The University of Southampton
University of Southampton Institutional Repository

The global Meridional Overturning Circulation's response to variable buoyancy forcing

The global Meridional Overturning Circulation's response to variable buoyancy forcing
The global Meridional Overturning Circulation's response to variable buoyancy forcing
The meridional overturning circulation (MOC) is a large-scale circulation throughout the global ocean and plays a significant role in the complex global climate system. However, our traditional understanding of the processes driving the MOC has been questioned in recent years. In particular, it has been suggested that surface buoyancy forcing plays little energetic role in driving the MOC. Furthermore, doubt has also been cast over the relationship between meridional overturning and meridional density gradients. In this thesis, we revisit these two ideas using a series of equilibrium and transient numerical ocean simulations.

We begin by diagnosing the global ocean's gravitational potential energy (GPE) budget at steady state in the coupled atmosphere-ocean model HadCM3, utilising the available gravitational potential energy (AGPE) framework to distinguish between the effects of adiabatic and diabatic processes on background GPE and the portion available for adiabatic conversion to kinetic energy (AGPE). We show that surface buoyancy forcing plays an active energetic role in diabatically sustaining large-scale ocean circulation, making available background GPE generated by diapycnal mixing. We then carry out a series of transient experiments using a global configuration of the NEMO ocean modelling framework, periodically varying North Atlantic surface buoyancy forcing over a broad range of timescales. It is shown that the ocean exhibits a highly nonlinear, frequency-dependent response to forcing of the same amplitude, which is well predicted by a scaled overturning streamfunction based on twice-integrated density differences at multi-decadal timescales and longer. Finally, we analyse the AGPE generated by surface buoyancy forcing during each forcing cycle, finding a strong relationship between the AGPE power input by surface buoyancy fluxes and the magnitude of the MOC response. We conclude by discussing the remote effect of North Atlantic buoyancy forcing on lower cell overturning in the Southern Ocean, considering regional changes in AGPE generation.
Butler, Edward D.
f733346a-4ee5-4781-9cf3-530a64d96a0b
Butler, Edward D.
f733346a-4ee5-4781-9cf3-530a64d96a0b
Oliver, Kevin
588b11c6-4d0c-4c59-94e2-255688474987

Butler, Edward D. (2015) The global Meridional Overturning Circulation's response to variable buoyancy forcing. University of Southampton, Ocean & Earth Science, Doctoral Thesis, 136pp.

Record type: Thesis (Doctoral)

Abstract

The meridional overturning circulation (MOC) is a large-scale circulation throughout the global ocean and plays a significant role in the complex global climate system. However, our traditional understanding of the processes driving the MOC has been questioned in recent years. In particular, it has been suggested that surface buoyancy forcing plays little energetic role in driving the MOC. Furthermore, doubt has also been cast over the relationship between meridional overturning and meridional density gradients. In this thesis, we revisit these two ideas using a series of equilibrium and transient numerical ocean simulations.

We begin by diagnosing the global ocean's gravitational potential energy (GPE) budget at steady state in the coupled atmosphere-ocean model HadCM3, utilising the available gravitational potential energy (AGPE) framework to distinguish between the effects of adiabatic and diabatic processes on background GPE and the portion available for adiabatic conversion to kinetic energy (AGPE). We show that surface buoyancy forcing plays an active energetic role in diabatically sustaining large-scale ocean circulation, making available background GPE generated by diapycnal mixing. We then carry out a series of transient experiments using a global configuration of the NEMO ocean modelling framework, periodically varying North Atlantic surface buoyancy forcing over a broad range of timescales. It is shown that the ocean exhibits a highly nonlinear, frequency-dependent response to forcing of the same amplitude, which is well predicted by a scaled overturning streamfunction based on twice-integrated density differences at multi-decadal timescales and longer. Finally, we analyse the AGPE generated by surface buoyancy forcing during each forcing cycle, finding a strong relationship between the AGPE power input by surface buoyancy fluxes and the magnitude of the MOC response. We conclude by discussing the remote effect of North Atlantic buoyancy forcing on lower cell overturning in the Southern Ocean, considering regional changes in AGPE generation.

PDF
Butler, Edward_PhD_June_2015.pdf - Other
Download (5MB)

More information

Published date: 25 June 2015
Organisations: University of Southampton, Physical Oceanography

Identifiers

Local EPrints ID: 378965
URI: https://eprints.soton.ac.uk/id/eprint/378965
PURE UUID: 7c8441eb-7dde-436c-afeb-2f43636d0a52

Catalogue record

Date deposited: 27 Jul 2015 10:47
Last modified: 17 Jul 2017 20:48

Export record

Contributors

Author: Edward D. Butler
Thesis advisor: Kevin Oliver

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of https://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×