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Observed characteristics at the eastern edge of the warm pool in the western Pacific Ocean

Observed characteristics at the eastern edge of the warm pool in the western Pacific Ocean. Christophe Maes 1 , Kentaro Ando 2 , Thierry Delcroix 3 , William S. Kessler 4 , Michael J. McPhaden 4 , and Dean Roemmich 5

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Observed characteristics at the eastern edge of the warm pool in the western Pacific Ocean

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  1. Observed characteristics at the eastern edge of the warm pool in the western Pacific Ocean Christophe Maes1, Kentaro Ando2, Thierry Delcroix3, William S. Kessler4, Michael J. McPhaden4, and Dean Roemmich5 (1) Institut de Recherche pour le Dévelopement (IRD), Laboratoire d’Etudes en Geophysique et Oceanographie Spatiales (LEGOS), centre IRD de Nouméa, BP A5, Nouméa, New-Caledonia (2) Institute of observational research for global change, Japan Agency for Marine and Earth-Science Technology (JAMSTEC), Yokosuka, Japan (3) IRD/LEGOS, Toulouse, France (4) Pacific Marine Environmental Laboratory, Seattle, Washington, USA (5) Scripps Institution of Oceanography, La Jolla, California, USA Christophe.Maes@ird.fr Related paper: Observed correlation of surface salinity, temperature and barrier layer at the eastern edge of the western Pacific warm pool, Maes et al., 2006, Geophys. Res. Lett., in press

  2. Background Eastern edge of the warm pool: 0.4 psu over 15° in longitude

  3. Scientific issues - Zonal migrations of the warm pool / ENSO signal and fisheries (Picaut et al., 1996, 1997; Lehodey et al., 1997; Matsuura and Iizuka, 2000) - Extension of the upwelling region / CO2 evasion (Inoue et al., 1996; Le Borgne et al., 2002) - Warm SST and atmospheric deep convection / climatic forecasts (Fu et al., 1986; Waliser, 1996; Clarke et al., 2000) The advective-reflective oscillator (Picaut et al., 1997) Observed mean zonal current This theory places the heart of the oscillatory nature of ENSO at the eastern edge of the warm pool

  4. Scientific issues Observed zonal currents from the TAO/TRITON array Sea surface salinity variability reconstructed from an indirect method * Currents are mainly based on ADCP measurements along the equator * SSS is deduced from indirect estimations using direct observations and sea level altimetry Trajectories of hypothetical drifters moved by zonal currents within 2°N-2°S Mean RMS 29°C (Maes et al., 2004) ► Convergence zone migrates between 140°E and 140°W ► Variability of parameters in air-sea interaction is nearly constant on each side of the front ▲ Central position of the SSS front

  5. Scientific questions In their synthesis on the oceanic zone of convergence at the eastern edge of the Pacific warm pool, Picaut et al. (2001) conclude that « it should be not a surprise if the hypothetical drifter trajectories, 29°C, and salinity front are not strictly collocated ». ►Can the eastern edge of the warm pool be define more accurately? The advective-reflective oscillator emphasizes a positive feedback of zonal currents that advect the warm pool toward the east during El Niño (Picaut et al., 1997). The coupling principle used in such an approach was based on a simple approximation of the observed SST and wind interaction: during El Niño, westerly winds penetrate from the western Pacific into the central part of the basin. It also implies that high SST must be maintained to force the atmosphere. ►What are the processes that maintain the high SSTs in the warm pool region?

  6. Data overview - Salinity and Temperature data are from the TAO/TRITON array, the VOS-SSS collected from merchant ships (centre de Nouméa) and the Argo floats from the CORIOLIS DAC (Brest) - Complementary data include OSCAR ocean currents, Reynolds’ SST, TAO/TRITON winds, OLR from the CDC center and the CMAP precipitations from CPC. Argo data processing: - profiles with only temperature data are discarded, - profiles with gaps in depth larger than 30 m in the top 150 m are discarded, - surface level is defined at 15 m and a cut-off at 1000 m is applied, - unrealistic values of T and S are discarded ( 4°C < T < 32°C ; 30 < S < 37 ) as well as negative values, - stability in density is checked using a difference criterion of 0.002 kg/m3 ; deeper than 50 to 100m, instabilities are removed using a scheme based on a linear interpolation with the nearest neighbour points, while in the mixed layer, density inversions are left unchanged if the values do not present some unrealistic behaviour.

  7. Data processing Data comparison between Argo and TAO/TRITON Co-localization is based on criteria of : +/-0.5° in latitude and +/-4° in longitude TAO/TRITON : daily mean SST 30 RMS SST* = 0.52 °C 28 0°-170°W SSS 5°N-156°E All the SSS data are combined through an objective Laplacian interpolation within the 3°N-3°S band RMS SSS* = 0.22 psu * Surface values = 15 m

  8. Results (1/5) SSS within the 3°N-3°S band 34 35 TAO/TRITON ThermoSalinoGraph ARGO floats Eastern edge of the warm pool is characterized by a permanent SSS front of 0.4 psu over 2-5° in longitude around the 34.6 value 28.5°C isotherm

  9. Results (2/5) Relationships between SSS, precipitations and zonal currents in the warm pool region (similar results with the OLR)

  10. Results (3/5) Relationships between SSS and SST in the warm pool SST higher than 28°C SSS composite

  11. Results (4/5) Relationships between high SSTs, zonal wind and mixed layer depth in the warm pool region

  12. Results (4/5) Relationships between high SSTs, mixed layer depth and barrier layer thickness in the warm pool region Salinity composite profiles Depth SST in the 28-29.75°C range SST larger than 29.75°C BLT = DEPTHSST-0.5°C - DEPTHMLD Criterion Mean RMS SST > 28°C 18 m 18 m SST > 29.75°C 30 m 19 m

  13. Return to the indirect estimates Obs. SSS front (composite) Indirect estimation of the SSS front based on TAO/TRITON and altimetry (Maes and Behringer, 2000) SSTs higher than 28°C (data from Reynolds)

  14. Results (5/5) Impact of the salinity field on sea level variability Thermosteric SL Dynamic Height Anomalies Halosteric SL Consistent with the eastward propagation of downwelling Kelvin waves from the eastern edge of the warm pool as depicted by the advective-reflective oscillator.

  15. Conclusions and perspectives • A salinity front of 0.4 over 2-3° in longitude is a permanent feature of the hydrography at the eastern edge of the equatorial Pacific warm pool region. The Argo data allow a better representation of the front itself as well as its vertical structure. • The tight relationship near the salinity front between the very warm SSTs and the fetch of the westerly wind bursts suggests that a positive feedback is at work mediated by the barrier layer thickness. • 3. A better understanding and accurately simulating the salinity variability is requested to better represent the SST field and variability in models, and so forth, the coupling between the ocean and the atmosphere. • 4. The ongoing effort to deploy Argo floats will provide an unique opportunity to study the formation processes for the barrier layer at the eastern edge of the warm pool.

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