METEOROLOGICAL EFFECTS

Even the most carefully prepared tidal predictions differ from the sea levels actually observed

because of the effects of the weather. The relative importance of the tidal and non-tidal

movements depends on the time of year, the latitude, and the proximity of extensive shallow

water areas. Typical standard deviations of observed levels from predicted tidal levels vary

from 0.03 metres at tropical ocean islands to 0.25 metres or more in stormy high latitudes,

where the continental shelves are shallow.

Patterns of geological coastline development and sediment deposition often operate to

produce flat low-lying fertile land adjacent to extensive shallow seas. The northern bay of

Bengal is an outstanding example. People tend to settle on this low-lying land. If storm surges

combine with high tides to produce severe flooding the most immediate danger is of

drowning. However, the subsequent dislocation of normal services such as water supplies

and sewerage gives rise to further dangers. Also, once flooded by sea water, previously fertile

lands are unsuitable for growing crops for several years because of the saline deposit which

remains after the floods have receded.

Physically the atmosphere acts on the sea in two distinctly different ways. Changes in

atmospheric pressure produce changes in the pressures acting vertically on the sea surface.

One millibar increase of atmospheric pressure decreases sea level by one centimetre. This is

called the inverted barometer effect. The drag of the wind on the sea surface increases as the

square of the wind speed, to a first approximation. This drag sets the water in motion – in

shallow water the flow is in the direction of the wind but in deeper water the transport is at

right angles (to the right in the northern hemisphere). When the water transport is impeded by

land boundaries there is an increase of water levels, the storm surge.

For scientific analysis and for systems designed to predict imminent surges, it is usual to

distinguish between tropical and extra-tropical surges.

Tropical Surges

These are generated by tropical storms which are small and very intense. These storms are

generated at sea, from where they move in an irregular way until they meet the coast. Here

they produce exceptionally large flood levels within the region of perhaps 10-50 km. Tropical

storms are difficult to monitor off-shore and their effects on a particular piece of coastline

cannot be estimated from the statistics of observed floods because they are so rare. A

combination of numerical and simple statistical models may be used to estimate the maximum

flood levels, but their exact location depends on the track of each individual storm.

Extra-Tropical Surges

These are generated by storms which extend over several hundred kilometres and which are

generally slow moving. They affect large areas of coast over periods which may extend to

several days. At their centre is a region of low atmospheric pressure. The effects of the earth’s

rotation must be taken into account when making numerical predictions of the storm’s

behaviour and the resulting potential for flooding.