundamen- Longwaves tal mode of the bay (Helmholtz mode, n = 0), about 20 min. Although their average height was 20 cm, in Seiches Harbor resonance some situations, these waves exceeded 30 cm, and in an extreme event, associated with an atmospheric Atmospheric pressure jumps pressure jump of 10 hPa/2 h, the oscillations reached 60 cm. That is comparable to the largest amplitude Brazilian coast low-frequency variation (T > 36 h) measured in the region between 1999 and 2008. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Arraial do Cabo, a city located on the east coast of the Rio de Ja- neiro (RJ) State, is the main point of manifestation of coastal Large-amplitude sea level oscillations with periods between 2 h upwelling phenomenon on the Brazilian coast, generally referred and a few minutes can be induced by atmospheric disturbances. to as the ‘‘Cabo Frio upwelling”. Because of this, the Instituto de These waves have been referred to as meteorological tsunamis or Estudos do Mar Almirante Paulo Moreira, a Brazilian Navy oceano- meteotsunamis and, under appropriate conditions, are able to graphic research institution, was established there in the 1970’s, cause destruction in coastal areas (Monserrat et al., 2006). Ciutad- which maintains a tidal station in the city port. Between the end ella Inlet (Menorca Island, Western Mediterranean), where the of 2001 and the beginning of 2005, a digital tide gauge was oper- phenomenon is called rissaga (Rabinovich and Monserrat, 1996), ated at this station, which allowed obtaining a time series with and Široka Bay, on the Island of First in the northern Adriatic (Šepic´ sampling intervals between 1 and 3 min and subsequently the et al., 2009), are examples of how destructive these waves can be. identification of several events in subtidal scale. There are only a few studies related to examination of high-res- The Bay of Arraial do Cabo, called Enseada dos Anjos (Fig. 1), has olution sea level oscillations along the Brazilian coast, mainly due basically a bottle shape, and is formed in an indentation in the con- to the lack of adequate measurements. Although the tide gauge tinent, limited by the islands of Cabo Frio on the east and Porcos on network is being improved with the installation of digital equip- the north. The bay’s main opening of about 1.4 km, faces northeast, ment at several sites along the coast, the main purpose of these but there are two other narrow passages, the first, 240 m wide, fac- gauges remains the low-frequency events, such as storm surges, ing north, and the second, 130 m wide, facing southwest. The length tides and low-frequency sea level variations. Rosauro (1986) stud- of the bay is approximately 3.5 km, but much of the southern por- ied the presence of seiches in the Lagoa dos Patos, Rio Grande do tion of the bay is shallow with depths less than 5 m, evidenced by Sul (RS). Melo Filho et al. (2005) described a possible meteotsun- the isobathymetric lines in Fig. 1. The intensive energy dissipation ami event at Cassino Beach, RS, which even caused loss of proper- in this region significantly influences longwave oscillations within ties and lives. However, this study was based only on reports, the bay. Moreover, the port where the tide gauge was installed is without measurements. Dragani et al. (2002) used data from 3 sta- protected by a breakwater, forming a sub-bay and making the cal- tions to study the occurrence of waves with periods less than 2 h in culation of its characteristic parameters very complex. Future the Argentinean coast, and Dragani (2007) conducted numerical numerical experiments should clarify the effects of bathymetry experiments to investigate their possible forcing mechanism. and configuration of the coast on the behavior of seiches in the bay. The continental shelf south of the bay is quite short, with 50-m depths along the coast of Cabo Frio Island. Along the 23°S parallel, E-mail addresses: [email protected], [email protected] there is a step in the bathymetry, best seen in 100 m isobath line,

