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Climate Impacts of the Atlantic Meridional Overturning Circulation

Periodic Reporting for period 1 - CliMOC (Climate Impacts of the Atlantic Meridional Overturning Circulation)

Okres sprawozdawczy: 2021-06-01 do 2023-05-31

CliMOC investigated the role of AMOC in the global climate, specifically for the European continent. AMOC stands for Atlantic Meridional Overturning Circulation, which is a system of global-scale ocean currents that redistribute heat, salinity and carbon around the world. The AMOC has important consequences for the global climate. Energetically, the AMOC transports more heat into the North Atlantic than in the Southern Hemisphere oceans, which causes an inter-hemispheric imbalance of heat transport. This imbalance is believed to make the Northern Hemisphere warmer by about 1°C than the Southern Hemisphere and to shift the tropical peak of annual mean precipitation (the so called Intertropical Convergence Zone or ITCZ) to be in the Northern Hemisphere at about 5°C. Hence, the ocean heat transport by the AMOC has important consequences on temperature and precipitation, including important rainfall systems such as the tropical monsoons and the extra-tropical cyclones.

However, some studies have challenged the notion that the AMOC (and specifically the Gulf Stream) plays a role in specifically making European winters milder than they would be in the absence of the AMOC. By using older modeling setups in which the AMOC was artificially removed by coupling an atmospheric model with a motionless ocean model, these studies found that it’s the Rocky Mountains that contribute to milder European winters by modifying the westerly wind flow at the mid-latitudes. In CliMOC we challenged this notion by proposing a series of novel modelling experiments and setup to investigate the climate response to changes in AMOC strength, with a particular focus on the European continent.

We concluded the CliMOC project by finding that our hypothesis was correct. The earlier modeling setups masked the real role of the AMOC in the current climate because the missing ocean dynamics in the motionless ocean model that was used in those studies strongly limited the coupling between the atmosphere and the ocean, plus the associated feedbacks. We designed and made available a modeling experiment setup with state-of-the-art global climate model EC-Earth3 in which we are able to artificially tune the strength of the AMOC. Initial experiments with this setup conducted within CliMOC show an important role of the AMOC for the European hydroclimate, which has of course important implications for future climate change. In fact, because all models within the Coupled Model Intercomparison Project (CMIP) robustly project an AMOC decline in the 21st century, it is of utmost importance to assess the impacts of an AMOC decline relative to those of greenhouse gas forcing.
Over the course of the project, the PI in collaboration with the HI and external collaborators developed and made available a model experiment protocol to investigate the impacts of a weakened AMOC on the European climate. This protocol has been implemented in the global climate model EC-Earth3 model and results of ad-hoc model experiments in which the AMOC has been artificially weakened have been investigated by the PI and collaborators. Our main findings show that when the AMOC is weakened, global temperatures cool, particularly in the Northern Hemisphere. Net precipitation decreases over most of Europe, although some regions experience a slight increase. We in particular examined precipitation anomalies at daily timescales in the Euro-Atlantic sector during winter. We discovered that the decrease in AMOC leads to a decline in precipitation over most of the Euro-Atlantic sector, while certain areas in northwestern Europe experience more wet days. We further explored the mechanisms behind precipitation change, finding that the enhancement of the mid-latitude jet stream and its eastward extension towards northwestern Europe contribute to the increase in wet days anomalies. The widespread drying over Europe is attributed to the deficit in moisture and the role of transient eddies. We analyzed changes in weather regimes and identified an increase in the persistence and frequency of NAO+ events, which are associated with enhanced precipitation over northwestern Europe and reduced precipitation over southern Europe. Overall, our research suggests that if the AMOC weakens, the European continent will experience widespread drying due to intensified but drier storms, while regions with wetter anomalies will arise from changes in the mid-latitude jet and an increase in NAO+ days. Given that the level of AMOC decline studied herein aligns with projected scenarios in climate change models, our findings have implications for understanding the role of AMOC weakening in future climate change. The PI also participated in a collaborative model intercomparison project (NAHosMIP) study in which several modelling centers conducted similarly designed ad-hoc experiments with other climate models.
In total:
Number of published peer-reviewed publications: 4
Number of conference presentations: 12
Number of outreach activities: 4
State-of-art knowledge of AMOC impacts on the European region was hampered by older experimental setups and limited availability of data at shorter timescales (i.e. daily) due to computational costs. CliMOC advanced the understanding of AMOC impacts on the Euro-Atlantic sector, especially for what concerns impacts on temperature, precipitation and wind circulation at short timescales. With the progress made within CliMOC we are now able to pinpoint the changes attributable to AMOC decline and be able to assess their effects with climate change. For example, changes in hydroclimate are already causing major societal issues due to rising greenhouse gases. Our work highlights the major role of the AMOC in making droughts in southern Europe even more severe than what would be expected from greenhouse gas forcing alone. Further, while an AMOC decline might mitigate (albeit temporarily) regional warming, herein we showed a direct effect of AMOC on Scandinavian blocking events and consequently on temperature extremes. However, how greenhouse gas forcing acts on Scandinavian blocking and how it feed backs on the AMOC response is still an open question. The work carried out in AMOC overall highlighted a primary role for AMOC in shaping future climate change, which was previously unknown.
Idealized modeling experiments