Ocean-Atmosphere Interactions and El Niño Lisa Goddard Advanced Training Institute on Climatic Variability and Food Security 2002 July 9, 2002
Coupled Behavior in tropical Pacific SST Winds Upper Ocean Structure (Thermocline)
Tropical Pacific Average State Walker Circulation
Pacific Ocean Temperatures along Equator
Coupled Behavior in tropical Pacific (or any equatorial ocean basin) SST Winds Upper Ocean Structure (Thermocline)
What is so special about the Pacific Ocean?
Equatorial Pacific spans nearly ½ of Earth s circumference Long time delay for negative feedback due to adjustment of off-equatorial perturbations Magnitude of coupled growth Potential predictability of future evolution
Wind Anomaly applied for 30 days Warm SSTa Dynes/cm**2 Response of upper-ocean structure - - +Warm SSTa
Evolution of upper-ocean structure (or thermocline) anomalies Perturbations move eastward on the equator; westward off the equator Perturbations move slower as latitude increases
Continuing Evolution of upper-ocean structure (or thermocline) anomalies Warm SSTa At western boundary, waves are reflected and channeled onto equator Delayed negative feedback Warm SSTa
Equatorial Pacific spans nearly ½ of Earth s circumference Long time delay for negative feedback due to adjustment of off-equatorial perturbations Magnitude of coupled growth Potential predictability of future evolution Large longitudinal shift in western Pacific convection Shifts in tropical rainfall and subsidence Shifts in mid-latitude storm tracks
Average Conditions during Oct-Nov-Dec Sea Surface Temperature Precipitation
Conditions during Oct-Nov-Dec 1997 Sea Surface Temperature Precipitation SST Anomalies Precip. Anomalies
Conditions during Oct-Nov-Dec 1998 Sea Surface Temperature Precipitation SST Anomalies Precip. Anomalies
November 1997 : peak El Niño SST Anomaly Outgoing Longwave Radiation (OLR) Anom. Low-level wind anomalies (925mb) Upper-level wind anomalies (200mb)
November 1998 : peak La Niña SST Anomaly Outgoing Longwave Radiation (OLR) Anom. Low-level wind anomalies (925mb) Upper-level wind anomalies (200mb)
Anomalous SST [gradients] Anomalous low-level winds Anomalous convergence/rainfall Anomalous upper-level winds Anomalous subsidence
Teleconnection of El Niño to other tropical ocean basins Indian Ocean (~ 1/3 size of Pacific) - dynamical forcing from tropical Pacific potential for coupled ocean-atmos. growth - thermo-dynamical forcing Atlantic Ocean (<1/3 size of Pacific) - N.Atlantic variability related to Pacific variability - Coupled growth possible in eastern equatorial Atlantic, but not explicitly related to Pacific variability
Coupled Ocean-Atmosphere Dynamics in the Indian Ocean during 1997-98 (Webster et al. 1999, Nature) In the Pacific Growing El Niño In the Indian Mature El Niño Peak El Niño Growing La Niña
Importance of regional SST forcing to regional atmospheric response EXAMPLE : The Indian Ocean and eastern Africa
Association between Pacific Ocean SSTa and anomalous rainfall over eastern Africa :Global Ocean / Global Atm. :Pacific Ocean / Global Atm. Goddard & Graham, 1999 JGR-Atmos.
Association between Indian Ocean SSTa and anomalous rainfall over eastern Africa :Global Ocean / Global Atm. :Indian Ocean / Global Atm. Goddard & Graham, 1999 JGR-Atmos.
Conclusions Coupled ocean-atmosphere interaction occurs in all tropical ocean basins. Tropical Pacific is central to coupled climate system because its large size allows for: - relatively long timescales, leading to potential predictability of El Niño; - large amplitude growth of coupled anomalies; - potential for sustained oscillations (El Niño/La Niña); - large spatial shifts in convection, and thus atmospheric heating, impacting global circulation.
Conclusions (cont.) Atmospheric circulation changes induced by El Niño / La Niña often modify SST in other tropical ocean basins. SST anomalies in the Indian and tropical Atlantic Oceans can play significant role in effecting climate variability of neighboring regions, that may be modified by the atmospheric response to SST anomalies in the tropical Pacific.