Sources and sinks of variability and predictability in the North Atlantic

Abstract

The North Atlantic has long been recognised to have a unique role in climate, owing to its ability via deep water formation to sequester large quantities of heat and carbon, and due to the associated meridional overturning circulation (MOC), which permits the northward transport of heat (impacting the climate of neighbouring regions such as Europe). In recent decades, a \data revolution" in both computational power and observational coverage and quality have revealed a significant amount of variability in the North Atlantic, but the origins of these variations and their predictability remain open questions. The simplest explanation is that variations in the North Atlantic are the passive response of the atmospheric forcing to the ocean. In this perspective, surface-borne anomalies are carried into the ocean interior along the ventilation pathways of its water masses. These pathways therefore motivate the start point for this thesis, which begins by presenting a new configuration of an ocean general circulation model (OGCM), and its application to their study. The configuration is used to describe the passive origins and fate of two ocean water masses present in the North Atlantic: North Atlantic Deep Water and North Atlantic Subtropical Mode Water. The configuration repurposes the tangent-linear and adjoint model (TAM)framework used for tracking perturbations and sensitivities, such that feedbacks are nullied and transport is passive. Using the forward and backward modes of the TAM in 400 year simulations, spatiotemporal and thermohaline probability distributions of water mass origins and fate are constructed. This highlights a disparity between the forward and backward modes suggesting an important role for water mass transformation (changes in thermohaline properties). Following this, the passive configuration is applied to exploring the passive and active nature of atmospherically forced heat content variability, and the North Atlantic is compared with other major ocean basins. A stochastic representation of atmospheric forcing is diagnosed from a coupled climate model and projected onto heat content sensitivity fields produced by the TAM. It is shown that surface layer heat content variability is primarily explained(> 92% agreement in all basins) by the behaviour of the passive ocean simulation, but that for full-depth heat content, the North Atlantic is uniquely poorly represented(~ 27%) by a passive ocean. The spatiotemporal origins of this discrepancy are explored, revealing signatures of the MOC which slow the development of the active ocean variability. Having established an active role of the large-scale ocean dynamics when stimulated by atmospheric stochastic forcing, the relative role of ocean mesoscale eddy turbulence is then considered in the development of uncertainty in the North Atlantic. A stochastic representation of these internal buoyancy fluxes is diagnosed from an eddy-permitting ocean model and projected onto TAM sensitivity fields of subpolar and subtropical MOC and heat content. This representation is applied alongside the stochastic atmospheric representation, so that their relative contributions can be quantified. It is shown that, in the subtropics, the atmosphere ultimately generates around 60% of MOC uncertainty when averaged over a month or decade(owing primarily to momentum and buoyancy uxes, respectively). For annually averaged MOC, however, ocean internal noise is responsible for 60% of the total after 60 years. In the subpolar regions, atmospheric forcing prevails for all metrics and time averages, explaining up to ~ 90% of the unpredictable variability in the case of monthly MOC. The spatial origins of this variability are diagnosed and compared with the spatial patterns which have been determined to most efficiently stimulate variance in an optimal stochastic framework, and are in general overall agreement. Motivated by the concept that nonlinear ocean fluctuations can drive large variations in the annual average subtropical MOC (such as those recorded by modern observational arrays), a nonlinear optimal perturbation technique is then applied to an eddy-permitting OGCM in an attempt to recreate one such event from the recent observational record (\the 2009 event") via an oceanic perturbation. This perturbation, obtained by an iterative optimisation approach, is able (from an anomaly of size ~ 0:1 K) to push the ocean into a temporary state of reduced overturning (3:2 Sv decline), while a linear context, no decline was able to be induced. The linear and nonlinear optimal perturbations differ in both their magnitude and targeted spatial scales, with the former ineffectively concentrating density anomalies within smaller-scale structures and the latter targeting large-scale patterns. The work presented in this thesis collectively reaffirms the unique position of the North Atlantic, demonstrating the long lifespan of its water masses, the interaction between its circulation and atmospherically forced anomalies, and a role for the ocean in forcing substantial deviations from a mean state.

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Identifiers

ORCID for Sybren Drijfhout: orcid.org/0000-0001-5325-7350

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