In general it is not possible to associate error bars to G.C.M. results. Therefore, in addition to the model comparison we decided to compare ORCA to direct observations. Besides comparisons between Eulerian model results and transports computed over hydrological sections (e.g., Blanke et al., 1999) we studied the ORCA-simulated South Equatorial Current (SEC) and compared it to the inversion of the A17 WOCE section data performed by Wienders et al. (2000).

The results are resumed in the following diagram:

The meridional WOCE A17 section has permitted a complete sampling of the SEC in latitude and depth near the South American western boundary. Three bands are generally considered as composing the SEC, divided by the eastward South Equatorial Countercurrent and South Equatorial Undercurrent. These three branches, respectively south, central and north, are recirculations associated with the subtropical, subequatorial and equatorial gyres respectively.

ORCA Lagrangian diagnostics give a transport of about 52 Sv for the southern branch of the SEC (SSEC) that compares well with the 49 Sv estimated from the A17 section (Wienders et al., 2000). This is also true for the central (CSEC) and northern (NSEC) branches (cf. figure). The warm water export to the North Atlantic estimated from data is 18 Sv (Wienders et al., 2000). It corroborates well the ORCA estimate of the Upper-Branch flow (18.0 Sv: 15.8 Sv from Pacific origins and 2.2 Sv of NADW upwelling inside the Indo-Atlantic basin).

A17 section was documented between January and March 1994. ORCA analysis suggests that seasonal variability is negligeable for SSEC whereas CSEC and NSEC show strong seasonal fluctuations. Work is in progress to evaluate the impact of the annual cycle on the warm water export to the North Atlantic.