ALBERT

Description: Previous studies on surface temperature reconstructions for the last 2000 years (2 k) revealed a long-term cooling trend for the last millennium in comparison to the previous millennium. However, knowledge on the decadal- to centennial-scale variability in sea surface temperature and the underlying governing mechanisms throughout the period is limited. We reconstructed high-resolution continuous sea surface temperature changes over the last 2 k in the northwest Pacific margin based on the alkenone unsaturation index. Our alkenone temperature record revealed enhanced and more rapidly changing climate variability during the last millennium (approximately 1200–1850 Common Era) than during the previous millennium. Cold and hot extremes also occurred more frequently during the last millennium. The enhanced and rapidly changing climate variability appears to be associated with frequent volcanic eruptions and grand solar minima. The reconstructed surface temperature variability tends to be associated with variations in the East Asia summer monsoon and the Pacific Decadal Oscillation, implying that these variations are also enhanced in the last millennium than in the previous millennium.

Description: El Niño/Southern Oscillation (ENSO) is the most dominant mode of climate variability on interannual time scales in the tropical Pacific sector and arises from a complex interplay between amplifying and damping feedbacks. Given its large socio-economic impacts caused by e.g. severe weather events such as floods and droughts in various regions of the world, it is very important to accurately predict how ENSO will change under global warming. Despite improvements have been made in simulating ENSO over the last decades, a realistic representation of ENSO and its projection under global warming remains a challenge. ENSO projections widely differ amongst climate models participating in the phase 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), which are the base of the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Although these models simulate ENSO, which in terms of simple indices are consistent with observations, the underlying dynamics are very different from the observed. In observations, an initial SST anomaly grows during ENSO events by wind-induced changes in the ocean dynamics. This tendency is counteracted by damping surface heat flux feedback, especially the atmospheric shortwave radiation and latent heat flux damping. In most climate models, however, the wind- SST feedback is too weak and the shortwave-SST feedback erroneously positive so that ENSO is a hybrid of wind-driven and shortwave-driven dynamics. In the most biased models, the shortwave-SST feedback contributes to the SST anomaly growth to a similar degree as the wind-SST feedback. As the models not only underestimate the wind-SST feedback but also heat flux damping, this error compensation explains why models with less than a half of the observed wind-SST feedback strength can still exhibit realistic ENSO amplitude. A broad continuum of ENSO dynamics exists in the climate models that may explain the large spread in 21st century ENSO projections. In the IMBE21C project, the effect of biased ENSO dynamics on ENSO projections will be investigated. With a new method, based on an ‘Offline Slab Ocean SST’, we can quantify the effects of the amplifying and damping feedbacks by separating the SST changes caused by the wind-driven ocean dynamics and by atmospheric heat fluxes. In this project we will use this method to quantify the forcing and damping in observed ENSO dynamics and to compare it with the modeled ENSO to identify and quantify the biases of the simulated ENSO dynamics. Further we will analyze global warming projections with respect to the influences of biased ENSO dynamics by dividing the models into groups with realistic and biased ENSO dynamics. Overall, IMBE21C aims at identifying sources of uncertainties in ENSO projections by innovative methods and will try to reduce them.