Decreasing overturning current: The overturning current in the Atlantic is apparently not only weakening at its northern end – an important tributary current off Florida has also weakened in the last century, as a study suggests. According to this, the volume of water flowing north in the Florida Current has decreased by 1.7 million cubic meters per second. This supports evidence of a general weakening of the North Atlantic circulation.
The North Atlantic Recirculating Current (AMOC) is a motor of global ocean currents and an important factor influencing the climate. Because it drives the Gulf Stream and North Atlantic Current, which bring warm water from the Caribbean to Europe. Off Greenland, the warm water sinks into the depths and flows back south as cold deep water.
But climate change is also affecting this circulation, which is so important for our European climate. Due to the increased influx of melt water and the decline in sea ice, the AMOC has already weakened by around 15 percent, as measurements show.
Headwaters of the Gulf Stream in view
It is now evident that one of the most important tributaries of the Gulf Stream could also be affected by this weakening – the Florida Stream. This current, which begins off the south coast of Florida, is one of the most important feeders of warm water for the Gulf Stream together with the smaller Antilles Current. Whether and to what extent the volume of water transported by this current has changed in the last hundred years has so far been unclear.
For his study, Christopher Piecuch of the Woods Hole Oceanographic Institution in Massachusetts reconstructed the volume of water in the Florida Stream since 1909. To do this, he evaluated water levels that have been measured regularly on the Florida coast and in the Bahamas since then. Using physical models, it is possible to indirectly determine how much water is flowing north with the Florida Current between the two measuring points.
Water transport has declined since 1909
The result: Since 1909, the transport volume of the Florida stream has decreased almost steadily. At the beginning of the 20th century, the average amount was more than 33 million cubic meters per second. From 1982 to the present day, however, the volume of water transported has averaged 31.8. “The 100-year trend shows a reduction of 1.7 million cubic meters per second,” reports Piecuch.
The greatest weakening of the Florida Current has occurred in the past two decades. During this time, the ten-year values for water transport were the lowest in the entire past 110 years, as the researcher determined. “The timing of these extremes cannot be explained by natural fluctuations alone,” says Piecuch. This trend towards a steadily stronger weakening is clearly recognizable despite natural fluctuations.
Result confirms weakening of the AMOC
Piecuch believes that his results support the assumption that the North Atlantic environmental current is weakening. Because if less cold water flows in the depths from the far north to near the equator, according to the models, this also slows the warm countercurrent on the sea surface – and this is exactly what can now be observed in the Florida Current.
The informative value of these results is still limited by the fact that they only go back a good 100 years. The scientist therefore hopes that future studies will extend this time series even further into the past – that would significantly reduce the statistical uncertainties. (Nature Communications, 2020; doi: 10.1038/s41467-020-17761-w)
Quelle: Woods Hole Oceanographic Institution
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