Tidal Patterns Center on Amphidromic PointsTidal Patterns Vary with Ocean Basin Shape and SizeTidal Patterns Vary with Ocean Basin Shape and SizeChapter 11 – TidesA tidal bore is formed when a tide arrives to an enclosed river mouth. This is a forced wave that breaks. Tidal range can be very largeTide - rhythmic oscillation of the ocean surface due to gravitational & centrifugal forces (‘inertia’) between the Earth, Moon and Sun.Tide Patterns - regular, cyclic patterns of low water-high water Tidal cycle – one low tide and one high tide consecutivelydiurnal tide - one low tide, one high tide a day; semidiurnal tide - high water-low water sequence twice a day; 2 high, 2 low, about the same levelsemidiurnal mixed tide - same as semidiurnal but 2 highs and 2 lowsdo not reach/drop to the same level; may be the result of a combination of tide typesTide Patterns - regular, cyclic patterns of low water-high waterdiurnal tidesemidiurnal tidesemidiurnal mixed tideSemidiurnal tidesDiurnal tidesMixed tidesd(ft)Mixed tide, Los Angeles Diurnal tide, Mobile, Alabama Semidiurnal tide, Cape Cod(m)14Higher high tide 410Lower high tide 6High tide 2High tide 34100–4Low tide –10 612 18 24 30 0 6 12 18 24 30 36 42 48 0 6 12 18 24 30 36 42 48a Time (hr) b Time (hr) c Time (hr)Lower low tideHigher low tideLow tide484236Most of the world’s ocean coasts have semidiurnal tides.Flood Tide: tide wave is propagating (onto shore) onshore –water level is risingHigh Tide: water level reaches highest pointEbb Tide: tide is moving out to sea – water level is dropping Low Tide: water level reaches lowest pointSlack tide: period when tide wave is reversing –low current velocityWater currents are generated by the tides, the speed of the incoming tide is about the same but in the opposite direction of the outgoing tide. Moving waters have to slow down and reverse, from flood toebb and vice versa (slack tide). This is a good time for navigation through narrow places, particularly those characterized by strong tides (East River, for example).Mean Tide Level = MTL - computed from measurements taken at a place over many years and averaging all water levels. Mean High-Water = MHW. Mean Low-Water = MLW. For mixed tides: Mean Higher High Water = MHHW Mean Lower Low Water = MLLW tidal range – difference betweenMHW and MLW(water level at high tide and water level at low tide)Examples of typical tides - USStudy of TidesEquilibrium Tidal Theory - ideal approach to understand basic principles, assumes an earth covered with waterAssumptions:1: entire Earth surface covered in water2: infinitely deep basin (no shoaling)3: tidal bulge fixed relative to the moonDynamical Tidal Analysis - realistic approach, studying the tides as they occur on earth, accounts for modification due to landmasses, geometry of ocean basins, earth’s rotation. Tides are caused by the difference in gravitational forces resulting from the change of position of the Sun and the Moon relative to points on Earth* centrifugal (‘inertia’) and gravitational forces*universal law of gravitationF= G m1m2/ R2G = universal gravitational constantm1, m2 = mass of bodiesR = distance between centers of mass of bodiesB & C = gravitational forcesB’ & C’ = centrifugal forcesFigures in textbook1,650 km (1,023 mi)Earth’s mass is 81 times the mass of the moonMoon(81/82) r(1/82) r Average Earth–moon distance (r)The moon’s gravity attracts the ocean toward it. The motion of Earth around the center of mass of the Earth – moon system throws up a bulge on the side of Earth opposite the moon. The combination of the two effects creates two tidal bulges.Add the SunA (and B & C) = gravitational forcesA’ (and B’ & C’) = centrifugal forcesThe Tide Producing Force(difference between gravitational forces and centrifugal forces at the earth surface) is proportional to GM/R3(with figures 11.5 & 11.6 in textbook)The Moon Tide and how we get a ‘wave’The Tidal Day = 24 hr 50 minLunar Cycle: the Earth –Moon system has a period of 29.5 daysDiurnal = 24 hr 50 minSemidiurnal = 12 hr 25 minThe Sun TideSpring & Neap TidesSpring and Neap tides at two places on EarthDeclination TideMoon’s declination cycle 28.5N – 28.5S = 29.5 daysSun’s declination cycle 23.5N (summer solstice) – 23.5S (winter solstice) = annualOrbital Influence on Tides Astronomical High Tide: Moon at perigee, Sun at perihelion, and Earth-Moon-Sun system at syzygy (full or new moon)Dynamic Tidal AnalysisA mathematical study of tides as they occur. It looks at the tide wave, which is similar to the tide wave of the ideal water covered earth, but varies from place to place.* Continents break up wave propagation * Tide wave moves continuously around the globe only in the Southern Ocean (Antarctica).* shallow-water wave: speed is controlled by depth of ocean* standing wave: oscillates because it is contained in ocean basins (wave ‘contained’ in ocean basin)* reflected by continents, refracted by changes in depth, anddiffracted (spread of energy sideways) as it passes through gaps * Coriolis affects the water movement because it is a largescale phenomenon.Amphidromic SystemAmphidromic PointCotidal LinesTidal Patterns Center on Amphidromic PointsTidal Patterns Center on Amphidromic PointsThe development of amphidromic circulation(a) A tide wave crest enters an ocean basin in the Northern Hemisphere. The wave trends to the right because of the Coriolis effect (b), causing a high tide on the basin’s eastern shore. Unable to continue turning to the right because of the interference of the shore, the crest moves northward, following the shoreline (c) and causing a high tide on the basin’s northern shore. The wave continues its progress around the basin in a counterclockwise direction (d), forming a high tide on the western shore and completing the circuit. The point around which the crest moves is an amphidromic point (AP).Tidal Patterns Vary with Ocean Basin Shape and SizeTidal Patterns Vary with Ocean Basin Shape and SizeHow do tides behave in confined basins?The tidal range is determined by basin configuration. (a) An imaginary amphidromic system in a broad, shallow basin. The numbersindicate the hourly positions of tide crests as a cycle progresses. (b) The amphidromic system for the Gulf of St. Lawrence between New Brunswick and Newfoundland, southeastern Canada. Dashed lines show the tide heights when the tide crest is passing.Tidal Patterns Vary with