CLIVAR/CCSM Tropical Biases Workshop (May 28-30, 2003, GFDL)

1. CLIVAR/CCSM Tropical Biases Workshop(May 28-30, 2003, GFDL) Chris Bretherton University of Washington ( Thanks to Ping Chang, Leo Donner, Jim Hack, Meghan Cronin, Peter Gent , Ken Sperber, and others)

2. Workshop Themes • Status of biases in major coupled models Double ITCZ and E Pacific ENSO and seasonal variability • Are observational uncertainties pacing progress? • Are idealized and reduced models helping?

3. NCAR CCSM GFDL CM2 CSIRO HadGEM1 MPI BMRC INGV CCSR, U. Tokyo FRSGC, Earth Simulator NSIPP (NASA Goddard) COLA IPRC, U. Hawaii Participating Centers and Models Observations, Process Modeling, Diagnostics Obs: Cronin, Kessler, Raymond, Mapes, L. Yu, CRM/Idealized models: Grabowski, Khairoutdinov, Sobel Diagnostics: NOAA/CDC, etc.

4. Observations Coupled Models • Double ITCZ bias • in ALL models! Sperber et al. (2003)

5. AGCMs forced by observed SST do not consistently have double ITCZ

6. Coupled Model SST biases: ‘too cold’ tongue warm subtropical NE, SE Pac, SE Atl

7. Status of Biases in Major Coupled Models • Recent development cycles of major models have not reduced double ITCZ and ‘too-cold tongue’ biases. • E Pacific biases have reduced in models with realistic stratocumulus cloud forcing. • However, warm biases off the S American and S African coast in coupled models persist. • No ‘smoking gun’ in model physics.

8. Sensitivity studies: Results model-dependent • AGCM parameterization sensitivities model-dependent • cumulus momentum transport • convective rainfall evaporation • super-parameterization • OGCM sensitivities also not smoking guns • Dependence of penetrating solar radiation on upwelling reduces ‘too-cold tongue’ but not enough. • Artificially changing surface winds ameliorates too-cold tongue in some models, but AGCMs don’t make consistent wind-stress errors. • Effects of higher resolution also mixed • Does not remove major systematic biases.

10. Possible causes of biases • Deep convective feedbacks with PBL, SST wrong--> double ITCZ • Bad coastal winds (continental effect, resolution,…) --> bad coastal upwelling --> stratus region too warm --> double ITCZ • Solar and longwave cloud forcing wrong --> stratus region too warm --> double ITCZ • Ocean mixing wrong --> cold tongue too cold --> double ITCZ • Air-sea interaction wrong --> PBL not properly stabilized over cold tongue --> trades & upwelling too strong --> cold tongue too cold • Biology not included --> penetrative radiation too large --> cold tongue too cold (Thanks, Meghan)

11. What does EPIC have to contribute? • EPIC-inspired idea for warm bias in SE/NE Pacific SSTs: GCMs have ‘stratofogulus’ in the EPIC region so maybe high surface relative humidity leads to insufficient ocean cooling from latent heat fluxes. Mean profiles at 20S 85 W: 16-21 Oct 2001 (obs, NCEP, ECMWF) Oct AGCM climatology (CAM2, AM2)

12. CAM2 surface energy budget compared with WHOI stratus buoy Excessive SW heating Too little heat into ocean (yet CCSM2 has warm SST bias!) Excessive LW cooling SHF+LHF surprisingly good

13. EPIC 95 W region does not show systematic atmospheric, SST biases in coupled models • This means coupled modelers probably won’t come running for the 95W datasets (unless their subsurface oceanography looks awful). • Nevertheless, there are worthy goals for us to aim for: - a well-prepared TAO-EPIC dataset - a nice 95W composite atmospheric cross-section - useful, easily tested constraints on ITCZ convection • Combined with applicable insights into component physical processes, these may allow EPIC to improve climate models so as to reduce these biases.