Future changes in North Atlantic sea surface temperatures (NASST) are uncertain. We assess how different future changes in NASST can affect the eddy-driven jet and the atmospheric circulation over the North Atlantic and Western Europe. We use a set of atmosphere-only simulations and show that NASST warming results in a negative North Atlantic Oscillation (NAO) phase, decreased European storm activity, and an equatorward shift of the eddy-driven jet. The response to the NASST anomaly is asymmetric, with the warming of the NASST affecting the winter season while the cooling of the NASST only affects the late winter season (poleward shift of the eddy-driven jet and positive phase of the NAO). We link the subseasonal asymmetry to troposphere-stratosphere interactions. The warming of the NASST strongly affects the troposphere, while the cooling of the NASST has a substantial impact on the stratosphere, with anomalies that are at the surface in late winter. In addition, we demonstrate that the change in subpolar gyre temperature affects the North Atlantic climate but contributes only moderately to the asymmetric response.

To understand the urgency of emissions reductions, policymakers and citizens need a full analysis of what is at stake. To understand the urgency of emissions reductions, policymakers and citizens need a full analysis of what is at stake.

Communicating how people are already affected by climate change is challenging, partly because global warming of 'just' 1 degrees C does not necessarily sound like a problem. However, the hottest UK summer days have warmed more than 3 times faster than the rate of global warming in the past century. Using a simple observation-based approach, we estimate that for the hottest UK day on record, when temperatures reached 40 degrees C in 2022, a comparable day a century ago would have peaked around 36 degrees C. Similarly, several historical days would now reach 40 degrees C if comparable events reoccurred, offering novel opportunities to communicate risks from ongoing global warming.

Earth’s energy imbalance and El Niño’s heat release from the tropical-Pacific subsurface drive year-to-year global surface temperature changes. We show that a one-year heat budget with these two drivers closes in every month of the Argo era, reproducing observed global surface temperature (correlation 0.964) and providing an explainable monitoring framework. A scenario-based outlook for the developing El Niño suggests a 1997/98-class release would break the 2023/24 record by 0.33 K.

Abstract Over the period 1979-2022, European surface air temperatures warmed around three times as fast as the global mean temperatures in both winter and summer. Here we define “excess” European warming as the difference between the rate of European regional warming and the rate of global warming and investigate the causes. Using a simple observation-based method, we estimate that around 40% ± 39% (in winter) and 29% ± 10% (in summer) of excess European warming is “dynamical” - attributable to changes in atmospheric circulation. We show that the rate of European warming simulated in CMIP6 models compares well with the observations, but only because these models warm too fast in the global mean; excess European warming is underestimated, particularly in winter. The CMIP6 models simulate well the magnitude of the thermodynamic component of excess European warming since 1979 in both winter and summer, they suggest only a weak dynamical contribution in the multi-model mean. The models suggest greenhouse gas-induced warming made the largest contribution to excess thermodynamic warming in winter, whereas changes in anthropogenic aerosols made the largest contribution in summer. They also imply a substantially reduced future rate of excess European warming in summer. However, the failure of current models to simulate observed circulation trends (either as a forced response or as a combination of forced response and internal variability) also implies large uncertainty in future rates of European warming.

Global warming results from anthropogenic greenhouse gas emissions which upset the delicate balance between the incoming sunlight, and the reflected and emitted radiation from Earth. The imbalance leads to energy accumulation in the atmosphere, oceans and land, and melting of the cryosphere, resulting in increasing temperatures, rising sea levels, and more extreme weather around the globe. Despite the fundamental role of the energy imbalance in regulating the climate system, as known to humanity for more than two centuries, our capacity to observe it is rapidly deteriorating as satellites are being decommissioned.

