from arid to wet and causes the huge fish stocks, normally associated with the nutrient-rich cold water, to migrate away. The phenomenon is normally accompanied by a change in atmospheric circulation, called the Southern Oscillation. It is associated with changes in precipitation in regions of North America, Africa, and the western Pacific, droughts and bush fires in Australia and droughts in southeastern Asia, India and southern Africa. It is one of the main causes of change in the world’s climate, and the 1997/8 event was the worst on record. The sea-surface temperatures in most tropical seas were particularly high, resulting in large-scale coral bleaching, particularly in Bahrain and the Maldives. Nearly every region on earth felt El Niño’s effect in some way.
BLEACHING
Bleaching occurs when corals, anemones, clams and some other animals like sponges, expel their symbiotic algae (zooxanthellae) or the pigments of those algae. This is thought to occur due to higher temperatures and excess ultraviolet light penetration due to failed monsoons, very calm seas or lack of cloud cover. Some bleaching may be a seasonal event in the Indo-Pacific and the Caribbean, when full recovery is normal. If the water temperatures quickly return to normal then the animals recover, otherwise they die. Bleaching is most pronounced in water less than 15m (50ft) deep and particularly affects fast-growing species such as Acropora. Slower growing massive species like Porites also bleach, but are more likely to recover in a couple of months.
Bleaching was particularly far-reaching during the 1997/8 El Niño-Southern Oscillation Phenomenon, with areas such as Bahrain, the Maldives, Sri Lanka, Singapore, and parts of Madagascar and Tanzania being seriously affected. Scientists recently found that after having expelled one type of Zooxanthellae, some corals can take up other types that are better suited to the higher temperatures, thus enabling them to survive as long as temperatures do not get too high.
El Niño is called a warm event. La Niña, which means The Little Girl is called a cold event. (The phenomenon is also known as Viejo, the Spanish word for old.) The opposite of El Niño, with unusually cold surface temperatures in the Eastern Tropical Pacific, it usually, but not always, follows an El Niño and did so in 1998. The effects on global climate are the opposite to those of El Niño.
WEATHER, CURRENTS AND TIDES
In regions where there is a distinct summer and winter, many divers would normally avoid diving at offshore sites in winter. Some areas have distinct seasons of travelling storms of great violence that form over warm oceans when several thunderstorms release heat. These tropical cyclones are known as hurricanes in the North Atlantic and eastern North Pacific, and as typhoons in the western North Pacific. The winds of these systems revolve around a centre of low pressure, ‘the eye,’ in an anticlockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere.
Tropical cyclones are a phenomenon of the tropical oceans. They originate in two distinct latitude zones, between 4° and 22° South and between 4° and 35° North. They are absent in the equatorial zone between 4° South and 4° North. Most tropical cyclones are spawned on the poleward side of the area known as the intertropical convergence zone (ITCZ).
Monsoon winds are primarily caused by the difference between temperatures over large landmasses and adjacent large oceans, notably Arabia, Asia, Australia, and the Indian subcontinent. Seasonal changes in temperature are large over land, but small over oceans. A monsoon wind blows from cold to warm regions and, in summer, from the sea towards land carrying humid air from the ocean. In winter it blows from the land toward the sea. As a consequence, where monsoons occur, one side of a landmass may get heavy rain and not be divable at one time of year and the opposite side of that landmass at another.
Some regions are known for consistently bad tropical cyclones or monsoons at certain times of year and the resorts shut down for that period. Regions where these events only occasionally cause problems, tend to stay open during the bad weather season, while offering cheaper rates. Divers who book resorts in these regions at this cheaper time of year should be aware that their holiday could be ruined.
Although not necessarily of tropical cyclone strength, bad weather can occur anywhere at any time of year. However, diving can be quite pleasant during inclement weather, if divers jump into the water and quickly descend below the swell. The real problems are in getting the boat out to the site and, worse, getting out of the water into a boat in a heavy swell.
MAJOR CURRENTS
Although local currents vary during the day due to winds, upwellings, downwellings and the heat of the sun, there are more consistent current patterns in the world’s oceans that affect the climate, conditions for diving and which migratory species can be found at a given time in a normal year.
Ocean currents are horizontal and vertical circulation systems of ocean waters that are produced by the earth’s rotation, gravity, wind friction, and the variations in water density that result from differences in temperature and salinity.
For instance, the currents that form the Gulf Stream bring warm waters northward, affecting the climates and waters of the Bahamas, Bermuda, eastern North America, the British Isles and the Atlantic coast of Norway. This leads to tropical species off Bermuda and some surprising species such as Ocean Sunfish and Leatherback Turtles visiting the west coast of the UK. Similarly, part of the South Equatorial Current that flows towards East Africa joins the Agulhas Current and relatively warm water flows southward at high speed along the east coast of South Africa. However, when this current is reversed, cold water flows north, bringing with it huge quantities of sardines, which in turn attract large predators.
The Lionfish (Pterois miles) is the Indian Ocean relative of the Pacific’s Pterois volitans. They are often treated as a single species.
Tube worms burrow into sediment or live coral. They retract instantly into their tubes if a shadow passes over them.
The Red Sea Bannerfish (Heniochus intermedius) is endemic to the Red Sea. Juveniles form shoals, but adults are solitary or in pairs.
TIDES
Tides are primarily caused by the combined effects of the centrifugal force of the spinning earth and gravitational attraction between the moon and the earth. The sun, despite its huge size, is so far away that its effect on the tides is only about half that of the moon. The cycle of one tide, to go from high water to low water and back to high water again, usually takes roughly 12 hours (semidiurnal). In some parts of the world it may take roughly 24 hours (diurnal), depending on whether the sun or the moon is dominant. Some areas experience a mixture of both diurnal and semidiurnal tides. The normal tidal day is 24 hours and 50 minutes. Around some islands and reefs you may, effectively, get four tides per day if the flow along one side of the obstacle is longer than along the other.
Spring tides, those of maximum range
