ALBERT

Description: Understanding and predicting future global monsoon changes is critically important owing to its impacts on about two-thirds of population. Robust post-eruption signals in the monsoon climate raise the question of their potential for a role in future climate. However, major volcanic eruptions are generally not included in current projection scenarios because they are inherently unpredictable events. By using sixty plausible eruption scenarios sampled from reconstructed volcanic proxies over the past 2,500 years, we revealed the volcanic impacts on the future changes of summer precipitation over global and sub-monsoon regions. Episodic volcanic forcing not only leads to a 10% overall reduction of the centennial global land monsoon (GLM) precipitation, but also causes larger ensemble spread (~20%) compared to no-volcanic and constant background-volcanic scenarios. Moreover, volcanic activity is projected to delay the time of emergence of anthropogenic GLM precipitation changes by five years on average over about 60% of the GLM area. Our results demonstrate the added value of incorporating major volcanic eruptions in monsoon projections.

Description: Extratropical SST variability associated with ocean dynamics has well-documented effects on the atmospheric boundary layer, but it is still unclear to what degree and through which mechanisms it influences the free troposphere. The mesoscale features of this SST variability (eddies, fronts) are not captured using typical climate model resolutions, making their large-scale effects difficult to study. We use the Norwegian Earth System Model version 2 (NorESM2) with two ocean setups - standard 1° resolution (MM) and eddy-permitting 0.125° resolution (MX) - both coupled to a 1° atmosphere. With a higher resolution ocean, MX is overall warmer and has different large-scale SST patterns compared to MM. MX exhibits stronger surface turbulent heat fluxes from the ocean to the atmosphere in regions where it is warmer than MM (notably, the western boundary currents, eastern tropical Pacific, and South Atlantic), indicating that SST differences drive the heat flux anomalies. In the extratropical free atmosphere, MX exhibits more energetic storm tracks and a poleward shift of the eddy-driven jets and storm tracks in all sectors and seasons except the North Atlantic during winter. These analyses, together with atmosphere-only experiments forced by SSTs from MX and MM, suggest that the large-scale SST patterns and fine-scale ocean features in an eddy-permitting ocean play a role in setting the jets and storm tracks. Future work will use SST pacemaker experiments to isolate and investigate the underlying coupled mechanisms for how mesoscale ocean variability affects the extratropical atmospheric circulation.