[SLS Pre-Defense] Sean Chen

“On the Sub-annual Variability of Deep Ocean: Benthic Storms in the Western North Atlantic”

Observations from the 1980-1990s revealed that the deep ocean exhibits variability on sub-annual timescales. Episodic strong near-bottom currents that are capable of resuspending fine sediments in the deep ocean, known as benthic storms, were first extensively observed during the High Energy Bottom Boundary Layer Experiment (HEBBLE) in the western North Atlantic. They have subsequently been observed in multiple regions with strongly eddying surface currents, such as under the Gulf Stream and in the Argentine Basin, and they are associated with sharp increases in particle concentrations over 30-60 days by one to two orders of magnitude, which contribute to the formation of strong benthic nepheloid layers of O(10-100 m) in thickness. These events were typically found to coincide with the presence of cyclonic circulations and meander troughs in the upper ocean. Deep cyclones were extensively observed beneath the meander troughs of the Gulf Stream in the Hatteras abyssal plain during the Synoptic Ocean Prediction (SYNOP) field campaign, with typical durations between 6-9 weeks from development to decay. It is hypothesized that deep cyclonic eddies that form within the meanders of the unstable surface currents could lead to strong episodic abyssal flows comparable to benthic storms and redistribute sediments in the deep ocean. We test this hypothesis through two model studies, one with an idealized domain of a periodic zonal channel and another with a regional domain of the western North Atlantic. Both model simulations show that strong near-bottom currents with speeds comparable to those observed in benthic storms can be associated with deep cyclogenesis under the meandering surface current. In the idealized model experiment, emphasis is placed on the mechanism for the increase of eddy kinetic energy at abyssal depths during deep cyclogenesis and the contrasting characteristics of fluid particle transport by deep cyclonic and anticyclonic eddies. In the realistic model simulation, emphasis is placed on the characteristic and dynamics of deep cyclones, their distributions and comparison with field observations, and how they may contribute to particle transport in the deep western North Atlantic. Overall, we find that deep cyclonic eddies that form beneath the meanders of a strong surface current, like the Gulf Stream, could lead to rapid responses in the abyssal environment and could have significant implications for particle cycling in the deep ocean. Future observations of the abyssal ocean should consider additional dedicated sampling for deep eddies and their associated impacts on sub-annual timescales.

Sack Lunch Seminar Series —
Informal seminar series within PAOC (Program in Atmospheres, Oceans, and Climate) that focuses on more specialized topics than the PAOC Colloquium. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest.

Contact: sacklunch-committee@mit.edu