1. The tropical Atlantic circulation: a comparision between ORCA025, ORCA05 and FLAME 1/12° Drakkar-Meeting Grenoble, 25.1.-26.1.2007 Sabine Hüttl

2. 1) How realistic is the mean tropical Atlantic circulation in ORCA025 compared to observations, ORCA05 and FLAME?  zonal section along equator  meridional section at 23°W  section along 5°S 2) How fits the variability of the interannually forced runs into the results from the ORCA05 runs and similar experiments within the FLAME framework ? Questions to answer: 23°W 0°N 5°S

3. Section along the equator FLAME 1/12°ORCA05 ORCA025 0.7 0.8 0°N  EUC core depths more realistic in ORCA than in FLAME (see Schott et al. 2003 and 2005, Brandt et al. 2006)  similar velocities and strength, stronger eastward vertical inclination of EUC in ORCA

4. Meridional section at 23°W observations by Brandt et al. 2006, 11 sections EUC: 13.8 Sv SEUC: 7.0 Sv EUC: 12.13 Sv SEUC: 2.61 SvNEUC: 2.43 Sv

5. Meridional section at 23°W EUC: 12.13 Sv SEUC: 2.61 SvNEUC: 2.43 Sv EUC: 18.44 Sv SEUC: 0.00 SvNEUC: 0.00 Sv

6. Meridional section at 23°W EUC: 19.89 Sv SEUC: 1.94 SvNEUC: 5.71 Sv EUC: 12.13 Sv SEUC: 2.61 SvNEUC: 2.43 Sv

7. Mean annual cycle of EUC transport 23°W G70 KAB001 KAB002 ORCA05 FLAME 1/12°  transport maximum in autumn (response to zonal wind, see Arhan et al. 2006) in all models, second maximum in spring: not in ORCA05, in G70 only weak max.  FLAME (and OCCAM ¼°) maxima occur one month earlier amplitude ann. cycle 8 Sv 10 Sv 14 Sv 8 Sv  ORCA05 shows too strong annual cycle  ORCA025 and FLAME in good accordance to results from OCCAM ¼° (see Hazeleger et al. 2003) mean 16.91 Sv 19.89 Sv 18.44 Sv 12.13 Sv transport in Sv

8. Interannual EUC transport variability 23°W G70 KAB001 KAB002 ORCA05 FLAME 1/12° Most observational and model studies agree, that interannual EUC variability is mainly wind-driven by local surface Ekman divergence. This implies:  differences between different experiments with different surface forcing/parameters:  KAB001 and KAB002 differ in amplitude (different strength of surface restoring)  KAB001/002 and G70 show different behaviour  between variability of the ORCA025-runs, ORCA05 and FLAME 1/12° even the sign is unclear ! interannual anomalies EUC 23°W in Svtransport in Sv

9. Western boundary regime 5°S observations by Schott et al., 2005 NBC: 26.5 Sv DWBC: 25.5 Sv ORCA025 KAB001 NBC: 28.65 Sv DWBC: 24.01 Sv NBC DWBC

10. Shallow western boundary current 5°S FLAME 1/12° ORCA05 ORCA025 KAB001  core next to western boundary is realistic only but respresented only in FLAME !  mean northward NBC from measurements by Schott et al., 2005: 26.5 Sv  differences between ORCA025-runs? (two-core structure around 200m in KAB001 looks strange…) NBC: 29.17 Sv NBC: 21.8 Sv NBC: 28.65 Sv

11. Shallow western boundary current at 5°S KAB001/002 G70 NBC: 28.65 Sv NBC: 30.61 Sv  both currents are too strong and show high velocities only near the surface  G70 shows stronger current core velocities (around 1.4 m/s) but core still located at the shore

12. Annual cycle & interannual variability NBC 5°S G70 KAB001 ORCA05 FLAME 1/12°  only KAB001 & ORCA05 show similar annual cycle (offset)  G70 and FLAME 1/12° totally different structures !  interannual variability of KAB001 and FLAME 1/12° shows some similarity for the last 10 years  ORCA05 and G70 do their own… transport in Sv NBC transport anomaly at 5°S in Sv

13. Conclusions 1) Mean tropical Atlantic circulation in ORCA025, ORCA05 and FLAME  EUC structure and velocities quite realistic in all ORCA runs  western boundary current (NBC) in all ORCA runs not seperated from the coast as observed !  higher resolution ?  mean transports of EUC and NBC in most ORCA runs higher than observations,  small-scale zonal currents only in ORCA025 and FLAME, but transports too low 2) Annual cycle and interannual variability EUC  phases of the annual cycle of the EUC quite robust between different ORCA025- runs and FLAME but amplitudes differ, ORCA05 not able to reproduce observed EUC annual cycle  interannual variability depends on strength of surface restoring and wind-forcing NBC  annual cycles between KAB001, G70 and FLAME not comparable !  interannual variability also different…