Description: It might be impossible to truly fathom the magnitude of the threat that global-mean sea level rise poses. However, conceptualizing the scale of the solutions required to protect ourselves against global-mean sea level rise, aids in our ability to acknowledge and understand the threat that sea level rise poses. On these grounds, we here discuss a means to protect over 25 million people and important economical regions in northern Europe against sea level rise. We propose the construction of a Northern European Enclosure Dam (NEED) that stretches between France, the United Kingdom and Norway. NEED may seem an overwhelming and unrealistic solution at first. However, our preliminary study suggests that NEED is potentially favorable financially, but also in scale, impacts and challenges compared to that of alternative solutions, such as (managed) migrations and that of country-by-country protection efforts. The mere realization that a solution as considerable as NEED might be a viable and cost-effective protection measure is illustrative of the extraordinary global threat of global-mean sea level rise that we are facing. As such, the concept of constructing NEED showcases the extent of protection efforts that are required if mitigation efforts fail to limit sea level rise.

Description: The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC). The pathways feeding the overflow remain, however, poorly resolved. Here we use multiple observational platforms and an eddy-resolving ocean model to identify an unrecognized deep flow toward the Faroe Bank Channel. We demonstrate that anticyclonic wind forcing in the Nordic Seas via its regulation of the basin circulation plays a key role in activating an unrecognized overflow path from the Norwegian slope – at which times the overflow is anomalously strong. We further establish that, regardless of upstream pathways, the overflows are mostly carried by a deep jet banked against the eastern slope of the Faroe-Shetland Channel, contrary to previous thinking. This deep flow is thus the primary conduit of overflow water feeding the lower branch of the AMOC via the Faroe Bank Channel.

Description: The occurrence of extreme weather events has increased during the two last decades. European heat waves are responsible for social, economic and environmental damage and are projected to increase in magnitude, frequency and duration under global warming, heightening the interest about the contribution of different drivers. By using the ERA5 Re-analysis product, we performed a two-sided composite analysis to investigate a potential relation between North Atlantic sea surface temperatures (SSTs) and the near-surface air temperature (T2m) over the European continent. Here, we show that in the presence of cold North Atlantic SSTs during summer, the distribution of European T2m shifts towards positive anomalies a few days later, increasing the likelihood for heat waves. During these events a predominant wave number three pattern in addition to regionally confined Rossby wave activity contribute to a trough-ridge pattern in the North Atlantic-European sector. Specifically, five of 17 European heat waves within the period of 1979 to 2019 could be related to a cold North Atlantic SST event a few days in advance. In the upstream analysis we identify eleven of 17 European heat waves co-existent with cold North Atlantic SSTs. In order to confirm the crucial role of North Atlantic SSTs for European heat waves, we analysed output from a coupled climate model, HadGEM3, with three different horizontal resolutions. The high-resolution run revealed the closest resemblance to the ERA5 data, suggesting that mechanisms on the mesoscales (〈50 km) play a role in the relationship between North Atlantic SSTs and European T2m. Results also highlight the importance of using a climate model with a high horizontal resolution for the purpose of studying the variability of European heat waves. Based upon our results, conducted with ERA5 Re-analysis and HadGEM3 data, North Atlantic SSTs provide potential predictive skill of European heat waves.

Description: This study highlights the relevance of North Atlantic SSTs and certain jet stream properties for the onset of high European temperatures by using the ERA-5/ERA20c reanalysis product and a targeted experiment with the OpenIFS model. We found that certain European heat wave events could be related to the simultaneous appearance of cold North Atlantic SST events, specific jet stream wave numbers and further to transient and recurrent Rossby wave activity. The coexistence of cold North Atlantic sea surface temperature (SST) and positive European surface temperature anomalies during several summer seasons, like in 1994, 2015 and 2018 motivated us to evaluate whether and how widespread and significant North Atlantic SST anomalies could be associated with European heat waves.Therefore we investigated the role of the jet stream in serving as a medium for a downstream signal propagation. A composite study reveals that cold North Atlantic SST anomalies in summer are accompanied by a more undulating jet stream and a preferred trough-ridge pattern in the North Atlantic-European sector. A wave analysis covering two-dimensional probability density functions of phase speed and amplitude after compositing cold SSTs show that cold North Atlantic SST events reveal a preference for a dominance of transient waves. In the presence of a trough during cold North Atlantic events, we obtain a slow-down of the transient waves, but not necessarily an amplification or stationarity. The deceleration of the transient waves result in a longer duration of a trough over the North Atlantic accompanied by a ridge downstream over Europe, favouring the conditions for the onset of European heat episodes. A study of the jet stream energetics via a kinetic energy power spectrum of meridional wind anomalies reveals that generally a trend shows up towards wave numbers 4 to 6. This is supported by an enhanced activity of specific wave numbers within this increased range during summer seasons of European heat wave events happening in the last two decades. An arising question poses whether the increased energy for a certain wave number originates from an SST forcing or different drivers. We investigate this by performing targeted OpenIFS model runs forced by different SST conditions.

