Natural long term cycles in the ocean and not climate change are behind the well publicised slow down in the Gulf Stream that has been observed in recent years, according to new research from Met Office scientists. The observed decrease in the so called Atlantic Meridional Overturning Circulation – of which the Gulf Stream is a part – over the past decade was preceded by a period where the circulation intensified, they report in a new paper.
From the UK Meteorological Office (Met Office)
Any substantial weakening of a major North Atlantic ocean current system would have a profound impact on the climate of north-west Europe, including the UK.
The Atlantic Meridional Overturning Circulation – part of which is known as the Gulf Stream – has been observed over the past 10 years, and has been seen to weaken over that time, raising the question of whether the weakening has been caused by climate change. New Met Office research published today instead suggests that the trend is likely due to variability over decades.
Laura Jackson of the Met Office Hadley Centre is the lead researcher. Commenting on the paper, published in the journal Nature Geoscience, she said: “The Atlantic Meridional Overturning Circulation plays a vital role in our climate as it transports heat northwards in the Atlantic and keeps Europe relatively warm.”
Reanalysis captures Gulf Stream changes
The Met Office research produced a new ocean ‘reanalysis’ combining a state-of-the-art model of ocean dynamics with ocean observations from satellites, and ocean floats sampling below the surface. This has captured year-to-year variations and recent decadal trends with unprecedented accuracy.
Laura Jackson said: “Our research produced a picture of how the ocean has evolved over the last couple of decades. The reanalysis reproduces the observed decrease in the Atlantic Meridional Overturning Circulation over the past decade, but finds that this was preceded by a period where the circulation intensified. This suggests that decadal timescale variability likely played a key role in the weakening of the circulation seen over the last decade.”
The researchers are keen to stress that this does not rule out the possibility that the observed weakening is a combination of decadal variability and a longer term decrease that would only be detectable after more years of observations.
The research also shows a link between the weakening in the Atlantic Meridional Overturning Circulation and decreases in density in the Labrador Sea (between Greenland and Canada) several years earlier. Laura Jackson added: “Continued monitoring of densities in the Labrador Sea may provide us with useful information about future changes in the Atlantic Meridional Overturning Circulation.”
The Atlantic meridional overturning circulation (AMOC) has weakened substantially over the past decade. Some weakening may already have occurred over the past century, and global climate models project further weakening in response to anthropogenic climate change. Such a weakening could have significant impacts on the surface climate. However, ocean model simulations based on historical conditions have often found an increase in overturning up to the mid-1990s, followed by a decrease. It is therefore not clear whether the observed weakening over the past decade is part of decadal variability or a persistent weakening. Here we examine a state-of-the-art global-ocean reanalysis product, GloSea5, which covers the years 1989 to 2015 and closely matches observations of the AMOC at 26.5° N, capturing the interannual variability and decadal trend with unprecedented accuracy. The reanalysis data place the ten years of observations—April 2004 to February 2014—into a longer-term context and suggest that the observed decrease in the overturning circulation is consistent with a recovery following a previous increase. We find that density anomalies that propagate southwards from the Labrador Sea are the most likely cause of these variations. We conclude that decadal variability probably played a key role in the decline of the AMOC observed over the past decade.
Laura C. Jackson, K. Andrew Peterson, Chris D. Roberts and Richard A. Wood; Recent slowing of Atlantic overturning circulation as a recovery from earlier strengthening; Nature Geoscience (2016) doi:10.1038/ngeo2715.
Met Office news release.