Fig. 1. Location and bathymetry of the Bay of Arraial do Cabo. The TG mark indicates the position of the tide gauge. and from that point, the platform tends to extend more to the 4L 4 3500 Tn ¼ pffiffiffiffiffiffi ) T0 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 19:2 min; ð1Þ broad, resulting in smoother depth gradients. The astronomical lo- ð2n þ 1Þ gh 9:8 15 cal tide is semidiurnal with typical spring-neap ranges from 1.0 m to 0.4 m. where L = 3500 m is the length of the bay and g is the gravitational The 2001 sea level data were selected to examine the spectral acceleration. For n = 1, 2, the corresponding periods are 6.4 and characteristics of the bay, because of the higher quality of the cor- 3.8 min. In the same figure, three other high-frequency peaks can responding record and lack of significant gaps. The spectrum was be noted: 12.4, 8.9 and 7.0 s. The first one is close to the first mode calculated with Fast Fourier Transform and using frequency harmonic (n = 1) of the bay. The two last are, probably, related to dependent degrees of freedom (df = 254 for periods (T) greater than the sub-bay dimensions (width = 870 m, length = 780 m), but the 66.3 min and df = 2046 for T < 66.3 min), to avoid noise in the high complexity of the bathymetry and of the coastal line of the bay 2 frequency domain. It follows an x energy distribution, typical for and sub-bay does not allow accurate estimations. longwave spectra (cf. Aida et al., 1972; Kulikov et al., 1983; For the same reason, the Q-factor, which measures the energy Rabinovich and Stephenson, 2004) and indicates the presence of decay of the system, had to be estimated from the observational several peaks with frequencies higher than 1 cph, highlighting the data (cf. Miles and Munk, 1961; Rabinovich, 2009) period of approximately 19.6 min as the most prominent (Fig. 2). Taking the mean depth h = 15 m, reasonable for the region, the fun- f0 0:0506 damental mode of the bay (Helmholtz mode, n = 0) can be calcu- Q ¼ ¼ 12; ð2Þ Df 0:0042 lated as R.N. Candella / Physics and Chemistry of the Earth 34 (2009) 989–997 991

Fig. 2. Sea level spectral analysis (with frequency dependent degrees of freedom) for 2001. The energy peaks related to main components of tides, the fundamental mode period (Helmholtz mode, n =0,T ﬃ 20 min) and other signiﬁcant periods are indicated.

Fig. 3. Location of meteorological stations used. Cananéia and Ubatuba are in the São Paulo State, while Arraial do Cabo and Galeão are in the Rio de Janeiro State. Linear distance between Cananéia and Ubatuba is 340 km. Ubatuba and Arraial do Cabo are 320 km apart.

where f0 is the peak frequency and Df is the width at its half 2. Data description and treatment maximum height. This factor is comparable to that determined by Rabinovich (2009) for Malokurilsk Bay, a bottle-like bay in one of The sea level records were obtained by an inductive type digital the Kuril Islands (NW Paciﬁc), where seiches with periods of tide gauge, with the sampling interval ranging from 1 to 3 min dur- 18.6 min and signiﬁcant heights are frequently recorded. ing the measurement period and with 1-cm vertical resolution. 992 R.N. Candella / Physics and Chemistry of the Earth 34 (2009) 989–997

Fig. 4. Maximum height for seiches (T < 180 min) over the period analyzed. Notable are the heights for the months of December 2004 (tsunami in Sumatra) and September 2002 (meteotsunami).

The original series was divided into two sub-sets, the first from and to identify the major events, the variance was calculated from November 20, 2000, to October 10, 2003, 08:50 UTC, and the sec- segments of 90 min with overlap of 45 min. ond from November 10, 2003, 13:42 UTC, to February 15, 2005, A toolbox developed by Torrence (1998) was employed to the due to a malfunction of the equipment. Initially, the data were wavelet analysis, with initial period of 5 min, 6 powers-of-two visually inspected for identification of spurious values, failure of with 8 sub-octaves each, and Morlet wavelet type. The toolbox equipment or spikes. The gaps were filled with hourly data from can be found in http://www.paos.colorado.edu/research/wave- an analog tide gauge located at the same station. This procedure re- lets/. sulted in the absence of high frequency oscillations in these occa- The meteorological data for Arraial do Cabo were obtained from sions but allowed the integrated analysis of major segments. The the automatic weather station, belonging to the Oceanographic data were linearly interpolated where necessary, resulting in two Institute, University of São Paulo (coordinates 22°56.10S; time series with 1-min sampling. A high-pass Butterworth filter 42°04.80W). The atmospheric pressure records of Cananéia and of order 8 and cutoff period of Tc = 180 min was then applied, Ubatuba, both in São Paulo State, also stations of the same Insti-

Fig. 5. Seiches height (a) wind direction (b) and atmospheric pressure (c) for September 2001. Maximum wave heights reached about 30 cm, after an atmospheric frontal passage marked with an ellipse. R.N. Candella / Physics and Chemistry of the Earth 34 (2009) 989–997 993

