The role of land-sea contrast in the circulation response to seasonal insolation and global warming Tiffany A. Shaw Columbia University University of Chicago Thanks to A. Voigt, I Simpson, R. Seager
What are the dynamical implications of thermal contrasts? What is the role of Asian land-sea contrasts in the seasonal circulation evolution? Why is the summertime circulation response to increased CO 2 not robust? http://www.geo.arizona.edu/antevs/ecol438/monsoon.gif
Dynamical Implications of thermal contrasts 1) Sea surface temperature gradients drive low-level flow (Lindzen & Nigam 1987) u gh s = 1 k r(ts ) 2 sin 2 See also Emanuel et al. (1994), Pauluis (24), Prive and Plumb (27)
Dynamical Implications of thermal contrasts 1) Sea surface temperature gradients drive low-level flow (Lindzen & Nigam 1987) u gh s = 1 k r(ts ) 2 sin 2 2) Moist entropy (s b = c p ln θ e* ) gradients drive flow when moist entropy is supercritical (Emanuel 1995) u s = (T s T t ) 2 sin k r(s b s bcrit ) Positive north-south thermal gradients imply eastward flow See also Emanuel et al. (1994), Pauluis (24), Prive and Plumb (27)
Dynamical Implications of thermal contrasts Thermally driven flow + friction = convergence s = r u s s Convergence due to energy input and gross moist stability (Neelin & Held 1987): = F net / m z Nie et al. (21)
Convergence drives global circulation: Monsoon-Desert relationship < H > L z s > Desert L s < Monsoon H Ocean Sverdrup balance v = f See Gill (198), Sardeskmuhk & Hoskins (1988), Wang & Ting (1999), Rodwell & Hoskins (21)
Thermodynamic & Dynamic Links Thermal contrast T or s b Rossby wave rotational flow, QG moisture & momentum transport (ITCZ, jet stream) Low-level convergence, Upper level divergence Poleward extent of Asian & N. American monsoons set by dynamical factors (Chou & Neelin 23)
July May 8 925hPa 1 6 4 2 Land Cyclone Opposite pattern aloft @ s b > u s > Oceanic Anticyclones Dynamical moisture 2 6 12 transport dominates 18 24 3 36 1e6 1e6 1 Data source: ERA-Interim
September - August 8 925hPa 1e6 6 4 2 Land Anticyclone Oceanic Cyclone @ s b < u s < 2 6 12 18 24 3 36 Opposite pattern aloft 1e6 Data source: ERA-Interim
See Shaw (214), Shaw & Voigt (215) Aquaplanet model simulations of the summertime circulation MPI-ESM model Fixed sea surface temperature (SST) Background SST follows Neale & Hoskins (21) Add SST perturbation to mimic land-ocean asymmetries 8 6 4 2 2 4 6 Sea surface temperature 31 3 29 28 27 8 6 4 2 2 4 6 SST Perturbation Asia Pacific N. America Atlantic Asia Pacific N. America Atlantic 15 1 5 5 1 8 6 12 18 24 3 36 26 8 6 12 18 24 3 36 15
Test hypothesis that Asian land-sea contrasts influence Pacific and Atlantic circulations Starting from a zonally-asymmetric aquaplanet control state: - Mimic increased solar insolation by warming Asia relative to control (A2-A1) - Mimic decreased solar insolation by cooling Asia (A4-A3) Asian SST amplitude A1 A2 A3 A4
Response to Asian warming 8 925hPa 1e6 6 Westward shift 4 2 Warmer @ s b > u s > Similar to July - May 2 6 12 18 24 3 36 1e6
Response to Asian warming 8 925hPa 1e6 6 Westward shift 4 2 Warmer @ s b > Increased Asian thermal contrast @ s b > u s > s < > Increased Asian Similar to July - May 2 6 12 18 24 3 36 convergence Amplified cycloneanticyclone, poleward jet shift 1e6
Response to Asian warming 8 925hPa 1e6 6 4 2 Warmer Westward shift Asian Monsoon evolution suppresses N. American Monsoon (see Rodwell & Hoskins 21) @ s b > u s > Similar to July - May 2 6 12 18 24 3 36 1e6
Response to Asian cooling 8 925hPa 1e6 6 4 2 Colder @ s b < u s < Similar to September - August 2 6 12 18 24 3 36 1e6
Opposite responses to seasonal solar insolation July - May - Δs b maximum over Asia September - August - Δs b minimum over Asia - @ s b > - @ s b < - Convergence toward land - Strengthened monsoon cyclone oceanic anticyclone circulation - Poleward jet shift - Divergence from land - Weakened monsoon cyclone oceanic anticyclone circulation - Equatorward jet shift
