Variations of the mean sea level in the southwest Atlantic Ocean

ContinentalShelfResearch,Vol. 8, No. 11, pp. 1211-1220,1988. Printedin GreatBritain.

0278~.343/88$3.00 + 0.00 © 1988PergamonPressplc.

Variations o f the m e a n sea level in the s o u t h w e s t Atlantic O c e a n NI~STOR W . LANFREDI,* ENRIQUE E . D ' O N O F R I O t a n d CARLOS A . M A Z I O t

(Received 2 November 1987; accepted 15 January 1988) Abstract--The lack of information on the variation of sea level in the southern hemisphere hinders its estimation. It is for this reason that in this paper an analysis of 64 years of hourly tide heights at the most trustworthy Argentina oceanic station has been made, using a symmetric lowpass filter based on the Kaiser-Bessel window, which permits elimination of periodic components (of 8.8 and 19 years). Convoluting the data with a synthesized filter, the spectre has been obtained by way of the Fast Fourier Transform method. With antitransformed data the long-term trend has been calculated yielding 16.09 cm/100 years, compatible with calculations made recently for variations on a global

scale.

INTRODUCTION

THE universal datum for heights is the mean sea level, whose value is not constant; its spectre contains space-temporal fluctuations which manifest themselves in all scales and frequencies. Its periods range from seconds (wind waves) through 10,000 years for eustatic changes related to the glacial-interglacial alternations. At its extreme, the spectre presents aperiodic low-frequency fluctuations of tectonic origin, a consequence of the relative movements among continents and oceanic basins. The variations of sea level arouse special interest, due ta the probable sensitivity which sea level shows to climatic changes and due to its effect on beach erosion. The behavior of sea level during the last century gave rise to a series of studies, revealing the difficulties to be faced due to the lack of data in the southern hemisphere, noting particularly a scarce number of complete annual records or series in South America and Africa. BARNETI"(1982) presents Montevideo (Uruguay) as the station with the longest series in the southwest Atlantic (1938-1970), with only seven annual records missing. Notwithstanding, Argentina owns tidal records more prolonged than these. This paper presents the most complete existing series, giving its prominent features in order to integrate it in future investigations of global sea level changes. G L O B A L SEA L E V E L C H A N G E S

In a classic sense, variations of sea level have been classified by means of the differences between the geoid and the surface determined from heights of the local sea level. * Comisi6n de Investigaeiones Cientificas de la Provincia de Buenos Aires, Facultad de Ciencias Naturales, U.N.L.P. CasiUa de Correo 45, 1900 La Plata, Argentina. t Servicio de Hidrografia Naval, Departamento Oceanografia, Av. Montes de Oca 2124, 1271 Buenos Aires, Argentina. 1211

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Periodic terms present in short-term fluctuations include oscillations induced by the local wind over the continental shelf, long waves, such as lunisolar tide, and related phenomena. Variations of aperiodic terms include changes due to meteorological causes, such as wind-produced sea level variations and the effect of atmospheric pressure. Due to the ocean surface-atmosphere interaction and to the great caloric capacity of the oceans, any climate change of global proportions would seem to be reflected in the oceanic conditions. It is accepted that mean sea level, on its planetary scale, reacts as a result of oceanic conditions, such as the mean temperature and the volume of the world ocean. The variations of oceanic thermal conditions may be infered from the mean surface temperature of the atmosphere. GATES et al. (1981), using a numerical model, have found that when no changes of the mean oceanic surface temperature are involved, the global mean temperature of the surface air mass may show no variations. Assuming this as a hypothesis, the stations analysed from 1903 to 1969 gave an average relative rise of mean sea level of 15 cm/100 years (BARNEIT, 1983). For the period between 1930 and 1975, another study gave a similar value, but it only covered the western half of the North Pacific Ocean. Notwithstanding, hydrographic data revealed a descent of the relative sea level. EMERY (1980) calculated that in the period 1940-1980 the sea level had risen at an annual rate of 3 mm; however, in the course of this period, the global mean temperature seems to have descended by about 0.2°C; this would indicate that the recent rise in sea level cannot be attributed to thermic expansion. It is assumed that short-term changes of the oceanic volume (HANsEN et al., 1981) are due to the global heating of the atmosphere, produced by atmospheric carbon dioxide, or some other cause, which would augment oceanic temperature; this effect induces an increase of the ocean volume and when no other influences intervene, the relative mean sea level would rise. This change in relative sea level will be global in nature, although the variations need not be uniform over the globe. Detection of this climate signal will be hindered by the vertical land motions that were originally of interest. BARNETI"(1983) explained the reason for not supporting the idea of a "global sea level rise". Among the causes which could induce a variation in relative sea level are changes in the position of the earth's rotation axis, supported by the notion that relative changes of sea level due to the thawing of Greenland and Antarctic icesheets are approximately equal; this is of no great importance, due to the change in longitude of the day and to the fact that it is possible to explain the change of these two astronomic parameters another way. The change observed in the sea's surface temperatures is related to variations in the vertical thermic structure, which could give rise to a change in relative sea level. An alternative would be to assume that the thermal expansion of the ocean will not affect these parameters and that the changes are due to processes not related with relative sea level. The effect of vertical displacement of the earth's crust generates a signal which cannot be adequately filtered, so it seems improbable that the movements of the crust are solely responsible for the values obtained for the relative mean sea level. It is consequently assumed that this signal from the crust has been negligible during the last 70 years in the stations used by the different investigators.