Significant changes have occurred during the last few decades across the North Atlantic climate system, including in the atmosphere, ocean, and cryosphere. These large-scale changes play a vital role in shaping regional climate and extreme weather events across the UK and Western Europe. This review synthesizes the characteristics of observed large-scale changes in North Atlantic atmospheric and oceanic circulations during past decades, identifies the drivers and physical processes responsible for these changes, outlines projected changes due to anthropogenic warming, and discusses the predictability of these circulations. On multi-decadal time scales, internal variability, anthropogenic forcings (especially greenhouse gases), and natural forcings (such as solar variability and volcanic eruptions) are identified as key contributors to large-scale variability in North Atlantic atmospheric and oceanic circulations. However, there remain many uncertainties regarding the detailed characteristics of these various influences, and in some cases their relative importance. We therefore conclude that a better understanding of these drivers, and more accurate quantification of their relative roles, are crucial for more reliable decadal predictions and projections of regional climate for the North Atlantic and Europe.

Climate records have been broken with alarming regularity in recent years, but the events of 2023–2024 were exceptional even when accounting for recent climatic trends. Here we quantify these events across multiple variables and show how excess energy accumulation in the Earth system drove the exceptional conditions. Key factors were the positive decadal trend in Earth’s Energy Imbalance (EEI), persistent La Niña conditions beginning in 2020, and the switch to El Niño in 2023. Between 2022 and 2023, the heating from EEI was over 75% larger than during the onset of similar recent El Niño events. We show further how regional processes shaped distinct patterns of record-breaking sea surface temperatures in individual ocean basins. If the recent trend in EEI is maintained, we argue that natural fluctuations such as ENSO cycles will increasingly lead to amplified, record-breaking impacts, with 2023–2024 serving as a glimpse of future climate extremes.

Abstract. A set of novel, idealised, single-forcing experiments were performed to isolate the impact of anthropogenic sulphur dioxide emissions on North Atlantic SST variability. The medium-resolution (60 km atmosphere, 0.25° ocean) and low-resolution (135 km atmosphere, 1° ocean) of the HadGEM3-GC3.1 model were used to investigate the impact of resolution on the forced response. The SST response at both resolutions is timescale dependent: a fast, large-scale surface cooling is followed by a slow, ocean-driven warming responses. Warming of the sub-polar North Atlantic is due to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) and is stronger at the medium resolution. This difference is related to surface density fluxes across the subpolar North Atlantic. The growth of Labrador Sea ice is stronger at low-resolution which inhibits air-sea interaction and reduces surface buoyancy forcing, leading to a weaker AMOC response. There is also evidence of a stronger AMOC positive feedback involving salt-advection at medium-resolution. These results show that the large-scale North Atlantic response to external forcing can be sensitive to regional differences, such as model climatology of Labrador Sea ice and its response to aerosol cooling.

The reality of human-induced climate change is unequivocal and exerts an ever-increasing global impact. Access to the latest scientific information on current climate change and projection of future trends is important for planning adaptation measures and for informing international efforts to reduce emissions of greenhouse gases (GHGs). Identification of hazards and risks may be used to assess vulnerability, determine limits to adaptation, and enhance resilience to climate change. This article highlights how recent research programs are continuing to elucidate current processes and advance projections across major climate systems and identifies remaining knowledge gaps. Key findings include projected future increases in monsoon rainfall, resulting from a changing balance between the rainfall-reducing effect of aerosols and rainfall-increasing GHGs; a strengthening of the storm track in the North Atlantic; an increase in the fraction of precipitation that falls as rain at both poles; an increase in the frequency and severity of El Niño Southern Oscillation (ENSO) events, along with changes in ENSO teleconnections to North America and Europe; and an increase in the frequency of hazardous hot-humid extremes. These changes have the potential to increase risks to both human and natural systems. Nevertheless, these risks may be reduced via urgent, science-led adaptation and resilience measures and by reductions in GHGs.

Directly linking greenhouse gas emissions or global warming to experiences of local climatic changes is a potentially important communication tool. Using observations, we develop a physically plausible “causal chain” visualisation to demonstrate the connections between global carbon dioxide emissions and local climate events. We highlight how increased flood risk in one river basin in the UK could be discussed with people directly affected by recent floods.

