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The tropical Atlantic circulation: a comparision between ORCA025, ORCA05 and FLAME 1/12°

The tropical Atlantic circulation: a comparision between ORCA025, ORCA05 and FLAME 1/12°. Drakkar-Meeting Grenoble, 25.1.-26.1.2007 Sabine Hüttl. 23°W. 5°S. 0°N. Questions to answer:.

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The tropical Atlantic circulation: a comparision between ORCA025, ORCA05 and FLAME 1/12°

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  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. 23°W 5°S 0°N Questions to answer: • 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 ?

  3. 0°N Section along the equator 0.8 0.7 FLAME 1/12° ORCA05 • 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 ORCA025

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

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

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

  7. Mean annual cycle of EUC transport 23°W mean 16.91 Sv 19.89 Sv 19.89 Sv 18.44 Sv 12.13 Sv amplitude ann. cycle 8 Sv 10 Sv 10 Sv 14 Sv 8 Sv G70 KAB001 KAB002 ORCA05 FLAME 1/12° transport in Sv • ORCA05 shows too strong annual cycle • ORCA025 and FLAME in good accordance to results from OCCAM ¼° (see Hazeleger et al. 2003)  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

  8. Interannual EUC transport variability 23°W G70 KAB001 KAB002 ORCA05 FLAME 1/12° transport in Sv interannual anomalies EUC 23°W in Sv • 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 !

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

  10. Shallow western boundary current 5°S NBC: 29.17 Sv NBC: 21.8 Sv NBC: 28.65 Sv ORCA025 KAB001 ORCA05 FLAME 1/12° • 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…)

  11. Shallow western boundary current at 5°S NBC: 28.65 Sv NBC: 30.61 Sv G70 KAB001/002 • 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 • only KAB001 & ORCA05 show similar annual cycle (offset) • G70 and FLAME 1/12° totally different structures ! transport in Sv G70 KAB001 ORCA05 FLAME 1/12° • interannual variability of KAB001 and FLAME 1/12° shows some similarity for the last 10 years • ORCA05 and G70 do their own… 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…

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