The Atlantic Ocean Circulation plays an important role in regulating Earth’s climate by redistributing heat through the global ocean. This large-scale ocean circulation consists of a northward transport
of relatively warm surface waters from the tropics to the North Atlantic Ocean. These relatively warm surface waters eventually reach the subpolar regions around Greenland and Iceland where they are
cooled by the atmosphere. As cold water is heavier
than warm water, the surface waters sink to depths of 2000 to 3000 meters before returning southward as a cold deep undercurrent. This Atlantic Meridional Overturning Circulation (AMOC) is crucial for
Europe’s climate, but it also plays an essential role in the ocean’s ability to absorb CO2 and supply oxygen as well as in regulating rainfall patterns in the tropics.
Since 2004, the AMOC strength is closely monitored along the 26N parallel. This
observational record is unfortunately too short to detect any long-term trend. However, climate reconstructions indicate that the circulation’s strength has fallen by 15% since 1950. A related sign of the
circulation weakening is the so-called “cold blob” over the North Atlantic Ocean. It is the sole region on Earth that experienced a cooling trend rather than a warming trend since the start of the last
century. A sign consistent with a declining transport by the AMOC.
The AMOC has been classified as a potential tipping element in the present-day climate. A tipping element is a system that can
(rapidly) shift from one state to another state as the result of a small change in an external forcing. In 1961, it was first realised that the AMOC may have
two stable states and that transitions between these states are possible i.e. the circulation can tip. Using a highly idealised model that represented the Atlantic Ocean circulation,
the feedback loop that causes the circulation to collapse was identified. A freshwater anomaly near the sinking regions will inhibit the sinking as freshwater is lighter than saline
water. This in turn reduces the northward transport of salinity to these regions, which freshens the regions even more. This in turn reduces the circulation strength again and so forth.
When this feedback is strong enough, the circulation will make a transition to a different stable state: the collapsed state.
The overall aim of the TAOC project is to determine reliable estimates of the probability that the Atlantic Meridional Overturning Circulation will undergo a collapse before the year 2100. To reach
this aim, we work along four objectives O1-O4, connected to the Work Packages WP1-WP4 in the project: (O1) to (further) develop novel computational methodology to determine transition
probabilities in high-dimensional multi-stable systems, (O2) to apply this new methodology to a hierarchy of ocean-climate models to determine AMOC transition probabilities and transition
paths, (O3) to simulate an AMOC collapse in one of the state-of-the-art climate models under at least one scenario of climate change, and (O4) to determine precursors of AMOC
transitions and develop a skillful prediction scheme for future AMOC behavior.
