How Much Has the North Atlantic Ocean Overturning Circulation Changed in the Last 50 Years?

Volume transports from six ocean reanalyses are compared with four sets of in situ observations: across the Greenland-Scotland ridge (GSR), in the Labrador Sea boundary current, in the deep western boundary current at 43 degrees N, and in the Atlantic meridional overturning circulation (AMOC) at 26 degrees N in the North Atlantic. The higher-resolution reanalyses (on the order of 1/4 degrees X 1/4 degrees) are better at reproducing the circulation pattern in the subpolar gyre than those with lower resolution (on the order of 1 degrees). Simple Ocean Data Assimilation (SODA) and Estimating the Circulation and Climate of the Ocean (ECCO)-Jet Propulsion Laboratory (JPL) produce transports at 26 degrees N that are close to those observed [17 Sv (1 Sv equivalent to 10(6) m(3) s(-1))]. ECCO, version 2, and SODA produce northward transports across the GSR (observed transport of 8.2 Sv) that are 22% and 29% too big, respectively. By contrast, the low-resolution reanalyses have transports that are either too small [by 31% for ECCO-JPL and 49% for Ocean Reanalysis, system 3 (ORA-S3)] or much too large [Decadal Prediction System (DePreSys)]. SODA had the best simulations of mixed layer depth and with two coarse grid long-term reanalyses (DePreSys and ORA-S3) is used to examine changes in North Atlantic circulation from 1960 to 2008. Its results suggest that the AMOC increased by about 20% at 26 degrees N while transport across the GSR hardly altered. The other (less reliable) long-term reanalyses also had small changes across the GSR but changes of +10% and -20%, respectively, at 26 degrees N. Thus, it appears that changes in the overturning circulation at 26 degrees N are decoupled from the flow across the GSR. It is recommended that transport observations should not be assimilated in ocean reanalyses but used for validation instead.