Constraining variability of the Atlantic Meridional Overturning Circulation at 25N from historical observations, 1980 to 2005

The Atlantic Meridional Overturning Circulation (MOC) plays an important role in<br/>maintaining western Europe’s moderate climate. Although expected to weaken under<br/>increasing greenhouse gas concentrations, the magnitude of the response remains<br/>uncertain. Monitoring of the Atlantic MOC strength at 25ºN commenced in 2004 as part<br/>of the UK Rapid Climate Change programme. Prior to this, detection of such a slowing<br/>of the MOC and its associated meridional heat transport was inhibited by the temporally<br/>sparse observations of its strength. This thesis aims to extend the time series of Atlantic<br/>MOC at 25ºN back to 1980 using historical observations, thus constraining the<br/>magnitude of its recent interannual variability.<br/>Analysis of transatlantic hydrographic sections at 25ºN show the annual average MOC<br/>to have varied in strength by approximately 25% over the last 50 years, with weaker<br/>overturning in 2004 than previously computed. Temporal resolution of the dataset is<br/>increased through decomposition of the Atlantic MOC at this latitude into its Ekman<br/>component computed from wind stress climatology, transport of the Florida Current<br/>from cable observations and mid-ocean geostrophic transport from CTD or moored<br/>temperature and pressure derived boundary dynamic height profiles. The resulting time<br/>series constrain interannual fluctuations of the Ekman, Florida Straits and mid-ocean<br/>baroclinic geostrophic transports at ± 1, ± 2, and ± 3 Sv respectively between 1980 and<br/>2005. In addition to interannual fluctuations in strength, evidence is found for a 2 to 3<br/>Sv weakening of the MOC between 1980 and 2005. The underlying changes of<br/>increased mid-ocean southward transport of thermocline waters and decreased<br/>southward flow of lower North Atlantic Deep Water are consistent in sign, although of<br/>reduced magnitude, with the transatlantic hydrographic sections observations. This<br/>thesis presents and discusses these findings along with the relative merits of different<br/>datasets for detecting such variability.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2007

Author dc:creator

• Longworth, H.R.