SLIDE 1
Kevin M. Grise
Department of Environmental Sciences University of Virginia
Recent Tropical Expansion: Natural Variability or Forced Response? - - PowerPoint PPT Presentation
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
SLIDE 2
SLIDE 3
10 Years Ago …
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.
ion an ne umn he ed he n, as e of es ed l.7 ing re ey est
- f
Ozone (NH only, Hudson et al.) Outgoing longwave radiation (Hu and Fu) Hadley circulation (Hu and Fu) Jet-stream separation (Reichler) Tropopause (Seidel and Randel) 70 60 50 Width of tropics (degrees lattitude) Width of NH tropics (degrees lattitude) 1980 1985 1990 1995 2000 2005 35 30 25
SLIDE 4
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)
SLIDE 5
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
- f the recent tropical expansion
SLIDE 6
Comparing Reanalysis and Model Trends
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
PSI500 1979–2005 USFC
ERA40 NCEP1 NCEP2 ERAI MERRA2 JRA55 CFSR
−60 −50 −40 −30 −20 −10 10 20 30 40 50 60 100 150 200 300 500 700 1000
X X
Pressure (hPa) Latitude
> > > > > > X PSI500
CMIP5 Models
Trends in PSI500 metric in older generation reanalyses well exceed those from model simulations (Johanson and Fu 2009).
Grise et al. (2019)
SLIDE 7
Comparing Reanalysis and Model Trends
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
PSI500 1979–2005 USFC
ERA40 NCEP1 NCEP2 ERAI MERRA2 JRA55 CFSR
−60 −50 −40 −30 −20 −10 10 20 30 40 50 60 100 150 200 300 500 700 1000
X X
Pressure (hPa) Latitude
> > > > > > X PSI500
CMIP5 Models
Trends in PSI500 metric are highly variable across reanalyses … in part because mean meridional circulation does not conserve mass. (Davis and Davis 2018)
Grise et al. (2019)
SLIDE 8
Comparing Reanalysis and Model Trends
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
PSI500 1979–2005 USFC
ERA40 NCEP1 NCEP2 ERAI MERRA2 JRA55 CFSR
−60 −50 −40 −30 −20 −10 10 20 30 40 50 60 100 150 200 300 500 700 1000
X X X X
Pressure (hPa) Latitude
> > > > > > X PSI500 X USFC
CMIP5 Models
What if we choose a metric that is more closely linked to surface observations?
Grise et al. (2019)
SLIDE 9
Comparing Reanalysis and Model Trends
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
PSI500 1979–2005 USFC
ERA40 NCEP1 NCEP2 ERAI MERRA2 JRA55 CFSR
CMIP5 Models CMIP5 Models
Much better agreement between reanalysis and model trends!
Grise et al. (2019)
SLIDE 10
Why are the tropics widening?
Attribution from CMIP5 Single Forcing Historical Runs
Greenhouse gases (Southern Hemisphere) Stratospheric ozone depletion (Southern Hemisphere, summer only) Sea surface temperature variability (especially Northern Hemisphere)
Grise et al. (2019)
PIC HIST GHG NAT AER OZ
AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5 Trend (Deg. Latitude per Decade)
Northern Hemisphere (1979–2005)
PIC
HIST
GHG NAT AER
OZ AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5
66% > 0 64% > 0 61% > 0 50% < 0 56% > 0 78% > 0 77% < 0 71% < 0 67% > 0 55% < 0 63% < 0 88% < 0 Outside PIC 17% 4% 3% 5% 0% 28% Outside PIC 2% 4% 9% 0% 13% 28%
Southern Hemisphere (1979–2005)
75% < 0* 19%* *DJF only *
ACCESS1.3 BCC-CSM1.1 CanESM2 CCSM4 CESM1 (CAM5) CSIRO-Mk3.6.0 FGOALS-g2 GISS-E2-R HadGEM2-ES
SLIDE 11
Grise et al. (2019)
PIC HIST GHG NAT AER OZ
AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5 Trend (Deg. Latitude per Decade)
Northern Hemisphere (1979–2005)
PIC
HIST
GHG NAT AER
OZ AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5
66% > 0 64% > 0 61% > 0 50% < 0 56% > 0 78% > 0 77% < 0 71% < 0 67% > 0 55% < 0 63% < 0 88% < 0 Outside PIC 17% 4% 3% 5% 0% 28% Outside PIC 2% 4% 9% 0% 13% 28%
Southern Hemisphere (1979–2005)
75% < 0* 19%* *DJF only *
ACCESS1.3 BCC-CSM1.1 CanESM2 CCSM4 CESM1 (CAM5) CSIRO-Mk3.6.0 FGOALS-g2 GISS-E2-R HadGEM2-ES
Natural variability helps to account for similar observed expansion rates in the two hemispheres. Anthropogenic forcing alone should yield greater expansion in the SH.
