PDFFirst Paragraph
Upper-ocean fronts play an important role in determining upper pycnocline water properties by providing an efficient conduit for communicating atmospheric forcing into the stratified interior. Mesoscale fronts exhibit strong lateral contrasts at 1–10-km scales, with large frontal density gradients supporting intense, largely geostrophic, surface-trapped jets. Instabilities associated with these strong flows can radiate near-inertial internal waves and produce eddies, making frontal-zone regions of energetic, small-scale activity. These instabilities, modulated by atmospheric forcing (e.g., by strong winds, surface buoyancy loss) can also produce strong vertical exchanges that rapidly inject weakly stratified mixed-layer waters into the pycnocline, a process referred to as “subduction.” In the global ocean, subduction imprints wintertime mixed-layer properties on a vertically homogenous layer embedded within the upper pycnocline. Termed “mode water,” these layers often extend over large regions, impacting pycnocline structure and thus playing an important role in determining large-scale circulation. The potential importance of frontal subduction has motivated numerous observational (e.g., Pollard and Regier, 1992; Rudnick and Luyten, 1996; Joyce et al., 1998) and numerical (Spall, 1995; Spall et al., 2000) studies that provide significant insight into frontal dynamics during weak forcing. Although wintertime subduction drives significant mode-water formation, difficulties associated with collecting high-resolution measurements in demanding wintertime conditions complicate observational efforts. This article summarizes recent efforts to understand how fronts respond to intense wind and buoyancy forcing.