Predictability of weather and climate

Abstract:

Plain Language Summary: The Quasi‐biennial Oscillation (QBO) dominates the variability of the tropical atmosphere between 16 and 50 km above the surface. It manifests most strongly as downward propagating zonal wind variations exceeding 25 m/s with an average period of ∼ ${\sim} $ 28 months. Twice in the past 10 years the QBO regular phase evolution has been disrupted after 60 years of no disruptions, motivating our analysis of the role of greenhouse gases, aerosols, ozone, volcanic eruptions, and solar variability for historical changes in the QBO. We find prominent roles for four of these five external forcings, and specifically both rising greenhouse gases and volcanic eruptions help induce disruption events.

Abstract:

Plain Language Summary: Climate model emulators are approximations of climate models that provide a quick and low‐cost alternative to exploring future climate scenarios. Traditional emulators generate large amounts of data covering the whole world, which still need to be condensed when exploring local and regional impacts. In this paper, we propose a new emulator based off image compression techniques. The setup allows one to “zoom” in and out from global to regional to local levels, providing user‐relevant information across scales. It furthermore conserves both large‐scale and local features and can be run in minutes. Given its versatile framework, the approach is easily extendable to new variables, and in this paper we demonstrate its ability to jointly capture temperature and relative humidity.

Abstract:

Over the past 20 years, extreme event attribution has developed rapidly, providing a wide range of methods to attribute weather events - from unconditioned probabilistic to strongly conditioned storyline approaches. Advancing the field now requires combining results from multiple methods, allowing more robust conclusions drawing from various lines of evidence. Yet, doing so remains challenging. We call for closer interaction within the attribution field to develop approaches with method comparison in mind. We highlight the need to explicitly define the research questions answerable by specific methods, and to clearly outline the limitations of each method.

Abstract:

This study evaluates the performance of high-resolution (grid sizes of 9-28 km for the atmosphere; 5-13 km for the ocean) global simulations from the EERIE project in representing the persistence of the Southern Annular Mode (SAM), a leading mode of Southern Hemisphere climate variability. Using the decorrelation timescale of the SAM index (τ), we compare EERIE simulations with CMIP6 models and ERA5 reanalysis. EERIE simulations reduce long-standing biases in SAM persistence, especially in early summer, with τ values of 9-20 d compared to CMIP6's 9-32 d and ERA5's 11 d. This improvement correlates with a more accurate climatological jet latitude (λ0). EERIE atmosphere-only AMIP runs outperform the coupled simulations in both τ and λ0, showing smaller biases and ranges of variability, underscoring the critical role of sea surface temperature (SST) representation in shaping atmospheric circulation. In these AMIP experiments, the atmospheric eddy feedback strength, combined with the damping timescale estimated via friction, correlates more strongly with τ than λ0. We speculate that the well-captured jet position (biases < 1° relative to ERA5), due to prescribed SSTs, limits λ0's explanatory power for τ differences, allowing other processes to dominate. Using a finer model grid (9 km vs. 28 km) of the same AMIP model reduces τ, though the mechanism remains unclear. Finally, motivated by the importance of oceanic eddies in the Southern Ocean, we conducted sensitivity experiments that filter transient mesoscale features from the SST boundary conditions. The results suggest that oceanic eddies may enhance summertime SAM persistence (by g1/4 2 d), though this signal is not statistically significant and is absent in the single 9 km run, pointing to a subtle role of mesoscale ocean-Atmosphere interaction that remains to be explored.

Abstract:

Plain Language Summary: We investigate how the variability in the Indian Ocean Sea Surface Temperature in autumn, known as the Indian Ocean Dipole (IOD), influences the precipitation regime and storm activity in the North Atlantic, Europe, and Mediterranean regions during the winter season. Our results indicate that IOD variability triggers December shifts in atmospheric pressure over these regions and alters precipitation patterns, influencing the frequency and intensity of precipitation events. The strongest impacts are observed at mid‐latitudes, with storm activity decreasing over the Eastern Atlantic and Western Mediterranean. These storm changes are tied to stronger temperature contrasts between the north and south part of the domain, which produce significant changes in the vertical wind shear. Our study further supports the idea that Indian Ocean variability may influence the early winter weather in Europe and the Mediterranean—an important insight for improving sub‐seasonal to seasonal forecasts.