Global Oceans: Meridional overturning circulation observations in the subtropical North Atlantic [in 'State of the Climate in 2011']

Recommendations for a coordinated observing system to begin to measure MOC were presented at the international conference OceanObs’09 in September 2009 (e.g., Cunningham et al. 2010; Rintoul et al. 2010) and subsequent planning workshops focused on expanding existing observations to include the subpolar North and South Atlantic (e.g., Garzoli et al. 2010). The most complete MOC observing system has been in place since April 2004, and spans the subtropical gyre in the North Atlantic near 26.5°N. The system is composed of UK-NERC RAPID-WATCH moorings, US-NSF Meridional Overturning Circulation Heat-Transport Array (MOCHA), and the US-NOAA Western Boundary Time Series program (see also Rayner et al. 2010; Chidichimo et al. 2010).

The estimates of MOC from the 26.5°N array include data from April 2004 to December 2010 (see also Rayner et al. 2010), shown in Fig. 3.21. Over this time period, the MOC had a mean transport of 18.1 Sv with a high of 31.6 Sv, a low of -2.6 Sv in December 2009 and a standard deviation of 4.7 Sv (using the twice daily values filtered with a 10-day cutoff as described in Cunningham et al. 2007). From early December 2009 through the end of April 2010, the MOC sustained low values with a mean of 9.8 Sv. At the end of the time series in December 2010, the MOC was again relatively low, with a transport of about 13 Sv. These two low MOC “events” were produced by a combination of changes occurring on different time scales (e.g., short-term Ekman and Florida Current transport changes) and long-term changes in the southward geostrophic flow. Overall, the Florida Current and Ekman (EK) transport were about 2 Sv less northward than usual and the southward thermohaline circulation was about 2 Sv stronger, leading to a year-long anomaly of about 5 Sv – 6 Sv in the MOC. With these two events present at the end of the multiyear time series, a linear regression of MOC versus time yields a decrease of -6 ± 0.3 Sv decade-1 (95% confidence). A linear trend estimated with the time series ending in December 2009 has a trend of only -4.8 Sv decade-1. Baringer et al. (2011) reported an insignificant trend through April 2009 of -0.8 ± 1.6 Sv decade-1. It is clear that 2010 was an unusual year for MOC transport across 26°N, but given the large variability of MOC estimates, it would be imprudent to ascribe too much to the last year of values in determining a decadal trend. After six years of data, however, a clear seasonal signal is beginning to emerge (Kanzow et al. 2010), with a low MOC in April and a high MOC in October with peak to peak range of 6.9 Sv. The seasonal cycle of the MOC appears to be largely attributable to seasonal variability in the interior rather than Ekman or Florida Current fluctuations; Kanzow et al. (2010) show that the interior seasonal cycle is likely due to seasonal upwelling through a direct wind-driven response off Africa.