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West African Rainfall and seasonal tropical Atlantic circulation in CFS

This study examines seasonal rainfall patterns and tropical Atlantic circulation in climate forecast models, focusing on the accuracy of representing the seasonal cycle with implications for regional forecasts like Nordeste Brazil. Comparisons between different models reveal errors in SST, winds, shortwave radiation, and more affecting the Atlantic climate variability. Insights are drawn from relationships between surface variables, ITCZ errors, and humidity discrepancies, shedding light on possible model biases. Challenges and potential solutions related to cloud specifications and oceanic influences are also discussed.

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West African Rainfall and seasonal tropical Atlantic circulation in CFS

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  1. West African Rainfall and seasonal tropical Atlantic circulation in CFS James Carton1, Ching-Yee Chang, Semyon Grodsky, and Sumant Nigam(UMD),Jiande Wang and Wanqui Wang (NCEP)(1carton@atmos.umd.edu) • Most CGCM efforts have concentrated their attention on the Pacific. But, the Atlantic has interesting and important climate variability. • Because of the dominance of seasonal variability in the Atlantic, errors in our representation of the seasonal cycle will project onto seasonal forecasts, e.g. Nordeste Brazil. Thanks: S. Saha (NCEP)

  2. How CFS mean state looks in comparison with other CGCMs:SST averaged 2S-2N CFS (Davey et al, 2002)

  3. Surface variables

  4. Seasonal SST CFS OBS Error is in the SH CFS-OBS

  5. Seasonal winds CFS SSMI OBS Strong coastal winds Northerly anomalies

  6. Seasonal rainfall Southern displacement ITCZ too intense

  7. Seasonal net shortwave CFS OBS CFS-OBS

  8. Seasonal latent heat CFS OBS CFS-OBS

  9. PIRATA moorings

  10. Seasonal short wave AMIP OBS CFS

  11. Seasonal relative humidity OBS AMIP CFS

  12. Seasonal latent heat OBS AMIP CFS

  13. AMIP Precipitation errorsCFS vs AMIP mod OBS • ITCZ error similarities: • Intensification • Southward displacement Next: w vs lat CFS-OBS AMIP-OBS What we saw before

  14. Vertical velocity with latitude (20W-0W) ITCZ convection too intense CFS OBS CFS-OBS

  15. AMIP Latent heat flux errorsCFS vs AMIP mod OBS • ITCZ error similarities: • Intensification • Southward displacement AMIP-OBS CFS-OBS What we saw before

  16. AMIP Relative humidity errorsCFS vs AMIP OBS AMIP-OBS CFS-OBS What we saw before

  17. Ocean

  18. Seasonal equatorial SST

  19. Effect of misplaced ITCZ?: enhanced semiannual cycle in HC JAN 17C 19C JAN

  20. Effect of high evaporation: high surface salinity in the SH (Jul-Sep) CFS 37 37 Amazon’s missing 37 37 Too salty

  21. Cause of Southern Hemisphere ITCZ Conceptual model of Yu and Mechoso (1999) for the ITCZ errors in the UCLA model, eastern Pacific Can this explain what’s going on in the western tropical Atlantic in CFS?

  22. Interannual variability

  23. Remote influence Seasonal correlation of (SST, winds)AMJ and NINO3DJF CFS OBS

  24. Issues AGCM • Excess SW heat gain implying too little cloudiness in the east. • Explains: SST error – upwelling is actually stronger than observed • Puzzles: • Why is the SST error primarily in the south? • Why is there so little low level cloud cover in CFS and AMIP? • Excess latent heat loss • Explains: high salinities, High precip in ITCZ. • Puzzle: Why is RH < 80% over the ocean in both CFS and AMIP? OGCM • Excess sea surface salinity • missing river discharge • Explains: may explain why the thermocline is too deep as the result of too much tropical water formation. CGCM • Boreal winter ITCZ southward displacement in the west • Explains: Semiannual thermocline slope variations

  25. Seasonal stratocumulus experiment • UCLA CGCM: perpetual winter and summer experiments with prescribed clouds does cool the SST in the southeast. Will seasonal specification of clouds address bias in meridional winds and ITCZ convection? Yu and Mechoso (1999)

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