![]() When this bend brings the currents into more perpendicular contact with the shore, it can amplify the surge, and when it bends the current away from the shore it has the effect of lessening the surge. The Earth's rotation causes the Coriolis effect, which bends currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. For example, a major storm with a 100 millibar pressure drop would be expected to have a 1.0 m (3.3 ft) water level rise from the pressure effect. This effect is estimated at a 10 mm (0.39 in) increase in sea level for every millibar (hPa) drop in atmospheric pressure. The rising water level will counteract the low atmospheric pressure such that the total pressure at some plane beneath the water surface remains constant. The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low atmospheric pressure and fall in regions of high atmospheric pressure. The surge will be driven into bays in the same way as the astronomical tide. Because the Ekman Spiral effects spread vertically through the water, the effect is proportional to depth. Strong surface winds cause surface currents at a 45° angle to the wind direction, by an effect known as the Ekman spiral. Intuitively, this is caused by the storm blowing the water toward one side of the basin in the direction of its winds. Wind stresses cause a phenomenon referred to as wind setup, which is the tendency for water levels to increase at the downwind shore and to decrease at the upwind shore. Mechanics Īt least five processes can be involved in altering tide levels during storms. Communities and governments can adapt by building hard infrastructure, like surge barriers, soft infrastructure, like coastal dunes or mangroves, improving coastal construction practices and building social strategies such as early warning, education and evacuation plans. Some theorize that as extreme weather becomes more intense and sea level rises due to climate change, storm surge is expected to cause more risk to coastal populations. There is a suggestion that climate change may be increasing the hazard of storm surges. Other factors affecting storm surge severity include the shallowness and orientation of the water body in the storm path, the timing of tides, and the atmospheric pressure drop due to the storm. The main meteorological factor contributing to a storm surge is high-speed wind pushing water towards the coast over a long fetch. ![]() It is measured as the rise in water level above the normal tidal level, and does not include waves. A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones.
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