Fine-scale alongshore variability over the inner continental shelf revealed by fiber-optic distributed temperature sensing

Abstract:

The physical dynamics of the inner continental shelf determine the exchange of heat, nutrients, and other tracers between the nearshore environment and deeper ocean. At locations where the coastline and bathymetry are not uniform, there is likely to be significant exchange associated with with complex three-dimensional flow structures. Resolving such features in observations remains a challenge, particularly below the surface where satellites and high-frequency radar do not measure. To obtain high-resolution measurements of subsurface alongshore temperature variability during a field study on inner-shelf exchange, a fiber-optic distributed temperature sensing (DTS) system was deployed at the Martha’s Vineyard Coastal Observatory off the southern coast of New England. The DTS system emits a laser down a fiber-optic cable and measures the intensity of Raman backscatter at different frequencies, which provides an estimate of the surrounding ocean temperature at 1-m intervals along the cable. In this study, a 5-km fiber-optic cable was deployed along the seafloor at the 17-m isobath for a duration of over three months. The fine-scale measurements of temperature along the cable reveal sharp fronts that propagate along the bottom and are associated with rapid near-bottom cooling. When these features are present, which is only when the water column is stratified, their appearance is linked with the tidal cycle. The direction of propagation, calculated at a location where the cable turns inshore, has a significant alongshore component and suggests that the propagating fronts originate at a tidal mixing front where strong tidal currents flow over a shallow shoal. The fronts decay as they travel, indicating that they influence the alongshore thermal structure of the water column. These DTS observations provide a new perspective on the complex three-dimensional circulation associated with the combination of strong tides, stratification and shallow bathymetry.