![]() ERL PMEL-56, Pacific Marine Environmental Laboratory, NOAA. Larsen, 1984: Tides and Tidal Currents of the Inland Waters of Western Washington. D., 2004: Secrets of the Tide: Tide and Tidal Current Analysis and Applications, Storm Surges and Sea Level Trends. Trinity University Press, San Antonio, 335 pp. White, J., 2017: TIDES: The Science and Spirit of the Ocean.I also have learned a great deal from these books and reports: Authoritative Educational information from NOAA.There is a lot of great information on the web about tides, especially from NOAA: The ellipticity is not happening at exactly the same frequency as the full moon, so the pattern varies throughout the year, and over different years. At the following spring tide during a full moon the moon was at the far side of its elliptical orbit, and so had a smaller tractive force. What these show is, for example in April 2016, the first spring tide was during a new moon that had a larger tractive force because the moon was at the close part of its elliptical orbit. This is shown in the figure by the "Lunar Tractive Force" lines. ![]() The reason for this is that the moon's orbit around the Earth is elliptical, and so about once a month it is relatively closer to the Earth. There is a clear variation of the size of spring tides, like in April 2016 there is a strong spring tide followed by a weak spring tide. Also shown on the figure is the timing of full and new moons, and these clearly line up with the spring tides. There are about two spring tides per month. This is not the whole tide, but just that part of the tide forced at twice-a-day frequency by the sun and moon. In Puget Sound the estuarine circulation turns out to be very large, and exerts a profound influence on water properties.Plotted here is a year of tides at Seattle. This pattern is called the “estuarine circulation” or the “exchange flow” and it is a characteristic of every estuary in the world. However, if you put a current meter at any place in the Sound (or any other estuary) you will find that after averaging over many tidal periods the mean is not zero, but instead there is a persistent inflow of deep water and outflow of shallower water. The tidal excursion in Admiralty Inlet is about 20 km, and in Main Basin it is about 1.5 km. Tidal currents mainly move water back and forth, over a distance called the tidal excursion. In terms of the residence time of water in the Sound, the important currents are the persistent ones. While tidal currents are quite apparent to boaters, their importance to Puget Sound water quality is primarily because of the turbulent mixing they cause. The tides are what cause the strongest currents in the Sound, peaking around 2.2 m s-1 in Admiralty Inlet, 3.4 m s-1 in Tacoma Narrows and over 3.8 m s-1 in Deception Pass. The tides bring in about 8 km3 of water each high tide, removing it roughly 12.4 hours later. In addition, high tide occurs about 1 to 2 hours later in Olympia than it does at Admiralty Inlet. However, the tidal range actually increases as you move landward, and the biggest tidal range is at the extreme southward end. The tides are forced by the tidal variation of sea level at the mouth of the Salish Sea – the seaward end of the Strait of Juan de Fuca. Tides in the Sound are large, with ranges between 3 and 4 m.
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