Cold filamentary intensification and oceanic surface convergence lines

The reflectance images of the oceanic surface are full of surfactant lines created by surface velocity convergences, with typical line spacings of 0.1–10 km, i.e., in the submesoscale range. Here we propose a dynamical explanation by the process of filamentary intensification that shrinks the transverse scale of rectilinear surface temperature extrema at a super‐exponential rate by the action of horizontal deformation flows associated with mesoscale eddies. This process is analogous to deformation‐induced frontogenesis with sharpening temperature gradients accelerated by its ageostrophic secondary circulation. In the particular case of cold filaments, the shrinking rate, surface convergence, and downwelling are especially strong because of the configuration of ageostrophic acceleration. This behavior is demonstrated theoretically with idealized two‐dimensional solutions and illustrated in a realistic regional circulation simulation near Peru.