Attribution of ocean climate change signals in the Atlantic.
Lead Research Organisation:
University of Oxford
Department Name: Oxford Physics
Abstract
The proposal aims to identify observational signals associated with changes in the overturning circulation and heat transport in the North Atlantic, exploiting the development of the monitoring system put in place by international programmes and the UK RAPID monitoring initiative. Our study aims to analyse both existing historical data obtained over the last 50 years and the latest emerging observations. Our approach is to apply a forward circulation model with an accompanying adjoint for the North Atlantic in order to identify how signals in the data are formed by external forcing and ocean physics. The adjoint provides a highly effective method of identifying the sensitivity of model signals and optimising the model towards selected data signals, which will be used together to identify how signals in the data are controlled. The study will focus on several themes: identifying how data signals along the western boundary and ocean interior are controlled using the adjoint approach; identifying the role of geostrophic eddies in limiting the adjoint approach; mapping hydrographic changes along the margins of the Atlantic from historical data and identifying the propagation of coastal wave modes from altimetric data.
Organisations
Publications
Czeschel L
(2010)
Oscillatory sensitivity of Atlantic overturning to high-latitude forcing
in Geophysical Research Letters
Goes M
(2009)
Eddy Formation in the Tropical Atlantic Induced by Abrupt Changes in the Meridional Overturning Circulation
in Journal of Physical Oceanography
Johnson H
(2007)
Reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria
in Climate Dynamics
Kanzow T
(2009)
Basinwide Integrated Volume Transports in an Eddy-Filled Ocean
in Journal of Physical Oceanography
| Description | Under anthropogenic greenhouse forcing, the Atlantic meridional overturning circulation (MOC) is expected to weaken over the next century due to high-latitude warming and freshening. However, the ongoing global warming as seen in the upper North Atlantic ocean has not been monotonic. Departures from a steady warming on decadal time scales are well established and it is widely accepted that the MOC plays an important role in driving these decadal near-surface temperature variations. Using an ocean circulation model we have shown that the modelled MOC exhibits oscillatory sensitivity to anomalous forcing at high latitudes. This leads to the counter-intuitive result that a positive Atlantic overturning anomaly may emerge as a result of, although some years after, anomalous warming at high latitudes. Our study suggests that the variability of the MOC is sensitive to changes in the buoyancy fluxes and SST in the North Atlantic over at least the last two decades. We find that linearity breaks down after roughly 20 years, providing a useful estimate on the upper limit of potential predictability of the MOC. This long-term memory makes it extremely challenging to detect secular change in the MOC through short observational time series. We have studied the sensitivity of the MOC at different latitudes to high-latitude forcing. We find that the maximum sensitivity is greater, and occurs earlier, when the MOC is evaluated at higher latitudes. Our sensitivity maps also show the strongest sensitivities are confined to the North Atlantic on multidecadal and shorter time scales, supporting the focus of international observational efforts on the North Atlantic. It has been suggested that the structure and strength of the MOC is governed by the input of mechanical energy to the system by winds and tides. However, it is not clear how this suggestion relates to the existence of multiple equilibria of the MOC, which depends on thermohaline feedbacks and is more consistent with a buoyancy-driven view of the circulation. We developed a conceptual box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation. The box model has two equilibrium solutions, one with sinking at high northern latitudes as in the present-day Atlantic, and one without. The circulation is mechanically driven, but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect. While the source of energy comes from mechanical forcing, the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks. |
| Exploitation Route | The main values are academic and in improving future climate models (though users, for example, at the Met Office Hadley Centre). This, in turn, should lead to non-academic impacts. Results are of value in assessing alternative MOC monitoring strategies, for example, in the South Atlantic. |
| Sectors | Environment |
| Description | Public lecture, University of Georgetown, Guyana |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Public/other audiences |
| Results and Impact | Around 40 members of the University of Georgetown (mostly students) and the general public attended a lecture on ocean circulation and climate which generated a lot of questions and discussion. Interesting discussions following the talk about a recently reported "whirlpool" off the coast of Guyana, which turned out to be media hype. |
| Year(s) Of Engagement Activity | 2012 |
| Description | St Hugh's College Senior Members talk |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | National |
| Primary Audience | Public/other audiences |
| Results and Impact | Around 30 alumni of St Hugh's College attended a talk on ocean circulation and climate, prompting excellent discussion afterwards. n/a |
| Year(s) Of Engagement Activity | 2013 |