Joseph Harari, Ricardo de Camargo & Michel Gordon
Long series of hourly tidal data of Recife and Belém were edited filtered and analysed indicating the correspondent long term variabilities of the tides and mean sea level.
The tidal records of Recife cover the period from 1946 to 1987, having monthly mean values plotted in Figure 1, with correspondent spectrum given in Figure 2. The monthly means present a trend of 0.52 cm/year, with a significative annual oscillation of 1.83 cm.
Mean tidal amplitudes and phases were computed considering analyses of yearly records (Table 1). Such computations allowed the monitoring of the time evolution of these parameters, as presented for the M2 amplitude (Fig. 3), which has a trend of 0.01 cm/year, with a periodicity of 21 years, its amplitude reaching 0.57 cm. S2 tidal component in Recife has the same trend in amplitude (0.01 cm/year) and the same periodicity, but with lower intensity (0.24 cm).
Data from Belém since 1961 until 1987 have a relatively smaller trend in the monthly mean series (0.39 cm/year - Fig. 4); on the other hand, the correspondent annual and semi - annual signals are prominent, having amplitudes of 3.90 and 3.46 cm, respectively (Fig. 5).
Mean amplitude and phase values of the principal tidal components in Belém are also presented in Table 1. The corespondent long term trends are very variable, decreasing - 0.06 cm/year for the M2 amplitude, with no associated periodic oscillation (Fig. 6), while S2 amplitude increases +0.002 cm/year, not having any significative periodic oscillation too.
Tidal and mean sea level evolutions have been monitored since the last decade, considering long records of coastal observations, some of them since the last century.
The global warming is considered as the main effect responsible for the mean sea level increase in Equatorial and tropical areas, but the rates vary in space, mostly due to local effects, such as hydrodynamical characteristics or erosion; these effects may change the treads in particular regions, even reducing them to negative values.
In the present study, the extremely high mean sea level trend in Recife is due to well known local effects related to erosion and geological influences.
On the other hand, tidal variations seem to be closely associated to the mean sea level changes, as shown in Table 2 for the variations of M2 amplitude, in Recife, Belém and some other ports along the Brazilian coast (Mesquita, Harari & França 1994; Harari & Camargo, in press).
As a conclusion, the global warming seems to be affecting not only the mean sea level, but also acts slowly modifying large scale characteristics of the tidal wave propagation.
We are grateful to Instituto de Pesquisas Hidroviárias (INPH),
for providing the tidal data of Recife and Belém, to Dr Alberto
dos Santos Franco, for his help in the tidal analyses, and to CNPq and
FAPESP, for their financial support.
Table 1 - Recife and Belém mean values of tidal components amplitude
H, in cm, and local phase G(local), in dagrees
Figure 1 - Monthly mean sea level values in Recife, 1946 to 1987.
Figure 2 - Spectrum of the monthly mean sea level in Recife, considering a time series of 504 points.
Figure 3 - Yearly values of the M2 amplitude in Recife.
Figure 4 - Monthly mean sea level values in Belém, 1961 to 1987.
Figure 5 - Spectrum of the monthly mean sea level in Belém, considering a time series of 324 points.
|Port||TMSL (cm/year)||TM2 (cm/year)|
|Cananéia (25º01'S - 47º56'N)||0.4||0.05|
|Santos (23º57'S - 46º19'W)||0.1||-0.02|
|Ilha Fiscal (22º53'S - 43º10'W)||1.3||0.03|
|Recife (08º03'S - 34º52'W)||0.5||0.01|
|Belém (01º25'S - 48º30'W)||0.4||-0.06|
Mesquita, A. R. de & Harari, J. & Franqa, C. A. S. - 1994- "Interannual variability of tides and sea level at Cananbia, Brazil, from 1955 to 1990" - Publ. esp. Inst. oceanogr., S Paulo, 11.
Harari, J. & Camargo, R. - in press - "Tides and mean sea level
variabilities in Santos (SP), 1944 to 1989" - submitted to Relat. int.
Inst. oceanogr., S Paulo, in 1994.