We characterise, and explore the drivers of, differences in the internal variability of the atlantic meridional overturning circulation (AMOC) across five NEMO-based CMIP6 class climate models. While the variability of AMOC variability is dominated by its lower dense limb in all models, there is large diversity in the timescale, multidecadal variability, and latitudinal coherence of AMOC across models. In particular, the UK models have much weaker AMOC multidecadal variability and latitudinal coherence. The model diversity is associated with differences in salinity-governed surface density variations which drive high-density water mass transformation (WMT) in the Greenland–Iceland–Norwegian Seas (GIN) and the Arctic. Specifically, GIN Seas WMT shows large multidecadal variability which has a major impact on AMOC variability in non-UK models. In contrast, the smaller variability in GIN Seas WMT in the UK models has limited impact on the lower latitude AMOC via the Denmark strait overflow mass transport. This leads to a latitudinally less coherent and weaker multidecadal variability of the AMOC lower limb. Such differences between UK and non-UK models are related to differences in model mean states and densification processes in the Arctic and GIN Seas. Consequently, we recommend further in-depth studies to better understand and constrain processes driving salinity changes in the Arctic and GIN Seas for more reliable representation of the AMOC in climate models.

Abstract Precipitation projections in transient climate change scenarios have been extensively studied over multiple climate model generations. Although these simulations have also been used to make projections at specific Global Warming Levels (GWLs), dedicated simulations are more appropriate to study changes in a stabilizing climate. Here, we analyze precipitation projections in six multi‐century experiments with fixed atmospheric concentrations of greenhouse gases, conducted with the UK Earth System Model and which span a range of GWLs between 1.5 and 5°C of warming. Regions are identified where the sign of precipitation trends in high‐emission transient projections is reversed in the stabilization experiments. For example, stabilization reverses a summertime precipitation decline across Europe. This precipitation recovery occurs concurrently with changes in the pattern of Atlantic sea surface temperature trends due to a slow recovery of the Atlantic Meridional Overturning Circulation in the stabilization experiments, along with changes in humidity and atmospheric circulation.

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in climate, transporting heat and salt to the subpolar North Atlantic. The AMOC’s variability is sensitive to atmospheric forcing, especially the North Atlantic Oscillation (NAO). Because AMOC observations are short, climate models are a valuable tool to study the AMOC’s variability. Yet, there are known issues with climate models, like uncertainties and systematic biases. To investigate this, pre-industrial control experiments from models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) are evaluated. There is large, but correlated, spread in the models’ subpolar gyre mean surface temperature and salinity. By splitting models into groups of either a warm-salty or cold-fresh subpolar gyre, it is shown that warm-salty models have a lower sea ice cover in the Labrador Sea and, hence, enable a larger heat loss during a positive NAO. Stratification in the Labrador Sea is also weaker in warm-salty models, such that the larger NAO-related heat loss can also affect greater depths. As a result, subsurface density anomalies are much stronger in the warm-salty models than in those that tend to be cold and fresh. As these anomalies propagate southward along the western boundary, they establish a zonal density gradient anomaly that promotes a stronger delayed AMOC response to the NAO in the warm-salty models. These findings demonstrate how model mean state errors are linked across variables and affect variability, emphasizing the need for improvement of the subpolar North Atlantic mean states in models.

Over the period 1979–2022, European surface air temperatures warmed around three times faster than global mean temperatures in both winter and summer. Here we define “excess” European warming as the difference between the rate of European regional warming and the rate of global warming and investigate the causes. We estimate that about 40% (in winter) and 29% (in summer) of excess European warming is “dynamical” - attributable to changes in atmospheric circulation. We show that the rate of European warming simulated in CMIP6 models compares well with the observations, but only because these models warm too fast in the global mean; excess European warming is underestimated, particularly in winter. The CMIP6 models simulate well the magnitude of the thermodynamic component of excess European warming since 1979 in both winter and summer, but do not simulate the dynamical contribution. The models suggest greenhouse gas induced warming made the largest contribution to excess thermodynamic warming in winter, whereas changes in anthropogenic aerosols made the largest contribution in summer. They also imply a substantially reduced future rate of excess European warming in summer. However, the failure of current models to simulate observed circulation trends also implies large uncertainty in future rates of European warming.

