Elizabeth Maroon

1. Experiments with an idealized coupled GCM: the tropical precipitation response to extratropical heating Elizabeth Maroon

2. Outline • Motivations • Theory • Model 1: Idealized Atmosphere • Results I • Model 2: Idealized Atmosphere + Ocean • Imposed Heating • Results II • Conclusions

3. Motivations • Determine how changes in the extratropics can affect the tropical circulation • Study the roles of the atmosphere and ocean in the location of the ITCZ

4. The tropical climate is dominated by the Hadley Cells, with ascent in the Intertropical Convergence Zone (ITCZ) near the equator and descent in the subtropics. From Vallis, Atmos & Oceanic Fluid Dynamics

5. How is the location of the ITCZ determined? • Philander et al (1996) • Takahashi and Battisti (2006) • Kang et al (2009) Tilt of the continental coasts Influence of the Andes Anomalous Hadley Circulation from differential heating

6. Kang et al (2009) proposed a mechanism where a differential heating in the extratropics moves the ITCZ. From Kang et al 2009

7. An idealized atmospheric model, as described in Frierson (2005), is used to explore ITCZ shifts. • Gray radiation • Radiative fluxes function of temperature only • No water vapor, cloud or radiative feedbacks • Large Scale Condensation • Liquid water condenses/falls out of grid box immediately upon saturation • No seasonal or diurnal cycle • T42 horizontal resolution, with 25 vertical levels • Slab Mixed Layer Ocean

8. The same extratropical imposed heating structure as used in Kang et al (2008) and Kang et al (2009) was added to the atmospheric slab model. Heating Cooling

9. Results for imposed heating (and control) matched those of Kang et al (2009).

10. The change in precipitation is large in the tropics, but not in the extratropics, where all the heating was imposed.

11. MOMMA • Coupled model described in Farneti and Vallis, (2009) based on GFDL CM2.0 • Modular Ocean Model + Moist Atmosphere • For instance… “Yo MOMMA’s ocean basin so rectangular it makes Spongebob look like Patrick Star…”

12. MOMMA • Coupled model described in Farneti and Vallis (2009) based on GFDL CM2.0 • Modular Ocean Model + Moist Atmosphere • Sector model – 120˚ longitude, 168˚ latitude, 3.75˚ x 3˚ resolution • Same gray radiation atmosphere model with 7 vertical levels • Uses Modular Ocean Model 4, 24 ocean levels, 2˚ x 2˚horizontal resolution

13. A slanted coastline affects the circulation of the ocean. • Used 2 setups (provided by NevenFučkar at IPRC) that mimic the slant of coastlines in the Atlantic. 120˚ 70˚ 87˚ 30˚ EQ 20˚ Normal Reversed

14. Using a coupled ocean drastically changes the tropical precipitation structure.

15. Tropical SSTs have a different structure due to the nature of a coupled system.

17. So what happens if the same imposed heating is applied in the coupled model?

18. In this model, the ITCZ does not go toward the expected hemisphere when thinking about the slanted coastline theory.

19. With a “normal” coastline and closed Drake Passage, the ITCZ is south of the equator, but with the imposed heating, the ITCZ is in the Northern Hemisphere.

21. The control experiment with the reversed coastline has the ITCZ in the NH. The added heating has the ITCZ southward. *** The imposed heating reversed coastline run (blue) is not completed and may not have spun up sufficiently yet. ***

22. Although the change in ITCZ location is not very great, the change in the tropical precipitation structure is present.

23. With the normal coast, the Meridional Overturning Circulation (MOC) is dominant in SH for the control and in the NH with the imposed heating.

24. For reversed coasts, we see the mixed layer depth being largest in the NH for the control and the imposed heating cases.

25. When comparing the imposed heating and control runs with the normal coastline, there is most change to OHT than to AHT. Little Change in AHT More Change in OHT

26. With the reversed coast, AHT changes much more than OHT. Little Change in OHT More Change in AHT

27. Possible explanation that will require more testing: • Nonlinearity in the response of the ocean and atmosphere to an imposed extratropical heating • Normal case: Imposed heating creates more cold sinking water in NH. Strong OHT overcomes the imposed heating. • Reverse case: The change in OHT is small large, NH has most cold sinking water. Atmosphere Heat Transport wins over Ocean Heat Transport. Will need to run model longer to insure that it has completely spun up.

28. Conclusions • The ITCZ does not always move to the warmer hemisphere in this idealized coupled model. Adding a coupled ocean has a substantial effect. • The hemisphere with dominant MOC may not necessarily the dictate ITCZ hemisphere. • The role of the ocean is important. • More study and analysis needed.

29. Much, much thanks to my advisors DarganFrierson and David Battisti • NevenFučkar • Marc Michelsenand Harry Edmon • Officemates • Grads ’10/Housemates/friends • Family Acknowledgements

30. Selected References • Farneti, R. and G. K. Vallis, 2009: An Intermediate Complexity Climate Model (ICCMp1) based on the GFDL flexible modelling system. Geosci. Model Dev., 2,73-88. • Frierson, D. M. W., I. M. Held, and P Zurita-Gotor, 2006: A Gray-Radiation Aquatplanet Moist GCM. Part I: Static Stability and Eddy Scale. J. Atmos. Sci., 63, 2548-2566. • Kang, S. M., I. M. Held, D. M. W. Frierson and M. Zhao, 2008: The Response of the ITCZ to Extratropical Thermal Forcing: Idealized Slab-Ocean Experiments with a GCM. J. Clim.,21, 3521-3532. • Kang, S. M. , D. M. W. Frierson and I. M. Held, 2009: The Tropical Response to Extratropical Thermal Forcing in an Idealized GCM: The Importance of Radiative Feedbacks and Convective Parameterization. J. Atmos. Sci., 66, 2812-2827. • Philander, S.G.H., D. Gu, D. Halpern, G. Lambert, N.-C. Lau, T. Li and R. C. Pacanowski, 1996: Why the ITCZ is Mostly North of the Equator. J. Clim., 9, 2958-2972. • Takahashi, K. and D. S. Battisti, 2006: Processes Controlling the Mean Tropical Pacific Precipitation Pattern, Part I: The Andes and the Eastern Pacific ITCZ. J. Clim., 20, 3434-3451. • Trenberth, K. E. and J. M. Caron, 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. J. Clim., 14, 3433-3443.

31. More Slab Results II

32. More Slab Results III

33. More Slab Results IV