Thursday, 1 October 2015

Prediction from Oxford University

What will the RAPID team find when they recover their ocean moorings this autumn?

By Helen Johnson, David Marshall, Helen Pillar and So Takao, University of Oxford

Since 2004 oceanographers from the National Oceanography Centre in Southampton and their US colleagues have been using data from ocean moorings on the eastern and western sides of the Atlantic Ocean at 26oN to monitor the strength of the Atlantic meridional overturning circulation (AMOC).  This has resulted in a remarkable and unprecedented 10-year time-series of this key climate index (black line in the figure).  

Observed and predicted time series of AMOC flow

The AMOC is closely related to ocean heat transport in the Atlantic and therefore of great importance for the climate of Western Europe as well as the entire globe.  The observations have revealed large variations in the AMOC on all time-scales.  There is an apparent decline in AMOC strength over the ten years, and significant wind-driven weakening in several recent winters.

This autumn the team will collect a further 18 months of data from their ocean moorings.  But what will this latest batch of data tell us about the strength of the AMOC?

At the University of Oxford, we have been working to reconstruct the time-series of AMOC variability, based on our knowledge of how winds, heat and freshwater fluxes over the Atlantic have changed over the last few decades, combined with our understanding of how sensitive the AMOC is to variations in these quantities. 

Our reconstructed AMOC time series (orange line) successfully reproduces most of the interannual variability in the observations. These short-timescale fluctuations are dominated by wind forcing. However, the decadal trend seen in the observations is not well captured by our reconstruction.  This longer-timescale variability results from the response of the ocean to heat fluxes over the subpolar North Atlantic over the last two decades at least, and as yet ocean models are unable to accurately represent the ocean’s adjustment on such timescales.

Our reconstructed AMOC time series extends 15 months beyond the end of the observations available to date.  We have reasonable confidence in that portion of the variability which is wind-driven (blue line) and therefore make two predictions:
  1. The observations will show a mean AMOC over this period which has been roughly equal to that over the previous few years (a small increase of 0.3 ± 0.2 Sv over the 2009-2014 mean).
  2. The RAPID data recovered in the autumn this year won't reveal any evidence of a large "dip" over the 2014-2015 winter; in contrast we expect to find that the AMOC reached a maximum in November-January.
Our predictions will be validated when the RAPID team publish their updated AMOC time-series early in 2016!  And as RAPID data continue to accrue, alongside observations from higher latitudes such as those made by the OSNAP programme, we will learn more about the climatically-important longer-term AMOC changes which are currently inaccessible via our reconstruction.  Watch this space!

How the predictions were made
We use an ocean model and its adjoint to determine the sensitivity of the AMOC to surface wind, heat and freshwater forcing over the entire globe and the preceding 15 years. We then project observed forcing anomalies onto these sensitivity patterns; only those forcing anomalies which project strongly in space and time onto the sensitivity fields will generate variability in the AMOC.  Since NCEP II reanalysis atmospheric forcing data is available until June 2015, our reconstructed AMOC time series extends 15 months beyond the end of the currently available observed AMOC time-series. 

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