A document by Josefine Axelsson. Click on the document to view its contents.

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC’s response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. We find no consistent changes in the overall AMOC strength across the simulations, which conforms with reconstructions assimilating proxy records. Similarly, the decadal variability of the AMOC does not change during the mid- and late-Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment.

Significant multi-centennial climate variability with a clear peak at approximately 200 years is found in a pre-industrial control simulation conducted with the EC-Earth3 climate model. The oscillation mainly emerges from the North Atlantic and appears to be closely associated with the Atlantic Meridional Overturning Circulation (AMOC). By examining the salinity advection feedback, we find that the perturbation flow of mean subtropical-subpolar salinity gradients in the subpolar area governs as positive feedback to the AMOC anomaly. Meanwhile, the mean advection of salinity anomalies and the vertical mixing or convection acts as negative feedback to restrain the AMOC anomaly. In a warmer climate, although the AMOC becomes weaker, such low-frequency variability still exists, indicating the robustness of the salinity advection feedback mechanism.

Observational data from the past century show a weakening trend in summer monsoon over the Indian subcontinent. This is possibly attributed to the reduced land–sea contrast resulting from the Indian Ocean warming under the rapid increase of greenhouse gases in the atmosphere. In contrast, speleothem records indicate that the Indian summer monsoon was stronger during the last interglacial (LIG) warm period than it is today. Using climate model simulations, we show that orbital forcing effect during the LIG, as well as related ocean feedbacks, led to warming in the Eurasian continent and cooling in the Indian Ocean basin. This amplified the land–sea contrast in the region and intensified the Indian summer monsoon. Although the LIG is often portrayed as a potential analogue of future climate, our study shows that the Indian monsoon responded differently to the LIG warming period than it does to current climate warming.

Solar energy, if carefully planned, can contribute to the attainment of global climate mitigation goals by reducing reliance on fossil fuel energy. It has been suggested that large-scale photovoltaic solar farms envisioned over the Sahara desert would reduce surface albedo, leading to increased rainfall and vegetation cover that would benefit the regional environment while meeting the world’s energy demand. However, adverse remote effects resulting from atmospheric teleconnections could offset such regional benefits. We use state-of-the-art Earth-system model simulations to evaluate the global impacts of Sahara solar farms. Our results indicate a redistribution of precipitation causing Amazon droughts and forest degradation, and global surface temperature rise and sea-ice loss, particularly over the Arctic due to increased polarward heat transport, and northward expansion of deciduous forests in the Northern Hemisphere. We also identify reduced El Niño-Southern Oscillation and Atlantic Nino variability and enhanced tropical cyclone activity. All these remote effects are in line with the global impacts of the Sahara land-cover transition ~6,000 years ago when Sahara desert was wetter and greener. The improved understanding of the forcing mechanisms of massive Sahara solar farms can be helpful for the future site selection of large-scale desert solar energy facilities.

Coastal risks are increasing due to the warming of the climate, resulting in rising mean sea levels and changes in storminess. Projections of future coastal flooding rely on global climate models based on greenhouse gas scenarios with inherent large uncertainties. The past warm climate of the Last Interglacial (LIG, ~127,000 years ago) is considered a partial analogue of a future warmer world. Therefore, understanding how coastal systems were affected by changes in atmospheric and relative sea levels during the LIG can inform us about possible future changes. In this contribution we will analyze extreme sea levels and coastal flooding during the LIG. The analysis is based on the hydrodynamic Global Tide and Surge Model (GTSM; Muis et al., 2016, doi: 10.1038/ncomms11969). To simulate storm surges during the LIG GTSM will be forced by 6-hourly wind and surface pressure fields from LIG simulations of IPCC-type climate models. Due to non-linear effects, tides and surge levels will be influenced by changes in mean sea level. Therefore, a key input variable is map of regional mean sea levels during LIG. However, there is still considerable uncertainty on sea level high-stands and regional patterns during the LIG. Using output from a Glacial Isostatic Adjustment model (GIA), we will model tides and surges for a set of plausible scenarios of relative sea levels and assess sensitivities.

We investigate hydrology during a past climate slightly warmer than the present: the Last Interglacial (LIG). With daily output of pre-industrial and LIG simulations from eight new climate models we force hydrological model PCR-GLOBWB, and in turn hydrodynamic model CaMa-Flood. Compared to pre-industrial, annual mean LIG runoff, discharge, and 100-year flood volume are considerably larger in the Northern Hemisphere, by 14%, 25% and 82%, respectively. Anomalies are negative in the Southern Hemisphere. In some boreal regions, LIG runoff and discharge are lower despite higher precipitation, due the higher temperatures and evaporation. LIG discharge is much higher for the Niger, Congo, Nile, Ganges, Irrawaddy, Pearl, and lower for the Mississippi, Saint Lawrence, Amazon, Paraná, Orange, Zambesi, Danube, Ob. Discharge is seasonally postponed in tropical rivers affected by monsoon changes. Results agree with published proxies on the sign of discharge anomaly in 15 of 23 sites where comparison is possible.