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North Atlantic Climate Change as a Response to Indian Ocean Warming. Jim Hurrell Climate Analysis Section, NCAR jhurrell@ucar.edu Marty Hoerling Climate Diagnostics Center, NOAA/ERL. Climate Interactions Between High and Low Latitudes and Stratosphere-Troposphere Interactions

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North Atlantic Climate Change as a Response to Indian Ocean Warming


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    1. North Atlantic Climate Change as a Response to Indian Ocean Warming Jim Hurrell Climate Analysis Section, NCAR jhurrell@ucar.edu Marty Hoerling Climate Diagnostics Center, NOAA/ERL Climate Interactions Between High and Low Latitudes and Stratosphere-Troposphere Interactions 9-10 September, 2002 Bergen, Norway

    2. The North Atlantic Oscillation Spatial Structure Temporal Evolution Dec-Mar SLP (hPa) Dec-Mar • •

    3. Leading patterns of North Atlantic climate variability Sea Surface Temperature 500 hPa Geopotential Height

    4. ATMOSPHERIC GCM RUNS: 1950-1999 NCAR CCM3 (T42,L18) GOGA: Monthly evolving globalSSTand sea ice (12 members) TOGA: Monthly evolving tropical SST (12 members) TAGA: Monthly evolving tropical Atlantic SST (5 members) (fixed solar insolation and trace-gas composition)

    5. r = 0.8 CCM3 GOGA ENSEMBLE NAO Response to Global Ocean Forcing 500 hPa Geopotential Height OBSERVED

    6. Winter SST Regressed on NAO time series OBS CCM3

    7. DJF 500 mb Height Trend (1950-1999) m OBS GOGA (x2) TOGA (x2)

    8. Spatial Anomaly Correlations (30°–80°N, 100°W-80°E) EE Ensemble Members –0.7 –0.5 –0.3 –0.1 0.1 0.3 0.5 0.7 Avg 0.55 0.49 Reproducibility

    9. Normalized Spatial RMS Amplitude (30°–80°N, 100°W-80°E) EE Ensemble Members 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Avg 0.63 0.61 Reproducibility

    10. JFM TOGA Height Trends (1950-1999) m 50 hPa 500 hPa 1000 hPa

    11. TAGA mm day-1 JFM Total Precipitation Trend (1950-1999) TOGA mm day-1

    12. Empirical Evidence for Indian Ocean Forcing Winter height anomalies linearly associated with enhanced rainfall over the Indian Ocean 50 hPa 500 hPa 1000 hPa Empirical rainfall anomaly of 10 mm day-1 centered at 0°,60°E

    13. 50 hPa 500 hPa 1000 hPa Dynamical Evidence for Indian Ocean Forcing Idealized SST anomaly: 1.0°C maximum 45°-90°E; 5°S-5°N

    14. JFM Trend in Tropical SST (1950-1999) (°C per 50 years) Indo-Pacific “Warm Pool” (60°-170°E; 15°S-15°N) ENSO Index

    15. IDEALIZED SST ANOMALY EXPERIMENTS • NCAR CCM3 (T42,L18) • Specify pattern and amplitude of 50-yr SST trend: • over tropics (20°S-20°N) • over Indo-Pacific (45°-170°E) • 25-member ensembles, based on seasonal evolution of trend • anomalies defined relative to climatological SST integration

    16. Tropic-wide Indo-Pacific 50 hPa 50 hPa 500 hPa 500 hPa 1000 hPa 1000 hPa

    17. Concerning observed and simulated North Atlantic climate changes • The basic structure of the NAO results from internal, nonlinear atmospheric dynamics • GOGA runs, however, suggest a low frequency component of the NAO is not merely stochastic climate noise, but a response to global SSTs • TOGA simulations reproduce much of the trend evident in the GOGA runs, suggesting a more passive role for higher latitude SST changes • The observed trends are not reproduced in TAGA simulations, suggesting tropical Atlantic SSTs alone do not drive the observed low frequency changes over the North Atlantic • Idealized SST anomaly experiments suggest the trend in North Atlantic climate can be understood as a remote response to the trend in tropical SSTs, with warming in the Indo-Pacific region driving much of the change • What are the dynamics of the teleconnection: how model-dependent are the results: what is the source of the SST trend?