: Changement climatique: Northern Annular Mode

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Northern Annular Mode

A see-saw latitudinal fluctuation in Northern Hemisphere sea level pressure or geopotential height between the Arctic and the mid-latitudes. The NAM has some links with the stratospheric polar vortex and is related to the fluctuation in strength and latitude of the mean westerlies. Its variance is maximum in winter and its pattern has a strong regional expression in the North Atlantic, being strongly correlated with the North Atlantic Oscillation index. The NAM is also known as the Arctic Oscillation (AO). In its positive phase, the NAM is characterized by anomalously low pressure over the Arctic and high pressure over the mid-latitudes/subtropics, with a strengthening of the zonally averaged westerly winds on their polar flank that confines colder air across the Arctic. The negative NAM phase is characterized by a more distorted wind pattern and jet meanders that increase storminess in the mid-latitude regions. (Source: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FullReport.pdf)

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