An overview of the North Atlantic Oscillation

Abstract

The North Atlantic Oscillation (NAO) is one of the most prominent and recur rent patterns of atmospheric circulation variability. It dictates climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during boreal winter, so variations in the NAO are important to society and for the environment. Understanding the processes that govern this variability is, therefore, of high priority, especially in the context of global climate change. This review, aimed at a scientifically diverse audience, pro vides general background material for the other chapters in the monograph, and it synthesizes some of their central points. It begins with a description of the spatial structure of climate and climate variability, including how the NAO relates to other prominent patterns of atmospheric circulation variability. There is no unique way to define the spatial structure of the NAO, or thus its temporal evolution, but several common approaches are illustrated. The relationship between the NAO and variations in surface temperature, storms and precipitation, and thus the econ omy, as well as the ocean and ecosystem responses to NAO variability, are described. Although the NAO is a mode of variability internal to the atmosphere, indices of it exhibit decadal variability and trends. That not all of its variability can be attributed to intraseasonal stochastic atmospheric processes points to a role for external forcings and, perhaps, a small but useful amount of predictability. The surface, stratospheric and anthropogenic processes that may influence the phase and amplitude of the NAO are reviewed. 1. INTRODUCTION Over the middle and high latitudes of the Northern Hemisphere (NH), especially during the cold season months (November-April), the most prominent and recurrent pattern of atmospheric variability is the North Atlantic Oscillation (NAO). The NAO refers to a redistribution of atmospheric mass between the Arctic and the subtropical Atlantic, and swings from one phase to another produce large changes in

Trends of precipitation over the Iberian Peninsula and the North Atlantic Oscillation under climate change conditions

“…The links between precipitation and NAO over the Iberian Peninsula (IP) have been shown in many papers (Hurrell, 1995;Lamb and Peppler, 1987;Rodríguez-Puebla et al, 1998;Qian et al, 2000;Goodess and Jones, 2002;Trigo et al, 2004;Lopez-Moreno and Vicente-Serrano, 2008;López-Bustins et al, 2008). The NAO is related to westerly winds, storm track and jet stream (Hurrell et al, 2003;Nieto and Rodríguez-Puebla, 2006). Therefore, the NAO becomes an important teleconnection pattern to explain the precipitation decreases in the IP, because the NAO is related to shifts in the storm tracks and consequent precipitation.…”

“…Thus, there is a positive trend in the North Atlantic Oscillation (NAO) under climate change. The NAO is a large-scale circulation pattern, statistically and physically robust, that characterizes northern hemisphere climate variability (Branstator, 2002;Hurrell et al, 2001Hurrell et al, , 2003Wallace, 2000). The NAO is a measure of the strength of the Icelandic Low and the Azores High and it accounts for much of the precipitation variability over the Euro-Atlantic area.…”

A mechanistic model of mid-latitude decadal climate variability

“…Conceptual models suggest explanations for this variability in terms of a delayed ocean response to a noisy atmospheric forcing associated with the North Atlantic Oscillation (NAO; Hurrell 1995). The NAO is an atmospheric mode that has a coherent spatial pattern (Hurrell et al 2003) and a time dependence that contains a large stochastic component (Wunsch 1999). The proposed delays have been related to a variety of ocean processes; examples include mean ocean advection (Saravanan and McWilliams 1998) or planetary wave propagation (Jin 1997;Neelin and Weng 1999;Czaja and Marshall 2001).…”

Sea-level changes in Iceland and the influence of the North Atlantic Oscillation during the last half millennium

“…The NAO exerts a strong influence over regional wind patterns, precipitation and temperature, mainly in the winter (e.g. Hurrell et al, 2003). The influence of (winter) NAO (wNAO) on Atlantic sea level during the instrumental era is well established (Andersson, 2002;Haigh et al, 2010;Kolker and Hameed, 2007;Miller and Douglas, 2007;Tsimplis et al, 2005Tsimplis et al, , 2006Woodworth et al, 2007;Woolf et al, 2003), but its significance in controlling dynamic sea-level variability over longer time intervals has not previously been explored.…”