NODAL CYCLE, QUASI-LUNAR CYCLE, AND FLORIDA CURRENT INFLUENCES ON HIGH TIDE LEVEL IN MIAMI FLORIDA

As sea level rises, Miami experiences flooding at a higher frequency, not only during rainfall events but also during the spring tides when the tidal ranges are greater. Greater ranges mean higher than normal sea level during high tides and lower than normal sea level during low tides. The spring tides occur twice a month at the new and full moons when the Earth, Moon, and Sun are in alignment. In the fall months from September to November around the equinox, spring tides in Miami are higher than usual because of the occurrence of the perigean spring tides, when the moon is the closest to the Earth. These fall spring tides in Florida are known as king tides, and they are one reason for nuisance flooding.

However, some years have higher king tides and more flooding than others. We identified two additional factors influencing the severity of the king tides. One factor is predictable, and the other factor is not. The seasonal factor is part of the astronomical tide, the combination of two long constituents - lunar nodal cycle and lunar perigee. The period of the lunar nodal cycle is 18.61 years and the period of lunar perigee is 8.85 years; however, lunar perigee affects the high tides twice during the cycle, every 4.4 years, and is known as quasi-lunar perigee. In our research, we estimated the amplitude of these two cycles using the Fourier transform and from the tide ranges. The ranges were estimated from differences between higher high and lower low tide. From knowing the amplitude, we are able to say how much sea level differs in years when the nodal cycle is at its crest and its trough.

The second factor that we are not yet able predict that affects king tides is the Florida Current that moves from the Gulf of Mexico north along the Florida coast, where it becomes the Gulf Stream current. When the Florida current is fast, it creates a bulge of water on its east side in the Atlantic Ocean. However, when it slows down, the bulge in the ocean becomes smaller, and the seawater is displaced onto the east coast of Florida, making sea level near the coast higher. We performed cross-correlation analysis to measure the lag and the strength of the relationship between the tide and Florida Current. Better predictability of the Florida Current would help in forecasting years with extreme tidal levels.