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Remote Influences on the Tropical Atlantic Climate System

Remote Influences on the Tropical Atlantic Climate System. R. Saravanan National Center for Atmospheric Research (Collaborators: Alessandra Giannini, NCAR; Ping Chang & Marcelo Barreiro, Texas A&M). Sources and Mechanisms of Remote Influence. ENSO (Covey & Hastenrath, 1978; …)

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Remote Influences on the Tropical Atlantic Climate System

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  1. Remote Influences on the Tropical Atlantic Climate System R. Saravanan National Center for Atmospheric Research (Collaborators: Alessandra Giannini, NCAR; Ping Chang & Marcelo Barreiro, Texas A&M)

  2. Sources and Mechanisms of Remote Influence • ENSO (Covey & Hastenrath, 1978; …) • Midlatitude atmospheric bridge (Rossby wave propagation) • Nobre & Shukla (1996), … • Tropical atmospheric bridge (Walker circulation) • (…; Saravanan & Chang, 2000; Chiang et al, 2000; …) • NAO • “Southern lobe” of the Atlantic SST tripole • Grotzner et al (1998), Czaja et al (2002) • South Atlantic? • Southern analogue of the NAO? • Influence on the Atlantic ENSO?

  3. Sutton et al, 2000

  4. SST, HFLX, Wind vs. NINO3, -NAO(Czaja et al, 2002)

  5. NCAR Community Climate Model (CCM3) • Atmospheric General Circulation Model • T42 horizontal resolution (2.8 degrees lat/lon) • 18 vertical levels • Prescribed SST boundary condition

  6. CCM3 integrations (5x45 years) • GOGA (Global Ocean - Global Atmosphere) • TOGA (Tropical Ocean – Global Atmosphere) • TAGA (Tropical Atlantic – Global Atmosphere) • TOGA - TAGA

  7. SST standard deviation (timescales < 6 yrs)

  8. SST – NINO3 correlation (MAM)(Saravanan & Chang, 2000)

  9. SST – Nordeste Precip sq. correlation (MAM)(Saravanan & Chang, 2000)

  10. MAM 200 hPa wind regression vs. (NINO3, NTA)(Saravanan & Chang, 2000)

  11. MAM Precipitation regression vs. (NINO3, NTA)(Saravanan & Chang, 2000)

  12. Conclusions • ENSO clearly is a major influence on TAV • Upper level flow resembles large-scale Gill-type response to equatorial heat source • Surface flow does not! • NAO is also a major influence

  13. Open Questions • What is the “horizontal propagation” mechanism for the ENSO influence on TAV? • Midlatitude Rossby wave propagation? • Walker cell mechanism? • What is the “vertical” mechanism for inducing SST anomalies? • Windspeed changes and associated latent heat flux anomalies? • Tropospheric warming/changes in static stability? • Changes in cloudiness? • Latent heating vs. sensible heating vs. radiative heating? • Are there interactions between the different remote influences? • “Preconditioning” of the response to ENSO • Is there a role for the subtropical South Atlantic?

  14. ENSO correlations(Klein et al, 1999)

  15. NINO 3.4 index(Kaplan SST data set; base period 1951-1980)

  16. OBS MODEL SST index evolution (Dec 1982 - Aug 1983)

  17. OBS MODEL SST index evolution (Dec 1988 - Aug 1989)

  18. NTA SST vs. NAO/NINO3(Czaja et al, 2002)

  19. SLP (JFM) vs. NINO3/NAO correlation(Czaja et al, 2002)

  20. Tropical SST indices(Klein et al, 1999)

  21. Tropical Atmospheric Bridge(Klein et al, 1999)

  22. Enfield & Alfaro, 1999 (Fig. 2)

  23. HFLX – N. Tropical Atlantic SST sq. correlation (MAM)(Saravanan & Chang, 2000)

  24. MAM Surface pressure regression vs. (NINO3, NTA)(Saravanan & Chang, 2000)

  25. Preconditioning role of Tropical Atlantic Variability(Giannini, Saravanan, and Chang, J. Climate, submitted) • If there is no prior tropical Atlantic SST anomaly during the onset phase of ENSO • ENSO teleconnection follows the expected course • Warm event is correlated with a decreased in Nordeste rainfall and vice versa • If there is a prior tropical Atlantic SST anomaly during the onset phase of ENSO • ENSO impact can be diminished, or even reversed, if the Atlantic anomaly is “discordant”

  26. Concordant cases Warm Cold JJA0 SON0 DJF1

  27. Discordant cases Warm Cold JJA0 SON0 DJF1

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