Recent Tropical Expansion: Natural Variability or Forced Response? - PowerPoint PPT Presentation

Recent Tropical Expansion: Natural Variability or Forced Response? Kevin M. Grise Department of Environmental Sciences University of Virginia US CLIVAR Working Group on the Changing Width of the Tropical Belt (Co-authors: Sean Davis, Isla Simpson, Darryn Waugh, Qiang Fu, Robert Allen, Karen Rosenlof, Caroline Ummenhofer, Kris Karnauskas, Amanda Maycock, Xiao-Wei Quan, Thomas Birner, Paul Staten) 32 nd Conference on Climate Variability and Change Phoenix, AZ January 7, 2019

What is Tropical Expansion?

10 Years Ago … ion an ne 70 35 umn Width of NH tropics (degrees lattitude) Width of tropics (degrees lattitude) he ed he n, as 30 60 e of es ed l. 7 Ozone (NH only, Hudson et al .) Outgoing longwave radiation (Hu and Fu) 25 50 ing Hadley circulation (Hu and Fu) re Jet-stream separation (Reichler) Tropopause (Seidel and Randel) ey est 1980 1985 1990 1995 2000 2005 of Seidel et al. (2008) Johanson and Fu (2009) Prevailing view was that the tropics had rapidly expanded since 1979 and that this expansion was much larger than anticipated from global climate models.

More recent studies are concluding that … The observed tropical expansion does not exceed • that in global climate model simulations. (Adam et al. 2014; Garfinkel et al. 2015; Davis and Birner 2017; Grise et al. 2018) A large fraction of the recent tropical expansion can • be attributed to natural climate variability instead of anthropogenic forcing. (Allen et al. 2014; Allen and Kovilakam 2017; Mantsis et al. 2017; Amaya et al. 2018)

More recent studies are concluding that … The observed tropical expansion does not exceed • that in global climate model simulations. (Adam et al. 2014; Garfinkel et al. 2015; Davis and Birner 2017; Grise et al. 2018) A large fraction of the recent tropical expansion can • be attributed to natural climate variability instead of anthropogenic forcing. (Allen et al. 2014; Allen and Kovilakam 2017; Mantsis et al. 2017; Amaya et al. 2018) Working Group Objective: To reassess contradictory claims in the literature about the magnitude and causes of the recent tropical expansion

Comparing Reanalysis and Model Trends PSI500 USFC 1.4 1.4 1.2 1.2 100 1 1 Trend (Deg. Latitude per Decade) Trend (Deg. Latitude per Decade) 0.8 0.8 150 200 0.6 0.6 1979–2005 Pressure (hPa) 0.4 0.4 300 0.2 0.2 > > X X > 500 > 0 0 > > 700 -0.2 -0.2 1000 − 60 − 50 − 40 − 30 − 20 − 10 0 10 20 30 40 50 60 -0.4 -0.4 X PSI500 Latitude CMIP5 Models -0.6 -0.6 -0.8 -0.8 OBS PIC HIST AMIP OBS PIC HIST AMIP Grise et al. ERA40 (2019) Trends in PSI500 metric in older generation NCEP1 NCEP2 reanalyses well exceed those from model ERAI simulations (Johanson and Fu 2009). MERRA2 JRA55 CFSR

Comparing Reanalysis and Model Trends PSI500 USFC 1.4 1.4 1.2 1.2 100 1 1 Trend (Deg. Latitude per Decade) Trend (Deg. Latitude per Decade) 0.8 0.8 150 200 0.6 0.6 1979–2005 Pressure (hPa) 0.4 0.4 300 0.2 0.2 > > X X > 500 > 0 0 > > 700 -0.2 -0.2 1000 − 60 − 50 − 40 − 30 − 20 − 10 0 10 20 30 40 50 60 -0.4 -0.4 X PSI500 Latitude CMIP5 Models -0.6 -0.6 -0.8 -0.8 OBS PIC HIST AMIP OBS PIC HIST AMIP Grise et al. ERA40 (2019) Trends in PSI500 metric are highly variable NCEP1 across reanalyses … in part because mean NCEP2 ERAI meridional circulation does not conserve mass. MERRA2 (Davis and Davis 2018) JRA55 CFSR

