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Exploring climate model data

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Climate variations and internal variability

The climate varies on a wide range of time-scales because of external forcings and internal variability. Externally driven variations are for example the diurnal and annual cycles caused by variation in the solar radiation reaching the surface. Internal processes in atmosphere generate internal variability are known to operate on time scales ranging from virtually instantaneous (e.g., condensation of water vapour in clouds) up to years (e.g., troposphere-stratosphere or inter-hemispheric exchange), where everyday variations in the weather we experience is the most prominent. Other components of the climate system, such as the oceans and the large ice sheets, tend to operate on longer time scales. In particular, at the decadal time-scale internal variability produced by coupled interactions between components are important. Example of major atmosphere-ocean interactions causing interannual and decadal scale internal variability are the El-Niño/Southern Oscillation and the North Atlantic Oscillation.

This decadal-scale internal variability is important to keep in mind when looking at climate scenarios. At this time-scale one cannot assume that the climate model is in phase with reality. This is important to keep in mind when analysing climate mode data (see link below).


El Niño involves warming of tropical Pacific surface waters from near the International Date Line to the west coast of South America, weakening the usually strong sea surface temperature gradient across the equatorial Pacific, with associated changes in ocean circulation. Historically, El Niño events occur about every 3 to 7 years and alternate with the opposite phases of below-average temperatures in the eastern tropical Pacific (La Niña). Changes in the trade winds, atmospheric circulation, precipitation and associated atmospheric heating also affects the extratropicals.

NAO – North Atlantic Oscillation

In the northern hemisphere the NAO is prominent throughout the year. It is primarily the pressure anomalies over the Atlantic sector, and therefore corresponds to changes in the westerlies across the North Atlantic into Europe. The NAO has the strongest signature in the winter months (December to March) when its positive phase exhibits an enhanced Iceland Low and Azores High. The NAO is the dominant pattern of near-surface atmospheric circulation variability over the North Atlantic, accounting for one third of the total variance in monthly MSLP in winter.

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The ENES3 project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 824084.