Propagation of Freshwater Lenses as Buoyant Gravity Currents

Freshwater lenses (FWL) deposited by rain create surface salinity and temperature anomalies that can persist for extended periods of time (> 1 day). The resulting patchiness in near-surface density and sea surface temperature influence upper ocean dynamics and air-sea fluxes of heat. For these reasons, understanding lens formation and evolution has been a focus of recent observational and modeling efforts. The work presented here integrates near-surface ocean and atmosphere time series with remote sensing of sea surface roughness (X-band radar) and precipitation (C-band radar) to describe the formation and temporal evolution of lenses within the equatorial Indian Ocean. Twenty-six FWLs are observed at different stages of their evolution from freshly deposited and actively spreading to older, passively advected features. Salinity anomalies reached -1.2 psu near the surface, while temperature anomalies were observed to be both cool (down to -0.8°C) and warm (up to +0.4°C). The largest density anomaly reached -0.5 kg/m³. Remotely-sensed, ship-based radar imagery allows for quantification of the observed propagation speeds of ten lenses, which follow internal gravity wave theory. These results offer a novel perspective on the evolution of FWLs whose dynamics need to be properly accounted for to assess lens longevity, including persistence of salinity and temperature anomalies, as well as influences to air-sea interactions.