Does the Mediterranean warm the North Atlantic? | Polarjournal
Arctic sea ice will continue to retreat as warming increases. Heat transport through the newly described oceanic heat channel could even accelerate this process. Photo: Julia Hager

The causes for the rapid warming of the North Atlantic and the Arctic Oceans seem obvious – the ocean absorbs heat energy from the atmosphere, which has become warmer especially in the high latitudes. At first, one would not think of an influence of the Mediterranean Sea as an additional contributor. A research team led by Dr. Stefanie Kaboth-Bahr from the University of Potsdam has now discovered that Mediterranean water flowing into the Atlantic could add to the warming in the North Atlantic.

In the recent study, which has now been published in the journal Nature Communications, the scientists looked back to the Pliocene, 3.1 to 2.7 million years ago, by analyzing cores from the Mediterranean Sea, the Gulf of Cadiz and the North Atlantic Ocean. They used these to investigate a possible link between climate events in the Mediterranean and temperature fluctuations in the far north under likely future climate conditions. During the Pliocene, environmental conditions were similar to those we can expect in the near future, i.e., with increased greenhouse gas concentrations and a smaller Greenland ice sheet.

The sediment cores were taken from aboard the drill ship R/V Joides Resolution. Photo: André Bahr, University of Heidelberg

“In fact, we see that especially between 2.9 to 2.7 million years ago, a strong Mediterranean outflow occurred simultaneously to surprisingly warm conditions on the surface of the high-latitude North Atlantic close to Iceland,” says paleoclimatologist Dr. Kaboth-Bahr from the Institute of Geosciences at the University of Potsdam. “We then fed our findings into a state-of-the art climate model. This enabled us to virtually step into the ocean and for the first time visualize that the plume of warm waters leaving the Mediterranean Sea via the Strait of Gibraltar in several hundred meters water depth eventually surfaces in the high latitudes of the North Atlantic, creating an oceanic ‘heat-channel’.”

Mediterranean water has been flowing through the Strait of Gibraltar into the Atlantic Ocean for about 5.3 million years. The strength of the outflow is determined by the African monsoon, which has an influence on ocean conditions in the Levantine Basin in the eastern Mediterranean. Here, warm and dry summers lead to the evaporation of large amounts of seawater, which increases the salinity at the sea surface. If this water cools during the winter months, its density further increases and it sinks. This triggers a circulation and the water moves at depth toward the west, where it flows through the Strait of Gibraltar into the Atlantic Ocean. However, if the monsoon rains are intense, a lot of fresh water is carried into the Levantine Basin via rivers such as the Nile, which leads to a decrease in the density of the surface water and prevents it from sinking – the circulation towards the Atlantic Ocean comes to a standstill.

Temperature distribution in the Mediterranean and North Atlantic Ocean at 800 meters water depth. The arrows mark the transport of Levantine Intermediate Water (LIW), Western Mediterranean Deep Water (WMDW) and Mediterranean Outflow Water (MOW). The numbers in black mark the sediment core sampling locations. Graphic: Kaboth-Bahr et al. 2021

For the eastern Mediterranean, climate models predict a sharp increase in aridity by 2100, which would increase salinity at the sea surface and boost the outflow of warm water into the Atlantic. Via the heat tunnel described by Dr. Kaboth-Bahr, this water would enter the North Atlantic Ocean, where it would contribute to the warming of surface waters.

The authors conclude that the heat tunnel may have delayed the growth of the Northern Hemisphere ice sheet with the onset of cooling in the late Pliocene 3.2 million years ago. Analogously, the mechanism could also contribute to the warming of the North Atlantic Ocean today. They believe this interaction between high-latitude and low-latitude climate variability has been underestimated and could accelerate global warming.

Julia Hager, PolarJournal

Link to the study: Kaboth-Bahr, S., Bahr, A., Stepanek, C. et al. Mediterranean heat injection to the North Atlantic delayed the intensification of Northern Hemisphere glaciations. Commun Earth Environ 2, 158 (2021).

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