Fig. 6. (a–e) Fields of atmospheric pressure at sea level extracted from the NCEP reanalysis, indicating the passage of a low-pressure system along the Brazilian south/ southeast coast. Although the center of the cyclone did not pass through the region of study, its effects extended to above the 20°S parallel. The black circles show the position of the tide gauge. 994 R.N. Candella / Physics and Chemistry of the Earth 34 (2009) 989–997 tute, are from analog instruments, digitized with hourly resolution. 2002. The analysis of the atmospheric pressure charts (Fig. 6a–e), For comparison, it was also used the barogram from the Tom Jobim obtained from the NCEP reanalysis (http://www.cdc.noaa.gov/ International Airport (ex-Galeão), located in Rio de Janeiro (http:// cdc/reanalysis/reanalysis.shtml), indicates the presence of a cy- www.wunderground.com/history/airport/SBGL/2002/11/10/Daily- clone centered on 35°S parallel, on September 6, 06:00 UTC, mov- History.html?req_city=NA&req_state=NA&req_statename=NA). The ing southeastward in subsequent days. Although the low-pressure locations of these meteorological stations are shown in Fig. 3. center did not pass over Arraial do Cabo, this and neighboring sites were under the cyclonic influence. In the records of three sites (Cananéia, Ubatuba and Arraial do Cabo, Fig. 7), there is a strong 3. Data analysis pressure gradient, more intense to the north, setting up a jump of up to 10 hPa/2 h. In the graph from the Tom Jobim International Adopting the classification suggested by Rabinovich and Airport (not shown) located in Rio de Janeiro at approximately the Monserrat, 1996, the three types of seiches can be found in the re- same latitude as Arraial do Cabo, the pressure jump is also clearly cords examined. The ‘‘impulsive” type signal corresponds to the seen and the two records are in good agreement. tsunami of December 2004 (Candella et al., 2008) and all the other According to Monserrat et al. (2006), meteotsunamis are gener- events are resonance or complex types. Actually, seiches are pres- ally associated with significant atmospherically generated long- ent in virtually all records, with typical height of about 20 cm and waves arriving from the open ocean, which means atmospheric main period of about 19 min, close to the period of the fundamen- forcing needs sufficient time to develop open-ocean waves. In tal mode. In Fig. 4, the heights of monthly maximum waves with other words, the atmospheric disturbance has to have the same periods less than 3 h are shown. The mean height is 22 cm, but speed (U) and direction as generated longwaves, i.e., to induce an event is clearly seen with heights exceeding 60 cm. these waves through Proudman resonance. The variance series enable us to identify some events with The linear distance between the first two sites (Cananéia and intensity above average. However, even in such situations, the Ubatuba) is about 340 km, which results in a displacement speed heights of seiches normally do not exceed 30 cm. One of these U = 31 m/s. The distance between Ubatuba and Arraial do Cabo is events occurred in September 2001 is presented in Fig. 5a. The 320 km and it was covered in 4 h resulting in U = 22 m/s. The cor- seiche heights reached 30 cm on two occasions. According to local responding resonance depths are 100 and 40 m, respectively, typ- weather conditions (Fig. 5b and c), the first event, marked with an ical for the continental shelf in this region. Thus, the observed ellipse, is apparently related to a cold front, as indicated by the results are in agreement with those described by Monserrat et al. change in wind direction from NE to SW (Fig. 5b) and by a slight (2006). atmospheric pressure oscillation (Fig. 5c). The second event oc- In Fig. 8a and b, it is possible to notice that the disturbance curred after another frontal passage and, although there are no started on September 7 at about 03:00 UTC and increased gradu- strong changes in the local atmospheric conditions, it could be re- ally until around 13:00 UTC, featuring a growing resonance effect. lated to the synoptic evolution of the system. The energy decay is abrupt (Fig. 8c), but its effects are still present for at least 12 h. 4. The September 2002 event The spectral characteristics of the meteotsunami and the background (the day before) were analyzed with FFT method for 2048 In the period analyzed, the largest seiches (T < 3 h), except those points, Hanning windowing of 512 points and 256 points of over- of the December 2004 Sumatra tsunami, occurred on September 7, lap (Fig. 9). They indicate that, for T < 60 min, the energy peaks

Fig. 7. Atmospheric pressure records for (a) Cananéia, (b) Ubatuba and (c) Arraial do Cabo for September 6 and 7, 2002. The pressure jumps are marked on each record, indicating that there has been a deepening of the system along the coast. R.N. Candella / Physics and Chemistry of the Earth 34 (2009) 989–997 995

Fig. 8. (a) Original sea level record at Arraial do Cabo for 6th, 7th and 8th September 2002; (b) filtered series (high-pass Butterworth filter, order 8, Tc = 180 min) for the same period; and (c) series of temporal variance (segments of 90 points, overlap of 45) applied to the filtered sea level. In the 8 graphics, the presence of seiches is quite visible.

