Last week, international scientists came together for the 8th GEWEX Open Science Conference: Extremes and Water on the Edge. In 26 sessions a wide range of interdisciplinary topics were covered. Plenary talks at the beginning and end of each day gave an overview of the state-of-the-art and pressing issues. One of these plenaries was given by Bart van den Hurk (KNMI) on “Exploiting the climate archives for meaningful events”. In his talk Prof. van den Hurk introduced the concept of compound events and the challenges that come with this new emerging topic.

Further, one of the sessions was dedicated to compound events. Four, mainly early career, scientists presented their work on compound events. A summary of these talks can be found below.

Multi-hazard Modeling in Coastal Systems (Hamed Moftakhari Rostamkhani, UCI)
In this study Dr Moftakhari Rostamkhani showed an innovative method to combine the bivariate probability distribution function and cumulative distribution function. He demonstrated the use of this method by analysing the dynamics of flooding from upstream freshwater discharge in coastal regions. With results from a two-dimensional shallow water flow solver (BreZo) Dr Moftakhari Rostamkhani assessed the dominance of the two main flood drivers, i.e. river discharge and coastal water levels, along the length of a river basin. His results suggest that flooding upstream is almost exclusively driven by river discharge. In contrast, downstream areas are influenced by a mix of both drivers with an increasing dominance of coastal water levels closer to the coast.

Assessment of the performance of the top-down approach to study compound events on the example of the Role of Atmospheric Rivers in compound events along the Dutch coast (Nina Ridder, KNMI)
In this study, Dr Ridder presented a comparison of the top-down and bottom-up approach by demonstrating their complementary nature that can be used for the assessment of multivariate climate extremes. This was done using the example of landfalling atmospheric rivers and their connection to the coincidence of heavy precipitation and high storm surges along the Dutch coast. Her results show that the bottom-up approach, which starts at the impact itself, in this case the compounding storm surge and heavy precipitation, can be a powerful tool to identify relevant climatological drivers of compound event. The top-down approach can then provide information on statistical features once the full system is understood and the main drivers have been identified.

Intensified departures from natural variability in coupled climate variables (Colin Mahony, UCI)
The term “time of emergence” signifies the point in time at which the signal of climate change starts to be distinguishable from the noise caused by natural variability. In his talk, Dr Mahony presented how the use of a multivariate perspective, i.e. taking into account dependencies between climate variables, can lead to an earlier time of emergence for these variables. He demonstrated this in a modelling study based on the correlation between summer precipitation and daytime temperature. The full study is summariesed in Mahony and Cannon (2018) published in Nature Communications.

Accelerated Warming of Temperatures during Droughts (Felicia Chiang, UCI)
In her talk Ms Chiang presented an assessment of the relationship between moisture conditions and temperatures during drought conditions. Based on CMIP5 data and defining a drought index as measure of relative meteorological dryness, Ms Chiang showed that droughts can be categorised in three different classes. Further, she demonstrated that both, temperature and atmospheric moisture shifts, are interacting and amplifying under the drought conditions defined in the presented study.

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