Comparing Reanalysis and Model Trends PSI500 USFC 1.4 1.4 1.2 1.2 100 1 1 Trend (Deg. Latitude per Decade) Trend (Deg. Latitude per Decade) 0.8 0.8 150 200 0.6 0.6 1979–2005 Pressure (hPa) 0.4 0.4 300 0.2 0.2 > > X X > 500 > 0 0 > > 700 -0.2 -0.2 X X 1000 − 60 − 50 − 40 − 30 − 20 − 10 0 10 20 30 40 50 60 -0.4 -0.4 X PSI500 Latitude X USFC CMIP5 Models -0.6 -0.6 -0.8 -0.8 OBS PIC HIST AMIP OBS PIC HIST AMIP Grise et al. ERA40 (2019) What if we choose a metric that is more NCEP1 NCEP2 closely linked to surface observations? ERAI MERRA2 JRA55 CFSR

Comparing Reanalysis and Model Trends PSI500 USFC 1.4 1.4 1.2 1.2 1 1 Trend (Deg. Latitude per Decade) Trend (Deg. Latitude per Decade) 0.8 0.8 0.6 0.6 1979–2005 0.4 0.4 0.2 0.2 0 0 -0.2 -0.2 -0.4 -0.4 CMIP5 Models CMIP5 Models -0.6 -0.6 -0.8 -0.8 OBS PIC HIST AMIP OBS PIC HIST AMIP Grise et al. ERA40 (2019) Much better agreement between reanalysis NCEP1 NCEP2 and model trends! ERAI MERRA2 JRA55 CFSR

Why are the tropics widening? Attribution from CMIP5 Single Forcing Historical Runs ACCESS1.3 Northern Hemisphere (1979–2005) Southern Hemisphere (1979–2005) BCC-CSM1.1 0.5 CanESM2 0.5 CCSM4 77% < 0 71% < 0 67% > 0 55% < 0 63% < 0 88% < 0 CESM1 (CAM5) Outside PIC 17% 4% 3% 5% 0% 28% 0.4 0.4 CSIRO-Mk3.6.0 FGOALS-g2 75% < 0* GISS-E2-R 19%* 0.3 0.3 HadGEM2-ES Trend (Deg. Latitude per Decade) 0.2 0.2 0.1 0.1 0 0 -0.1 -0.1 -0.2 -0.2 -0.3 -0.3 -0.4 -0.4 66% > 0 64% > 0 61% > 0 50% < 0 56% > 0 78% > 0 Outside PIC 2% 4% 9% 0% 13% 28% -0.5 -0.5 AMIP HIST GHG OZ AMIP PIC HIST GHG NAT AER OZ OBS PIC NAT AER OBS * *DJF only Grise et al. (2019) Greenhouse gases (Southern Hemisphere) Stratospheric ozone depletion (Southern Hemisphere, summer only) Sea surface temperature variability (especially Northern Hemisphere)

Why are the tropics widening? Attribution from CMIP5 Single Forcing Historical Runs ACCESS1.3 Northern Hemisphere (1979–2005) Southern Hemisphere (1979–2005) BCC-CSM1.1 0.5 CanESM2 0.5 CCSM4 77% < 0 71% < 0 67% > 0 55% < 0 63% < 0 88% < 0 CESM1 (CAM5) Outside PIC 17% 4% 3% 5% 0% 28% 0.4 0.4 CSIRO-Mk3.6.0 FGOALS-g2 75% < 0* GISS-E2-R 19%* 0.3 0.3 HadGEM2-ES Trend (Deg. Latitude per Decade) 0.2 0.2 0.1 0.1 0 0 -0.1 -0.1 -0.2 -0.2 -0.3 -0.3 -0.4 -0.4 66% > 0 64% > 0 61% > 0 50% < 0 56% > 0 78% > 0 Outside PIC 2% 4% 9% 0% 13% 28% -0.5 -0.5 AMIP HIST GHG OZ AMIP PIC HIST GHG NAT AER OZ OBS PIC NAT AER OBS * *DJF only Grise et al. (2019) Natural variability helps to account for similar observed expansion rates in the two hemispheres. Anthropogenic forcing alone should yield greater expansion in the SH.

