A 10 year project to observe and analyze regular data about ocean circulation and how it impacts on Britain’s climate has provided new insight into Atlantic Meridional Overturning Circulation (AMOC), a major system of currents in the North Atlantic.

10 years is too short a time to be meaningful but it is an important milestone. Since 2004, the project team has been monitoring the AMOC at 26.5N degrees, near where it carries its maximum heat, using instruments moored at 30 locations across the Atlantic between the Canary Islands and the Bahamas - so-called fixed arrays. The arrays’ instruments measure the temperature, salinity and pressure of the ocean, from which the AMOC’s strength and structure can be calculated.

Twice-daily estimates of the AMOC have been made every day since the start of the project, providing a significant increase in recorded data. Prior to this, measurements had only been taken during five separate ship surveys – one survey every ten years since the 1950s. The data collected is then calibrated and analyzed to see what changes have taken place.

Until recently it had been widely thought that the AMOC couldn’t be measured in such a consistent way – and there have been some surprising findings.

Firstly, it had been thought that the strength overturning of the AMOC would weaken due to climate change. However, the ocean sensors have detected that the AMOC is now declining faster than anticipated (Smeed et al. 2014), which could potentially have a long-term impact on Britain’s climate. Secondly, the results revealed that the AMOC was significantly more variable than had been previously thought. Thirdly, the data also appeared to confirm that the AMOC had a direct impact on Britain’s winter weather, which could be specifically seen with respect to the harsh winter of 2010/11.

The measurements showed that the strength of the AMOC in 2009/10 was much lower, which affected sea surface and atmospheric temperature – and seemed to directly affect Britain’s weather months later.

The slowdown in the AMOC in 2009/10 also raised sea levels in New York by 13 cm - 4 times the global average sea level rise.

Citation: D. A. Smeed, G. D. McCarthy, S. A. Cunningham et al.,"Observed decline of the Atlantic meridional overturning circulation 2004–2012" Ocean Sci., 10, 29-38, 2014