Variability of tidal constants in the Río de la Plata estuary associated to the natural cycles of the runoff

Abstract

We analyze the only long-term (47-years long) hourly tide-gauge record available for the Río de la Plata estuary for variability in the amplitude and phase of the dominant principal lunar semi-diurnal (M2) constituent. The tidal record was first filtered to isolate the signal at the frequency of M2. Then harmonic analysis was applied to the filtered data for periods of one year with progressive lags of one month. This way, monthly time series of M2 amplitude and phase were obtained, which were then analyzed by means of several spectral techniques. Both M2 amplitude and phase present a long-term trend/inter-decadal cycles and significant pseudo-periodicities at frequencies which are known as typical of the El Niño ? Southern Oscillation (ENSO) cycles. A joint multi-channel singular spectral analysis of the tidal constants and the runoff to the estuary (which variability is dominated by the ENSO cycles effect on precipitation over southern South America) reveals co-movements at periods around 2.5, 3.5 and 6.5 years, as well as long-term trends/inter-decadal cycles. Positive (negative) runoff anomalies are associated with negative (positive) amplitude anomalies and negative (positive) phase anomalies. The detected variability in the tidal constants seems to be the result of the interaction between the tidal wave propagating upstream and the downstream currents due to the runoff, rather than a result of the variability in the mean sea level caused by the runoff.