Why are the tropics widening?
Attribution from CMIP5 Single Forcing Historical Runs
SLIDE 12
Outlook for 21st Century
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
- 35
- 34
- 33
- 32
- 31
- 30
- 29
Latitude
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 29 30 31 32 33 34 35
Latitude
Northern Hemisphere Southern Hemisphere
Year
SH tropical expansion emerges from natural variability during 21st century. NH tropical expansion may not!
Grise et al. (2019)
CESM Large Ensemble
Gray: Control Blue: Historical + RCP8.5 (mean)
SLIDE 13
Conclusions
- Observed tropical expansion since 1979 is modest
(< 0.5˚ latitude per decade) and within the bounds
- f climate model simulations.
- Models suggest that anthropogenic forcing
(increasing greenhouse gases and stratospheric
- zone 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.
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SLIDE 15
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
−60 −50 −40 −30 −20 −10 10 20 30 40 50 60 100 150 200 300 500 700 1000
U ψ P U ψ P
Pressure (hPa) Latitude
> > > > > >
E E
P–E P–E
Adapted from Grise and Polvani (2016)
SLIDE 16
Methodology: Metrics for Tropical Expansion
PSI EDJ P-E SLP USF STJ TPB OLR ∆OLR PSI EDJ P-E SLP USF STJ TPB OLR ∆OLR 0.61*
(0.07)
0.65*
(0.08)
0.48*
(0.21)
0.68*
(0.08)
0.28
(0.16)
0.34*
(0.10)
0.46*
(0.16)
0.68*
(0.07)
0.53*
(0.14)
0.52*
(0.11)
0.61*
(0.30)
0.80*
(0.06)
0.15
(0.18)
0.25
(0.16)
0.36*
(0.17)
0.51*
(0.11)
0.64*
(0.08)
0.62*
(0.11)
0.32
(0.23)
0.66*
(0.13)
0.32*
(0.17)
0.36*
(0.15)
0.46*
(0.15)
0.55*
(0.11)
0.73*
(0.07)
0.75*
(0.07)
0.69*
(0.06)
0.62*
(0.30)
0.15
(0.20)
0.14
(0.21)
0.12
(0.25)
0.37*
(0.21)
0.73*
(0.07)
0.74*
(0.08)
0.70*
(0.06)
0.98*
(.006)
0.33*
(0.18)
0.42*
(0.15)
0.51*
(0.15)
0.65*
(0.09)
0.06
(0.20)
- 0.13
(0.17)
.003
(0.17)
0.10
(0.18)
0.08
(0.18)
0.60*
(0.14)
0.48*
(0.16)
0.38*
(0.21)
0.10
(0.17)
- 0.16
(0.15)
- 0.01
(0.16)
0.07
(0.15)
0.06
(0.16)
0.59*
(0.13)
0.53*
(0.10)
0.48*
(0.12)
0.20
(0.24)
- 0.12
(0.30)
0.09
(0.26)
0.16
(0.23)
0.19
(0.23)
0.28*
(0.15)
0.35*
(0.15)
0.71*
(0.09)
0.54*
(0.13)
0.20
(0.21)
0.36*
(0.17)
0.47*
(0.13)
0.49*
(0.14)
0.27
(0.24)
0.31
(0.20)
0.58*
(0.12)
NH SH
Annual
Waugh et al. (2018)
Interannual variability of these five metrics are highly correlated with one another.