Description: We investigate how European heat waves and their associated heat stress on humans have changed over the 20th century. We find that the heat stress has increased, even in regions where heat waves have not become warmer. As heat stress increases over wide areas of Europe there is also an increase in the total population affected by the heat stress. Heat waves pose a serious health risk to humans by reducing our ability to shed heat. We have studied the occurrence and intensity of heat waves as well as a heat stress index based on simplified wet-bulb globe temperature using data from ERA-20C reanalysis 1900-2010. Over the 110 years of data we find an overall warming of the air temperatures and dew point. The 98th percentile of both air temperature has increased by more than 1.5°C over large areas of Europe. We find an overall increase in heat wave days per year as well as an increase of air temperature during heat waves over most of Europe. As such, many densely populated areas exhibit increased heat stress during heat waves. For example, the mean heat stress during heat wave days over Paris has increased by one level, from “alert” in 1900-1930 to “caution” in 1980-2010. The fraction of the population exposed to heat waves has increased by 10%/century in central Europe and 25%/century over the Mediterranean. We find more heat waves during 1920 - 1950, which may be related to the positive phase of the Atlantic Multidecadal Variation (AMV). This suggests that the heat stress during European heat waves may also be influenced by internal climate variability, and large-ensemble model simulations may be used to disentangle the effects of natural variability and anthropogenic forcing.

Description: We discuss the development of a new coupled climate model at GEOMAR, capable of resolving the mesoscale. The ocean component is a ORCA05 NEMO configuration with regional refinements in e.g. the tropical Atlantic (INALT), Southern Ocean (ORION), or North Atlantic (VIKING) where the horizontal resolution is 10 km or finer. The atmosphere component is OpenIFS which can be run at T511 (39 km), T1023 (20 km) or T2047 (10 km) resolution. A climate model that resolves the mesoscale can capture oceanic eddies and tropical storms, and thus be used to understand how these phenomena respond to global warming. Latest results include experiments using NEMO ocean and ECHAM atmosphere, as well as atmosphere-only experiments with OpenIFS to understand the impact of horizontal resolution in the atmosphere. We report on the latest results and discuss the challenges and goals of future model development

Description: The occurrence of European heat events has increased during the last two decades. European heat events are responsible for social, economic and environmental damage and are projected to increase in magnitude, frequency and duration under global warming, strengthening the interest about the contribution of different mechanisms. Using the ERA5 reanalysis product, we go beyond case studies relating European heat events with cold North Atlantic sea surface temperatures (SSTs), and perform a systematic approach with a composite analysis to investigate the link between North Atlantic SSTs in a domain south of Greenland and the 2m air temperature (T2m) over central Europe. Composites of different North Atlantic SST states show that events with a negative tendency of North Atlantic SSTs are often followed by positive European T2m anomalies during summers when the North Atlantic SSTs are persistently low for several months. Enhanced lower–tropospheric baroclinicity in the North Atlantic is followed during these events by a slantwise ascent and an enhanced upper–tropospheric waveguide, promoting a downstream development of an European ridge. A combination of a wave number 3 pattern and regionally confined Rossby wave activity contribute to a trough–ridge pattern in the North Atlantic–European sector. A composite of European heat events further confirms the lagged statistical relationship between cold North Atlantic SSTs with a negative tendency and positive European T2m anomalies. A negative tendency of North Atlantic SSTs precedes 15 of 18 European heat events, and cold North Atlantic SST conditions are present during 14 of 18 European heat events.

Description: The Atlantic Niño is the leading mode of interannual sea-surface temperature (SST) variability in the equatorial Atlantic and assumed to be largely governed by coupled ocean-atmosphere dynamics described by the Bjerknes-feedback loop. However, the role of the atmospheric diabatic heating, which can be either an indicator of the atmosphere’s response to, or its influence on the SST, is poorly understood. Here, using satellite-era observations from 1982–2015, we show that diabatic heating variability associated with the seasonal migration of the Inter-Tropical Convergence Zone controls the seasonality of the Atlantic Niño. The variability in precipitation, a measure of vertically integrated diabatic heating, leads that in SST, whereas the atmospheric response to SST variability is relatively weak. Our findings imply that the oceanic impact on the atmosphere is smaller than previously thought, questioning the relevance of the classical Bjerknes-feedback loop for the Atlantic Niño and limiting climate predictability over the equatorial Atlantic sector.

Description: We developed a new version of the Alfred Wegener Institute Climate Model (AWI-CM3), which has higher skills in representing the observed climatology and better computational efficiency than its predecessors. Its ocean component FESOM2 (Finite-volumE Sea ice-Ocean Model) has the multi-resolution functionality typical of unstructured-mesh models while still featuring a scalability and efficiency similar to regular-grid models. The atmospheric component OpenIFS (CY43R3) enables the use of the latest developments in the numerical-weather-prediction community in climate sciences. In this paper we describe the coupling of the model components and evaluate the model performance on a variable-resolution (25-125 km) ocean mesh and a 61 km atmosphere grid, which serves as a reference and starting point for other ongoing research activities with AWI-CM3. This includes the exploration of high and variable resolution and the development of a full Earth system model as well as the creation of a new sea ice prediction system. At this early development stage and with the given coarse to medium resolutions, the model already features above-CMIP6-average skills (where CMIP6 denotes Coupled Model Intercomparison Project phase 6) in representing the climatology and competitive model throughput. Finally we identify remaining biases and suggest further improvements to be made to the model.