Climate dynamics

Abstract:

The occurrence of blocking weather patterns over Europe is analysed in a large ensemble of simulations of a climate model with perturbed physical parameters. The experiments were performed with HadGEM3-GC3 for the UK Climate Change Projections, and comprise a set of 15 coupled simulations supported by a larger suite of 505 atmosphere-only simulations. Despite the systematic perturbation of 47 different physical constants in the atmosphere-only experiments, only three were found to have any impact on European blocking frequencies. These reveal the sensitivity of European blocking to orographic drag in winter and to convective entrainment in summer. However, these sensitivities cannot be traced through to the coupled simulations, due to the smaller and more realistic range of perturbations used and likely also to coupled dynamical effects. Overall, we find that although physical sensitivity to the parameterisations exists, adjustment of the parameters is no replacement for further structural improvement in the representation of these processes in the model.

Abstract:

Anthropogenic greenhouse gas emissions significantly impact the middle and upper atmosphere. They cause cooling and thermal shrinking and affect the atmospheric structure. Atmospheric contraction results in changes in key atmospheric features, such as the stratopause height or the peak ionospheric electron density, and also results in reduced thermosphere density. These changes can impact, among others, the lifespan of objects in low Earth orbit, refraction of radio communication and GPS signals, and the peak altitudes of meteoroids entering the Earth's atmosphere. Given this, there is a critical need for observational capabilities to monitor the middle and upper atmosphere. Equally important is the commitment to maintaining and improving long‐term, homogeneous data collection. However, capabilities to observe the middle and upper atmosphere are decreasing rather than improving.

Abstract:

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.Supplementary informationThe online version contains supplementary material available at 10.1007/s00382-025-07591-1.