Geographical distribution and spatio-temporal scale dependence of air-sea coupling via the vertical mixing mechanism

Reanalysis and current generation global coupled climate models have shown a dominant role of atmospheric forcing for the ocean such as stronger winds that increase turbulent heat flux and consequently cools sea surface temperatures (SSTs) . While satellite observations and eddy-resolving global coupled climate models have additionally shown clear imprints of mesoscale ocean forcing on the atmosphere, such as warm SST anomalies that can destabilise the overlying atmosphere and enhance surface winds by transferring momentum from aloft to the surface (vertical mixing mechanism; VMM). With winds blowing along downwind or crosswind SST gradients, this can subsequently induce wind stress divergence or curl respectively, particularly over mesoscale ocean features. The dominance of forcing from either atmosphere or ocean indicate a spatial scale dependency for the coupling variability of air-sea interactions.

 

Using a global coupled 5-km ICON-ESM simulation, we derive a global distribution map of the air-sea coupling associated with the VMM, and investigate the spatial and temporal scale dependency of the vertical mixing mechanism. Choosing various regions over the ocean, we evaluate the frequency-wavenumber cross-spectra between downwind SST gradients and windstress divergence in order to identify the dominant temporal and spatial scales between them. For example, we found that over the tropical Pacific, such interactions are prevalent on spatial scales of about 300-3000km and longer than 10-days timescale, while over the Gulf stream region, they are dominant at scales of roughly 100-1000km and longer than 5-days timescale. This is the first time that the dominant spatial and temporal scales for the vertical mixing mechanism in various regions of the world's ocean is quantified.