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Abstract:

Abstract. This study investigates Quasi-Biennial Oscillation (QBO) teleconnections and their modulation by the El Niño–Southern Oscillation (ENSO) using a multi-model ensemble from the Atmospheric Processes And their Role in Climate (APARC) QBO initiative (QBOi). Analyzing observed QBO–ENSO teleconnections is challenging because it is difficult to separate the respective influences of QBO and ENSO outside the QBO region due to aliasing in the historical record. To isolate these signals, simulations were conducted with annually repeating prescribed sea-surface temperatures (SSTs) representing idealized El Niño and La Niña conditions (the QBOi EN and LN experiments, respectively), and results are compared with the QBOi control experiment (CTL) under ENSO-neutral conditions. The strength of the Holton-Tan relationship between the phase of the QBO and the strength of the polar vortex seen in observations is reproduced in fewer than three models in CTL and by one model in EN. In LN, three out of nine models reproduce the observed Holton–Tan relationship, but with less than half of the observed amplitude. In the Arctic winter climate, sudden stratospheric warmings (SSWs) occur more frequently in EN than in LN; however, unlike in observations, there is no discernible difference in SSW frequency between QBO westerly (QBO-W) and QBO easterly (QBO-E) phases. The Asia-Pacific subtropical jet (APJ) shifts significantly equatorward during QBO-W compared to QBO-E in observations, but this shift is not robust across models, regardless of ENSO phases. In the tropics, the sign and spatial pattern of the QBO precipitation response vary widely across models and experiments, indicating that any potential QBO signal is strongly modulated by the prevailing ENSO phases. Overall, the QBOi models exhibit unrealistically weak QBO wind amplitudes in the lower stratosphere, which may explain the weak polar vortex and APJ responses, as well as the weak precipitation signals in the tropics. In contrast, the QBO teleconnection with the Walker circulation during boreal summer and autumn shows consistent signals in both observations and most models. Specifically, the QBO-W phase is characterized by upper-level westerly and lower-level easterly anomalies over the Indian Ocean–Maritime Continent relative to QBO-E, although the amplitude and timing of these anomalies remain model-dependent. Notably, the influence of QBO phase on the Walker circulation appears insensitive to the ENSO phase.

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:

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 91̽��s 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.