Si-Yuan Sean Chen

Deep cyclones (DCs) are mesoscale, deep‐reaching features that develop near meander troughs of large‐scale currents, such as the Gulf Stream. Although, in some aspects, they could be viewed as oceanic analogs of synoptic‐scale cyclones in the mid‐latitude atmosphere, their dynamics and impacts are not fully understood. Notably, the roles of different vorticity sources in deep cyclogenesis and the relationship of DCs with “benthic storms” remain to be elucidated. Here we develop a regional configuration of a primitive‐equation model with 1/20° horizontal resolution and 10‐m vertical resolution over the entire water column to study DCs in the western North Atlantic. In our simulation, DCs form both in the Hatteras Abyssal Plain, where DCs were observed during the 1980‐90s SYNOP campaign, and in the Sohm Abyssal Plain, where observations are lacking. Their spatial scales, lifetimes, pressure drops, swirl velocities, and drift speeds compare favorably with observational estimates. During deep cyclogenesis, the pressure fall at abyssal depth reflects a small imbalance between the effects of the sea level drop and of the density increase in the overlying water column within tightening meander troughs. Below 1,500 m, the main source of cyclonic vorticity is vortex stretching, associated mainly with the curvature‐induced ageostrophic flow, while vortex tilting is a sink of smaller magnitude. Near‐bottom currents in DCs reach speeds comparable to those observed during benthic storms and, when present, dominate the basin‐scale bottom energy dissipation. Overall, the study highlights the importance of DCs for sub‐annual variability and material transport in the abyssal interior.

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 eastern North 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 ∼250m 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 produces 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 ≥1000km horizontally and 𝒪(100)m vertically within 3–4 months, suggesting that they provide an efficient mechanism for 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 storage operations. 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.

Deep cyclones (DCs) were observed in the western North Atlantic under meander troughs of the Gulf Stream (GS) west of the New England seamounts during the Synoptic Ocean Prediction (SYNOP) field campaign. Although subsequent dedicated observations have been sparse, DCs appear common underneath major surface-intensified western boundary current extensions. Recent model studies with idealised domains suggest that DCs are important sources of eddy kinetic energy in the deep ocean, key sites of energy dissipation, and potential contributors to episodes of strong near-bottom currents and sediment resuspension known as “benthic storms”. In the western North Atlantic, DCs that form within GS meander troughs could play a role in the Atlantic meridional overturning circulation by providing a path for recirculation of water between the Deep Western Boundary Current (DWBC) and the adjacent oceanic basins. However, most numerical ocean models lack the vertical resolution that is needed to simulate in detail both the vertical structure of DCs and the near-bottom flows.In this study, we configure the MIT general circulation model to produce eddy-rich simulations of western North Atlantic circulation at a horizontal resolution of 1/20o and with high vertical resolution (550 levels with uniform Dz = 10 m). Emphasis is placed on the role of DCs in the time-mean abyssal circulation and on their contribution to Lagrangian transport, particularly to the exchange of water between the DWBC and the adjacent basins and between the bottom mixed layer and the stratified interior. In the simulations, deep cyclones are found to form west of the New England seamounts, consistent with field observations from SYNOP. They also form in the Sohm abyssal plain – east of the seamounts – although observations are lacking to confirm or refute this result. In our simulations, the DCs typically persist for 30-90 days and move eastward at a speed of ~1.5 cm s-1 in tandem with the GS meander troughs near the surface. The time-averaged horizontal velocity field in the Sohm abyssal plain depicts a large-scale cyclonic circulation cell that is reminiscent of the Northern Recirculation Gyre proposed from sparse observations of deep current meters. Preliminary results of Lagrangian transport suggest that water parcels in the DWBC may enter the interior through entrainment of DCs near 68ºW (west of the New England seamounts). Further Lagrangian analyses of the simulated deep circulation are ongoing to elucidate the temporal and spatial scales of particle transport associated with the DCs and benthic storms.

The bottom boundary layer (BBL) is the portion of the water column which is directly affected by the drag of ocean currents on the seafloor. It is often assumed to coincide with the bottom mixed layer in which potential temperature is approximately uniform. Bottom mixed layers have an observed thickness of O(10 m), with maxima of O(100 m) in some basins. As a result, they are in general not represented in numerical models of large-scale circulation, which typically assume a vertical resolution of a few hundred meters near the bottom. The coarse vertical resolution near the bottom that is assumed in many numerical models implies that these models may not accurately represent the velocity shears near the bottom and the dissipation rates of kinetic energy by bottom drag, which depends on the near-bottom velocity. Here we present results from idealized numerical experiments of the circulation in a double (subtropical-subpolar) gyre, which are aimed at determining the effects of near-bottom vertical resolution on simulated ocean circulation and energetics. Results from experiments that resolve the BBL are compared to those from experiments that do not. In all experiments, the horizontal grid is fine enough to resolve the mesoscale eddy field. In our presentation, emphasis will be placed on energy dissipation by bottom drag in experiments with different near-bottom vertical resolutions. The implications of our results for the simulation of large-scale ocean circulation will then be clarified.

