Ephemeral Surface Chlorophyll Enhancement at the New England Shelf Break Driven by Ekman Restratification

The Mid-Atlantic Bight (MAB) hosts a large and productive marine ecosystem supported by high phytoplankton concentrations. Enhanced surface chlorophyll concentrations at the MAB shelf-break front have been detected in synoptic measurements, yet this feature is not present in seasonal means. To understand why, we assess the conditions associated with enhanced surface chlorophyll at the shelf break. We employ in-situ and remote sensing data, and a 2-dimensional model to show that Ekman restratification driven by upfront winds drives ephemerally enhanced chlorophyll concentrations at the shelf-break front in spring. Using 8-day composite satellite-measured surface chlorophyll concentration data from 2003-2020, we constructed a daily running mean (DRM) climatology of the cross-shelf chlorophyll distribution for the northern MAB region. While the frontal enhancement of chlorophyll is apparent in the DRM climatology, it is not captured in the seasonal climatology due to its short duration of less than a week. In-situ measurements of the frontal chlorophyll enhancement reveal that chlorophyll is highest in spring when the shelf-break front slumps offshore from its steep wintertime position causing restratification in the upper part of the water column. Several restratification mechanisms are possible, but the first day of enhanced chlorophyll at the shelf break corresponds to increasing upfront winds, suggesting that the frontal restratification is driven by offshore Ekman transport of the shelf water over the denser slope water. The 2-dimensional model shows that upfront winds can indeed drive Ekman restratification and alleviate light limitation of phytoplankton growth at the shelf-break front. Plain Language Summary The ocean south of New England contains high concentrations of phytoplankton that form the base of the marine food web and provide critical support to the region's fisheries. The offshore edge of the relatively shallow continental shelf, the shelf break, is the boundary between the cooler and fresher water on the continental shelf (shelf water) and the warmer and saltier water offshore (slope water). This water boundary at the shelf break is thought to support high chlorophyll concentrations. Enhanced shelf-break chlorophyll concentrations are not always present, however. We use data from satellites, ships, gliders, and moorings to determine what drives the episodically enhanced surface shelf-break chlorophyll concentrations. We find that the shelf-break surface enhancements of chlorophyll concentrations are short-lived events, and are associated with periods when the shelf-slope water interface slumps, as a surface layer of the lighter shelf water moves over the denser slope water. This process creates a shallow surface layer that has ample light to support photosynthesis. Both data and a computational model show that eastward winds are the primary driver of the episodic frontal slumping and localized enhanced surface chlorophyll.