Supplemental Material
Journal of Climate
Anthropogenic Aerosols Dominate Forced Multidecadal Sahel Precipitation Change throughDistinct Atmospheric and Oceanic Drivers
https://doi.org/10.1175/JCLI-D-19-0829.1© Copyright 2020 American Meteorological Society
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1Supplementary Material for: Anthropogenic aerosols
dominate forced multidecadal Sahel precipitation change
through distinct atmospheric and oceanic drivers.
EffectBackground StateCAM5 and CanAM4

2000s-1970s Direct
Atmospheric Response

CLIM SSTExp 1 - Exp 2

PERT SSTExp 3 - Exp 4

2000s-1970s Ocean MediatedResponse

CLIM EMIS

Exp 1 - Exp 3

PERT EMIS

Exp 2 - Exp 4

AVERAGE

Exp 1 - Exp 4

CTRL SUM

Exp 1 - Exp 2 + Exp 1 - Exp 3

PERT SUM

Exp 3 - Exp 4 + Exp 2 - Exp 4

1970s – 1950s Direct
Atmospheric Response

CLIM SSTExp 2 - Exp 5

PERT SST(Exp 6 - Exp 7) - (Exp 3 - Exp 4)

1970s – 1950s Ocean
Mediated Response

CLIM EMIS

Exp 3 - Exp 6

PERT. EMIS.

(Exp 5 - Exp 7) - (Exp 2 - Exp 4)

AVERAGE

Exp 4 - Exp 7

CTRL SUM

Exp 2 - Exp 5 + Exp 3 - Exp 6

PERT SUM(Exp 6 - Exp 7) - (Exp 3 - Exp 4) +(Exp 5 - Exp 7) - (Exp 2 - Exp 4)

2000s-1970s Total Direct
Atmospheric plus Ocean
Mediated Response

1970s-1950s Total Direct
Atmospheric plus Ocean
Mediated Response

Linearity Check, 1970s-1950s

Exp 4 - Exp 7 vs. (Exp 3 - Exp 6) + (Exp 2 Exp 5)

Linearity Check, 2000s-1970s

Exp 1 - Exp 4 vs. (Exp 1 - Exp 2) + (Exp 1 Exp 3)

Supplementary Table S1. Description of how the CAM5 AGCM simulations are differenced inorder to obtain the direct atmospheric and ocean mediated responses for the 2000s minus 1970s
and the 1970s minus 1950s. Refer to Table 2 for the AGCM experiment list.
2

Supplementary Figure S1: Estimate of the minimum ensemble size required to find astatistically significant JAS precipitation response to anthropogenic aerosol forcing with astudent’s t-test in CESM1 (left column) and CanESM2 (right column) for the 1970s minus 1950s(top row) and the 2000s minus 1970s (bottom row).
The minimum ensemble size required to find a statistically significant difference between two
populations N is estimated as

𝑁 = 2(𝑡0 𝑠𝑝 2)
𝛥𝑥

where t0 is the threshold for statistical significance of the student’s t-test, sp is the pooled standard
deviation of the populations, and Δx is the difference of the means of the populations. Thisformula is obtained by inverting the Student’s t-test for two populations with equal size and is the3version of the metric described by Wehner (2000). Following Deser et al. (2012), we approximate
t0 as 2 and calculate𝑠𝑝 2)𝛥𝑥

𝑁 ≈ 8(Wehner, M. F. (2000). A method to aid in the determination of the sampling size of AGCM
ensemble simulations. Climate Dynamics, 16(5), 321–331.
https://doi.org/10.1007/s003820050331
Deser, C., Phillips, A., Bourdette, V. et al. Clim Dyn (2012) 38: 527.
https://doi.org/10.1007/s00382-010-0977-x

4

Supplementary Figure S2: Climatological JAS precipitation for 2000-2009 in a) the CESM1 LEAll forcing simulation, b) the CanESM2 LE All forcing simulation, c) the CAM5 EXP1simulation, and d) the CanAM4 EXP1 simulation. The observed climatology of GPCC for 20002009 is plotted in blue contours on 3mm/day intervals.

5Supplementary Figure S3: As in Figure 5, but with a) CAM5 C20C+ and b) CanAM4 AMIP
Sahel precipitation anomalies rather than the CESM1 and CanESM2 LE AER time series.

6

Supplementary Figure S4: As in Fig. 4 for Black Carbon (BC) burden.

7

Supplementary Figure S5: CESM1-LE All forcing minus All-but-Biomass Burning ensemble
average JAS mean Sulphate burden (left column) and Black Carbon burden (right column)
anomalies for the 1970s minus 1950s (top row) and 2000s minus 1970s (bottom row).

8

Supplementary Figure S6: CESM1 LE JAS precipitation response to biomass burning aerosols(left column) and combined biomass burning and other anthropogenic (primarily energy-sector)
aerosols (right column) for the 1970s minus 1950s (top row) and the 2000s minus 1970s (bottomrow). Stippling surrounded by a thin red contour indicates grid points whose responses are
statistically significant at the 95% level.

9Supplementary Figure S7: As in Fig. 6, but for the 2000s minus 1970s. Note that (a) and (b) are
the same as Fig. 1d and Fig. 2d respectively.

10

Supplementary Figure S8: Moisture Convergence (δMC) (Left column) and P-E (right column)
over Africa for the 2000s minus the 1970s for the Total (Top row), direct atmospheric (Middle),and ocean mediated (Bottom) responses in the CAM5 simulations. Red boxes indicate the region
for which the covariance fractions are computed.

11

Supplementary Figure S9: As in Supplementary Figure 7, but for CanAM4.

12

Supplementary Figure S10: The combined CAM5 total response to the direct atmospheric andocean mediated responses (left column), the sum of the individual components (middle column),
and the difference between the two (right column) in the1970s-1950s (top row) and 2000s-1970s(bottom row) AGCM experiments for the 1970s minus 1950s. Note due to the configuration ofthe AGCM experiments the difference here is the same difference as for the two different
methods for calculating either the direct atmospheric or the ocean mediated responses throughalgebraic rearrangement of the simulations. Stippling surrounded by a thin red contour indicatesgrid points whose responses are statistically significant at the 95% level. The blue boxes indicate
the averaging region used in Fig. 12.

13

Supplementary Figure S11: As in Fig. S10, but for CanAM4.

14