Future Stokes drift climate under worst-case climate scenario

Figure 1. Panel a: Average Stokes depth, historical period. Panel b: Relative change to Stokes depth under RCP8.5.

In a study recently published on the AGU Journal of Climate, Breyvik and colleagues investigate the future Stokes drift climate using a global wave climate projection (2071–2100) forced with EC-Earth winds under the RCP8.5 scenario. The future climate run is compared against a historical run (1976– 2005) and analysed in terms of Stokes transport and surface Stokes drift to identify the impact on Stokes drift of changes to the wind, wind sea and swell climate. The authors also studied consequences for upper-ocean mixing and circulation by investigating the introducing turbulent Langmuir number and the Stokes depth. As the surface Stokes drift is largely dictated by high-frequency waves, it is to a great degree controlled by changes to the local wind field, whereas the Stokes transport is more sensitive to swell.

The study found that both the surface Stokes drift and transport are expected to increase in the Southern Ocean by about 15%, and decrease in the North Atlantic by about 10%. The Stokes depth and the turbulent Langmuir number are set to change by about 20% and 10%, respectively (Figure 1). The changes to the Stokes depth suggest a deeper impact of the Coriolis-Stokes force in the Southern Ocean and a decrease in the northern extratropics. Changes to the KPP Langmuir enhancement factor suggests potentially increased mixing in the Southern Ocean and a reduction in the North Atlantic and the North Pacific.

Also important is the mixed layer depth. For example, in wintertime the deep mixed layer deepens and the additional mixing induced by Langmuir turbulence is unlikely to make much difference. This means that although the mixing can be expected to decrease in the northern extratropics and to increase in the Southern Ocean, as seen in Figure 2, the actual magnitude of these changes must be further investigated by integrating fully coupled models with an active wave model component under different emission scenarios. However, the differences found for the enhancement parameterization, are large enough to expect increased mixing in the Southern Ocean and reduced mixing in the northern extra-tropics towards the end of the 21st century under the RCP8.5 scenario. Similarly, the projected changes of 10% to the Stokes depth suggest that the changing wave climate may also affect the circulation directly through changes to the near-surface Coriolis-Stokes forcing.

The open access article can be found on: https://doi.org/10.1175/JCLI-D-18-0435.1

Figure 2. Panel a: Average of the FLT Langmuir enhancement factor (for the historical period (1976–2005). Panel b: Relative change [%] to FLT under RCP8.5 for the future period (2071–2100) versus the historical period. Only regions statistically different with significance level 0.01 are colored in panel b.