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.