The impact of the AMV on Eurasian summer hydrological cycle

Abstract

Impact studies of the Atlantic Multidecadal Variability (AMV) on the climate system are severely limited by the lack of sufficiently long observational records. Relying on a model-based approach is therefore mandatory to overcome this limitation. Here, a novel experimental setup, designed in the framework of the CMIP6-endorsed Decadal Climate Prediction Project, is applied to the CMCC climate model to analyse the remote climate impact of the AMV on the Northern Eurasian continent. Model results show that, during Boreal summer, an enhanced warming associated to a positive phase of the AMV, induces a hemispheric-scale wave-train response in the atmospheric circulation, affecting vast portions of Northern Eurasia. The overall AMV-induced response consists in an upper-tropospheric anomalous flows leading to a rainfall increase over Scandinavia and Siberia and to an intensified river runoff by the major Siberian rivers. A strengthening of Eurasian shelves' stratification, broadly consistent with the anomalous river discharge, is found in the proximity of the river mouths during positive-AMV years. Considering that Siberian rivers (Ob', Yenisei and Lena) account for almost half of the Arctic freshwater input provided by terrestrial sources, the implications of these findings for decadal variability and predictability of the Arctic environment are also discussed. The variability of Northern Eurasian climate has direct implications for the hydrological cycle of the Arctic 1,2. Almost 10% of the total freshwater input into the Arctic Ocean derives from terrestrial sources, influencing seaice formation and sea surface salinity at different timescales 3,4. High-latitude precipitation is one of the drivers of the Arctic rivers discharge, even if the direct link between rainfall and river streamflow is still unclear 5,6 , due to the concurrence of several additional factors which may have implications for the freshwater inflow: snowmelt, permafrost degradation, reservoir and dam regulation, fire frequency and human activities 7. However, understanding the origins of the regional scale, decadal variability of precipitation remains a challenging issue, which may disclose some degree of predictability, potentially improving the quality of climate predictions. Several studies have focused on the winter period, invoking a role for the Siberian High, Northern Hemisphere Annular Mode and sea surface temperature (SST) variability in the North Atlantic and Pacific Ocean 8-12. Berg et al. 13 claim that, at daily timescale, increasing surface temperature, the Clausius-Clapeyron law prevails in the modulation of the large-scale precipitation, whereas moisture transport and availability are the keys to explain summer rainfall. Different physical processes require different approaches for winter and summer analysis, given also the strong climate seasonality of the high-latitude regions 5,14. On the interannual timescale, summer precipitation is still driven by the convergence of large-scale moisture fluxes over eastern Siberia since the ev...

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Variations in northeast Asian summer precipitation driven by the Atlantic multidecadal oscillation

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The CMCC Decadal Prediction System

“…Indeed, significant skill is only found over Sahelian Africa, Northern Eurasia and over the Western and Central Europe. This spatially confined skill may derive from the AMV, which is known to be a source of predictability for these regions, influencing rainfall variability on annual-to-decadal timescales (Doblas-Reyes et al, 2013;Ehsan et al, 2020;Ruggieri et al, 2020). On the other hand, no significant skill for precipitation is found over the rest of the globe, probably also due to the relatively small size of our ensemble (Yeager et al, 2018), to the very high spatial variability (Goddard et al, 2013) and absence of a strong trend in precipitation (Gaetani and Mohino 2013;Bellucci et al, 2015).…”