Jen’s talk is postponed to a later seminar.

Jens Terhaar Senior Scientist, Woods Hole Oceanographic Institute/University of Bern

Title: Atlantic overturning inferred from air-sea heat fluxes indicates no decline since the 1960s

Abstract: The Atlantic Meridional Overturning Circulation (AMOC) is crucial for global ocean carbon and heat uptake, and controls the climate around the North Atlantic. Despite its importance, quantifying the AMOC’s past changes and assessing its vulnerability to climate change remains highly uncertain. Understanding past AMOC changes has relied on proxies, most notably sea surface temperature anomalies over the subpolar North Atlantic. Here, we use 24 Earth System Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to demonstrate that these temperature anomalies cannot robustly reconstruct the AMOC. Instead, we find that air-sea heat flux anomalies north of any given latitude in the North Atlantic between 26.5°N and 50°N are tightly linked to the AMOC anomaly at that latitude on decadal and centennial timescales. On these timescales, air-sea heat flux anomalies are strongly linked to AMOC-driven northward heat flux anomalies through the conservation of energy. On annual timescales, however, air-sea heat flux anomalies are mostly altered by atmospheric variability and less by AMOC anomalies. Based on the here identified relationship and observation-based estimates of the past air-sea heat flux in the North Atlantic from reanalysis products, the decadal averaged AMOC at latitudes between 26.5°N and 50°N has not weakened from 1963 to 2017 although substantial variability exists at all latitudes.

Dorcas Daniella Ebedi Nding PhD student, Ecole des Hautes Etudes en Santé Publique/AIMS Cameroon

Title: Extremes events and socio-economic impacts in central Africa: a CMIP6-based analysis of projections

Abstract: Extreme events such as floods, droughts, and heatwaves profoundly impact all socio-economic sectors in Central Africa. Previous studies often focus on how extreme events respond to global warming without contextualizing these results within socio-economic sectors. In contrast, the present study uses data from 13 global climate models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to explore the impacts of global warming-induced changes in extreme events across various socio-economic sectors. Six indices defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) are employed for the purpose. Analyses are conducted over two time frames, namely 2030–2059 for the near future and 2071–2100 for the far future in comparison to the historical period 1985–2014, under the low SSP126 and high SSP585 emission scenarios. The results indicate enhancements in dry spells and weakening in wet spells in response to global warming. Simultaneously, an increase in annual total precipitation is expected, in association with intensification in heavy precipitation days and daily precipitation intensity. Temperature-based indices exhibit a decreasing trend in the total number of cold days per year and an increasing trend in the number of hot days, with more intense changes under the unmitigated SSP585 scenario. The low-emission, highly mitigated SSP126 scenario demonstrates its effectiveness in limiting the worsening of projected conditions compared to SSP585. Discussing the potential socio-economic risks associated with these changes highlights the urgent need to formulate robust policies to mitigate underlying hazards, as they could lead to challenges such as food insecurity, heat and humidity-related illnesses, population impoverishment, market inflation, and social instability.

Pedro Jose Roldan-Gomez Researcher, Barcelona Supercomputing Center

Title: Irreversibility in CMIP6 overshoot scenarios: from heat transport and sea ice to temperature and precipitation changes

Abstract: Overshoot scenarios, in which the forcing reaches a peak before starting to decline, show non-symmetric changes during the CO2 increasing and decreasing phases, producing persistent changes on climate. Irreversibility mechanisms, associated among others with lagged responses of climate components, changes in ocean circulation and heat transport and changes in the ice cover, bring hysteresis to the climate system. This work analyzes simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to explore the relevance of these mechanisms in overshoot scenarios with different forcing conditions (SSP5-3.4OS and SSP1-1.9) and the impact on regional climates. These analyses show that, even if the global temperature may be recovered after the overshoot, at regional level the situation post-overshoot strongly differs from the situation pre-overshoot, with spatial patterns characterized by large-scale temperature asymmetries. These asymmetries, found between Northern (NH) and Southern Hemisphere (SH) and between high and medium latitudes of the NH, contribute to shift the Intertropical Convergence Zone (ITCZ). These large scale changes have an impact on regional climates, particularly relevant for temperature extremes in extratropical regions and for precipitation extremes in tropical regions around the ITCZ. Differences between pre- and post-overshoot states may be associated with persistent changes in the heat transport and with a different thermal inertia depending on the region, leading regionally to a different timing of the temperature maximum. Other factors like changes in aerosol emissions and ice melting may be also important, particularly for polar areas. The relative contribution of these mechanisms strongly depends on the model, being certain models mainly impacted by changes in the Atlantic Meridional Overturning Circulation (AMOC) and some others showing a more relevant impact of sea ice changes.