(Solomon et al. 2016; Davis and Birner 2017; Waugh et al. 2018)
SLIDE 17
Attribution of Recent Tropical Expansion
Southern Hemisphere
PIC
HIST
GHG NAT AER OZ
AMIP
OBS
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 81% < 0 61% < 0 58% > 0 68% > 0 75% < 0 83% < 0
Outside PIC 13% 11% 6% 0% 19% 15%
Trend (Deg. Latitude per Decade) PIC
HIST
GHG NAT AER OZ
AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5
77% < 0 71% < 0 67% > 0 55% < 0 63% < 0 88% < 0 Outside PIC 17% 4% 3% 5% 0% 28%
1979–2005 (Annual)
Trend (Deg. Latitude per Decade)
1979–2005 (DJF)
Models suggest that increasing greenhouse gases and stratospheric ozone depletion (DJF) contributed to recent tropical expansion in the Southern Hemisphere.
ACCESS1.3 BCC-CSM1.1 CanESM2 CCSM4 CESM1 (CAM5) CSIRO-Mk3.6.0 FGOALS-g2 GISS-E2-R HadGEM2-ES
SLIDE 18
Tropical Width: Response to Increasing GHGs
Grise and Polvani (2016)
Seasonality
Annual DJF MAM JJA SON −10 −5 5
Shift (Degrees Latitude)
Annual DJF MAM JJA SON −10 −5 5
Shift (Degrees Latitude)
Response to 4xCO Forcing: Hadley Cell Edge
Northern Hemisphere Southern Hemisphere 2
NH circulation response has complicated seasonality! Largest tropical expansion driven by CO2 expected in NH during SON season
SLIDE 19
width (i.e., difgerence between the Northern
- f the tropics will be defjned here as the latitude
satellite era (around 1979). While difgerences do has fmuctuated interannually with a standard deviation (σ) of ~0.8°–1° latitude (Figure 1c), with individual years typically varying within an ~4º latitude range (~4σ, Figure 1a). Figure 1b illustrates scenario (see Figure 1 caption) and difger only in a round-ofg level perturbation introduced to the surface temperature fjeld in 1920. Thus, the difgerences between ensemble members arise
(a) Reanalyses
1930 1950 1970 1990 2010 58 60 62 64 66 68 70 Tropical Width (° lat)
ERA-Interim JRA55 MERRA2 ERA20C 20thC
(a) Time series of annual mean tropical width for fjve difgerent
- thereafter. (c) Interannual standard deviation, σ, of the annual mean width,
Simpson (2018)
SLIDE 20
OBS PIC HIST
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
OBS PIC HIST
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
OBS PIC HIST AMIP
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 1.2 1.4
Trend (Deg. Latitude per Decade)
ERA40 NCEP1 NCEP2 ERAI MERRA2 JRA55 CFSR
PSI500 1979–2005 USFC 1979–2016
a b c d
SLIDE 21
PIC HIST GHG NAT AER OZ
AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5 Trend (Deg. Latitude per Decade) PIC
HIST
GHG NAT AER OZ
- 0.3
- 0.2
- 0.1
0.1 0.2 Trend (Deg. Latitude per Decade) PIC
HIST GHG
NAT AER OZ
- 0.2
- 0.15
- 0.1
- 0.05
0.05 0.1 0.15 0.2 Trend (Deg. Latitude per Decade) PIC HIST GHG NAT AER OZ
- 0.2
- 0.15
- 0.1
- 0.05
0.05 0.1 0.15 0.2 Trend (Deg. Latitude per Decade)
ACCESS1.3 BCC-CSM1.1 CanESM2 CCSM4 CESM1 (CAM5) CSIRO-Mk3.6.0 FGOALS-g2 GISS-E2-R HadGEM2-ES
1950–2005 Northern Hemisphere 1979–2005 Southern Hemisphere Southern Hemisphere (DJF)
PIC
HIST
GHG NAT AER OZ
AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5 PIC
HIST
GHG NAT AER OZ
AMIP
OBS
