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

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

Tony Lee, NASA JPL/CalTech

Bryden et al. (2005)

Maximum strength of Atlantic MOC at 25N

from the ECMWF meeting in 2006, plot generated by Armin Koehl

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

26N 48N

Seasonal & non-seasonal MOC at 900 m

26N 48N

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).

(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 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

Summary

• 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.