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Variations of m e a n sea level

Table 1.

Estimates o f mean global sea level increases (after BARNett, 1982)

Author THORARIt~SSOS GUTEMBERG KUENEN LISITZIN WEXLER FAIRBRIDGE and KREBS EMERY GOR~TZ et al. BAR~rETr

Rate (cndcentury)

Date

5 11 + 8 1 2 - 14 11.2 +-- 3.6 11.8 12 30 12 15.1 + 1.5

1940 1941 1950 1958 1961 1962 1980 1982 1983

Estimations of the mean global increase of sea level (Table 1) carried out since 1940, raise the interesting fact of the correspondence of the obtained values, approximately 15 cm per century, except the result obtained by EMERY (1980); these estimations show that the values are strongly biased in space and in time. It would seem that one of the greatest limitations in these studies is the lack of data in the southern hemisphere and in the mid-ocean region, and it means that one must accept a significant risk when interpreting the results in terms of global changes. SEA LEVEL CHANGE

IN T H E S O U T H W E S T

ATLANTIC

(ARGENTINA)

The first tidal records available in Argentina date from 1905, obtained at the station Buenos Aires, in the Rio de La Plata. They are not considered in this study due to the important increase of the river sedimentation, which could generate local trends in sea level, comparable with eustatic or isostatic changes. In the southwest Atlantic, Argentina holds a net of tidal gauges restricted to the coastal area (Fig. 1). In an early study on the subject, 14 stations were analysed with records covering 12 and 4 years. In this paper only the longest series is used, corresponding to Puerto Quequ6n, with 64 years of registered hourly heights, without gaps or equipment failures, starting as in 1918. Some of these net gauges were operated along decades, and a handful of them provides the only series longer than half a century in these latitudes. The first work, including part of the above records, has been made by BALAV (1958), where he analyses the seasonal cycle and offers an hypothesis to explain it. The proposed causes were the wind and the response to changes in atmospheric pressure. PATULLOet al. (1955) explained that the variations of sea level between latitudes 40°N and 40°S were produced by steric changes related to variations in the volume of water. The levels between the equator and latitude 40° are high during summer in the respective hemisphere. North of 40°N, sea level is high in winter: December or January. South of 40°S there are so few gauges that the phase is not known. The two assumptions enunciated by Patullo are fulfilled in our case, as it is found that to the south of 40°S maximum mean levels are registered in summer and during the month of February, with minimums observed in September (such is the case in the Puerto Madryn station at 42°S).

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40 ° 40 c

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Map showing the location of the tidal station.

EXISTING

DATA

BASE

AND

RESULTS

In order to study sea level variation, we selected the most trustworthy station in terms of precision of data and without gaps, Puerto Quequ6n. Table 2 shows its monthly and annual means. Sea level data have been taken routinely in Puerto Quequ6n since 1918-1950 by the Direcci6n Nacional de Construcciones Portuarias y Vias Navegables. In 1951, the Servicio de Hidrografia Naval installed a new tide gauge to replace the old one. Instrument position changes are accommodated because the old and new locations are tied to a common geodetic reference point, this ensures the data quality throughout the observational period. The method to obtain monthly sea level was identical throughout the period in question; Fig. 2 displays a graphic representation of the annual mean sea levels. The initial information for this study was from annual sea levels calculated as means of the monthly mean values. The goal was to obtain the long-term trend of the change of sea level, for which it was necessary to decontaminate the periodic components from the temporal series. This would eliminate the spectral contributions which disguise the searched for trend. The periods of these perturbing spectral contributions are 19, 18.8 years and quasi-annual. The numerical tool divised to eliminate the frequencies corresponding to the above

Variations of mean sea level

1215

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12 6 0 1928

1918

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1938

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1968

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1978

mean sea levels (1918--1981).

periods was a symmetrical 17-elements low-pass filter, by way of a K a i s e r - B e s s e l window.

