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C20C simulations at ICTP : Overview and analysis of teleconnections

C20C simulations at ICTP : Overview and analysis of teleconnections. Franco Molteni, Fred Kucharski and Annalisa Bracco Abdus Salam Int’l Centre for Theoretical Physics Third C20C Workshop, Trieste, 19-23 April 2004. C20C ensemble simulations at ICTP.

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C20C simulations at ICTP : Overview and analysis of teleconnections

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  1. C20C simulations at ICTP : Overview and analysis of teleconnections Franco Molteni, Fred Kucharski and Annalisa Bracco Abdus Salam Int’l Centre for Theoretical Physics Third C20C Workshop, Trieste, 19-23 April 2004

  2. C20C ensemble simulations at ICTP AGCM with simplified param.: SPEEDY T30 L8 (ver. 40) Prescribed SST from HadISST • 50-member ensemble for 1949-2002, global obs. SST (monthly means of full output) • 10-member ensemble for 1870-2002, global obs. SST (monthly m. from 1870 + selected daily means from 1949) • 9-member ensemble for 1949-2002, obs. SST in the tropics (25N-25S) + 50-m slab mixed-layer elsewhere (selected monthly means)

  3. Results: • A “minimal” model climatology • Regression maps vs. Nino3.4 index • EOFs of 500_hPa geop. height for different regions of the NH extratropics • Time series of PNA PC-1 vs Nino3.4 • Interdecadal trends of NH Z_500 from the 1950’s: • Ensemble mean vs. re-analysis • Taylor diagrams (amplitude – correlation with E.M.) • Single-run trends • Interdecadal variations in PDF of NH (PC1, PC2)

  4. The ICTP simplified AGCM (SPEEDY): features and advantages • PE dynamical core (GFDL), simplified parametrizations of radiation, convection, large-scale cond., vertical diff. and surface fluxes of momentum, heat and moisture (ICTP). • 5, 7 or 8-level versions, T30 (3.75 deg.) hor. resolution. • Simple program structure: makes easy to develop and test code updates. • Computationally efficient (~25 min-CPU/year on Xeon 2.4 GHz): a 50-year simul. can be run in < 1 day on 1 proc. • Large ensembles of simulations may be run to address climate predictability issues on multi-decadal time scales. • Results (mostly) comparable to state-of-the-art AGCMs.

  5. SPEEDY progress: the 500-hPa height climatology in DJF

  6. DJF rainfall climatology

  7. Regressions vs. the Nino3.4 index from HadISST (DJF 1950-2002)

  8. Speedy Re-analysis EOF1 WNH (210W 30E) EOF2

  9. Speedy Re-analysis EOF1 Eu-Atl (90W 60E) EOF2

  10. Speedy Re-analysis EOF1 PacNA (210W 60W) EOF2

  11. Nino3.4 vs. PNA PC-1 in DJF Nino 3.4PC1 ReAn. (cor = 0.46) Ens. Mean PC1 (cor = 0.84) Ens. Member PC1 (cor = 0.45 – 0.65)

  12. Interdecadal trends of SST and 500-hPa height in DJF 1977/2001-1952/76 What part of the height trend can be interpreted as a response to the SST trend, and what part is due to internal atmospheric variability ?

  13. How robust is the 50-yr trend of ENSO indices ? Linear trend of the bivar. ENSO index estimated from:

  14. Observed vs. ensemble-mean trend of 500-hPa height (1977/2001 – 1952/1976) Note : ens. mean contour scale is half of re-analysis scale

  15. Taylor diagrams for interdecadal trendsr = rms amplitude / obs. amplitudex = norm. projection on ens. mean y = component orthogonal to ens. meancos(a) = pattern correlation with ens. mean

  16. How close are trends simulated in single experiments to the observed trend ? Best match in the 50-member ensemble (same scale!)

  17. Interdecadal variations of (PC1, PC2) PDF for the NH : ReAnalysis P(x,y) - P(x)*P(y) P(x,y) – P(x)*P(y) 1977-2001 P(x,y) P(x,y) – P(x)*P(y) 1952-1976

  18. Interdecadal variations of (PC1, PC2) PDF for the NH : Speedy P(x,y) - P(x)*P(y) P(x,y) – P(x)*P(y) 1977-2001 P(x,y) P(x,y) – P(x)*P(y) 1952-1976

  19. Conclusions (1) • Regression patterns vs Nino3.4 are fairly well simulated by SPEEDY in DJF, but the u-stress in the Nino4 region is too weak. Also, an excessive intensification of summer monsoon rainfall over SE Asia is produced. • EOFs of monthly-mean 500-hPa height in various sectors of the NH in DJF show a close resemblance to the observed EOFs (less so for 2nd Eur-Atl EOF). • The Nino3.4 index is highly correlated to the leading PC of the PNA sector in the ensemble mean (84%).

  20. Conclusions (2) • Interdecadal differences in DJF 500-hPa height are fairly well reproduced in terms of pattern correlation (67%), but with an amplitude ranging from 40% to 60% of the observed signal when the ensemble mean is considered. • Trend patterns in a few individual ensemble members show an amplitude close to the observed one. • Interdecadal differences in the PDF of 500-hPa height PCs can be reproduced to a good extent for the last half of the 20th century, with stronger signature of regime-like behaviour in the 3rd quarter than in the last 25 years.

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