The Contribution of Mesoscale Ocean Dynamics to Atlantic Meridional Heat Transport

The potential importance of eddies in Atlantic meridional heat transport (MHT) has been extensively studied, but definitions of what constitutes the “eddy” heat transport vary considerably. A temporal mean-eddy decomposition may consider time variations in the zonal-mean meridional overturning to be an “eddy” process. Another framework considers eddy heat transport to be a spatial covariance in the mid-ocean velocity and temperature; however, this definition conflates the influence of basin-scale gyres with that of mesoscale eddies.

In the present study, the contribution of mesoscale ocean dynamics to Atlantic MHT is quantified using a spatial decomposition that distinguishes between mesoscale and larger gyre-scale processes, applied to output from eddy-permitting forced and coupled ocean model simulations. While the zonal-mean overturning circulation is the largest contributor to the Atlantic MHT south of ~45°N, mesoscale dynamics are a substantial influence on time-mean MHT in three critical latitude ranges: the tropical instability wave band (1-5°N), the peak in northward Ekman transport (10-18°N), and the North Atlantic Current (40-45°N). In the two tropical bands, nearly all of the upper limb of the overturning circulation is confined to the upper 200 meters, and the very warm northward time-mean flow is counteracted by southward mesoscale MHT. At 40-45°N, there is a minimum in time-mean gyre MHT between the subtropical and subpolar gyres, and northward mesoscale MHT compensates for relatively low gyre MHT. The contribution of mesoscale dynamics to the time variability of Atlantic MHT is relatively small on intraseasonal and seasonal timescales. However, the mesoscale MHT exerts a leading-order influence on the interannual variability of MHT north of the Gulf Stream separation, highlighting the importance of realistic eddy-permitting simulations and eddy parameterizations for accurate representations of climate variability in the Atlantic basin.