w(n) = lo[Fct[1.0- (n/N/2)2]½]/Io [n~t] with 0 ~< Inl ~< N/2, w h e r e N is the n u m b e r of samples of the signal. oo

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N.W. LANFREDIet

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(modified first class and zero order of Bessel function), a is a parameter equal to the semiproduct of the duration of the signal by the band width. The decision to use the Kaiser-Bessel window was a response to the recommendation by HARRIS (1978), who, after having analysed numerous types of windows and compared their merits, concluded that the Kaiser-Bessel's is the best. The adopted cutoff frequency was 0 . 5 1 2 y - 1 ( T = 9 years) in -18.7 dB. Figure 3 shows the values assigned to each of the filter elements, and in Fig. 4 the response to power function is displayed, its low-pass characteristic standing out clearly, with a very neat cutoff near the zero frequency. The primary data were convoluted with the synthesized filter, thus obtaining a series of data which were submitted to a spectral analysis by means of the Fast Fourier Transform (FFT) method, with a rectangular window. Knowing the response to power function of the filter, the signal has been restored in the domain of frequencies, recoloring the spectre in order to correct the contributions in amplitude and phase, eliminating those whose response was below -18 dB. The obtained signal was treated with the Fourier antitransform:

n-~ (2rot s(t) = ~, R,,, exp j \~V-A

) + (~)m

m =-m

0 -tO

- 20

w

(d b) _-:

- 50

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' 01~

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Fig. 3. Values assigned to each element of the filter. 0 20

0.15

O. 10

w(n) O. 05

0.00

-0.05

Fig. 4. Response to power function.

n

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Variations of m e a n sea level

I "II

15~

,AI

'o i E

\l

L~

o

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191a Fig. 5.

~

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Linear regression calculated with filtered data of the annual m e a n sea levels.

with s a time signal of T duration with a sampling at A intervals, in such a manner that N (number of samples) = T/A is even, and thus N = 2n. R m and d~m stand for amplitude and phase of the mth harmonics, bearing the signal into the time domain. Returning to the least squares method the lineal regression was calculated. Since for Quequrn the study started with data of 64 consecutive annual mean sea levels (1918-1981), the linear regression was calculated with 48 data (1926-1973) because the filter produces a loss of 8 data at each end of the considered series. The results gave a mean value of 97.99 cm and a slope of 16.09 crn/100 years, with a correlation coefficient r = +0.52 (Fig. 5). The stability of Puerto Quequrn was confirmed by the high correlation that exists between Mar del Plata (Club) and Quequrn, a nearby station. Some discrepancies were found with the data of the tide gauge off Mar del Plata (Puerto), situated in the interior of the port, as a consequence of a local situation. CONCLUSIONS

The following conclusions can be drawn with regard to sea level variations at Quequrn (Argentina): (a) This paper reduces the lack of data in the southwest Atlantic. (b) The gaps in the remaining coastal stations do not permit to obtain reliable records to estimate variations to the south of latitude 42°S for more than 30 years. (c) A sea level change occurred in the period 1918-1981. This pattern was associated with a linear increase in sea level at a rate of 16.09 cm/100 years. REFERENCES BALAY M. (1958) Variations saisonnieres du niveau m o y e n de la Mer Argentine. Revue Hydrographique International, 35, 117-145. BARNETI" T. P. (1982) O n possible changes in global sea level and their potential causes. SIO Reference Series 82-10, p. 34. BARNETT T. P. (1983) Global sea level: Estimating and explaining apparent changes. Proceedings of the Symposium on Coast and Ocean Management, 3, 2777-2783.

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EMERY K. O. (1980) Relative sea levels from tide gauge records. Proceedings of the National Academy of Sciences, 77, 6968-6972. FAIRBRIDGE R. W. and O. A. KREBS, Jr (1962) Sea level and the southern oscillations. GeophysicsJournal, 6, 532-545. GATES W. L., K. H. COOK and M. E. SCHLESINGER(1981) Preliminary analysis of experiments on the climatic effects of increased CO2 with an atmospheric general circulation model and a climatological ocean. Journal of Geophysical Research, 86, 6385-6393. GORNITZ V. S., P. LEBEDEFFand J. D. HANSEN(1982) Global sea level trend in the past century. Science, 215, 1611-1614. HANSEN J. D., A. JOHNSON, S. LACIS, P. LEBEDEFF, D. LEE and G. RUSSEL (1981) Climate impact of increasing carbon dioxide. Science, 213, 957-966. HARRIS F. (1978) On the use of windows for harmonic analysis with Discrete Fourier Transform. Proceedings of the IEEE, 66, 51-83. PATULLOJ. G., W. MUNK, R. REVELLEand R. STRONG (1955) The seasonal oscillation in sea level. Journal of Marine Research, 14, 88--155.