Nonlinear effects of wind on the circulation of the Atlantic Ocean

Surface wind stress can lead to variability in the Atlantic overturning meridional circulation on decadal time scales. Credit: Jenna Bash/Unsplash

The Atlantic Meridian Overturning Circulation (AMOC) is a system of ocean currents that transports warm, salty water from the tropics to the North Atlantic. As water cools, it becomes denser and sinks, in a process known as overturning. Cold deep water then flows back towards the equator. This transport process plays a vital role in the 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. fill this knowledge gap by using MPI-ESM (Max Planck Institute for Meteorology Earth System Model). They performed 250-year simulations in which the average wind pressure received by the ocean was changed so that it was either half or twice that of a reference simulation.

They found that under reduced wind forcing, the AMOC force decreases sharply. Reduced northward heat and salt transport and subsequent greater sea ice extent in winter and reduced surface density have halted production of the cold deep waters that typically form in the Atlantic Subpolar North and Nordic Seas.

Under increased wind stress forcing, the authors found that effects opposite to those of reduced wind stress forcing initially occur and the strength of the AMOC increases. Over time, however, the AMOC weakens and stabilizes at a strength similar to that seen in the baseline simulation. The researchers attribute this non-linear effect to a decrease in surface density (after the initial increase) in the North Atlantic and a weakening of subpolar deep water formation.

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

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More information:
K. Lohmann et al, Response of the North Atlantic and Atlantic Southerly Overturning Circulation to Reducing and Increasing Wind Forcing, Journal of Geophysical Research: Oceans (2021). DOI: 10.1029/2021JC017902

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