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Global Hydrological Response to External Forcing

This study examines the global hydrological response to external forcing, such as warming and differential warming between hemispheres. The results show increased horizontal moisture fluxes, poleward expansion of subtropics, and the movement of tropical rainbelts to the warmer hemisphere. The precipitation changes by the end of the 21st century are inconsistent among models. The study also investigates the effects of variations in moisture supply, stabilization of troposphere, and ITCZ displacement.

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Global Hydrological Response to External Forcing

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  1. Key ingredients in global hydrological response to external forcing Response to warming => Increased horizontal moisture fluxes => Poleward expansion of the subtropics Response to differential warming of the 2 hemispheres => tropical rainbelts move to warmer hemisphere

  2. Percentage change in precipitation by end of 21st century: PCMDI-AR4 archive White areas => less than two thirds of the models agree on the sign of the change

  3. PCMDI/IPCC % increase in midlatitude maximum in poleward flux of vapor vs global mean temperature

  4. Precipitation and evaporation “Aqua_planet” climate model (no seasons, no land surface) Instantaneous precip (lat,lon) Time means

  5. One can see effects of poleward shift of midlatitude circulation And increase(!) in strength of Hadley cell

  6. 21st century A1B Equilibrium 2x 20th century

  7. r2=0.72 r2=0.85

  8. Sarah Kang, Princeton Aqua planet/slab ocean Model A: Frierson et al 2006 -- idealized moist GCM (no clouds -- water water vapor feedback) Model B: AM2

  9. Idealized GCM with different convection schemes

  10. A parameter in the convection scheme is varied continuously in each model Idealized GCM: Modified Betts-Miller relative humidity to which one relaxes when convecting AM2: Relaxed Arakawa Schubert minimum allowed entrainment rate

  11. Compensation at equator AM2 Idealized GCM

  12. AM2 AM2 IGCM IGCM

  13. IGCM AM2

  14. For the idealized GCM: A simple energy balance model with diffusion of moist static energy, fitting the diffusivity to the symmetric control, predicts compensation of 20-30% The tropical precipitation response is determined by The degree of compensation The gross moist stability Increasing RH decreases gross moist stability but not compensation

  15. Solid: idealized GCM precip response at 7S Dashed: fit assuming degree of compensation and gross moist stability Increasing RH

  16. Fraction of rain in ITCZ that falls as “large-scale” precip in AM2

  17. Compensation at equator Latitude of precip max => Interesting benchmark for GCMs

  18. In AM2 Degree of compensation strongly dependent on Entrainment limiter Why? Cloud feedback and water vapor feedback

  19. Single realization of CM2 (greenhouse gases; aerosols, solar, volcanoes, land use)

  20. Mean of 8-member ensemble

  21. Range of SRES scenarios

  22. Sahel summer rain: (1980-2000) minus (1960-1940) (mm/month) 8 member ensemble

  23. Observed annual mean precipitation trend 1950-2000 Simulated annual mean precipitation trend in CM2 1950-2000

  24. +2K SST perturbation: annual mean precip NCAR CCSM GFDL CM2

  25. QUMP: 129 different mixed layer models (courtesy of Matthew Collins, Hadley Center) % Sahel precip response to 2xCO2 CM2.0

  26. Ind NA Regress: P(%) = I * Ind + N * NA = U * Ind + N * (NA – Ind) ( U = I – N ) Ind => Stabilization of troposphere? NA => ITCZ displacement? Moisture supply?

  27. Regressing observed rainfall vs observed Ind and NA => P = - 0.12 Ind + 0.38 (NA - Ind)

  28. Observed evolution of Ind and NA - Ind, 11yr running means 1954 1940 1919 1975 1985

  29. SW override experiments (Jian Lu) Want to study how change in absorbed SW at surface affects precip But: take two realizations of same model and override absorbed SW of one with the absorbed SW from the other => big difference So: change in precip dP due to 2K increase in SST = dP due to change in SST with fixed uncorrelated SW (small) +dP due to change in uncorrelated SW (wrong sign) +dP due to difference in effects of correlation at different SSTs

  30. + dSST; fixed uncorr SW + Decorrelation at 2K + d(uncorr SW); fixed SST - Decorrelation in cntrl

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