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PHENOMENAContents
NON-TIDAL INFLUENCES Wind-driven currents, and Atmospheric pressure effects (Storm surges) Because the tide usually dominates the spectrum of water level and current fluctuations along the ocean coasts, it is common to think of non-tidal fluctuations mostly in connection with inland waters. The tide in the deep ocean, however, can be quite insignificant and the tidal streams completely negligible from the standpoint of navigation. Wind-driven surface currents in the deep ocean, on the other hand, are of major importance to navigation. Water levels along ocean coasts are just as surely affected by atmospheric pressure and wind as are water levels along the shores of inland bodies of water. This is the case of storm surge. As the name suggests, storm surges are pronounced increases in water level associated with the passage of storms. Much of the increase is the direct result of wind set-up and the inverted barometer effect under the low pressure area near the center of the storm. There is, however, another process by which the surge may become more exaggerated than would be anticipated from these two effects alone. As the storm depression travels over the water surface, a long surface wave travels along with it. If the storm path is such as to direct this wave up on shore, the wave may steepen and grow as a result of shoaling and tunneling, (see Shallow-water effects). The term «negative surge» is sometimes used to describe a pronounced non-tidal decrease in water level. These could be associated with offshore winds and travelling high pressure systems, and are not usually as extreme as storm surges. Negative surges may, however, be of considerable concern to mariners, since they can create unusually shallow water if they occur near the low tide stage. The range, however, is generally small compared to that of the tide on the coast, and the importance may not be fully realized until an extreme of the non-tidal fluctuations coincides with a corresponding extreme (high or low) of the tidal fluctuation. In using tidal predictions, such as those in the Canadian Tide and Current Tables, it should be borne in mind that they contain no allowance for non-tidal effects, other than for the average seasonal change in mean water leve. Canadian Tidal Manual
Seiches A seiche is the free oscillation of the water in a closed or semi-enclosed basin at its natural period. Seiches are frequently observed in harbours, lakes, bays. And in almost any distinct basin of moderate size. They may be caused by the passage of a pressure system over the basin or by the build-up and subsequent relaxation of a wind set-up in the basin. Following initiation of the seiche, the water sloshes back and forth until the oscillation is damped out by friction.
Seiches are not apparent in the main ocean basins, probably because there is no force sufficiently coordinated over the ocean to set a seiche in motion. The tides are not seiches, being forced oscillations at tidal frequencies. If the natural period, or seiche period, is close to the period of one of the tidal species, the constituents of that species (diurnal or semidiurnal) will be amplified by resonance more than those of other species. The constituent closest to the seiche period will be amplified most of all, but the response is still a forced oscillation, whereas a seiche is a free oscillation. A variety of seiche periods may appear in the same water level record because the main body of water may oscillate longitudinally or laterally at different periods, it may also oscillate both in the open and closed mode if the open end is somewhat restricted, and bays and harbours off the main body of water may oscillate locally at their particular seiche periods. Seiches generally have half-lives of only a few periods, but may be frequently regenerated. The largest amplitude seiches are usually found in shallow bodies of water of large horizontal extent, probably because the initiating wind set-up can be greater under these conditions. Canadian Tidal Manual
Tsunamis A tsunami is a disturbance of the water surface caused by a displacement of the sea-bed or an underwater landslide, usually triggered by an earthquake or an underwater volcanic eruption. The surface disturbance travels out from the center of origin in much the same pattern as do the ripples from the spot where a pebble lands in a pond. In some directions the waves may almost immediately dissipate their energy against a nearby shore, while in other directions they may be free to travel for thousands of kilometers across the ocean as a train of several tens of long wave crests. Being long waves, they travel at the speed (gD)½, giving them a speed of over 700 km/h (almost 400 knots) when travelling in a depth of 4 000 m. The period between crests may vary from a few minutes to the order of 1 h, so that in a depth of 4 000 m the distance between crests might range from less than a hundred to several hundred kilometers.
The wave heights at sea are only the order of a metre, and over a wavelength of several hundred kilometers this does not constitute a significant distortion of the sea surface. When these waves arrive in shallow water, however, their energy is concentrated by shoaling and possibly tunneling, causing them to steepen and rise to many meters in height. Not only are the tsunami waves high, but they are also massive when they arrive on shore, and are capable of tremendous destruction in populated areas. Because of the relative gentleness of tsunamis in deeper water, ships should always leave harbour and head for deep offshore safety when warned of an approaching tsunami.
The origin of the word is, in fact, from the Japanese expression for «harbour wave.» This name has been adopted to replace the popular expression «tidal wave,» whose use is to be discouraged since there is nothing tidal in the origin of a tsunami. Another expression sometimes used for these waves is «seismic sea wave,» suggesting the seismic, or earthquake, origin of most tsunamis. A tsunami warning system for the Pacific has been established by the United States, with its headquarters in Honolulu, Hawaii. Other countries, including Canada, that border on the Pacific have since been recruited into the system. Canada's direct contribution consists of two automatic water level gauges programmed to recognize unusual water level changes that could indicate the passage of a tsunami, and to transmit this advice to Honolulu. The gauges are at Tofino on the west coast of Vancouver Island, and at Langara Island off the northwest tip of the Queen Charlotte Islands group. The tsunami warning center at Honolulu receives immediate information from seismic recording stations around the Pacific of any earthquake that could possibly generate a tsunami; it calculates the epicenter and intensity of the quake and the arrival time of the as yet hypothetical tsunami at the water level sensing stations in the network; it initiates a «tsunami watch» at all water level stations in the path, for a generous time interval around the ETA of the hypothetical tsunami; and it issues tsunami warnings to the appropriate authorities in threatened locations if the water level interpretation indicates that a tsunami has indeed been generate. Canadian Tidal Manual
Melting and freezing When seawater freezes, it is only the water that forms into ice crystals. The salt becomes trapped between the crystals in a concentrated brine that eventually leaches out, leaving mostly pure ice floating on the surface, surrounded by sea water of increased salinity and density. Since the ice displaces its own weight in this denser water, it does not displace as much volume as it occupied before freezing. Because of this, freezing has an effect similar to that of evaporation - it lowers the water level and increases the surface salinity and density. Surface water must therefore flow toward a region of freezing, while the cold salty water that is formed must sink and flow away from the region. In the polar regions, particularly in the Antarctic, freezing produces cold salty water that sinks and flows along the ocean bottom for thousands of kilometers. When sea ice melts, mostly fresh water is released, and this decreases the salinity and the density of the surrounding water. Melting thus has an effect similar to that of precipitation - it raises the water level and decreases the surface salinity and density. Surface water must therefore flow away from a region of melting ice. The speed of currents associated with freezing and melting in the ocean are never great. Canadian Tidal Manual
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