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 68% > 0 61% > 0 58% < 0 77% < 0 69% > 0 66% > 0 64% > 0 61% > 0 50% < 0 56% > 0 78% > 0 77% < 0 71% < 0 67% > 0 55% < 0 63% < 0 88% < 0 81% < 0 61% < 0 58% > 0 68% > 0 75% < 0 83% < 0 98% < 0 93% < 0 58% > 0 86% > 0 81% < 0 94% < 0 79% < 0 67% > 0 73% > 0 94% < 0
a b c d e f
0.3 Outside PIC 51% 43% 3% 9% 31% Outside PIC 13% 11% 6% 0% 19% 15% Outside PIC 57% 50% 0% 9% 19% Outside PIC 6% 14% 3% 18% 6% Outside PIC 17% 4% 3% 5% 0% 28% Outside PIC 2% 4% 9% 0% 13% 28%
USFC metric
SLIDE 22
PIC
HIST
GHG NAT AER OZ AMIP OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5 Trend (Deg. Latitude per Decade) PIC
HIST GHG
NAT AER OZ
- 0.2
- 0.15
- 0.1
- 0.05
0.05 0.1 0.15 0.2 Trend (Deg. Latitude per Decade) PIC
HIST GHG
NAT AER OZ
- 0.2
- 0.15
- 0.1
- 0.05
0.05 0.1 0.15 0.2 Trend (Deg. Latitude per Decade)
ACCESS1.3 BCC-CSM1.1 CanESM2 CCSM4 CESM1 (CAM5) CSIRO-Mk3.6.0 FGOALS-g2 GISS-E2-R HadGEM2-ES
1950–2005 Northern Hemisphere 1979–2005 Southern Hemisphere Southern Hemisphere (DJF)
PIC
HIST GHG
NAT AER
OZ
- 0.3
- 0.2
- 0.1
0.1 0.2 Trend (Deg. Latitude per Decade) PIC
HIST GHG
NAT AER OZ
AMIP
OBS
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.1 0.2 0.3 0.4 0.5 PIC
HIST
GHG NAT AER OZ
AMIP
OBS
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 79% > 0 86% > 0 67% < 0 73% < 0 75% > 0 98% < 0 96% < 0 61% > 0 82% > 0 81% < 0 96% < 0 86% < 0 61% > 0 68% > 0 100% < 0 77% > 0 57% > 0 52% > 0 59% < 0 63% > 0 73% > 0 83% < 0 79% < 0 55% > 0 55% > 0 69% < 0 88% < 0 81% < 0 61% < 0 52% > 0 73% > 0 69% < 0 80% < 0
a b c d e f
Southern Hemisphere (DJF)
0.3 Outside PIC 43% 43% 3% 9% 25% Outside PIC 13% 11% 9% 5% 6% 18% Outside PIC 74% 64% 6% 18% 19% Outside PIC 15% 14% 6% 0% 0% Outside PIC 19% 14% 9% 0% 6% 30% Outside PIC 0% 14% 6% 0% 6% 5%
PSI500 metric
SLIDE 23
SLIDE 24
1980 1985 1990 1995 2000 2005
Year
- 2
- 1
1 2 3
Tropical Expansion (Deg. Latitude)
OBS AMIP Run 4 AMIP Run 9 OBS AMIP 0.1 0.2 0.3 0.4 0.5 1979-2009 Trend (Deg. Latitude per Decade)
SLIDE 25
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
Year
- 39
- 38
- 37
- 36
- 35
- 34
- 33
Latitude
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
- 35
- 34
- 33
- 32
- 31
- 30
- 29
Latitude
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 29 30 31 32 33 34 35
Latitude
Northern Hemisphere Southern Hemisphere
a b
Southern Hemisphere (DJF)
c
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
Year
29 30 31 32 33 34 35
Latitude
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
Year
- 35
- 34
- 33
- 32
- 31
- 30
- 29
Latitude
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
Year
- 38
- 37
- 36
- 35
- 34
- 33
- 32
Latitude
a b c
Northern Hemisphere Southern Hemisphere Southern Hemisphere (DJF)
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
Year
- 39
- 38
- 37
- 36
- 35
- 34
- 33
Latitude
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
- 35
- 34
- 33
- 32
- 31
- 30
- 29
Latitude
1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 29 30 31 32 33 34 35
Latitude
a b c
Northern Hemisphere Southern Hemisphere Southern Hemisphere (DJF)
CESM Large Ensemble (USFC) CMIP5 Models (USFC) CESM Large Ensemble (PSI500)
SLIDE 26
Outlook for 21st Century
Grise et al. (2019)