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Use of SEA LEVEL data in ICMs to understand ENSOs

Use of SEA LEVEL data in ICMs to understand ENSOs. Perigaud (JPL) , Boulanger (LODYC) and Illig (JPL). ICMs=Intermediate Coupled Models of Ocean /Tropical Atmos. Either “ FORCED ” or “ COUPLED ” Expts FORCED = Data are used to force the ICM ocean or atmos.

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Use of SEA LEVEL data in ICMs to understand ENSOs

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  1. Use of SEA LEVEL data in ICMs to understand ENSOs Perigaud (JPL) , Boulanger (LODYC) and Illig (JPL) ICMs=Intermediate Coupled Models of Ocean/Tropical Atmos. Either “FORCED”or “COUPLED” Expts FORCED = Data are used to force the ICM ocean or atmos. COUPLED = ICM only = “Data-FREE” experiments

  2. Atmosphere/Land models CZ: Cane and Zebiak, 1987 QTCM: Neelin and Zeng, 2000 Statistical Atmos: Suy et al., 1995 Pacific Ocean models CZ: Cane and Zebiak, 1987 LODCA: Dewitte, 2000 TRIDENT: Boulanger, 2001 ICMs = Intermediate Coupled Models Mean States of ICM Oceans and Atmosphere are prescribed in the coupling Our choice of reference: 1980-1996 mean

  3. Atmos Model Period=3.6years COUPLING Ocean Models COUPLED models can reproduce ENSO-like oscillations. Nino4 Nino3

  4. ==> Use TOPEX to improve INITIAL CONDITIONS of forecasts. 15 years agoSEA LEVEL = “FORCED” (wind)+ “FREE” (wave theory) First ideas about the role of OCEAN in the coupled OA: Ocean is forced and has “feedback” on atmos. Kelvin Rossby waves--> “Delayed oscillator” The Ocean has a long memory compared to the chaotic atmos.

  5. Sea level Anom from XBT since 1980 + TP/Jason/OST XBT used to recenter TOPEX anom relative to 1980-1996 mean First: TOPEX trusted for variability --> used to corrrect XBT variations

  6. Fcd1 Fcd2 Forced Exps agree with data Atmos Model Data Ocean Models

  7. Forced Forced Forced Results SL used to initialize forecasts Coupled COUPLED Forecasts are NOT improved. Why? Not because of inappropriate initialization, Not because of data inaccuracy. Is it because of model errors? COUPLED

  8. 12 years ago…2nd thoughts about how to use data in ICMs.Data used to improve model parameterizations Results: new ICMs Start UNDERSTANDING WHY results improved or degraded Start finding NEW processes at work in the coupled O/A

  9. NORTH SOUTH TOPEX: South <0 but North >0 ---> many models wrong for North. Role SL off-equator North SL >0 South SL <0 OA models simulate signals which are symmetric with respect to the equator. Models are wrong!.. Models overestimate equatorial waves and drastically underestimate the meridional component of the currents and winds.  Statistical Atmos

  10. Zonally averaged Sea Level 1999 year 1996 ICM COUPLED Exp TOPEX:Poseidon The ICM succeeds to simulate the correct (dis)charge in the North.

  11. 8 years ago… using ICM to detect new coupled processes at work in reality 2 examples: 1) Role of off-equatorial sea level: Model SL decomposed at each time step into Rossby meridional modes 1 to 20. Amplitude of modes higher > 5 put to 0. 2) Opening the Western Boundary with the Indian ocean: Rossby modes not fully reflected in Kelvin waves, Reflection of Rossby arriving at 130E (2N-8N) put to 0.

  12. Role tested in FORCED Exps Role of off-equator Role of western boundary Both roles NEGLIGIBLE

  13. Role tested in Coupled Expts Role of off-equator Role of western boundary Results: Big role in COUPLED, Negligible in FORCED, Where is the truth?

  14. Role in the actual coupled Ocean/Atmos? 5 years ago…Using data and ICMs in “almost-data-free” Expts. 1980-2000 SL data used to prescribe the off-equator only, all other variables being computed by model. Role of off-equator 1980-2000 SL anomalies at 130E used to compute the geostrophic transport due to ITF-wind-driven Rossby waves at 130E, 2N-8N, all other variables being computed by model. Role of western boundary

  15. “Fully” coupled experiments with off-equatorial SL = 0 are very sensitive to initial conditions and to param, Role Off-Eq in “~ Coupled” expts “Almost-free” coupled expts with 1980-2000 SL data controlling the off-equator only. All simulate a series of warm and events with irregular amplitudes and durations. Results are robust for all Initial Conditions and parameterizations.

  16. Comparison with observed ENSOs Role Off-equator All fully coupled experiments fail to reproduce the observed time series. Controlled only by off-equatorial SL data, “almost-free” coupled experiments all reproduce the observed series of warm and cold events since 1980.

  17. We learnt that: * COUPLED Ocean/Atmos is sensitive to processes that cannot be detected in FORCED Oceans like: * Off-equator, North/South antisymmetric recharge, West, East, * 1cm change in the basin-wide SL average * Time-scales interaction (decadal-intraseasonnal-WWB)… Starting 15 years ago fromSea Levelknown as“FORCED” (wind)+ “FREE” (wave) “Almost data-free” ICM experiments can be used to: *DETECT and UNDERSTAND processes that matter in the actual Ocean/Atmos system. * BENEFIT from FURTHER PROGRESS in ALTIMETRY: (Sea state bias, Inverse Barometric Correction, Tides…).  Need CONTINUITY 1cm accuracy SL obs over decades. NOW and FOREVERCOUPLED = “Almost-Free”

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