Fig. 9. Sea level spectra to the meteotsunami (September 7, 2002) and background (September 6). The energy peaks, to frequencies higher than about 60 min, are almost the same. Bellow this frequency, the energy is present in a broad band between the periods of 80 and 115 min in the first spectra. The meteotsunami spectrum shows more energy than the background at, practically, all frequencies. are almost the same in both lines, which agrees with the theory (cf. Rabinovich and Stephenson, 2004). Also remarkable is the presence Honda et al., 1908; Miller, 1972; Rabinovich and Monserrat, 1996; of energy in a broad band between the periods of 80 and 115 min 996 R.N. Candella / Physics and Chemistry of the Earth 34 (2009) 989–997 in the first spectra. These peaks were present in some other mete- Necessary conditions to generate extreme high-frequency sea orologically induced events, and are probably connected to meso- level oscillations induced by the atmosphere were described by scale characteristics of the cold front. As expected, the Monserrat et al. (2006). Some of these conditions were fulfilled meteotsunami spectrum shows more energy than the background during the September 2002 event. A 10-hPa jump in atmospheric at practically all frequencies. pressure in less than 2 h, coupled with the passage of a cold front, The wavelet analysis (Fig. 10) confirms that most of the energy was able to generate waves with significant heights and periods during the event were distributed in the two main range of periods between approximately a few tens of minutes and 2 h in the open cited before. It also shows that, after an initial disturbance early on ocean. Unfortunately, there are no other quality high-resolution re- September 7, the energy kept ringing in the resonance frequencies cords capable of identifying those waves at other sites along the for about 24 h, although the energy at the longer periods is concen- coast, but the direction of the atmospheric disturbance from trated in the first 12 h. southwest to northeast, parallel to the coast, suggests a possible An estimation of the local topographic response of the bay to Proudman resonance. the longwaves can be obtained with (Rabinovich and Stephenson, The pressure jump recorded in September 2002 was remark- 2004). able. Rabinovich and Monserrat (1996) reported intensive seiches in bays on Shikotan Island (Kuril Islands) initiated by a deep cy- 1 1 S ðxÞ 2 S ðxÞ 2 clone with a pressure drop of 35 hPa in 2 days, resulting in an aver- HðxÞ¼ b ¼ x b ð3Þ S0ðxÞ A age gradient of 0.7 hPa/1 h. Dragani (2007) showed that atmospheric gravity waves are the where Sb(x) is the background spectrum, represented by the most probable forcing mechanism to generate ocean waves in the energy distribution on the day before the event, S0 is the spectrum range of periods encountered here. The characteristics of weather 3 2 of the event, and A is a constant taken as 10 cm Hz. The results evolution are quite different between Buenos Aires (30–40°S) and (Fig. 11) show some prominent peaks, including the fundamental Arraial do Cabo (23°S), with much more severe conditions in the period, which evidence the resonant characteristics of the bay. southern region. However, it seems reasonable that, sometimes, those rough conditions can propagate farther northward and sig- 5. Discussion and conclusions nificantly affect the atmosphere and, therefore, the sea level at the latitude of Arraial do Cabo. Unfortunately, there are no data The September 2002 event illustrates the possibility of large to confirm the presence of such atmospheric gravity waves during amplitude seiches on the Brazilian coast induced by meteorological the event analyzed. factors. Although such events in the past were virtually unnoticed, The entry of the waves into the bay was probably not done since there was no property damage or loss of lives, these waves through the southwest-facing opening. Numerical experiments have the potential to be highly destructive. Combined with other (not shown here) indicate that high-frequency sea level fluctua- factors, they can significantly influence the port operations and tions arriving through this opening have little influence on the sig- rainwater run-off, and cause flooding. nal recorded by the tide gauge. Although bays with narrow

Fig. 10. Sea level wavelet analysis for the period September 1–10, 2002, indicating the occurrence of strong ﬂuctuations focused on a period of 19 min and in the band between 60 and 120 min on September 7. R.N. Candella / Physics and Chemistry of the Earth 34 (2009) 989–997 997

