Climate change reverses Atlantic Ocean circulation

The Atlantic Ocean circulation (termed meridional overturning circulation, MOC) is an important component of the climate system. Warm currents, such as the Gulf Stream, transport energy from the tropics to the subpolar North Atlantic and influence regional weather and climate patterns. Once they arrive in the North the currents cool, their waters sink and with them they transfer carbon from the atmosphere to the abyss. These processes are important for climate but the way the Atlantic MOC responds to climate change is not well known yet. This study investigates the distribution of isotopes ( 231 Pa, 230 Th) in the Atlantic Ocean that are generated from the natural decay of uranium in seawater and are distributed with the flow of deep waters across the Atlantic basin. The results show that the ocean circulation was very different in the past and that there was a period when the flow of deep waters in the Atlantic was reversed. This happened when the climate of the North Atlantic region was substantially colder and deep convection was weakened. At that time the balance of seawater density between the North and South Atlantic was shifted in such a way that deep water convection was stronger in the South Polar Ocean. Recent computer models simulate a reversal of the deep Atlantic circulation under such conditions while with the new data generated some of the details of the circulation reversal become apparent. This situation occurred during the ice age, 20.000 years ago. Although this was far back in time the results are relevant for our climate today and in the near future. The new study shows that the Atlantic MOC in the past was very sensitive to changes of the seawater salt balance. Similar changes in seawater salt concentration are expected to occur in the North Atlantic in the course of climate warming over the next 100 years.  Therefore the data published in this study offer the climate modelling community with the opportunity to calibrate their models and improve their capacity to reliably predict future ocean and climate changes.