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Double ITCZ Problem in IPCC AR4 GCM’s

Double ITCZ Problem in IPCC AR4 GCM’s. Marcus D. Williams April 15, 2009. Outline. The role of the ITCZ in the tropics and it’s importance to tropical circulations Reasons for the Double-ITCZ problem in the IPCC AR4 coupled GCMs

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Double ITCZ Problem in IPCC AR4 GCM’s

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  1. Double ITCZ Problem in IPCC AR4 GCM’s Marcus D. Williams April 15, 2009

  2. Outline • The role of the ITCZ in the tropics and it’s importance to tropical circulations • Reasons for the Double-ITCZ problem in the IPCC AR4 coupled GCMs • A graphical look at the overestimation of precipitation compared to observations • Biases in the Models ocean-atmosphere feedback • Summary

  3. The importance of studying the ITCZ • The ITCZ is the region where the low-level air converges and rises into the upward branch of the Hadley/Walker circulation. The latent heat released in the ITCZ drives the Hadley/Walker circulation. • Hadley/Walker circulation is the largest circulation in the troposphere. A model that fails to simulate the Hadley/Walker circulation correctly cannot be expected to provide good forecast. • The location and intensity of the ITCZ affect the surface wind field, which is a critical factor in air-sea interaction-- a core component of El Nino. • A large part of the difficulties in forecasting El Nino has to do with the GCMs’ failure to simulate the ITCZ correctly. • Most GCMs have difficulties in correctly simulating the seasonal variation of the MJO intensity. This is largely a result of the models’ failure in correctly simulating the seasonal movement of the ITCZ. • The ITCZ study has led to new insight into the monsoon and monsoon onset. • The current GCMs have considerable amount of difficulty in simulating the ITCZ precipitation correctly.

  4. Reasons for Double ITCZ problem • Study preformed on 22 IPCC AR4 coupled GCMs and 12 Atmospheric Model Intercomparison Project (AMIP) model runs. Analysis based on 20 yr (1979-2009) simulations. • Most models have some degree of the double ITCZ problem characterized as excessive precip in the off Equatorial Pacific and insufficient precip over the Equatorial Pacific. • Excessive precip over the Tropics usually causes overly strong trade winds, excessive latent heat flux (LHF), and insufficient Shortwave radiation flux (SWF), leading to significant cold SST bias in much of the Tropical oceans. • The double-ITCZ problem was found to cause three bias in the ocean-atmosphere feedback over the equatorial Pacific. • Excessive Bjerknes mechanism feedback • Overly positive SST-LHF feedback • Insufficient SST-SWF feedback

  5. Models output SST and precip Annual mean SST (shading) and precipitation (contour) for IPCC AR4 CGCMs. Contour starts at 5 mm/day with an interval of 2 mm/day.

  6. Annual mean SST AVHRR Pathfinder 2006 Annual Mean SST

  7. SST observations

  8. Precipitation data

  9. Zonal mean precipitation

  10. Bjerknes Feedback • Caused by excessive sensitivity of precipitation to SST and overly strong time mean surface wind speed • SST gradient between the warm pool and cold tongue generates an east-west asymmetry in the atmospheric convection, precipitation, clouds, and water vapor • Creates an east-west asymmetry in the total diabetic heating • Forces the SLP gradient thus strengthening the trade winds (Walker circulation) • Trade winds feed into the SST gradient by inducing upwelling

  11. Zonal wind stress Overly strong wind stress

  12. Linear regression of wind stress

  13. SST-surface LHF feedback • Caused by excessive sensitivity of surface air humidity to SST • Perturbation in SST affects the surface wind speed, surface air humidity, and sea-air humidity difference. • Modifies the surface LHF, which in turn modifies the SST through evaporation • SST-LHF feedback has a different sign for different regions

  14. Latent Heat Flux (LHF) Latent Heat flux too strong

  15. Linear regression of LHF

  16. SST-surface SWF feedback • Caused by insufficient sensitivity of cloud amount to precipitation • Feedback has a different sign over the warm pool and cold tongue. • Negative over the warm pool because of convection reducing SWF to the surface and cools the SST • Positive over the cold pool because of the static stability over the layer causing the SST to warm

  17. Zonal mean Shortwave radiation flux SWF underestimated in most models

  18. Linear regression of SWF

  19. Summary

  20. Summary • Any positive feedback tends to enhance the east-west SST gradient • Negative feedback weakens horizontal SST gradient, which weakens the Walker circulation • Strong positive feedbacks tend to shift the whole system westward, leading to an excessive SST cold tongue and double ITCZ pattern • The model biases where found to be intrinsic errors in the atmospheric models • Some cases the model biases interfere with or enhance the feedback loop • The ocean drives the atmosphere, and in turn the atmosphere forces the ocean so it’s difficult to determine the real root of the biases

  21. References • Lin JL (2007) The Double-ITCZ Problem in IPCC AR4 Coupled GCMs: Ocean-Atmospheric Feedback Analysis. Journal of Climate 20: 4497-4525 • Misra V et al. (2008) The Equatorial Pacific Cold Tongue Bias in a Coupled Climate Model. Journal of Climate 21: 5852-5869

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