OC103 Lesson #20: Tides What are Tides? A tide is a broad, single wave that stretches across large areas of an ocean basin. Tides occur due to the gravitational pull of the Moon and Sun on the ocean water. In terms of the characteristics that we used to describe other kinds of waves, the key features of tides are: • High tide = Wave crest • Low tide = Wave trough • Tidal range = Wave height, which for tides can be <1 m to >15 m • Tidal period = Wave period, which, depending on the location, is either 12 hours and 25 minutes, or 24 hours and 50 minutes For example, the plot below shows the predicted tides for Newport, Oregon, for 7 days around a Memorial Day weekend. The tides are essentially very broad waves (with crests and troughs) that pass the Oregon Coast about twice per day. Notice from the plot that the tidal period at Newport is 12 hours and 25 minutes (that is, it is 12 hrs and 25 min between 2 high tides or between 2 low tides), so it takes slightly longer than 1 day to go through 2 full tidal cycles in this area (see how the first low tide of each day occurred at midnight on the 1st day on this plot, but gets later and later each night (50 minutes later, to be exact), until it occurs at about 5AM on the last day on this plot). Notice also that the tidal range (the height difference between high and low tides) varies from just over 2 feet early on May 27th (between ~4 and ~6 feet above Mean Lower Low Water) to almost 12 feet early on June 2nd (between ~9 feet above and ~2 feet below MLLW). Tides are caused by a combination of factors, but they are highly predictable once we have about a year of observations in a location. In explaining what causes tides, we will also explain why tidal ranges and periods vary with location (i.e., why some locations have large ranges and some small, and some locations have only 1 tidal cycle per day, while most have 2), and also why the tidal range at any single location varies predictably from day to day.Origin of the Tides Unlike wind-driven surface waves, tides are caused by two principal factors: • Gravity: All masses are drawn to each other, exerting gravity based on their mass and distance. The gravity field of the Moon pulls on Earth’s ocean water and creates a bulge of water pointing toward it (see top panel of figure below). The gravitational attraction of the Sun also creates a water bulge pointing toward it, but this bulge is much smaller because the Sun is so much farther away (which more than offsets the fact that the Sun is much larger than the Moon). • Centrifugal Force: Centrifugal force sends ocean water away from the center of rotation of Earth and the Moon, creating a tidal bulge pointing away from the center of rotation (and thus, away from the Moon; see middle panel of figure at right). The centrifugal force between the Sun and Earth also creates a bulge pointing away from the Sun, but again, it is much smaller than the bulge pointing away from the Moon. The net result is 2 main bulges of ocean water caused by the Moon (1 from Gravitational Attraction + 1 from Centrifugal Force; see lower panel of figure at right), plus 2 much smaller bulges caused by the Sun.Tidal Periods The water bulges are not aligned quite as depicted in the figure on the previous slide because the orbit of the Moon is not aligned with Earth’s Equator, but is tilted at about 28.5 degrees, so the tidal bulge is also at a 28.5° angle to the Equator (see left side of figure below). As the solid Earth rotates once around every 24 hours beneath this double-bulged cloak of ocean water, most points on Earth rotate through some part of both bulges (each causing a high tide) and both low areas between the two bulges (each causing a low tide), thus causing 2 high tides and 2 low tides per day (almost). Earth does not quite rotate completely through both bulges in 24 hours because the Moon moves forward in its orbit each day, taking its water bulges with it, so it takes 50 additional minutes for a spot on Earth’s surface to catch up with the water bulges caused by the Moon. That is why it takes 24 hours and 50 minutes, instead of an even 24 hours, for Earth to rotate through both bulges and get through two complete tidal cycles. Some locations have only one tidal cycle per day (called diurnal tides, see upper panel of figure at right). Diurnal tides usually occur in areas in the far north and south because those areas encounter only one of the water bulges during a rotation, and completely miss the other bulge. So many locations near the poles have only 1 tide per day. Many locations on Earth have two unequal tides each day (called mixed tides, see lower panel of figure at right). For mixed tides, locations at mid-latitudes rotate directly under one of the bulges, but only catch the edge of the other bulge, so at those locations one of the daily tidal cycles of a mixed tide has higher high tides and lower low tides than the other daily cycle (see middle panel on right side of figure above and lower panel of figure at right). Some areas rotate under parts of both bulges that are about equal in magnitude, so those places have two nearly equal tides per day (semidiurnal tides; see lower panel on right side of figure above). Oregon, being at mid-latitudes, has mixed tides. That is why Newport has two low and two high tides each day, and one of those tides has a larger tidal range than the other (I am using "day" a little loosely here since it actually takes 24 hours and 50 minutes for a complete diurnal tidal cycle or two complete semidiurnal or mixed tidal cycles).Tidal Range The tidal prediction from Newport showed that the range between the high and low tides changes considerably during the week. This is because the tidal bulges caused by the Sun and Moon are moving relative to each other as those two bodies go through their separate orbits. And every two weeks, the Earth, Moon, and Sun all align. We see this as a new Moon when the Moon is between the Earth and Sun because the lit face of the Moon is hidden from our view, or as a full moon when the Moon is on the opposite side of Earth from the Sun because the entire lit side is visible to us (see the left side of the lower panel of figure below). During these times of alignment, the gravitational and centrifugal forces all act in concert and create constructive interference (which we covered in the waves lesson), so the high tides are at their