TIL: Weird Double Whirlpool Systems Sail Across The Australian Seas

Do you remember this week's phenomenon post on whirlpools? Like we said on Monday, whirlpools are spinning water vortexes that appear when two opposite moving sea currents (tidal flows) meet in an area with the "perfect" morphology (narrow passages made by straits or rocks on the seabed). Although whirlpools and eddies are not a rare phenomenon, spotting two of them joined in a U-shape form, spinning at the opposite direction one another at the same time is something rare and unique! [2, 3, 6, 7]

Image from: wikipedia.org - Author: Walter Baxter - License: CC BY-SA 2.0

Across the oceans whirlpools and eddies often show up, mixing up the waters, transferring nutrients and warmer water masses towards the poles and almost always travelling westwards. These spinning vortexes were theoretically believed to appear in pairs ever since 1970, but it was not until recently (late 2017) that scientists from the University of Liverpool did spot not one, but nine such pairs, called modons, by analyzing satellite data. The paper (Geophysical Research Letters) was released on December 4th, 2017. The paper was "inspired" by a pair of modons observed in the Tasman Sea (between Australia and New Zealand). [2, 3, 4, 6, 7]

The irony in this is that satellites have been recording their movement for at least 25 years now, but nobody ever noticed them until now. Looking back in satellite images from 1993, scientists saw eight modons around Australia and one in the Atlantic, at the southeast of Africa. [3]

What are modons?

Typical eddies move usually westwards at speeds of 1-2 cm/sec (not as fast as you might have imagined). Modons have the ability to travel almost 10 times faster than a typical eddy and cover distances of even more than 1,000 km. They can last for about 6 months before splitting up and gradually die off. [2, 3]

Eddies "travelling" along coastal areas may meet up with friction because of the coastline. This leads to another mass of water starting to spin in the opposite direction. The bottoms of the two whirlpools will then join in U-resembling shape and the couple will live together for a few months, enjoying a cruise over the planet's oceans. Another idea for a modon formation is that two whirlpools meet in the ocean, collide and become entwined. But both ideas need further studying before deciding which one is true. [8]

They are thought to be the ocean equivalent to smoke rings. Oceanographer Chris Hughes (one of the leading scientists of the study) referred to them as "half a smoke ring". He also thinks that a modon's high speed might be the result of the balance between the two eddies, which could possibly explain why the modons move eastwards instead of westwards. [5]

Smoke ring?

A smoke ring is a circular vortex of smoke (read more on wikipedia). Modons are an aquatic equivalent of a smoke ring. They are actually called half smoke rings because the eddy system forms half a circle, as the other half is cut off by the surface of the water. [9]

Image from: wikipedia.org - Author: Traitor - License: CC BY-SA 3.0

Importance

Eddies stir up the ocean waters, transferring nutrients and changing the temperatures of the oceans, as they bring warmer water towards the poles and then colder waters towards near the equator latitudes. They are even thought to "transfer" marine life from one area to another. [2, 4, 7]

As beneficial as they might be, eddies "feed" deeper waters, since they bring valuable nutrients for the phytoplankton; they can cause damage as well. Oceanographer Glen Flierl (University of Massachusetts) underlines indications that eddies crashing on the continental shelf might "srap off" zooplankton and fish larvae, which leads to a marine life population decline. [7]