Dense Water Cascades in the Rockall Trough

This study identifies the occurrence of one cascade off the Malin shelf in April 2020, the first observation of cascading into the Rockall Trough since 1996. Cascades are buoyancy driven plumes of downslope flowing water created by a build-up of dense water on the continental shelf. They are a fundamental element in shelf to deep ocean exchange, as well as key drivers of the global thermohaline circulation. This study utilizes glider data, collected during 2020–2023, to search for evidence of cascading off the Malin Shelf and into the adjacent Rockall Trough. Additionally, meteorological data and the Price-Weller-Pinkel (PWP) dynamical mixing model were used to assess and simulate winter conditions on the shelf break. With an input of meteorological data, the resultant density profiles from the PWP model suggested that cascading should occur every year due to winter cooling, though this model does not account for any horizontal water influx. Of the observed years, 2020 and 2021 were shown to be the most likely to trigger a cascade. Contrary to the model results, the glider data shows only one cascade (2020) during the observed years, indicating that the shelf is strongly influenced by advection. Assessment of directional winter wind stress data shows that westerly winds dominate the shelf, with occasional years of strong easterly winds, as is the case in 2021. This paper theorizes that easterly winds act to enhance the influx of low salinity coastal water onto the shelf, thereby reducing shelf water density and inhibiting cascading events. This explains the lack of cascading in 2021, while its absence in 2022 and 2023 can be explained by less intense winter cooling. Therefore, ideal cascading conditions on the Malin Shelf are strong westerly winds, weak easterly winds, and high winter heat loss.