A unique feature of the Arctic Ocean is its thick, lower-salinity layer at the sea surface. Researchers often refer to this somewhat misleadingly as a freshwater layer. It has always been replenished by numerous rivers and the influx of low-salinity surface water from the Pacific Ocean. But in recent decades researchers have observed an increase in the proportion of freshwater in the Arctic Ocean, while the water in the North Atlantic is becoming more saline. One reason for the freshening of the Arctic Ocean water could be the fact that it snows more now in Siberia in the winter and rains more through the rest of the year. The rivers therefore transport more meltwater and rainwater into the Arctic Ocean.
It is still uncertain what consequences the increasing amounts of freshwater will bring. However, scientists believe that this change could influence the overturning of water masses in the North Atlantic, which is a crucial factor for the climate, and could thus also impact the strength of the Gulf Stream. Like all of the Arctic Ocean water masses, the low-salinity surface water is transported southward through the Davis Strait, the Fram Strait, or the Norwegian Sea into the North Atlantic, and dilutes the water masses there. Under certain conditions, therefore, sufficiently large amounts of freshwater could cause the North Atlantic water, despite its low temperature, to be no longer heavy enough to sink to the depths necessary to flow back toward the equator as deep water. The engine in the North Atlantic that drives global ocean circulation would then run more slowly, and important currents like the Gulf Stream would be weakened.
Examples from climate history illustrate the possibility of such a chain reaction occurring. When the prehistoric Lake Agassiz in North America was abruptly emptied 8200 years ago, releasing an immense volume of freshwater through the St. Lawrence Estuary into the North Atlantic, the overturning circulation of Atlantic water masses slowed down. As a result, the warm Atlantic current came to a standstill, or at least weakened, causing the North Atlantic region to cool considerably within a few years.
And researchers now know that an interruption in the overturning of North Atlantic water also has a global impact. In the past, for example, a shift in the position of the rain belt over the tropics was related to warming in the Southern Ocean and in Antarctica.
Climate models predict that the Gulf Stream will weaken in the future as a result of increased emissions of greenhouse gases in the atmosphere, and that this will lead to cooling in the North Atlantic. Climate researchers have found that this is already happening. The subpolar part of the Atlantic Ocean is the only marine region in the world that has not warmed since the beginning of the 20th century, but has cooled down. The temperature changes suggest that the Gulf Stream has weakened by 15 per cent.
fig. 3.8 > Scientists cite falling surface temperatures in the marine region southeast of Greenland as evidence for a weakening of deep-water formation in the North Atlantic. The logic behind this is: Because, as a result of global warming, less water is being overturned in the North Atlantic, the North Atlantic Current, which transports heat from the US east coast to northern Europe, has weakened. And when less heat flows in, the sea cools down. © after NASA