Warm Spiral Streamers over Gulf Stream Warm-Core Rings

We examine the generation mechanism of warm spiral filaments over Gulf Stream warm-core rings. Satellite sea surface temperature imagery often shows warm spirals forming after a body of warmer water ---- Gulf stream or a new warm-core ring ---- impinges onto a relatively old warm-core ring and delivers warmer water into the old ring. Field measurements in April 2018 revealed vertical structure of a warm spiral filament and showed that the warm spiral is a shallow lens of low-density water winding over an old warm-core ring. Observations also show subduction on both sides of the warm spiral filament, and the subduction flows carry surface ring and shelf waters downward along the side walls of the warm filament. Idealized models initialized with observed water mass densities reproduce spiral filaments over warm-core rings and suggest that negative density anomaly of the intruding spiral water is necessary for the formation of the spiral pattern. The density anomaly provides an azimuthal density gradient that, through thermal-wind balance, drives a radial velocity toward the center of the ring. The anticyclonic ring flow and the radial motion of the warm intruding water together forms the surface spiral pattern. The model also reproduces the subduction process on both sides of the spiral filament, which results from frontogenesis caused by the inward-flowing warm water expanding into the ring. The upwelling part of the frontogenesis-associated secondary circulation brings the subsurface warm water to the surface, widening the warm spiral, thus enhancing its surface signal.