The Lofoten Maelstrom, or Moskstraumen as it is called by local Norwegians, is located at 67 deg. 48 min. N, 12 deg. 50 min. E between The Lofoten Point (Lofotodden) and the island Værøy - south west of the main chain of The Lofoten Islands. The Norwegian version of its name comes from the small island Mosken in the middle of it. The more general term, Maelstrom, appears to be of Dutch origin from malen, to grind or whirl, and strom or stroorn, a stream or current.

So, what is it then? Depicted by many authors such as Jules Verne and (most notably) Edgar Allan Poe in Decent Into The Maelstrom - as an immense and terrifying whirlpool capable of swallowing and destroying entire ships and then spitting out mere pieces of driftwood, these fictional accounts are by all means exaggerated. Although even before that, fantastical descriptions appeared in popular geographic journals and literature in the 17th and 18th centuries. More recent observations and studies have provided much more factual information but it remains one of the most powerful oceanic whirlpools is the world, and perhaps no less amazing for being real at all.

The Merriam-Webster's Collegiate Dictionary states that a whirlpool is "water moving rapidly in a circle so as to produce a depression in the centre into which floating objects may be drawn". Think about pulling the plug out of your bath – now think much much bigger!

Now for the technical bit. In general terms a whirlpool is basically a vortex of vertical axis, with a downward velocity component near its centre, a two-dimensional vortical structure occurring in a turbulent developing shear layer. Such a flow may be approximated by a series of vortices of identical rotational direction advected downstream. (In first approximation the advection velocity is equal to half the main stream velocity.) Each vortex is affected by the movement of the fluid due to the other vortices. Considering a pair of vortices, they will turn around each around while being advected downstream. (Far away the vortex pair is seen as a unique vortex of strength equal to twice the strength of one vortex.) In real turbulent flows, the vortices may collapse and form a new larger vortex of stronger strength and identical rotational direction, this is called vortex pairing. Vortex pairing may be repeated downstream and can produce very-large scale eddy (whirlpool) structures. The process is sometimes called an inverted cascade of vortices.

In coastal zones, whirlpools are produced by the interaction of rising and falling tides. They are often observed at the edges of straits with large tidal currents. The vortex, here manifesting as a whirlpool, is a coherent structure typical of shear flows where there is a velocity difference across the shear layer. It affect the surrounding flow and water can be seen going back and forth across the shear layer between vortices.

In the specific case of the Maelstrom, the strait between the islands is about 5 to 8 kilometres across and 40 to 60 meters deep which is very noticeably shallower than the surrounding sea. The tide fills up the Vestfjord twice a day, and the difference in height between high and low tides can be up to 4 meters.

Tide is defined as the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. Tidal current thus refers to the accompanying horizontal movement of the ocean water. The Maelstrom is a reversing tidal current – that which moves into and out of restricted passages – in this case, the strait between the islands. This current usually travels at speeds of about 7 knots (3.6 meters per second), however a contributing factor to the notoriety of this current in particular is the very strong local winds which can push the speed up to about 9 knots and make the water even more treacherous.

Midway between the high and low tide, as the current changes direction, whirlpools begin to form. The process begins when the water encounters resistance with a landmass or water with different properties. As the current meets this resistance, smaller currents break off from the main flow and become independent rotating structures (as described above). These whirlpools reach sizes of up to about 10 meters in diameter and 4 to 5 meters in depth, with speeds of up to 6 nautical miles per hour.

While these features may be quite a sight to see and provide an interesting and dynamic environment for scientist and researchers to conduct experiments and observations on, it is clear that it is by no means the terrible monster depicted in lore. While the strong currents, winds and the whirlpools themselves can pose a danger to smaller ships the strait is by no means shunned. The current that fills the strait at high tide is very cold and dense, dragged up from lower levels of the sea and thus it is very nutrient rich - which brings with it something further – fish. Since people have lived on the islands that the current passes by they have also fished the water, to great reward. This can be accomplished if the boats set out after the main occurrence of tide change has passed and return before the next one arrives. Although by no means a major tourist attraction, there are, for the more adventurous, companies that offer small tour boats that will take visitors out into the strait.

One of the first documented appearances of it is on Mercator’s Atlas of 1595.

Other major oceanic whirlpools include those of Garofalo along the coast of Calabria in southern Italy, and of Messina in the strait between Sicily and peninsular Italy, the whirlpools near the Hebrides and Orkney islands, and in the Naruto strait between Awaji and Shikoku islands.

For those that wish to find out more (very detailed) details, two interesting studies can be read, one in Nature, Vol.388 (28 Aug 1997) entitled “Sources of the Maelstrom” and another journal article "A high resolution tidal model for the area around the Lofoten Islands, northern Norway" by H. Moe, A. Ommundsen and B. Gjevik that was published in Continental Shelf Research 2002, Vol. 22.

Resources:
http://www.math.uio.no/maelstrom/ (has pictures of the islands)
http://www.uq.edu.au/~e2hchans/whirlpl.html (University of Queensland)

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