Nonlinear effects of wind on Atlantic ocean circulation

The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that transports warm, salty water from the tropics to the northern Atlantic. As the water cools, it becomes denser and sinks, in a process known as overturning. The cold deep water then flows back toward the equator. This process of transportation plays a critical role in Earth's climate.

Although scientists know that surface wind stress can affect AMOC variability from year to year, the impact of wind stress on decadal timescales is less clear. In a new study, Lohmann et al. address this knowledge gap using MPI-ESM (Max Planck Institute for Meteorology Earth System Model). They ran 250-year simulations in which the mean wind stress received by the ocean was modified so it was either half or double that of a reference simulation.

They discovered that under reduced wind stress forcing, the AMOC strength strongly decreases. Reduced heat and salt transport to the north and, subsequently, a larger winter sea ice extent and reduced surface density shut down production of the cold deep water that usually forms in the subpolar North Atlantic and the Nordic Seas.

Under enhanced wind stress forcing, the authors found that effects opposite those of reduced wind stress forcing happen initially and the AMOC strength increases. Over time, however, the AMOC weakens and stabilizes at a strength similar to what was observed in the reference simulation. The researchers attribute this nonlinear effect to a decrease in surface density (after the initial increase) in the North Atlantic and weakening of subpolar deepwater formation.

The results indicate that future intensification or weakening of jet streams in the Northern Hemisphere could affect North Atlantic circulation and climate. Further analyses with other climate models will provide additional support for the new findings, the authors say.