Fig. 11. Topographic transfer function for the Bay of Arraial do Cabo. The main periods are indicated. openings tend to have a high Q-factor (Rabinovich, 2009), it must References be remembered that, in this case, there are two other openings in the bay (Fig. 1), including the largest one toward the northeast, Aida, I., Hatori, T., Koyama, M., Nagashima, H., Kajiura, K., 1972. Long-period waves in the vicinity of Onagawa Bay: (I) Field measurements in Onagawa and Okachi which allows high-energy damping. bays. J. Oceanogr. Soc. Japan 28, 207–219. The same numerical experiments indicate that the northeastern Candella, R.N., Rabinovich, A.B., Thomson, R.E., 2008. The 2004 Sumatra tsunami as passage is the only entrance able to admit fluctuations such as recorded on the Atlantic coast of South America. Adv. Geosci. 14, 117–128. those identified in this work. Thus, the interaction of arriving Dragani, W.C., Mazio, C.A., Nunez, M.N., 2002. Sea-level oscillations in coastal waters of the Buenos Aires Province, Argentina. Cont. Shelf Res. 22, 779–790. waves with the bathymetry and the coastline, such as refraction Dragani, W.C., 2007. Numerical experiments of the generation of long ocean waves and diffraction, would be primarily responsible for the presence in coastal waters of the Buenos Aires Province, Argentina. Cont. Shelf Res. 27. of longwaves in the bay. Inside the bay the natural period of oscil- doi:10.1016/j.csr.2006.11.009. Honda, K., Terada, T., Yoshida, Y., Isitani, D., 1908. An investigation on the secondary lations was excited by the arriving wave energy, producing seiches undulations of oceanic tides. J. College Sci., Imper. Univ. Tokyo, 108. with wave heights up to 60 cm. The analysis of 10 years of record Kulikov, E.A., Rabinovich, A.B., Spirin, A.I., Poole, S.L., Soloviev, S.L., 1983. (1999–2008) from an analog tide gauge (in preparation) indicates Measurement of tsunamis in the open ocean. Mar. Geod. 6 (3–4), 311–329. Melo Filho, E., Calliari, L.J., Franco, D., Strauch, J.C.S., 2005. Indícios da ocorrência de that these wave heights are of the same order of magnitude as um tsunami meteorológico na Praia do Cassino RS. Vetor (FURG) 15, 79–85. the largest low-frequency wave (T > 36 h, which occurred in May Miles, J., Munk, W., 1961. Harbor paradox. J. Waterways Harbor Division, ASCE 87, 2001) ever recorded by the gauge. 111–130. Miller, G.R., 1972. Relative Spectra of Tsunamis. Hawaii Inst. Geophys. (p. 7, HIG-72- Within the analyzed period, there was no other atmospheric in- 8). duced similar event, neither equally strong atmospheric distur- Monserrat, S., Vilibic´, I., Rabinovich, A.B., 2006. Meteotsunamis: atmospherically bances, nor high frequency fluctuations in sea level with such induced destructive ocean waves in the tsunami frequency band. Nat. Hazards Earth Syst. Sci. 6, 1035–1051. height, but the lack of a long-time series prevents us from a conclu- Rabinovich, A.B., 2009. Seiches and harbour oscillations. In: Kim Y.C., (ed.), sion about the recurrence of such events. Handbook of Coastal and Ocean Engineering World Scientific, Singapore. Rabinovich, A.B., Monserrat, S., 1996. Meteorological tsunamis near the Balearic and Acknowledgments Kuril Islands: descriptive and statistical analysis. Nat. Hazards 13, 55–90. Rabinovich, A.B., Stephenson, F.E., 2004. Longwave measurements for the coast of British Columbia and improvements to the tsunami warning capability. Nat. This research was supported by National Council for Scientific Hazards 32, 313–343. and Technological Development (CNPq) under the Grant 471250/ Rosauro, N.M.L., 1986. Seiches e suas ocorrências na lagoa dos Patos. Revista Brasileira de Recursos Hídricos 4 (2), 91–131. 2007-4. The author would like to thank Fred Stephenson (Canadian Šepic´, J., Vilibic´, I., Belušic´, D., 2009. Source of the 2007 Ist meteotsunami (Adriatic Hydrographic Service, Sidney, BC, Canada) for the language review Sea). J. Geophys. Res. 114, C03016. doi:10.1029/2008JC005092. and José Roberto Bairão Leite (University of São Paulo) for the Torrence, C., 1998. WAVETEST Example Matlab Script for WAVLET. Available from: . atmospheric pressure data from Cananéia e Ubatuba.