Fig. 8. PS of observed SST (solid black), observed SST – GHG MMM
(dotted-dashed black), observed SST – ALL MMM (dotted black) and
associated 95% confidence intervals (black shading) in NA (a), GT (b),
and NARI (c), compared to the PS of piC simulations. Similar to Figure
4, mean PS by model are colored by average SST, where blue is colder
than observed, grey is observed, and red is warmer than observed.
However, deficiencies in simulating SST cannot explain the difference in
simulated externally forced precipitation variability between CMIP5 and
CMIP6. The only notable difference in simulated SST between the two
ensembles is that CMIP6 warms NA (and therefore NARI) less than CMIP5 in
the GHG simulations (Figure 7j and l). As in simulated Sahel
precipitation, warming of NA and NARI in CMIP6 ALL simulations is larger
than the smoothed sum of simulated SST change in the individual-forcing
simulations (burgundy dashed curve), which, aside from volcanic
eruptions, remains below the confidence interval for the CMIP6 MMM (dark
blue shaded area) from 1950 onward (this discrepancy is, again, robust
to differences in model availability for the different sets of forcing
agents). Thus, a non-linear interaction between forcing agents in CMIP6
balances the additional SST warming in CMIP5 in the ALL simulations, and
the difference in coupled simulations of Sahel rainfall between CMIP5
and CMIP6 must derive from changes in the fast response to forcing, SST
teleconnections, or both.
e. The NARI teleconnection in Coupled Simulations
Now that we have examined SST in the coupled simulations, we may
determine whether the teleconnection strength estimated from amip-piF
simulations is consistent with coupled simulations. This is verified by
the fact that the amip-piF teleconnection strength falls within the
range of teleconnection strengths calculated from individual piC
simulations in CMIP5 (\(0.5\pm 0.6\)) and CMIP6 (\(0.4\pm 0.6\)), but
the ranges are large (possibly because the increased presence of
atmospheric and oceanic IV and decreased variance of NARI in the
individual piC simulations obscures the teleconnection). As a second
test, we compare the confounded teleconnection strength in the amip-hist
simulations (\(0.93\pm 0.41\)) to that of bootstrapped MMMs in the
coupled ALL simulations in CMIP5 (\(0.66\pm 0.28\)) and CMIP6
(\(1.5\pm 0.3\)). The confounded teleconnection strength in amip-hist
simulations is consistent with the confounded estimate in CMIP5, but is
smaller than and inconsistent with the confounded estimate in CMIP6.
This may be because NARI variability in the coupled simulations is
smaller relative to the magnitude of external radiative forcing than it
is in the amip-hist simulations. If this is the cause for the apparent
inconsistency, we may still confirm the NARI teleconnection strength in
CMIP6 simulations by showing that the implied fast response to forcing
is consistent with the fast response from the amip-hist simulations.
f. Fast and Slow Responses to
Forcing in Coupled Simulations (\(\overrightarrow{f}\) and\(F\rightarrow SST\rightarrow P\))
Under the assumption that the dominant simulated path of SST influence
on the Sahel is captured by a linear relationship with NARI, we estimate
the slow response to forcing in coupled simulations as the simulated
NARI MMM scaled by the teleconnection strength derived from uncoupled
simulations (\(0.87\frac{\text{mm}}{day\ C}\), Section 4.c), so that a
warm (cold) NARI predicts a wet (dry) Sahel. In Figure 9, simulated NARI
(as in Figure 7, right column) is displayed on the left ordinates in
light blue (CMIP6) and turquoise (CMIP5). The right ordinates are scaled
by the teleconnection strength so that, when read on the right
ordinates, simulated NARI represents the estimated slow component of the
precipitation response to forcing. Also on the right ordinates are the
total simulated precipitation responses to forcing (as in Figure 2) in
CMIP5 (right column) and CMIP6 simulations (left column), colored by
forcing agents. The simulated precipitation responses to forcing
(colors) match the estimated slow response to forcing (turquoise)
reasonably well: the main differences appear after about 1970 in CMIP5
and 1990 in CMIP6.