Robust Sub-decadal Variability in the North Atlantic region

The North Atlantic Oscillation (NAO) is the dominant mode of North Atlantic sector climate variability in winter. During a positive (negative) NAO phase the sea level pressure difference between the Azores high and the Icelandic low is anomalously strong (weak). The NAO affects, for example, surface air temperature, precipitation, surface wind and heat flux in the North Atlantic region and also over large parts of Eurasia. We find in 20^th and early 21^st century instrumental observations a robust sub-decadal variability of the NAO index that goes along with a large-scale sea surface and subsurface temperature pattern, which explains a significant part of the total variability. We also find this mode of variability in an ensemble of CMIP5 model control simulations. The model results suggest as an underlying mechanism the ocean adjustment to an NAO-related wind-driven large-scale variability of the surface heat and momentum fluxes, where the ocean overturning variability in the North Atlantic region plays an essential role. It sets the timescale of the variability by providing a delayed negative feedback to the NAO. The sub-decadal variability is associated with a deep-reaching dipolar circulation anomaly in the model ensemble. The results suggest that large-scale ocean-atmosphere interactions is crucial to generate enhanced sub-decadal variability in the North Atlantic sector, which may increase multi-year climate predictability over the region.