Globally averaged surface air temperature (SAT) during the 20th and 21st centuries displays a gradual warming and superimposed year-to-year and decadal-scale fluctuations. The upward trend contains the climate response to an anthropogenic increase of heat-trapping atmospheric greenhouse gases. The temperature ups and downs around the trend - that are particularly pronounced in the Arctic - mostly reflect natural variability. Natural climate variations are of two types, internal and external. The former is produced by the climate system itself, e.g. due to variations in ocean circulation. An example of the latter is solar-induced climate variability. Decadal-scale variability is of large societal relevance. It is observed, for example, in Atlantic hurricane activity, Sahel rainfall, Indian and East Asian Monsoons, Eurasian winter coldness and in the Arctic SAT and sea ice. The understanding and skillful prediction of decadal-scale climate variability that modulates the regional occurrence of extreme weather events will be of enormous societal and economic benefit. InterDec is an international initiative aiming at understanding the origin of decadal-scale climate variability in different regions of the world and the linkages between them by using observational data sets and through coordinated multi-model experiments. How can a decadal-scale climate anomaly in one region influence very distant areas of the planet? This can happen through atmospheric or oceanic teleconnections. Fast signal communication between different latitudinal belts within days or weeks is possible through atmospheric teleconnection, whereas communication through oceanic pathways is much slower requiring years to decades or even longer. Understanding these processes will enhance decadal climate prediction of both mean climate variations and associated trends in regional extreme events. Scientists from different European countries, from China and Japan will closely collaborate to disentangle the secrets of the inter-relations of decadal-scale variability around the globe.

The loss of Arctic sea-ice is one of the most compelling manifestations of man-made climate change. Profound environmental change is already affecting Arctic inhabitants and ecosystems. Increasing scientific evidence, including many key papers by the PI, suggests the impacts of sea-ice loss will be felt way beyond the poles. Linkages between Arctic sea-ice loss and extreme mid-latitude weather have become an area of increasing scholarly enquiry and societal interest. Yet, significant knowledge gaps remain that demand urgent attention; in particular, the robustness of response to sea-ice loss - and its underpinning physical causes - across different climate models. The Polar Amplification Model Intercomparison Project (PA-MIP) will significantly advance the state-of-the-art in understanding and modelling the climate response to Arctic and Antarctic sea-ice loss. It will enable deeper understanding of the causes and global effects of past and future polar change, and the physical mechanisms involved. PA-MIP is a novel and unique collaboration of UK and international scientists. To promote fruitful collaboration and drive research excellence, this proposal supports two key activities: a secondment scheme and a synthesis workshop, both with direct benefit to NERC-funded science.