3.4 Model-simulated WWEs, past and future
The same analysis method is applied to ten ensemble members produced by
the CESM. Daily fields of 500 hPa geopotential height anomalies are
mapped onto the characteristic patterns in the master SOM that were
derived from reanalysis output. The closest matching node is identified
for each simulated daily field. It should be noted that a master SOM
created using model fields produces a very similar matrix of patterns
(Fig. S9 ). Moreover, our previous work (Francis et al., 2020)
demonstrated that three models participating in the Climate Model
Intercomparison Project – version 5 (CMIP5) successfully captured
monthly distributions of days belonging in each node as well as
frequencies of LDEs during the years included in the historical period
(1979-2005).
In terms of changes in WWEs over the historical period, we find CESM
simulations agree reasonably well with changes observed in the
reanalysis output (Fig. S10) . Given the large interannual
variability in WWEs (Fig. 6a ) and the fact that the 1995-2005
period is relatively early in the era of the climate-change signal
generally and the emergence of AAW specifically (node #1), it is not
surprising that changes over two relative short periods do not align
perfectly.
To further demonstrate the utility of this approach, next we explore
projected future changes in WWEs, assuming RCP 8.5 forcing conditions in
CESM. Our findings presented in Fig. 7 suggest robust changes
in the number of WWEs will occur from the beginning of the
21st century (2006-2030) to the end (2076-2100) during
winter and summer seasons. Nodes #1 and #3 exhibit robust increases in
both seasons, while WWEs originating in node #12 are projected to
decrease significantly. These changes can be attributed to a combination
of changing frequency of occurrence of each node (Fig. S11 ) as
well as changes in the probability (number of WWEs/number of days in a
node) of a WWE originating in each node (Fig. S12 ). Patterns
with positive height anomalies over northern and northeastern North
America are projected to not only occur more frequently (Fig.
S11 ), but also to spawn a higher frequency of WWEs in a warmer world.
To check for intermodel consistency, we also analyze output from three
of the models included in the CMIP5 generation. We find they project
changes in WWEs similar to those from the CESM runs (Fig. S13 ).
These findings point toward an increased occurrence of regime-based WWEs
in the future, as patterns with positive height anomalies in high
latitudes become more common and are also more likely to initiate WWEs,
while those with negative anomalies in Arctic regions will occur less
frequently and are less likely to spawn WWEs.
Finally, we calculate the annual mean Euclidean distances separating the
two nodes participating in a WWE, based on CESM output from 2006 to 2100
(Fig. S14 ). This metric provides an indication of the severity
of a WWE, as a larger Euclidean distance would represent a more dramatic
shift in the pattern. We find that 7 of 12 nodes exhibit statistically
significant trends; five with declining trends and two with increasing
trends. The preponderance of negative trends may result from the ongoing
decrease in the poleward temperature/height gradient as high latitudes
warm faster than lower latitudes and the corresponding reduction in
airmass contrast. This explanation is supported by the fact that
patterns with low height anomalies in the Arctic (nodes #11 and #12)
exhibit increasing trends, reflecting larger airmass contrasts when the
Arctic is anomalously cold.