The Antarctic ice sheet undergoes significant changes throughout the year, and its behavior is generally well studied. But some local anomalies have remained unexplained for decades. One of these is the Maud Rise Trough or Weddell Sea Trough, which is seen sporadically in the same place. An international team of oceanologists has finally managed to unravel the formation mechanisms.

Maud’s rising funnel was first observed in the 1970s when the American weather satellite Nimbus 5 began continuously monitoring Antarctic ice. Wormwood appeared every winter from 1974 to 1976, leading scientists to assume it occurred annually. But since then, the open water area between the solid perennial ice that covers most of the Weddell Sea has formed sporadically. Wormwood either did not appear at all or its area was insignificant.

The next full-fledged formation of the funnel took place in 2016 and 2017. Although less than half a century smaller, the area is still impressive: about 80 thousand square kilometers – roughly equal to the area of ​​the Kuril Islands or Sakhalin combined with two Swiss. As before, in the following years the maggots almost completely disappeared. This behavior of a natural phenomenon significantly complicates its study. Wormwoods rarely form in perennial ice, and even more rarely can they be this large and exist for long periods of time (weeks, even months).

New scientific studies were published in the journal Science DevelopmentsThe one under review sheds light on the nature of the funnel of the Mode’s rise. Its authors are employees of the universities of Gothenburg (Sweden), California (San Diego, USA) and Southampton (Great Britain). They used several data sources for their analysis: satellite optical and radar images, sensors on autonomous rafts, and tags on marine animals. As it turned out, it all boils down to a number of factors that successfully come together in one place in the World Ocean.

The primary process responsible for the formation of the giant trough in the Weddell Sea is Ekman convection. This is the effect of the movement of water masses at a certain angle to the wind direction. This is caused by the Coriolis force, which causes moving objects to bend to the left in the Southern Hemisphere (and to the right in the Northern Hemisphere). Moreover, as the depth increases, the angle only increases, creating a spiral vortex. Contrary to what intuition suggests, the ice cap isn’t blocking Ekman’s transfer. In fact, it was first discovered thanks to icebergs and ice fields during Nansen’s arctic expedition.

In the Weddell Sea, Ekman transport is what draws relatively warm brine from beneath the long-term ice. In the Maud Rise area, which is an underwater plateau with small, gently sloping peaks on it, the depth current is therefore getting stronger.

Finally, the third factor, without which the giant wormwood tree is impossible: a change in the prevailing direction and strength of the winds blowing over this area. A combination of these factors likely played out in 2016 and 2017 in a manner similar to what happened in the 1970s.