Observation of decadal change in the Atlantic meridional overturning circulation using 10 years of continuous transport data

Abstract

The meridional overturning circulation (MOC) represents the main mechanism for the oceanic northward heat transport in the Atlantic, and fluctuations of this circulation are believed to have major impacts on northern hemisphere climate. While numerical ocean and climate models and paleo‐records show large variability in this circulation, the use of direct observations of the MOC for detecting climate‐timescale changes has proven difficult so far. This report presents the first observational record of MOC measurements that is continuous and sufficiently long to exhibit decadal‐scale changes, here a decrease by 20% over the observational period (Jan. 2000–June 2009) and large interannual changes in the flow and its vertical structure. Data are from a mooring array at 16°N (Meridional Overturning Variability Experiment, MOVE). The observed change agrees with the amplitude of multi‐decadal natural fluctuations seen in numerical ocean and climate models. Knowledge of the existence and phasing of such internal cycles provides multi‐decadal climate predictability. Recently, some numerical model simulations have produced results that show a weakening of the MOC since the 1990's and observational confirmation of this now is a high priority.

Atlantic Meridional Overturning Circulation at 14.5° N in 1989 and 2013 and 24.5° N in 1992 and 2015: volume, heat, and freshwater transports

“…10b). This structure is consistent with the vertical structure of the NADW transport observed in the western basin by MOVE at 16 • N (Send et al, 2011). Send et al (2011) showed that the annual mean NADW transport has two southward maxima located at 2000 and 4000 m, corresponding to the LSW and DSOW, respectively, and a minimum in between centered at 3000 m, corresponding to the ISOW.…”

“…This structure is consistent with the vertical structure of the NADW transport observed in the western basin by MOVE at 16 • N (Send et al, 2011). Send et al (2011) showed that the annual mean NADW transport has two southward maxima located at 2000 and 4000 m, corresponding to the LSW and DSOW, respectively, and a minimum in between centered at 3000 m, corresponding to the ISOW. The high oxygen concentration in this area also implies the presence of relatively recently formed waters of northern origin (LSW, ISOW, and DSOW), which is also evident from higher transient tracer concentrations (e.g., chlorofluorocarbon, CFC; Molinari et al, 1992).…”

“…The MOVE array is composed of a current meter mooring at the western boundary and two end-point geostrophic moorings at each end of the western basin at 16 • N, which measures the DWBC and interior geostrophic transport between 1200 and 4950 m in the western basin. The assumption behind this mooring configuration is that the southward flow in the NADW layer in the western basin should represent (almost) entirely the southward return limb of the AMOC, and thus the AMOC intensity (Send et al, 2011). Note that the sign of the MOVE transport in Fig.…”

“…15b, as a representation and update of Fig. 2 in Send et al (2011). The MOVE array is composed of a current meter mooring at the western boundary and two end-point geostrophic moorings at each end of the western basin at 16 • N, which measures the DWBC and interior geostrophic transport between 1200 and 4950 m in the western basin.…”

“…Smeed et al (2014) found that a strengthened southward thermocline water transport is responsible for the weakened AMOC at 26 • N during 2004-2012, which was balanced by a decrease in the southward flow of the lower NADW. However, Send et al (2011) found that at 16 • N the interannual variability in the transport of the Labrador Sea Water (LSW; as one major component of the upper NADW) is responsible for the interannual variability in the AMOC strength.…”