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

• Main Author: Longworth, H.R.
• Format: Thesis
• Language: English
• Published: 2007
• Subjects: North Atlantic Deep Water; North Atlantic
• Online Access: https://eprints.soton.ac.uk/48835/; https://eprints.soton.ac.uk/48835/1/Longworth_PhD_2007.pdf

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