Atlantic Meridional Overturning Circulation slowdown cooled the subtropical ocean

Abstract

[1] Observations show that the upper 2 km of the subtropical North Atlantic Ocean cooled throughout 2010 and remained cold until at least December 2011. We show that these cold anomalies are partly driven by anomalous air-sea exchange during the cold winters of 2009/2010 and 2010/2011 and, more surprisingly, by extreme interannual variability in the ocean's northward heat transport at 26.5°N. This cooling driven by the ocean's meridional heat transport affects deeper layers isolated from the atmosphere on annual timescales and water that is entrained into the winter mixed layer thus lowering winter sea surface temperatures. Here we connect, for the first time, variability in the northward heat transport carried by the Atlantic Meridional Overturning Circulation to widespread sustained cooling of the subtropical North Atlantic, challenging the prevailing view that the ocean plays a passive role in the coupled ocean-atmosphere system on monthly-to-seasonal timescales.

Figure 1

(a) Measurement locations and region of interest. Bathymetry (0 and 2000 m contours) and topography (grey) of the North Atlantic region with the northern and southern limits of the volume shown by black lines at 26.5°N and 41°N. At 26.5°N, the Florida cable is given by the short segment between Florida and the Bahamas, and the principal RAPID 26.5°N array mooring positions are indicated by dots. (b) Area-averaged subsurface temperature anomalies in the subtropical North Atlantic (5–82°W, 26.5–41°N) calculated relative to the 1991–2010 seasonal cycle using monthly means from the EN3 v2a gridded objective analysis of quality-controlled subsurface temperature observations [Ingleby and Huddleston, 2007], (http://www.metoffice.gov.uk/hadobs/en3/). Black lines indicate the area-averaged depth of selected isotherms across the same region. Mean mixed layer depth (white) and the 95th percentile (grey) for which only 5% of grid boxes have a deeper mixed layer defined by Kara et al. [2000]. The MLD is the depth over which air-sea exchanges drive turbulent mixing of the ocean. (c) Six month low-pass filtered ocean heat content anomalies relative to 1991–2010 above 2000 m (solid black), above the 4°C isotherm (red) and above the 14°C isotherm (dashed black). OHC uncertainties (grey) are generated by model-based estimates associated with changes in sampling density and locations and are for 10 day values. They are approximated by the OHC uncertainties shallower than 2000 m and 500 m respectively (section S3).

Figure 2

(a) Deseasonalized 6 month low-pass time series of the meridional heat transports [PW] from the RAPID/MOCHA array at 26.5°N (black) [Johns et al., 2011] and from the Argo/altimetric estimate at 41°N (black dashed) [Hobbs and Willis, 2012]. The RAPID/MOCHA at 26.5°N data have a temporal resolution of 0.5 days, and 41°N data are 3 monthly. The error (grey) for daily values of heat transport at 26.5°N is 0.2 PW and at 41°N for monthly values is 0.22 PW. Horizontal black lines are the mean of the time series from 1 April 2004 to 31 March 2009. Both time series are deseasonalized by calculating a mean seasonal cycle for the period 1 of April 2004 to 31 March 2009 (section S1). (b) Heat transport divergence (41–26.5°N) set to zero mean in the period 1 April 2004 to 31 March 2009, where values above zero indicate cooling of the subtropical Atlantic. The uncertainty (grey) is the root-sum-square error from 26.5°N and 41°N.

Figure 3

(a) Time series of observed relative heat content above the 4°C isotherm determined from the EN3 v2a Argo data set 6 month low-pass filtered. Uncertainties for OHC above the 4°C isotherm are approximated by the uncertainty in OHC above 2000 m. (b) Integrated ocean heat transport divergence between 26.5°N and 41°N. (c) Integrated ocean-atmosphere surface flux from ERA-interim reanalysis. (d) OHC budget residual (Figures 3a − (3b + 3c)). OHC 10²²J.

Figure 4

(a) Relative heat content change RHC warmer than the 14°C isotherm. (b) Heat content change due to volume changes RHC_V. (c) Heat content change due to temperature changes RHC_T and integrated surface heat fluxes from ERA-interim (red). RHC = RHC_V + RHC_T. OHC 10²²J. RHC uncertainties (grey) are generated by model-based estimates associated with changes in sampling density and locations. For RHC warmer than 14°C, the RHC error is approximated by the OHC_U uncertainty shallower than 500 m (section S3). The total fractional uncertainties in RHC_V and RHC_T are 0.83 OHC_U and 0.55 OHC_U according to Palmer and Brohan [2011]. The uncertainties are for 10 day values.