The possibility that Arctic sea ice loss weakens mid-latitude westerlies, promoting more severe cold winters, has sparked more than a decade of scientific debate, with apparent support from observations but inconclusive modelling evidence. Here we show that sixteen models contributing to the Polar Amplification Model Intercomparison Project simulate a weakening of mid-latitude westerlies in response to projected Arctic sea ice loss. We develop an emergent constraint based on eddy feedback, which is 1.2 to 3 times too weak in the models, suggesting that the real-world weakening lies towards the higher end of the model simulations. Still, the modelled response to Arctic sea ice loss is weak: the North Atlantic Oscillation response is similar in magnitude and offsets the projected response to increased greenhouse gases, but would only account for around 10% of variations in individual years. We further find that relationships between Arctic sea ice and atmospheric circulation have weakened recently in observations and are no longer inconsistent with those in models.

As climate change accelerates, societies and climate-sensitive socioeconomic sectors cannot continue to rely on the past as a guide to possible future climate hazards. Operational decadal predictions offer the potential to inform current adaptation and increase resilience by filling the important gap between seasonal forecasts and climate projections. The World Meteorological Organization (WMO) has recognized this and in 2017 established the WMO Lead Centre for Annual to Decadal Climate Predictions (shortened to “Lead Centre” below), which annually provides a large multimodel ensemble of predictions covering the next 5 years. This international collaboration produces a prediction that is more skillful and useful than any single center can achieve. One of the main outputs of the Lead Centre is the Global Annual to Decadal Climate Update (GADCU), a consensus forecast based on these predictions. This update includes maps showing key variables, discussion on forecast skill, and predictions of climate indices such as the global mean near-surface temperature and Atlantic multidecadal variability. it also estimates the probability of the global mean temperature exceeding 1.5°C above preindustrial levels for at least 1 year in the next 5 years, which helps policy-makers understand how closely the world is approaching this goal of the Paris Agreement. This paper, written by the authors of the GADCU, introduces the GADCU, presents its key outputs, and briefly discusses its role in providing vital climate information for society now and in the future.

The El Niño Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the Pacific, and its global impacts have been extensively studied (e.g., Taschetto et al., 2020;Yeh et al., 2018). In particular, El Niño, the positive ENSO phase, tends to promote a negative North Atlantic Oscillation (NAO) and cold winters in northern Europe (Brönnimann, 2007). This connection occurs through both tropospheric and stratospheric pathways (Domeisen et al., 2019;Hardiman et al., 2019). In the tropospheric pathway, Rossby waves generated by tropical convection and atmospheric circulation anomalies in the Pacific and Atlantic propagate into the north Atlantic and affect the NAO (

<p>Pacific Decadal Variability (PDV) is characterised by a pattern similar to the interannual El Niño Sothern Oscillation (ENSO), with anomalously warm sea surface temperatures (SSTs) in the tropical Pacific surrounded by a horseshoe of cool SSTs. PDV is one of the most influential decadal climate modes, contributing to the ‘slowdown’ of global temperature rise at the beginning of the 21<sup>st</sup> century and affecting precipitation extremes worldwide. However, differences between climate impacts of PDV and ENSO are more uncertain.</p><p>Here, the interannual and decadal impacts of PDV are investigated using ERA5 re-analyses and forced Pacific experiments from the CMIP6 Decadal Climate Prediction Project (DCPP) which impose the PDV SST pattern in the coupled Met Office climate model (HadGEM3.1). Both model and observations show a decreased Pacific Walker circulation and an increased Hadley response which combine to generate large scale Rossby waves, impacting global atmospheric circulation and precipitation patterns.</p><p>Impacts on the winter North Atlantic Oscillation (NAO) are opposite on the two timescales in both the model and observations. On interannual timescales a positive PDV produces a weakening of the stratospheric polar vortex and a negative NAO, whereas on decadal timescales a strengthening of the polar vortex and positive NAO is seen. This appears to be related to the balance between an increased meridional temperature gradient which tends to shift the jet poleward on decadal timescales, and increased  upward propagation of planetary waves which reduces the polar vortex and shifts the jet equatorward on interannual timescales. Reasons for these differences will be explored further.</p>

<p>It is well established that the positive phase of El Niño Southern Oscillation (ENSO) tends to weaken the Northern Hemisphere stratospheric polar vortex (SPV), promoting a negative North Atlantic Oscillation (NAO). Pacific Decadal Variability (PDV) is characterised by a pattern of sea surface temperatures similar to ENSO, but its impacts are more uncertain: some studies suggest similar impacts of ENSO and PDV on the SPV and NAO, while others find the opposite. We use climate model experiments and reanalysis to find further evidence supporting opposite interannual and decadal impacts of Pacific variability on the extratropics. We propose that the decadal strengthening of the SPV in response to positive PDV is caused by a build-up of stratospheric water vapour leading to enhanced cooling at the poles, an increased meridional temperature gradient and a strengthened extratropical jet. Our results are important for understanding decadal variability, seasonal to decadal forecasts and climate projections.</p>