Distinct sources of interannual subtropical and subpolar Atlantic overturning variability

The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat and carbon. Establishing the causes of historical variability in AMOC strength on different timescales can tell us how the circulation may respond to natural and anthropogenic changes at the ocean surface. However, understanding observed AMOC variability is challenging because the circulation is influenced by multiple factors that co-vary and whose overlapping impacts persist for years. Here we reconstruct and unambiguously attribute intermonthly and interannual AMOC variability at two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While, on interannual timescales, AMOC variability at 26° N is overwhelmingly dominated by a linear response to local wind stress, overturning variability at subpolar latitudes is generated by the combined effects of wind stress and surface buoyancy anomalies. Our analysis provides a quantitative attribution of subpolar AMOC variability to temperature, salinity and wind anomalies at the ocean surface

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017

Sensitivity of the Atlantic meridional overturning circulation to surface forcing

The determination of the mechanisms setting the strength and structure of the large scale circulation is a fundamental and long-standing problem in physical oceanography. In this thesis, we seek to explore the mechanisms contributing to the steady state and variability of the large scale flow, with a focus on better understanding the dynamics of the Atlantic meridional overturning circulation (AMOC). In the first part of this thesis, we explore the linear sensitivity of the monthly mean subtropical AMOC to surface fluxes of buoyancy and momentum. Our approach is to use a numerical adjoint. Key insights are provided into the memory of the AMOC to historic atmospheric forcing. We find that significant memory to wind forcing is confined to timescales of less than a year. In contrast, we identify significant memory to surface buoyancy forcing spanning multi-decadal timescales and characterised by a large scale oscillation in the sign of sensitivity between the eastern and western North Atlantic basin. An important result is that to understand the origins of seasonal variability in the modelled AMOC, we must examine the response to a multidecadal history of atmospheric forcing. In the second part of this thesis, a new tool is presented that enables a clean diagnosis of the force balance controlling the circulation regime for a Boussinesq fluid. Specifically, the tool is based on the development of the "rotational momentum" equations and sets of scalar "velocity potentials" and analogous "force functions". The latter allow the projection of all forces onto the acceleration of the vertical shears and external modes of overturning to be visualised in isolation. The rotational momentum decomposition is applied to the modelled circulation in idealised Atlantic and global configurations of the MITgcm, with a focus on elucidating the dynamics of the simulated AMOC. We discuss the key role played by the rotational buoyancy forcing right on the western boundary.</p

Sensitivity of the Atlantic meridional overturning circulation to surface forcing

The determination of the mechanisms setting the strength and structure of the large scale circulation is a fundamental and long-standing problem in physical oceanography. In this thesis, we seek to explore the mechanisms contributing to the steady state and variability of the large scale flow, with a focus on better understanding the dynamics of the Atlantic meridional overturning circulation (AMOC). In the first part of this thesis, we explore the linear sensitivity of the monthly mean subtropical AMOC to surface fluxes of buoyancy and momentum. Our approach is to use a numerical adjoint. Key insights are provided into the memory of the AMOC to historic atmospheric forcing. We find that significant memory to wind forcing is confined to timescales of less than a year. In contrast, we identify significant memory to surface buoyancy forcing spanning multi-decadal timescales and characterised by a large scale oscillation in the sign of sensitivity between the eastern and western North Atlantic basin. An important result is that to understand the origins of seasonal variability in the modelled AMOC, we must examine the response to a multidecadal history of atmospheric forcing. In the second part of this thesis, a new tool is presented that enables a clean diagnosis of the force balance controlling the circulation regime for a Boussinesq fluid. Specifically, the tool is based on the development of the "rotational momentum" equations and sets of scalar "velocity potentials" and analogous "force functions". The latter allow the projection of all forces onto the acceleration of the vertical shears and external modes of overturning to be visualised in isolation. The rotational momentum decomposition is applied to the modelled circulation in idealised Atlantic and global configurations of the MITgcm, with a focus on elucidating the dynamics of the simulated AMOC. We discuss the key role played by the rotational buoyancy forcing right on the western boundary.This thesis is not currently available in ORA

Momentum Budget Evaluation in ASTE Release 1 Part I: Full momentum budget

The purpose of these notes is to describe how to perform accurate momentum budget analyses using output from the first release of the Arctic and Subpolar gyre sTate Estimate [ASTE R1 Nguyen et al., JAMES, 2021]. The goal of these analyses is to partition, at the grid-point level, the rate of change of momentum into all of its contributing terms in the momentum equation, such as wind and Coriolis forces, horizontal advection, resolved diffusion of momentum, parameterized diffusion of various kinds, etc.This work was supported by NSF-OPP-1603903, NSF-OPP-1708289, and NSF-OCE-1924546. Additional funding was provided from NASA Physical Oceanography's ECCO project through a JPL/Caltech subcontract. Computing resources were provided by the University of Texas at Austin Texas Advanced Computing Center (TACC) and NASA Advanced Supercomputing Division at the Ames Research Center

The Deep Ocean Observing Strategy: Addressing Global Challenges in the Deep Sea Through Collaboration

The Deep Ocean Observing Strategy (DOOS) is an international, community-driven initiative that facilitates collaboration across disciplines and fields, elevates a diverse cohort of early career researchers into future leaders, and connects scientific advancements to societal needs. DOOS represents a global network of deep-ocean observing, mapping, and modeling experts, focusing community efforts in the support of strong science, policy, and planning for sustainable oceans. Its initiatives work to propose deep-sea Essential Ocean Variables; assess technology development; develop shared best practices, standards, and cross-calibration procedures; and transfer knowledge to policy makers and deep-ocean stakeholders. Several of these efforts align with the vision of the UN Ocean Decade to generate the science we need to create the deep ocean we want. DOOS works toward (1) a healthy and resilient deep ocean by informing science-based conservation actions, including optimizing data delivery, creating habitat and ecological maps of critical areas, and developing regional demonstration projects; (2) a predicted deep ocean by strengthening collaborations within the modeling community, determining needs for interdisciplinary modeling and observing system assessment in the deep ocean; (3) an accessible deep ocean by enhancing open access to innovative low-cost sensors and open-source plans, making deep-ocean data Findable, Accessible, Interoperable, and Reusable, and focusing on capacity development in developing countries; and finally (4) an inspiring and engaging deep ocean by translating science to stakeholders/end users and informing policy and management decisions, including in international waters