Wider impacts of Subpolar North Atlantic decadal variability on the ocean and atmosphere (WISHBONE)

The Subpolar North Atlantic (SNA), which is the region of the Atlantic Ocean between 45-65N latitude, is a highly variable region. Surface temperatures and surface salinity here have varied on a range of time-scales, but the changes are dominated by large and slow changes on decadal or longer timescales. This decadal timescale variability appears to form a key component of a larger climate mode, the Atlantic Multidecadal Variability, which has been linked to a broad range of important climate impacts, including rainfall in the North African and south Asian monsoons, floods and droughts over Europe and North America, and the number of hurricanes. The SNA is also one of the most predictable places on Earth at decadal timescales, which suggests the potential for improved predictions of regional climate and high-impact weather years ahead.

However, the origins of this variability in the SNA, and the processes controlling its impacts, are far from fully understood. There is significant evidence to suggest that anomalous heat loss from the subpolar North Atlantic Ocean to the atmosphere can instigate a cascade of changes across the North Atlantic basin in both the ocean and atmosphere. For example, changes in the SNA can change the strength of the ocean circulation to the south, affect the northward transport of heat and freshwater in the North Atlantic, and subsequently affect the upper ocean temperatures and salinity across the whole North Atlantic basin, and into the Arctic. Changes in the subpolar North Atlantic surface temperature are also thought to affect the atmospheric circulation - i.e. wind patterns - in both summer and winter. However, observational records are very short, and so there are significant problems with understanding causality, and considerable uncertainty about how well many of the important processes are represented in current climate models.

WISHBONE will make use of new advanced climate simulations and forecast systems to make progress in understanding the impact of the subpolar North Atlantic on the wider North Atlantic basin. It will also test specific hypotheses related to understanding the specific role of heat loss over the subpolar North Atlantic in driving changes throughout the basin including the role of surface anomalies in driving wind patterns.

WISHBONE is a collaboration between the National Centre for Atmospheric Science at the University of Reading, The National Oceanography Centre Southampton, The University of Oxford, and The University of Southampton from the U.K., and The National Center for Atmospheric Research, from the U.S.

Changes in the Subpolar North Atlantic (SNA) are a significant driver of decadal changes in weather and climate across large regions of the globe, including floods and droughts over the UK/Europe, the number of hurricanes, rainfall over South Asian monsoon regions. As the magnitude of anthropogenic climate change grows, the importance of the SNA for shaping regional climate change is likely to grow - for example if there is a significant slowdown in the Atlantic Meridional Overturning Circulation (AMOC), as has been anticipated.

It follows that improved understanding of changes in the SNA and the impacts of these changes on the wider climate system is essential to improve assessments of current and future risks arising from high impact weather and climate events. Such risk assessments are in urgent demand both from governments - for example the UK government's 5-yearly Climate Change Risk Assessments (CCRA) - and increasingly from businesses in a wide range of sectors (including insurance, Energy, Fisheries and agriculture). For example, the international Task Force on Climate Related Financial Disclosures (https://www.fsb-tcfd.org/) recently issued an urgent call for a step change in the capacity of businesses to quantify their physical and other climate-related risks. As business and governmental interests are not confined to its home country, the global nature of SNA impacts and the broad exposure to risks could have huge financial consequences. Therefore, businesses, industry and governments will benefit from increased understanding of the modulation of the regional climate interannual-to-decadal timescales, and improved predictions that this brings.

Decadal climate forecasting is an emerging technology which has the potential to provide valuable early warnings of climatic events, and more generally to improve quantification of weather and climate related risks, at lead times up to 10 years ahead. The UK Met Office has been a pioneer in the development of decadal forecasting capabilities and leads the World Meteorological Organisation (WMO) activity for the global dissemination of these experimental forecasts. The potential beneficiaries of these forecasts thereby include national meteorological services in all WMO countries, and their customers. Therefore, the outcomes of WISHBONE will benefit the Met Office and Climate prediction services by improving understanding of the SNA's role in driving regional climate variability and high-impact weather, in evaluating models and predictions, and the identification for improvements in future climate models and prediction systems (e.g. for CMIP7). Furthermore, due to the global dissemination of the prediction experiments via the WMO, WISHBONE's results could have a worldwide impact.

The public interest in climate change and related issues is greater than ever. There is particular interest in understanding past and future changes in the local climate that people experience - for example in the UK - and how these local changes relate to changes on larger regional and global scales. As indicated above, the SNA exerts a significant influence on climate change in the UK and elsewhere; therefore there is public interest in understanding the nature of these influences and how they may change in the future. WISHBONE will engage the public on the causes of regional variability and help bring greater trust to climate predictions.