Response to CO 2 increase: Thermal contrasts around Asia If land warms (fast adjustment) - Δs b maximum over land - @ s b > - Convergence toward land - Strengthened monsoon cyclone oceanic anticyclone circulation - Poleward jet shift
Response to CO 2 increase: Thermal contrasts around Asia If land warms (fast adjustment) - Δs b maximum over land - @ s b > - Convergence toward land - Strengthened monsoon cyclone oceanic anticyclone circulation - Poleward jet shift If ocean warms - Δs b maximum over ocean - @ s b < - Divergence from land - Weakened monsoon cyclone oceanic anticyclone circulation - Equatorward jet shift Modern analogs: July May versus September August Except CO 2 is well mixed è competition
Idealized climate change with aquaplanet & CMIP5 models 8 6 4 2 2 4 6 SST Perturbation Asia Pacific N. America Atlantic Asia Pacific N. America Atlantic 8 6 12 18 24 3 36 15 1 5 5 1 15 - Increased solar insolation (warm Asia) - Decreased solar insolation (cool Asia) - Land warming response to CO 2 increase (warm Asia) - Ocean warming response to CO 2 increase (warm outside black land boxes) - Global warming (warm everywhere)
8 Response to Asian warming 1e6 8 Response to Asian cooling 1e6 6 6 4 2 4 2 Stronger monsoon-anticyclone circulation, Poleward jet shift 2 1e6 6 12 18 24 3 36 925hPa 2 6 12 18 24 3 36 Weaker monsoon-anticyclone circulation, Equatorward jet shift 1e6
8 Response to Asian warming 1e6 8 Response to Asian cooling 1e6 6 6 4 2 4 2 Stronger monsoon-anticyclone circulation, Poleward jet shift 2 1e6 6 12 18 24 3 36 925hPa 2 6 12 18 24 3 36 Weaker monsoon-anticyclone circulation, Equatorward jet shift 1e6 Response to ocean warming 8 6 4 2 2 6 12 18 24 3 36 1e6 1e6 Similar circulation responses consistent with @ s b <
8 Response to Asian warming 1e6 8 Response to Asian cooling 1e6 6 6 4 2 4 2 Stronger monsoon-anticyclone circulation, Poleward jet shift 2 1e6 6 12 18 24 3 36 925hPa 2 6 12 18 24 3 36 Weaker monsoon-anticyclone circulation, Equatorward jet shift 1e6 Response to ocean warming 8 1e6 Response to global θ e warming 8 1e6 6 6 4 2 4 2 2 6 12 18 24 3 36 1e6 Similar response to 4xCO 2 2 6 12 18 24 3 36 1e6 Shaw & Voigt (215)
Non robust Asia-Pacific circulation response to increased CO 2 8 Color = RCP8.5 (28-299) historical (1986-25) 925hPa JJA 4e6 6 2e6 4 2 Non robust circulation response 2e6 Stippling = > 8% model agreement 2 6 12 18 24 3 36 4e6 Data source: CMIP5 models
Tug of war between radiative forcing and SST warming in CMIP5 models Increased CO2 fixed SSTs (AMIP4xCO2) 8 4e6 6 2e6 4 2 925hPa JJA Modern analog: July-May 2 6 12 18 24 3 36 2e6 4e6
Tug of war between radiative forcing and SST warming in CMIP5 models Increased CO2 fixed SSTs (AMIP4xCO2) 8 6 4e6 2e6 4 925hPa JJA 2 Increased SSTs fixed CO2 (AMIP4K) 8 Modern analog: July-May 6 6 12 18 24 2 3 36 2e6 4e6 4e6 2e6 4 2 2e6 Shaw & Voigt (215) 2 Modern analog: Sept. Aug. 6 12 18 24 3 36 4e6
Tug of war between radiative forcing and SST warming in CMIP5 models Sum of response to radiative forcing and SST warming 8 4e6 6 4 2 8 2e6 2e6 925hPa JJA Fully-coupled model response 4e6 2 6 6 12 18 24 3 36 See decomposition of recent trends (Deser & Phillips 29) and future response of tropical precipitation (Bony et al. 213) 4 2 4e6 Non robust circulation response 2 6 12 18 24 3 36 2e6 2e6 4e6
Tug of war on the Pacific jet stream jet lat. Pacific 7hPa 2 2 4 SST warming Radiative forcing Poleward shift Equatorward shift RCP85 AMIP4K AMIPFuture AMIP4xCO2 Shaw & Voigt (215)
Summary What are the dynamical implications of land-sea moist entropy contrasts? Land-sea moist entropy contrasts drive rotational flow and convergence in the presence of friction What is the role of Asian land-sea thermal contrasts in the seasonal circulation evolution? Asian land-sea moist entropy contrasts control the seasonal cycle of circulation globally (see Rodwell & Hoskins 21) Why is the Asia-Pacific summertime circulation response to climate change not robust? Tug of war on circulation between radiative forcing and sea surface warming due to opposite land-sea moist entropy contrast, analogous to seasonal evolution