Summary :
The European continent experienced a considerably increased number of heat events within the last two decades, most likely driven by a surface warming due to anthropogenic greenhouse gas emissions. The investigation of heat events and associated drivers is essential due to numerous impacts, e.g., on human health. The European heat event in 2015 was accompanied by unusually cold sea surface temperatures (SSTs) south of Greenland, which led to the influence of North Atlantic SSTs being considered along with other potential drivers. This thesis incorporates three different research chapters, all contributing to disentangle the role of North Atlantic SSTs for European heat events.

The first chapter is based on a composite study with reanalysis data and highlights that events of unusually cold North Atlantic SSTs with a negative tendency precede a surface temperature maximum over central Europe a few days later, increasing the probability for European heat events. 

The second chapter utilises seven coupled climate models with different horizontal resolutions in the atmosphere and ocean model component. The model simulations generally confirm the identified relationship between cold North Atlantic SSTs with a negative tendency and European heat events. The relationship is better reproduced with an increased horizontal resolution, particularly of the ocean model component. 

The third research chapter demonstrates the importance of SSTs and warm conveyor belt activity in the North Atlantic sector for the development of the 2015 European heat event. Additionally, subseasonal-to-seasonal reforecasts are used to estimate the forecast skill of North Atlantic warm conveyor belt activity and the 2015 European heat event. 

All three research chapters emphasise to account for North Atlantic SSTs in future assessments of the European summer climate and the development of European heat events. 

Krüger, J, Kjellsson, J, Kedzierski, RP and Claus, M. 2023. Connecting North Atlantic SST Variability to European Heat Events over the Past Decades. Tellus A: Dynamic Meteorology and Oceanography, 75(1): 358–374. DOI: https://doi.org/10.16993/tellusa.3235

Summary: "The intensity and frequency of European heat events has increased in the recent two decades, which has strengthened the interest about identifying potential driving mechanisms. This study investigates five different composites of North Atlantic sea surface temperatures. We find that events with an atmospherically-driven fall of cold North Atlantic sea surface temperatures are accompanied by a subsequent surface air temperature maximum over central Europe, indicating a higher probability for heat events. In addition to the surface temperature contrast, we identify a distinct atmospheric trough-ridge pattern in the North Atlantic-European sector. An additional composite of European heat events supports this time-lagged relationship between cold North Atlantic SSTs with a negative tendency and European heat events. "

Duteil, O., Park, W. Future changes in atmospheric synoptic variability slow down ocean circulation and decrease primary productivity in the tropical Pacific Ocean. npj Clim Atmos Sci 6, 136 (2023). https://doi.org/10.1038/s41612-023-00459-3

Summary: "This study investigates the impact of future changes in atmospheric synoptic variability (ASV) on ocean properties and biogeochemical cycles in the tropical Pacific Ocean using coupled and forced atmosphere–ocean model experiments. Future climate projections show an annual mean decrease in ASV in subtropical gyres and an increase in the tropical band. Maintaining ASV to current values lead to a deepening of the mixed layer in subtropical regions and a shalllowing at the equator associated with a sea surface temperature decrease. The changes in ASV impact the large-scale ocean circulation and the strength of the subtropical and tropical cells, which constrain the equatorial water upwelling and the tropical net primary productivity. Ultimately, this study highlights the significance of ASV in understanding the impacts of climate change on ocean dynamics and biogeochemical processes, as half of the primary productivity decline due to climate change is caused by changes of ASV in the tropical Pacific Ocean."

Richardson, K., Steffen, W., Lucht, W., Bendtsen, J., Cornell, S. E., Donges, J. F., Drüke, M., Fetzer, I., Bala, G., von Bloh, W., Feulner, G., Fiedler, S. , Gerten, D., Gleeson, T., Hofmann, M., Huiskamp, W., Kummu, M., Mohan, C., Nogués-Bravo, D., Petri, S., Porkka, M., Rahmstorf, S., Schaphoff, S., Thonicke, K., Tobian, A., Virkki, V., Wang-Erlandsson, L., Weber, L. and Rockström, J.  (2023) Earth beyond six of nine planetary boundaries. Sci. Adv., 9 (37), https://doi.org/10.1126/sciadv.adh2458

Summary from https://www.stockholmresilience.org: "The planetary boundaries concept presents a set of nine planetary boundaries within which humanity can continue to develop and thrive for generations to come. In September 2023, a team of scientists quantified, for the first time, all nine processes that regulate the stability and resilience of the Earth system." [more...]

Latif, M., Bayr, T., Kjellsson, J.  Lübbecke, J.F., Martin, T., Nnamchi, H.C.,  Park, W.,  Savita, A., Sun, J. and  Dommenget, D.: Strengthening atmospheric circulation and trade winds slowed tropical Pacific surface warming. Commun Earth Environ 4, 249 (2023). https://doi.org/10.1038/s43247-023-00912-4

Summary: "The globally averaged sea-surface temperature (SST) has steadily increased in the last four decades, consistent with the rising atmospheric greenhouse gas concentrations. Parts of the
tropical Pacific exhibited less warming than the global average or even cooling, which is not
captured by state-of-the-art climate models and the reasons are poorly understood. Here we
show that the last four decades featured a strengthening atmospheric circulation and
stronger trade winds over the tropical Pacific, which counteracted externally-forced SST
warming. Climate models do not simulate the trends in the atmospheric circulation irrespective of whether an external forcing is applied or not and model bias is the likely reason."

Summary: "Low-level jets (LLJs) typically occur at night due to strong surface cooling. This study provides the first observational evidence that LLJs can also be driven by convectively generated cold pools (CPs), areas of cool and dense air beneath precipitating clouds. CPs can favor temperature inversions with reduced friction of the winds with the surface, as a prerequisite for generating LLJs also during daytime."

Summary: "The pan-Atlantic decadal oscillation exerts far-reaching impacts on regional and global climate fluctuations. This study uncovers a pattern of ocean circulation across the entire Atlantic basin that underlies the surface climate oscillation."