Understanding Zonal Mean Precipitation Changes in a Warmer Climate

1. Understanding Zonal Mean Precipitation Changes in a Warmer Climate Dargan M. W. Frierson University of Washington, Department of Atmospheric Sciences PCC Summer Institute, 9-14-11 Collaborators: Yen-Ting Hwang (Uw), Jack Scheff (UW)

2. How Can Precipitation Change? • I’ll argue that precipitation changes can be usefully separated into two components • Changes in intensity of features • “Wet gets wetter” with warmth • Changes in position of features • Poleward shifts of midlatitude storms with warmth • Tropical precipitation shifts towards the hemisphere with more heating

3. Changes in Precip in 21st Century Simulations • Multi-model mean precip change • With stippling based on a weak significance criteria

4. 21st Century Simulations Summary Most confidence in high latitude moistening Lots of subtropical drying, occasionally with confidence Tropical moistening (but little confidence) Why?

5. Changes in Water Vapor in AR4 Simulations Changes in water vapor content (% increase) versus temperature change (K) for change over 21st century (A1B scenario): Water vapor increases by 10-25% with 1.5-3 K warming. Spread among models is mostly due to spread in amount of warming. Held and Soden 2006

6. Why Wet Get Wetter More moisture in the atmosphere q more moisture flux vq wet get wetter This explains tendency for high latitudes and tropics to moisten

7. Precipitation Changes with Warming • Global average P rises much more slowly though • Significantly less than water vapor content increase, around 1-3% per K • So some regions have to dry… Per K increase of P varies by over a factor of 3 in models! See Angie Pendergrass’s poster for why: black carbon prescriptions are the primary culprit Held and Soden 2006

8. Why Global P Changes Slowly • Weak global average precip change because… • From surface budget perspective: • From the atmospheric energy budget perspective: Net surfacelongwave Solar Sensible heat flux Ocean heat storage Evaporation is constrained by solar radiation input, etc Precipitation is constrained by the ability of the atmosphere toshed heat Net radiative cooling of the atmosphere Sensible heatflux

9. Why Dry Regions Persist/Expand • So some regions have to dry as well… • Subtropics are the main place where this happens • Part of this is because more moisture is fluxed away from there (flux gets more negative) Actual (solid) and thermodynamic prediction(dashed) of P-E change with global warming Held & Soden 2006

10. Poleward Shift of Eddies • Eddy kinetic energy changes from Yin 2005 • Black contours are current mean, colors are predicted change • Poleward (and upward) shift with global warming

11. Poleward Shifts of Midlatitude Storm Tracks • Midlatitudeprecip moves along with the storm tracks: • Moistens high latitudes • Dries on the equatorward side of the storm track Moistening on poleward side Drying on equatorward side From Scheff and Frierson (submitted to J. Climate): Storm track shifts are the primary cause of significant drying

12. Extra-tropical summary • Wet regions get wetter (high latitudes) • Dry regions persist/expand poleward • And land surfaces tend to get more arid unless precip goes up b/c potential evaporation increases (see Jack Scheff’s poster) • What about the tropics? • We generally expect moistening but we don’t have much confidence there

13. Hadley Circulation The Hadley Cells transports energy poleward, and moistureinto the deep tropics Energy flux by upper branches Moisture flux by lower branches Dima and Wallace (2003)

14. Zonally averaged precip and evap Hadley cell is key to converging moisture towards the equator Evap and precip annual means (NCEP Reanalysis 2)

15. Seasonal Shifts of Tropical Precipitation Tropical rain follows the warmth in the seasonal cycle (TRMM climatology)

16. Seasonal Hadley Cell/Precip January (top) and July (bottom) Hadley cells & precip Precip shift southward Energy transport from warm to cold Energy transport from warm to cold Precip shift northward

17. Energetics and Tropical Precipitation Shifts • Claim: whenever one hemisphere is heated more, some of that heat is fluxed to the other hemisphere • Resulting in an ITCZ shift in the other direction • Even if the forcing is purely extratropical, the ITCZ will still respond (see Ph.D. thesis of Sarah Kang) • Eddies bring some of the response into lower latitudes • Approach: correlate precip shift with cross-equatorial energy transport • Then use the atmospheric energy budget to attribute the shift to different heating terms

18. Change in Precip, AR4 Slab Ocean Models Moistening intropics and mid/high latitudes mm/year Drying in subtropics Plot by Yen-Ting Hwang

19. Change in Precip, AR4 Slab Ocean Models Huge variance in tropics though! mm/year 60 cm/yr difference in precip! Plot by Yen-Ting Hwang

20. Change in Precip, AR4 Slab Ocean Models mm/year Big differences in SH too: 10 cm/yr Plot by Yen-Ting Hwang

22. Precip shift versus cross-eq energy flux Anticorrelated: Hadley cell governs both If we can explain the energy flux changes, we can explain the ITCZshifts

23. Change in Precipitation in Extreme Cases CCCMA (most S-ward) MPI (most N-ward) mm/year Seen across most longitudes, and over continents as well Plot by Yen-Ting Hwang

24. Surface Albedo + Cloud Feedbacks CCCMA has more net heatingin SH: ITCZ shifts south MPI has more heating in NH: ITCZ shifts north Plot by Yen-Ting Hwang

25. Feedbacks Surface albedo Cloud Shortwave Lots of ice melting Low clouds form

26. 1-D Energy Balance Model for Energy Fluxes • Prescribe latitudinal structure of forcings/feedbacks: • Surface albedo changes • Cloud radiative feedbacks • Ocean heat uptake • Aerosol scattering/absorption • Predict: • Energy fluxes • Temperature changes • Clear sky outgoing radiation changes • Assumes constant diffusivity! See Frierson and Hwang (in press) for more details

27. EBM Prediction for Slab Models R = 0.91

28. Importance of Extratropical Forcing EBM forced by terms outside of the tropics only (poleward of 20o N/S) R = 0.86 Extratropical forcing explains the range in ITCZ shifts

29. Relevance to Precipitation Climatology? • Why does tropical precipitation peak more in the Northern Hemisphere? • Let’s study from the energetics perspective… GPCP Climatology

30. Why is the tropical precip mostly in the north? Total energy flux from CERES: More energy inputinto the Southern Hemisphere!!

31. Ocean heat flux The ocean heat flux is northward though… EQ

32. My argument • Ocean heat flux (MOC) drains all the cold water out of the Atlantic • Atmospheric heat fluxes respond & flux energy back southward • Tropical precip goes to the north • The ocean causes the rain to be in the north • If true, this is one of the most important roles of the ocean in the current climate?? • See recent work by NevenFučkar (U Hawaii) in a coupled GCM

33. Conclusions • In a warmer climate, wet gets wetter and dry regions persist/expand • Midlatitudeprecip: • High latitudes moisten • Subtropics dry • Poleward shifts are important • Tropical precipitation shifts: • Uncertain in 21st century primarily due to climate feedbacks • Oceanic MOC causes tropical precipitation to be in the north? See recent papers by Ting, Jack & I for details…