Implications of Winter NAO Flavors on Present and Future European Climate

Efi Rousi,Christina Anagnostopoulou,Uwe Ulbrich,Henning W. Rust

doi:10.3390/cli8010013

Abstract

The North Atlantic Oscillation (NAO), a basic variability mode in the Northern Hemisphere, undergoes changes in its temporal and spatial characteristics, with significant implications on European climate. In this paper, different NAO flavors are distinguished for winter in simulations of a Coupled Atmosphere-Ocean GCM, using Self-Organizing Maps, a topology preserving clustering algorithm. These flavors refer to various sub-forms of the NAO pattern, reflecting the range of positions occupied by its action centers, the Icelandic Low and the Azores High. After having defined the NAO flavors, composites of winter temperature and precipitation over Europe are created for each one of them. The results reveal significant differences between NAO flavors in terms of their effects on the European climate. Generally, the eastwardly shifted NAO patterns induce a stronger than average influence on European temperatures. In contrast, the effects of NAO flavors on European precipitation anomalies are less coherent, with various areas responding differently. These results confirm that not only the temporal, but also the spatial variability of NAO is important in regulating European climate.

Highlights

North Atlantic Oscillation (NAO) is a basic variability mode known to have a strong influence on weather and climate in the Euro-Atlantic sector of the Northern Hemisphere, especially in winter
With NAO+, higher winter precipitation accompanies higher temperatures over northern Europe (Mediterranean), the largest changes in rainfall may occur over the Atlantic Ocean [4]
The data used to study the NAO pattern consist of winter (December–January–February) daily anomalies

Summary

Introduction

North Atlantic Oscillation (NAO) is a basic variability mode known to have a strong influence on weather and climate in the Euro-Atlantic sector of the Northern Hemisphere, especially in winter. It is often quantified with the use of standardized indices, for example from the difference of normalized sea-level pressure (SLP), or geopotential height (gph), between the subtropical anticyclone near the Azores or Lisbon and the subpolar low pressure system near Iceland [1]. With NAO+, higher (lower) winter precipitation accompanies higher temperatures over northern Europe (Mediterranean), the largest changes in rainfall may occur over the Atlantic Ocean [4].

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