Prediction from the University of Southampton

Near-real time estimates of the AMOC using Altimetry 

By Eleanor Frajka-Williams

This AMOC estimate relies on satellite measurements of sea level anomaly (SLA).  While it is not a true prediction or forecast, it does provide an alternate method to estimate ocean transports in near-real time, and in the interim before in situ data are recovered.  The method may further be applicable at other latitudes, enabling investigation of circulation changes across the Atlantic.

The method relies on an inverse relationship between sea level anomalies in the western half of the basin at 26N and the thermocline displacement.  When sea level is higher than usual, the thermocline is deeper than usual, which results in a stronger southward transport in the upper mid-ocean layer (0-1100 m, between the Bahamas and Africa) and a weaker MOC.

Method

The SLA is first spatially smoothed (5 deg latitude, 10 degrees longitude) and the pixel nearest 70W and 28N is extracted. Time series of SLA and Rapid UMO transport are binned monthly, then a seasonal cycle is removed.  A linear least squares regression is calculated between monthly, deseasonalised time series of SLA and UMO transport.  To reconstruct full variability, a seasonal cycle from the Rapid UMO time series is added to the UMO estimate from SLA.

Figure 1: UMO transport from the Rapid array and UMO* (proxy for UMO from SLA at 70W, 28N).  Both time series are deseasonalised and quarterly binned.

Using SLAs near 70W and 28N in the Atlantic, we can estimate the variability in the UMO transport (UMO*).   Combining this with near real time estimates of meridional Ekman transport at 26N from NCEP (scaled to match past Rapid estimates from ERA-Interim) and near real-time estimates of Florida Current transport from NOAA, we can calculate an estimated AMOC* = Ek + FC + UMO*.

Figure 2: AMOC components including Florida Current (FC, blue), Ekman transport (Ek, green) and UMO (magenta).  In the top two panels, the black lines are monthly averages and coloured are quarterly averages.  In the third panel, the black line is the Rapid estimate, and the magenta line is from SLA with the Rapid seasonal cycle added.  In the most recent 18 months, there were no strongly negative anomalies in Ekman transport (whereas there were 3 events in the preceding years, 2009-2014).

Reconstructed overturning circulation

The reconstructed AMOC is shown in Fig. 3. The original method in Frajka-Williams (2015) was designed to estimate interannual variations only; here, the quarterly reconstruction still has an r=0.45 (significant at 95%).  Using this method to estimate the AMOC, it appears that the previously observed decline of the AMOC (2004-2012, Smeed et al., 2014; 2004-2014, Frajka-Williams et al., 2015) is continuing, with a slope of -0.4 Sv/yr.

Figure 3: AMOC reconstruction.  Black shows the estimate from Rapid and red from SLA.  The dashed line is the linear trend calculated from the SLA-based estimate of the AMOC, with a slope of -0.4 Sv/yr.

The full method is based on the paper appearing in Frajka-Williams (2015). Here we have updated the estimates for a quarterly time series rather than just looking at variations on timescales longer than 1 year.

References

Frajka-Williams, E., 2015, Estimating the Atlantic MOC at 26N using satellite altimetry and cable measurements.  Geophys. Res. Lett. 42:3458-3464.  http://dx.doi.org/10.1002/2015GL063220.

Smeed, D. A., McCarthy, G., Cunningham, S. A., Frajka-Williams, E., Rayner, D., Johns, W. E., Meinen, C. S., Baringer, M. O., Moat, B. I., Duchez, A., Bryden, H. L.. 2014, Observed decline of the Atlantic meridional overturning circulation 2004 to 2012. Oc. Sci. 10:29-38. http://dx.doi.org/10.5194/os-10-29-2014.

Frajka-Williams, E., Meinen, C. S., Johns, W. E., Smeed, D. A., Duchez, A. D., Lawrence, A. J., Cuthbertson, D. A., Bryden, H. L., McCarthy, G. D., Rayner, D., Moat, B. I. 2015, Compensation between meridional flow components of the AMOC at 26N. Oc. Sci. Disc. 12:2705-2741. http://dx.doi.org/10.5194/osd-12-2705-2015.