Workshop on the Signal -to -Noise Paradox in Climate Forecasts What: Nearly 40 participants from universities (Oxford, Reading, Exeter, Leeds, Lincoln), the National Centre for Atmospheric Science (NCAS), and operational forecasting centers (Met Office, ECMWF) in the United Kingdom, as well as from the Hebrew University in Israel and NCAR in the United States, gathered-primarily in person-to discuss our current understanding of the so-called signal-to-noise paradox in climate forecasts and to develop ideas to resolve it. When: 25-26 September 2023 Where: Oxford, United Kingdom

Recent studies suggest seasonal forecasts for European winters are now skilful, but they also identify a “signal‐to‐noise paradox”, wherein models predict the real world more skilfully (higher correlation) than the evolution of their ensemble members. Here, we analyse seasonal hindcasts from the Met Office GloSea5 seasonal forecast system to identify sources of predictability and seek insight into the signal‐to‐noise problem. For the first time, we use an optimal detection method to identify predictable signals over the North Atlantic region within the forecast system on subseasonal time‐scales. We find two primary predictable modes: a Pacific North America (PNA)‐like mode and a North Atlantic oscillation (NAO)‐like mode. The latter is the leading predictable mode in December–January, and its spatial pattern closely resembles the NAO. The PNA‐like mode dominates in January–February. Whereas the PNA‐like mode is driven by Pacific Ocean sea‐surface temperatures, the NAO‐like mode is driven at least partly by Indian Ocean sea‐surface temperatures, not solely due to the common trend. We develop a novel method of comparing the magnitude of these modes in the forecast system and observations that complements previous approaches. This suggests that the signal‐to‐noise problem in GloSea5 is primarily a feature of the December–January NAO‐like mode, with the observed mode being three times larger than in the model. The magnitude of the PNA‐like mode is better captured by the forecasts, although there is still evidence of a weaker signal‐to‐noise problem. This suggests particular mechanisms may lead to the lower signal to noise seen in NAO hindcasts, rather than a global weakness of the forecast system in responding to initialization and external forcing. Our results, though specific to GloSea5, provide insights into the causes of the signal‐to‐noise problem in seasonal forecasts of European winters. They also imply there is significant potential for improving such forecasts and suggest how such improvements may be achieved.

Climate models are a valuable tool to study the interaction between ocean and atmosphere. Nevertheless, they are known to suffer from various biases and uncertainties. In the subpolar North Atlantic typical biases among models from the Coupled Model Intercomparison Project phase 6 (CMIP6) are found in the mean surface temperature and salinity, and in the mean sea ice concentration. These biases will affect the air-sea interaction.In this study, we are investigating the diversity of CMIP6 models with respect to their response of the Atlantic Meridional Overturning Circulation (AMOC) to the North Atlantic Oscillation (NAO) in pre-industrial control experiments. This response is sensitive to the mean spiciness of the North Atlantic. Thus, we focus on two categories of models: Models that are spicy (warm-salty) and models that are minty (cold-fresh) within the subpolar gyre of the North Atlantic. Spicy models tend to have a lower sea ice cover in the Labrador Sea (LS) and larger LS heat loss during a positive NAO, compared to minty models. Also, spicy models have a weaker stratification in the LS. Sub-surface density changes 1 to 3 years after the NAO are larger in the spicy models and establish a zonal density gradient that can cause a stronger delayed AMOC response that is also more coherent across latitudes.Although some metrics seem to be more realistic in the spicy models, other characteristics seem less realistic compared to the minty models, like the mixed layer depth relative importance between the eastern and the western subpolar North Atlantic. This could be a sign for how some mean states or processes might be right for the wrong reasons and stresses the need for model improvement.

