Seasonal effects on estuarine tidal currents and implications for prediction

Seasonality in tidal currents has not been well researched, in part because the great difficulty in collecting multi-year observations of estuarine currents. Yet understanding this seasonal variability is critically important to accurately predict tidal currents for marine navigation and other uses. Here we perform an analysis of monthly tidal current variability over 11 years at two locations in Tampa Bay, Florida. Tidal current constituents are correlated with environmental parameters including local winds, freshwater discharge into the Bay, and salinity to determine potential forcing mechanisms. Comparison with local winds demonstrates that periods of strong land-sea breeze can increase the amplitude of the diurnal K₁ tidal current constituent up to 30% when compared to periods of weak land-sea breeze. Sub-tidal, weather-scale wind forcing with periods of 2 to 7 days is well correlated with periods of increased non-tidal residual flow, likely resulting from both direct wind forcing as well as modification of along-estuary water level gradients. Analysis of freshwater discharge and salinity show that the depth averaged semidiurnal M₂ tidal current constituent can increase by more than 10 cm/s, an increase of 25%, during times of high discharge and increased stratification. This observed variability has significant influence on the accuracy of tidal current predictions depending on when observations are collected and when predictions are made. For instance, predictions of maximum flood or ebb currents can vary by more than 40 cm/s when observations are collected in the fall and predictions are made for the winter months. This study has direct implications for how current observations are collected and analyzed to generate predictions for safe and efficient marine navigation.