Outlook for 21 st Century Northern Hemisphere 35 CESM Large Ensemble 34 33 Latitude 32 SH tropical expansion emerges 31 from natural variability during 30 21 st century. 29 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 Southern Hemisphere -29 NH tropical expansion may not! Gray: Control Blue: Historical + RCP8.5 (mean) -30 -31 Latitude -32 -33 -34 -35 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 Grise et al. (2019) Year

Conclusions Observed tropical expansion since 1979 is modest • (< 0.5˚ latitude per decade) and within the bounds of climate model simulations. Models suggest that anthropogenic forcing • (increasing greenhouse gases and stratospheric ozone depletion) played a role in recent tropical expansion in the Southern Hemisphere. Forced trends in the Northern Hemisphere are much • smaller, and natural variability must be taken into account to explain similar observed expansion rates in the two hemispheres.

Methodology: Metrics for Tropical Expansion 100 150 200 Pressure (hPa) 300 > > ψ ψ > 500 > > > 700 E E U U 1000 P P − 60 − 50 − 40 − 30 − 20 − 10 0 10 20 30 40 50 60 P–E P–E Latitude Adapted from Grise and Polvani (2016) Ψ Poleward bound of Hadley circulation (500 hPa streamfunction) P–E Poleward bound of subtropical dry zones U Transition from surface easterlies to surface westerlies P Subtropical sea-level pressure maximum E Eddy-driven jet (850 hPa zonal-mean zonal wind max)

Methodology: Metrics for Tropical Expansion Annual NH 0.68 * 0.51 * 0.55 * 0.37 * 0.65 * 0.38 * 0.48 * 0.71 * ∆ OLR SH (0.07) (0.11) (0.11) (0.21) (0.09) (0.21) (0.12) (0.09) 0.46 * 0.36 * 0.46 * 0.12 0.51 * 0.48 * 0.53 * 0.58 * OLR (0.16) (0.17) (0.15) (0.25) (0.15) (0.16) (0.10) (0.12) Interannual 0.34 * 0.25 0.36 * 0.14 0.42 * 0.60 * 0.35 * 0.31 TPB (0.10) (0.16) (0.15) (0.21) (0.15) (0.14) (0.15) (0.20) variability of these 0.28 0.15 0.32 * 0.15 0.33 * 0.59 * 0.28 * 0.27 five metrics are STJ (0.16) (0.18) (0.17) (0.20) (0.18) (0.13) (0.15) (0.24) highly correlated 0.68 * 0.80 * 0.66 * 0.62 * 0.08 0.06 0.19 0.49 * USF with one another. (0.08) (0.06) (0.13) (0.30) (0.18) (0.16) (0.23) (0.14) (Solomon et al. 2016; 0.48 * 0.61 * 0.32 0.98 * 0.10 0.07 0.16 0.47 * SLP Davis and Birner 2017; (0.21) (0.30) (0.23) (.006) (0.18) (0.15) (0.23) (0.13) Waugh et al. 2018) 0.65 * 0.52 * 0.69 * 0.70 * .003 -0.01 0.09 0.36 * P-E (0.08) (0.11) (0.06) (0.06) (0.17) (0.16) (0.26) (0.17) 0.61 * 0.62 * 0.75 * 0.74 * -0.13 -0.16 -0.12 0.20 EDJ (0.07) (0.11) (0.07) (0.08) (0.17) (0.15) (0.30) (0.21) 0.53 * 0.64 * 0.73 * 0.73 * 0.06 0.10 0.20 0.54 * PSI (0.14) (0.08) (0.07) (0.07) (0.20) (0.17) (0.24) (0.13) PSI EDJ P-E SLP USF STJ TPB OLR ∆ OLR Waugh et al. (2018)