Fig. 2. Observed (black, left ordinates) and simulated (colored, right
ordinates) Sahelian precipitation anomalies, forced with ALL (a, blue),
AA (b, magenta), NAT (c, brown/red), and GHG (d, green). The CMIP6 MMMs
are presented with solid curves surrounded by shaded areas demarking the
bootstrapping confidence interval, while the CMIP5 MMMs are presented
with dotted curves. The yellow shaded area is the confidence interval of
randomized bootstrapped MMMs of CMIP6 piC simulations, and represents
the magnitude of noise in the CMIP6 MMMs. Hemispherically asymmetric
volcanic forcing from Haywood et al noted in panel (c). A negative sign
denotes an eruption that cooled the northern hemisphere more than the
southern hemisphere while a positive sign denotes the opposite, aligning
with the sign of the expected Sahelian precipitation response to the
eruption. Panel (a) additionally shows the CMIP6 ALL MMM when restricted
to models, rather than institutions, that provide AA simulations (blue
dashed curve), and a 20-year running mean of the sum of the AA, NAT, and
GHG MMMs for CMIP5 (lavender dashed curve) and CMIP6 (burgundy dashed
curve). The label shows the number of institutions used for each CMIP6
MMM (N), the correlation of the CMIP6 MMM with observations (r), and the
standardized root mean squared error of the CMIP6 MMM with observations
(sRMSE).
In the AA experiments (panel b), CMIP6 is anomalously wetter than CMIP5
in the 1970s and around 2000, but otherwise looks similar to CMIP5:
precipitation declines in the mid-century and then recovers after the
clean air acts, preceding the timing of observed variability by about 10
years. There are some differences in the NAT experiments between CMIP5
and CMIP6 (panel c), but the largest variations in both ensembles are
interannual episodes that are clearly associated with volcanic
eruptions. In the GHG experiments (panel d), CMIP6 shows anomalous
wetting after 1970 that wasn’t present in CMIP5.
Similar changes can be seen in the ALL simulations (panel a): while
CMIP5 reaches peak drought in 1982 – close to the observed
precipitation minimum – CMIP6 dries very little and only until 1970,
after which it displays an anomalously wetter climate than CMIP5 through
the end of the century. But while the precipitation responses to
different forcing agents appear to add linearly in CMIP5 (compare the
lavender dashed curve to the blue dotted curve), the late century
wetting in CMIP6 is larger than the sum of GHG and AA wetting (burgundy
dashed curve; including NAT does not help.) This effect is robust to
differences in model availability for the different sets of forcing
agents (see figure caption and light blue dashed curve). Thus, in the
ALL simulations, CMIP6 displays slightly less drying from AA compared to
CMIP5, more wetting from GHG, and additional wetting after 1990 from a
non-linear interaction between forcings.
As a result of these changes, the response to forcing in CMIP6 is a poor
match to observations. Figure 3 displays the correlation (panel a,
“r”) and sRMSE (panel b) between observations and simulated MMMs
(dots) and bootstrapped MMMs (curves) from CMIP6 (ALL in blue, AA in
magenta, NAT in brown, and GHG in green solid curves) and CMIP5 (ALL and
AA in blue and magenta dotted-dashed curves; other simulations omitted
for clarity) from 1901 to the end of the simulations (2003 for CMIP5 and
2014 for CMIP6). The dotted curves present the randomized bootstrapping
distributions for the CMIP6 piC simulations, and the vertical dashed
lines mark the one-sided p=0.05 significance level given by these
distributions. Recall that correlation measures similarity in timing
between simulations and observations where 1 is a perfect match, and
sRMSE measures the amplitude of differences between the simulations and
observations where 0 is a perfect match.