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Projects

Unravelling the missing links in the latitudinal coherence of the Atlantic Meridional Overturning Circulation

Scientific Objectives:

The Atlantic meridional overturning circulation (AMOC) is a large-scale oceanic circulation comprised of currents that carry warm, shallow water northwards and return cold deep-water southwards. The AMOC is crucial in maintaining the relatively mild winter climate of Northwest Europe and AMOC changes strongly impact European temperature and precipitation variability.

Meridional coherence is the idea that the AMOC functions as a coherent flow across the Atlantic basin (e.g. changes observed in the AMOC at one latitude will also be observed at other latitudes). It is generally accepted that the AMOC is meridionally coherent on long (decadal+) timescales [1]. On shorter timescales, it is not well understood where and how the meridional coherence breaks down, how the associated driving mechanisms change, and how these influence climate.

The primary objective of this project is to use new techniques combining in situ and satellite data to investigate these questions. You will then use these observations to assess state-of-the-art climate model projections. This will lead to a better understanding of the influence of the North Atlantic on European climate.

Methodology: 

RAPID is an international campaign that has been monitoring the AMOC since 2004 using a moored instruments array along a line at 26ºN. Techniques have been recently developed using satellite altimetry and gravity to estimate the AMOC strength at the RAPID line [2,3]. You will use the techniques developed at 26ºN to obtain direct estimates from satellite [2,3], providing a basis for extending the AMOC to the north and south of the RAPID line. This will enable you to reconstruct the first whole basin AMOC time series spanning 34ºS to 58ºN from satellite data alone. You will learn how to use both gridded and along-track sea level products. Gridded products (altimetry and gravimetry) will be used to provide a first estimate of a satellite-based AMOC across different latitudes. Higher resolution along-track satellite data (e.g. Jason, Sentinel-3A/B, Sentinel-6, CryoSat2) will be used to compare SSH data with RAPID moorings particularly near the coast (<50km), where the SSH signal is not very well resolved with gridded satellite products [2]. The approach will incorporate both standard altimetry products as well as higher resolution SAR altimetry and there is the potential in the latter part of the studentship for inclusion of data from the innovative SWOT mission.Other mooring arrays such as MOVE (16ºN), OSNAP (58ºN) and SAMBA (34ºS) will then be used to provide validation for the satellite-derived estimates of the AMOC. For latitudes in between the mooring arrays, density profiles from Argo floats (autonomous ocean profilers) will be used to fill data/knowledge gaps and provide further validation for the satellite-derived AMOC estimates.

You will use the resulting latitudinal estimates of the AMOC to investigate meridional coherence of the AMOC variability from annual to decadal time-scales. The overarching objective will be to assess the nature and evolution of the AMOC meridional coherence: where it holds, where it falls apart, its drivers and wider implications these results have for the North Atlantic climate system. Based on this new understanding, you will evaluate the ability of climate simulations (e.g. CMIP6) to adequately represent the observed AMOC characteristics and their connection to European climate.

Training: 

The student will be hosted at the National Oceanography Centre (NOC), Southampton, and registered at the University of Leeds. The student will be working closely with supervisors at the NOC, University of Leeds, and University of Southampton.

Specifically, you will be trained in:

  • You will be part of the RAPID team and have access to the expertise of a diverse and international group of scientists within the NOC, the University of Southampton, and the University of Leeds.
  • Use of satellite altimetry and gravimetry including in the coastal zone – data handling, analysis and interpretation.
  • Ocean observation techniques, which will include the opportunity to take part in a research expedition. More information on our research expeditions can be found here: https://noc.ac.uk/science/previous-expeditions
  • The use of high performance computing for analysing ensembles of climate simulations.
  • Communication skills (oral, written, presentation and academic networking). The RAPID group has several international collaborators, and you will be encouraged to participate and present at national and international workshops and conferences.

References:

[1] Bingham, R. J.,  Hughes, C. W.,  Roussenov, V., and  Williams, R. G. (2007),  Meridional coherence of the North Atlantic meridional overturning circulation, Geophys. Res. Lett.,  34, L23606, doi:10.1029/2007GL031731.

[2] Sanchez-Franks, A., Frajka-Williams, E., Moat, B. I., and Smeed, D. A.: A dynamically based method for estimating the Atlantic meridional overturning circulation at 26° N from satellite altimetry, Ocean Sci., 17, 1321–1340, https://doi.org/10.5194/os-17-1321-2021 , 2021.

[3] Landerer, F. W., Wiese, D. N., Bentel, K., Boening, C., & Watkins, M. M. (2015). North Atlantic meridional overturning circulation variations from GRACE ocean bottom pressure anomalies. Geophysical Research Letters42(19), 8114-8121