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Abstract:

Since 2004, many methods for event attribution have been developed. Early studies showed that attribution statements are sensitive to the framing of research questions but few large comparisons have been undertaken.Here, we firstly motivate the need for multi-method extreme event attribution, highlighting conceptual differences between methods. In a second part, we present a case study of midlatitude storm Babet (2023) to compare three common storyline attribution methods, alongside a severity-based probabilistic method. We discuss three widely relevant questions which highlight the complementarity and the differences between methods: (1) How has climate change impacted the frequency of the event? (2) How has climate change impacted the event severity? (3) Were the dynamics of the event influenced by climate change and if yes, how?We show that methods differ in the extent to which they reproduce observed weather patterns. This influences attribution statements, and can even change the sign of results for events with uncertain climate signals. We argue that limitations and strengths of methods need to be clearly communicated when presenting event attribution reports to ensure findings can be used reliably by a wide range of stakeholders.

Abstract:

The Quasi-Biennial Oscillation (QBO) is the dominant mode of tropical stratospheric variability that modulates global circulation and climate. Although a long-term weakening of QBO amplitude has been observed under global warming, the relative roles of different tropical oceans remain unclear. We perform sensitivity experiments forced by sea surface temperature perturbations over the tropical Pacific, Atlantic, and Indian Oceans, as well as their combined warming, to separate individual and joint effects. Pacific warming produces the strongest weakening and slowest descent of the QBO, whereas Atlantic warming slightly strengthens the amplitude and extends the vertical structure. Indian Ocean warming slightly weakens the amplitude and accelerates the descent. When all three oceans warm simultaneously, the QBO exhibits a weaker amplitude and faster descent, consistent in sign with the combined single-basin responses but with a reduced magnitude owing to diminished zonal and inter-basin SST gradients. Momentum budget analyses further show that basin-dependent competition between equatorial wave forcing and tropical upwelling underlies these contrasting responses.

Abstract:

ABSTRACT Dengue fever outbreaks impose a severe healthcare burden in Vietnam; therefore, the development of a Dengue early warning system is key to improve public health planning and mitigate this burden. This study assesses the ECMWF medium‐range (up to 10 days) forecast skill for relative humidity in Vietnam—a key factor for vector‐borne disease transmission—in re‐forecasts between 2001 and 2020. Analysis focused on the rainy season (May–October) with ERA5 reanalysis as a reference dataset. Re‐forecast data were pre‐processed using a lead‐time dependent quantile mapping technique to reduce the bias between forecasted and observational data, and skill was assessed using climatology and persistence as a reference. Rank histograms showed that the humidity forecast is reliable up to 10 days, and continuous ranked probability skill score (CRPSS) values show that the forecast is more skilful than the climatology up to 10 days. Nonetheless, when using persistence as a reference, CRPSS values are lower in South Vietnam, which was associated with the inaccurate representation of 2 m dew point temperature in the tropical regions, and the fact that persistence is a hard reference to beat in the tropics, hindering model forecast skill. Results from this study demonstrate that ECMWF ensemble forecasts of relative humidity are suitable to use as inputs for a Dengue early warning system up to 10 days in advance.