<p>Evidence from model simulations has suggested that anthropogenic aerosols may have forced multidecadal variability in a range of North Atlantic variables including sea surface temperatures, ocean circulation, and sea ice. However, many questions remain concerning the importance of anthropogenic aerosols in driving past changes in the North Atlantic climate system. The pathways via which changes in aerosol and aerosol precursor emissions, and oxidant levels, influence climate are complex. They involve both chemical and physical processes, and likely include changes in clouds, radiation, surface temperatures, atmospheric and oceanic circulation, and Arctic sea ice. This complexity is an important factor in the large uncertainty surrounding the role of anthropogenic aerosol in North Atlantic climate change, and was one of the major motivations for the UK’s North Atlantic Climate System Integrated Study (ACSIS). ACSIS was a multidisciplinary research programme conducted over the period 2016-2022, delivered by a consortium of seven UK institutions. This presentation draws together findings from the programme to provide an overall synthesis of what was learned in ACSIS about the role of anthropogenic aerosol in North Atlantic climate change. Remaining uncertainties, the potential for observational constraints, and opportunities for future work will also be discussed.</p> <p> </p> <p>ACSIS made extensive use of simulations conducted for CMIP6, particularly historical simulations, and attribution experiments included in AerChemMIP and DAMIP. Additional sensitivity experiments with HadGEM3-GC3.1 and UKESM1 were used to quantify the effects of uncertainty in aerosol forcing in the absence of the additional uncertainty associated with model differences, to decompose the aerosol forcing, and to better illustrate the role of aerosol in recent changes.</p> <p> </p> <p>As aerosol emissions increased (1850-1985), North Atlantic CDNC increased. Emissions of ozone precursors, and resulting changes in OH, contributed to this trend. This led to downwelling surface shortwave decreases across the North Atlantic, which drove colder surface temperatures, increased sea ice extent, and increased mean sea level pressure. In contrast, the eastern subpolar gyre warmed, likely due to increased ocean heat convergence due to the increase in the AMOC.</p> <p> </p> <p>As local aerosol emissions fell (1986-2014) much of the reverse occurred. Downwelling surface shortwave increased across the North Atlantic, predominantly over land, driving warmer surface temperatures and reduced sea ice extent. The eastern subpolar gyre cooled. However, the role of aerosol in this later period is less clear due to a dominance of temperature-mediated cloud feedbacks over aerosol forcing, AMOC related feedbacks, and a changing aerosol forcing pattern.</p>

Satellite-derived products and reanalyses show consistent increases in downward surface solar radiation (SSR) and decreases in cloud cover over North America and Europe from the 1980s to 2010s. These trends show a strong seasonality, with the largest changes in boreal summer. A set of timeslice experiments with an atmospheric general circulation model (AGCM) forced with prescribed changes in sea surface temperature/sea ice extent (SST/SIE), greenhouse gas (GHG) concentrations, and anthropogenic aerosol (AA) emissions, together and separately, is performed to assess the relative roles of different forcings in these observed trends. The model reproduces the main observed features over Europe and North America, including the seasonality of trends, suggesting a dominant role of forced changes in the recent trends in SSR and cloud cover. Responses to individual forcings indicate that recent decadal trends in SSR over Europe are predominantly driven by AA emission reductions, with an additional influence from SST/SIE and GHG changes. In contrast, changes in AA, SST/SIE, and GHG contribute more equally to simulated decadal trends in SSR and cloud cover over North America, although SST/SIE play the most important role. In our simulations, responses of SSR to AA emission reductions are primarily governed by aerosol-radiation interactions. Responses to SST/SIE and GHG changes are predominantly due to cloud cover changes, which are driven by atmospheric circulation and humidity changes. This process level understanding of how different forcing factors influence decadal trends in SSR and cloud cover is valuable for understanding past changes and future projections in global and regional surface energy budgets, surface warming, and global and regional hydrological cycles.