Freshwater Lens Fronts Propagating as Buoyant Gravity Currents in the Equatorial Indian Ocean. Issue 8 (29th July 2021)

• Record Type: Journal Article
• Authors: Moulin, Aurélie J.; Moum, James N.; Shroyer, Emily L.; Hoecker‐Martínez, Martín
• Is Part Of: Journal of geophysical research. Volume 126:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-07-29
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2021JC017186
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Freshwater lenses (FWLs) deposited by rain exhibit local anomalies in surface salinity and temperature. The resulting patchiness in near‐surface density and sea surface temperature influence upper ocean dynamics and air‐sea fluxes of heat. 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 disturbances (X‐band radar) to describe properties and kinematics of FWLs in the equatorial Indian Ocean. Twenty‐eight FWLs were observed with diverse temperature‐salinity properties and structure. Fresh salinity anomalies were as large as −1.35 psu at 3 m depth. Associated temperature anomalies ranged from −0.80 to +0.59°C. Ship‐based radar imagery allowed quantification of propagation speeds of 10 FWL fronts. In the reference frame of the moving fluid, the observed speeds are consistent with the linear long wave speed of g ′ h . These results offer a novel perspective on the evolution of FWLs as gravity currents whose dynamics need to be properly accounted for to assess lens longevity, including persistence of salinity and temperature anomalies, as well as influences on air‐sea interaction. Plain Language Summary: Rainfall over the ocean creates thin surface lenses of fresher, cooler, lighter water compared to the surrounding water. Because their density difference with the ambient is relatively large, they persist at the surface for periods of hours to days. Basic understanding of the evolution of the physical and dynamical properties of these lenses helps to predict how freshwater is distributed in the ocean. This is important in assessing their collective effect on heat and moisture exchanges with the atmosphere. To help answer some of these questions, we measured their physical characteristics, including the sharp fronts which often form at their edges and which can create a disturbance with a signature visible on shipboard marine radar. We observed 28 freshwater lenses, including 10 lens fronts. Since these fronts could be tracked on radar, we could estimate their propagation speed which was well represented by a theoretical estimate based on a long wave speed driven by the density anomaly across the front. This finding complements clues from subsurface data that freshwater lenses have similarities to gravity currents and allow some assumptions to predict their evolution and persistence. More work is needed to see how complete the analogies to gravity currents are. Key Points: Thin, buoyant, near‐surface lenses of freshwater formed by local precipitation were observed in the central equatorial Indian Ocean 50% of observed lenses had sharp lateral interfaces and physical structure similar to gravity currents, including a bulbous leading edge Lens front propagation speeds derived from X‐band radar signatures were equal to their respective long interfacial wave speed g ′ h