Fig. 23: Annual-mean changes in the energy budget, relative to piControl
(delta approach). (a) Top of Atmosphere (TOA) net total (longwave (LW) +
shortwave (SW); solid) and net shortwave (dashed) radiation time series
after application of an 11-year running-mean, for the historical (black)
and scenario (colours) simulations. Positive is downward. (b) Changes in
the effective albedo diagnosed as upwelling divided by downwelling
shortwave radiation at the TOA (planetary; solid) and at the surface
(dashed). (c) and (d) same as (a) and (b) but as zonal means averaged
over the period 2071-2100 (scenario simulations only), plotted against
sin(latitude) to reflect equal-area global contributions. Extreme
negative values of effective surface albedo changes, reaching about
-20% in both polar regions, are truncated in (d) to increase the
visibility of changes in lower latitudes. Only the respective first
ensemble member of the historical and SSP370 scenario simulations is
shown.
While according to our simulations an increased planetary albedo has
prevented a stronger warming of the climate system until present-day,
the planetary albedo is projected to decrease and thus amplify the
future warming in all scenarios (Fig. 23b, coloured solid curves and
Fig. 23b, coloured dashed curves). The global-mean effective surface
albedo, which seems to have played no major role until around 1980, is
projected to decline by more than 1% (the absolute simulated effective
surface albedo is ~13%) until the end of the century in
the SSP585 scenario simulation and is thus a significant part of the
projected positive shortwave feedback. Interestingly, the global-mean
net shortwave radiation is projected to increase faster than the total
radiation (Fig. 23a). This implies that, while reduced outgoing longwave
radiation (OLR) has caused the warming until present-day, the OLR is
projected to increase toward the end of the century: Due to the
strong shortwave feedback, the positive influence of increasing
temperatures on OLR is projected to outweigh the direct negative
influence of increased greenhouse-gas concentrations on OLR. This
behaviour has been found for most CMIP3 and CMIP5 models (Donohoe et al.
2014).
The surface albedo is decreasing particularly strongly in the regions
with declining sea-ice extent, that is, the Southern Ocean (60°S-70°S)
and the Arctic (north of 70°N) (Fig. 23d, dashed curves). These changes
are clearly reflected in the planetary albedo (Fig. 23d, solid curves),
which however also reveals non-surface-related albedo changes in lower
latitudes caused by cloud feedbacks. In particular, toward the end of
the century the planetary albedo is projected to increase in the tropics
between 15°S and 15°N (negative feedback) and to decrease in the
subtropics (positive feedback) (Fig. 23d, solid curves).
While the surface-driven changes in the planetary albedo projected
toward the end of the century are substantial in both polar regions, the
positive net total TOA radiative imbalance is particularly pronounced
over the Southern Ocean (Fig. 23c, solid curves). This is consistent
with the relatively weak Antarctic and strong Arctic warming (Fig. 14a),
leading to strongly enhanced upwelling longwave radiation in the Arctic
but not in the Antarctic. This asymmetry in terms of polar amplification
and TOA fluxes is consistent with the fact that the Southern Ocean
temperature responds much more slowly to changes in atmospheric thermal
forcing because of the spatial structure of the global Meridional
Overturning Circulation (MOC), with circumpolar upwelling of unperturbed
water masses in the south and downwelling in the north (Armour et al.
2016, Rackow et al. 2018).
5.6 Changes in ENSO
Since ENSO is the dominant mode of interannual variability (Timmermann
et al., 2018), with pronounced global impacts through far-reaching
teleconnections such as the atmospheric bridge (Alexander et al. 2002),
an important question is whether the character of ENSO will change under
climate change. To address this question, we resort to the five SSP370
projections with AWI-CM until the end of the 21stcentury. According to these simulations, when compared to the
probability distribution of Niño 3.4 SST anomalies for 1870–2014,
strong warm and cold SST anomalies become more likely by the end of this
century (Fig. 24). The increase of strong cold SST anomalies dominates,
so that the clear positive asymmetry between El Niño and La Niña (Fig.
5), as diagnosed from the skewness of modelled Niño 3.4 SST anomalies
(1870-2014: 0.15 ± 0.16, observed: 0.36), is reduced under climate
change (2071-2100: 0.04 ± 0.17). A reduced positive asymmetry under
global warming has been found for most CMIP5 models (for the overlapping
Niño3 region; Ham et al., 2017). However, despite sharing the
atmospheric model with AWI-CM, in that study MPI-ESM-LR and MPI-ESM-MR
showed a strong increase of DJF Niño3 skewness with the RCP4.5 scenario,
which might again hint at the different ocean model formulation in
AWI-CM and MPI-ESM and should be evaluated in more detail in the future.
Interestingly, the seasonal cycle that has been subtracted to compute
the SST anomalies within the Niño 3.4 box consistently changes under
climate change in all ensemble members (Fig. 25): when subtracting the
different annual means, a stronger positive (negative) peak in April/May
(August–October) is evident, suggesting that seasonality within this
region will likely increase until the end of the century. Concerning
phase locking of Niño 3.4 SST anomalies to the seasonal cycle, the
variability increases throughout the entire year (Fig. 26), shifting the
characteristic U-shape upwards by the end of the century.