Remote Influences on the

Tropical Atlantic Climate System

R. SaravananNational 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; …)– 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?

Sutton et al, 2000

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

NCAR Community Climate Model (CCM3)

• Atmospheric General Circulation Model

• T42 horizontal resolution (2.8 degrees lat/lon)

• 18 vertical levels

• Prescribed SST boundary condition

CCM3 integrations (5x45 years)

• GOGA (Global Ocean - Global Atmosphere)

• TOGA (Tropical Ocean – Global Atmosphere)

• TAGA (Tropical Atlantic – Global Atmosphere)

• TOGA - TAGA

SST standard deviation (timescales < 6 yrs)

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

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

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

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

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

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?

ENSO correlations(Klein et al, 1999)

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

SST index evolution (Dec 1982 - Aug 1983)

OBS MODEL

SST index evolution (Dec 1988 - Aug 1989)

OBS MODEL

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

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

Tropical SST indices(Klein et al, 1999)

Tropical Atmospheric Bridge(Klein et al, 1999)

Enfield & Alfaro, 1999 (Fig. 2)

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

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

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”

Concordant cases

JJA0

SON0

DJF1

Warm Cold

Discordant cases

JJA0

SON0

DJF1

Warm Cold