A long-standing question in paleoclimate research concerns the fate and consequences of the glacial water released into the ocean from the Laurentide Ice Sheet (LIS) during the last deglaciation. In this presentation, we will describe detailed simulations of the pathways of glacial meltwater released from the LIS which have been obtained from an eddy-resolving, regional configuration of the general circulation model of the MIT (MITgcm) coupled with a sea-ice model. Emphasis will be placed on glacial meltwater discharged from Hudson Strait into the Labrador Sea and on its interaction with the North Atlantic Current (NAC). Our regional configuration of the MITgcm represents the glacial Atlantic between 34.5oN and 67oN at a horizontal resolution of 1/20o, with 61 vertical levels (21 levels in the upper 100 m), and with continental shelves removed (sea level lowered by 130 m). The relatively fine spatial grid permits the simulation of the mesoscale eddy field and of the baroclinic structure of the buoyant current produced by the meltwater inflow. Surface forcing is provided by the atmospheric conditions during the last glacial maximum which have been simulated by a global climate model (Community Climate System Model v.3). Our preliminary results show that the meltwater current from Hudson Strait flows to the SE along the continental slope of Labrador and Newfoundland and sheds anticyclonic eddies which carry offshore meltwater and are entrained by the NAC near the Grand Banks. In turn, the meltwater influences the NAC through its effect on seawater density, suggesting a new mechanism by which glacial water fluxes may change large-scale circulation in the North Atlantic. In our presentation, attention will be paid on the influence of the meltwater on the strength and structure of the NAC near and downstream of the Grand Banks.

Benthic storms are episodes of intensified near‐bottom currents capable of sediment resuspension in the deep ocean. They typically occur under regions of high surface eddy kinetic energy (EKE), such as the Gulf Stream. Although they have long been observed, the mechanism(s) responsible for their formation and their relationships with salient features of the deep ocean, such as bottom mixed layers (BMLs) and benthic nepheloid layers (BNLs), remain poorly understood. Here we conduct idealized experiments with a primitive‐equation model to explore the impacts of the unforced instability of a surface‐intensified jet on near‐bottom flows of a deep zonal channel. Vertical resolution is increased near the bottom to represent the bottom boundary layer. We find that the unstable near‐surface jet develops meanders and evolves into alternating, deep‐reaching cyclones and anticyclones. Simultaneously, EKE increases near the bottom due to the convergence of vertical eddy pressure fluxes, leading to near‐bottom currents comparable to those observed during benthic storms. These currents in turn form BMLs with thickness of O(100 m) from enhanced velocity shears and turbulence production near the bottom. The deep cyclonic eddies transport fluid particles both laterally and vertically, from near the bottom through the entire BML and may contribute to the formation of the lower part of BNLs. A sloping bottom reduces both the intensity of the near‐bottom currents and the extent of vertical transport. Overall, our study highlights a significant response of the abyssal environment to near‐surface current instability, with potential implications for sediment transport in the deep ocean.

Benthic storms are episodes of strong bottom currents and sediment resuspension that occur at abyssal depths in the ocean. They are often observed in regions with strong, eddying surface currents, such as the western North Atlantic and the Argentine Basin. Deep cyclonic and anticyclonic eddies induced by surface current instabilities have been postulated to accelerate abyssal currents and generate benthic storms. Here, using a primitive-equation model with high vertical resolution, we conduct idealized numerical experiments of the unforced instability of a surface-concentrated, eastward-flowing jet in a zonal channel. We find that the jet, initially in strict thermal wind balance, becomes spontaneously unstable as a result of baroclinic instability, meanders, and eventually develops a complex eddy field with regions of strong ageostrophic flow. Associated with jet meandering, a train of cyclonic and anticyclonic eddies form along the jet and migrate in the same direction as the parent current but much slower. These eddies extend over the whole water column (4000 m depth), consistent with the tendency for eddy barotropization in geostrophic turbulence. They accelerate bottom currents, which are found to be more vigorous in a flat channel than in a channel with a meridional bottom slope. Over the course of the numerical integrations, bottom mixed layers with quasi-uniform potential temperature develop at various locations in the channel. These layers have thicknesses of O(100) m on a flat bottom and O(10) m on a sloping bottom. Our study yields preliminary insights into the fundamental role of surface current instabilities on near-bottom processes, calling for further investigations on downward energy transfer and near-bottom dissipation.

Long-lived colonial cold-water corals have the potential to provide robust continuous archives of environmental change. These high-resolution records of the subsurface ocean are particularly valuable, especially at understudied intermediate water depths. Yet, to understand the anthropogenic impacts on the sub-surface ocean and better predict future changes, it is critical to establish the natural variation of temperature and circulation of the ocean system prior to the Industrial Revolution.Here we combine temperature proxy and radiocarbon data from specimens of two taxa of cold-water coral that grew in intermediate water depths (~1500 m) in the tropical North Atlantic. In 2013, specimens of the bamboo coral Lepidisis spp. and scleractinian coral Enallopsammia rostrata were collected from sites currently situated in the boundary of North Atlantic Deep Water and Antarctic Intermediate Water to reconstruct the temperature and circulation history of the region. We demonstrate that bamboo corals can be used to reconstruct ambient seawater radiocarbon content when independently dated by organic node annual band counting. Radiocarbon was also analysed in Enallopsammia rostrata to develop age models for both the radial section and from discrete corallites (polyps) along a branch. Dating results show that this coral is about 500 years old, allowing us to generate a temperature record as far back as the Little Ice Age. Trace metal ratios were analysed along the growth axis of the coral, and the Li/Mg ratio was used as a temperature proxy. We find that the Li/Mg derived temperature of the most recent polyps is consistent with modern ambient temperature. The overall temperature record shows a general increasing trend since the Little Ice Age, while the radiocarbon record indicates no significant change until the late 20th century. Combining these records allows us to reconstruct potential ocean circulation changes in the central tropical North Atlantic over last 500 years.