On Subannual Variability in the Abyssal Ocean

Abstract

The abyssal ocean is a critical yet understudied component of the climate system and is of growing economic interest. This thesis combines field observations and numerical modeling to advance our understanding of subannual variability in the abyssal ocean and its broader implications.

First, hydrographic measurements from the Clarion-Clipperton Zone of the tropical Northeastern Pacific are used to characterize the structure and variability of the bottom mixed layer (BML) in a region targeted for deep-sea mining. The observations reveal a spatially and temporally variable BML with a mean thickness of ~250 m and influenced by interactions with mesoscale eddies and abyssal thermal fronts. A simplified model of sediment transport suggests that such variations in BML structure could significantly influence the dispersal of sediments resuspended by seabed mining activities.

Second, idealized model experiments are conducted to explore the genesis of benthic storms – episodes of strong near-bottom flows and sediment entrainment – underneath an unstable, surface-intensified jet resembling the Gulf Stream east of Cape Hatteras. In these experiments, the baroclinic instability of the jet gives rise to deep cyclonic and anticyclonic eddies through eddy barotropization and to high levels of eddy kinetic energy at abyssal depths through the convergence of vertical eddy pressure fluxes. The near-bottom currents are comparable in magnitude to those observed during benthic storms, with vertical shears strong enough to produce BMLs up to O(100) m thick. Deep cyclonic eddies transport particles from near the bottom over the entire BML and could contribute to benthic nepheloid layers. The results suggest that the abyssal response to the intrinsic instability of surface-intensified currents could contribute significantly to subannual variability near the seafloor.

Third, a model simulation of western North Atlantic circulation is performed to study the deep cyclones (DCs) observed beneath Gulf Stream meander troughs. The characteristics of the simulated DCs compare well with field observations. The negative pressure tendency during cyclogenesis arises from a small imbalance between the sea surface depression and the vertically-integrated increase in seawater density. Vortex stretching is the primary source of cyclonic vorticity, while vortex tilting is a non-negligible sink. The deep pressure tendency, vorticity fluxes, and ageostrophic flows are diagnosed, and their similarities and differences with mid-latitude synoptic cyclones in the atmosphere are discussed. Near-bottom currents in DCs dominate the basin-scale bottom energy dissipation and transport fluid over ≥1000 km horizontally and O(100) m vertically within 3~4 months, suggesting that they provide an efficient mechanism for tracer and material transport in the abyssal interior.

Collectively, this thesis highlights the importance of transient, mesoscale processes in contributing to subannual variability in the abyssal ocean, particularly near the seafloor. The findings have broader relevance for monitoring the environmental impacts of human activities, including deep-sea mining and carbon sequestration. While further questions remain for future investigation, this work underscores the need for sustained in-situ observations in the abyssal ocean and calls for the implementation of high vertical resolution in numerical ocean circulation models.