Link with CLIVAR GSOP through the CLIVAR/GODAE Global Ocean Synthesis Evaluation effort Tony Lee, NASA JPL/CalTech As a follow-up for the 1st workshop for ocean reanalysis product intercomparison in the fall of 2006 at ECMWF, a 2nd workshop was held in the fall of 2007 at MIT. More focused analysis and intercomparison: (1) Atlantic Meridional Overturning Circulation (AMOC); (2) water-mass characteristics. Synthesis application: global sea level. Status of coupled approach. Uncertainty in surface fluxes. Data errors. Error covariance.

Comparison of Atlantic Meridional Overturning Circulation (MOC) for 7 assimilation products Tony Lee, NASA JPL/CalTech Thank the following groups for providing MOC fields: ECCO (MIT/AER, U. Hamburg/G-ECCO, & JPL), K-7, Mercator, & ECMWF Picture from http://www.noc.soton.ac.uk

Maximum strength of Atlantic MOC at 25N from the ECMWF meeting in 2006, plot generated by Armin Koehl Bryden et al. (2005)

MOC strength at 900 m (near the depth of MAX MOC strength)

Seasonal & non-seasonal MOC at 900 m

Also presented a decomposition of AMOC into 3 dynamical components that are directly related to observations of density, sea level, and wind stress. Will not discuss here. See the following references for more info: Lee & Marotzke (1998), Baehr, Hirschi, Beismann, & Marotzke (2004), Cabanes, Lee, & Fu (2007).

Summary statistics for MOC strength at 900 m Black column: r.m.s. difference of MOC anomaly among products Red column: r.m.s. difference of time-mean MOC among products Blue column: r.m.s. variability of MOC itself, averaged over 6 products (Unit: Sv) 26N 48N Total 1.02, 2.57, 2.50 0.72, 2.15, 2.29 Seasonal 0.53 0.37 Non-seasonal 0.87 0.62

Summary Take home messages: Good consistency for the temporal variability of 900-m MOC strength at 26N (48N) for the 7 products. The r.m.s. difference among the products (overall <= 1 Sv) is smaller than the variability of the MOC itself (~ 2.5 Sv). The consistency is better for seasonal (<= 0.5 Sv) than for non-seasonal anomalies. The consistency of the time mean is much poorer than that for the anomaly. Take home messages: Uncertainty of observational estimates need to be smaller than 1 Sv to effectively constrain/distinguish these products. Need to understand why these products agree so well in estimated MOC variability? Hypothesis - consistency in wind forcing Cabanes, Lee, and Fu (2008): interannual variability of AMOC in subtropical North Atlantic is largely controlled by Ekman pumping near the western boundary; most wind products consistently show much larger Ekman pumping near the western boundary.