Influence of the wintertime North Atlantic Oscillation on European tropospheric composition: an observational and modelling study

Richard J Pope,Wuhu Feng,Martyn P Chipperfield,Norbert Glatthor,Richard Siddans,Sandip S Dhomse,Stephen R Arnold,Barry G Latter,Brian J Kerridge

doi:10.5194/acp-18-8389-2018

Abstract

Abstract. We have used satellite observations and a simulation from the TOMCAT chemistry transport model (CTM) to investigate the influence of the well-known wintertime North Atlantic Oscillation (NAO) on European tropospheric composition. Under the positive phase of the NAO (NAO-high), strong westerlies tend to enhance transport of European pollution (e.g. nitrogen oxides, NOx; carbon monoxide, CO) away from anthropogenic source regions. In contrast, during the negative phase of the NAO (NAO-low), more stable meteorological conditions lead to a build-up of pollutants over these regions relative to the wintertime average pollution levels. However, the secondary pollutant ozone shows the opposite signal of larger values during NAO-high. NAO-high introduces Atlantic ozone-enriched air into Europe, while under NAO-low westerly transport of ozone is reduced, yielding lower values over Europe. Furthermore, ozone concentrations are also decreased by chemical loss through the reaction with accumulated primary pollutants such as nitric oxide (NO) in NAO-low. Peroxyacetyl nitrate (PAN) in the upper troposphere–lower stratosphere (UTLS) peaks over Iceland and southern Greenland in NAO-low, between 200 and 100 hPa, consistent with the trapping by an anticyclone at this altitude. Model simulations show that enhanced PAN over Iceland and southern Greenland in NAO-low is associated with vertical transport of polluted air from the mid-troposphere into the UTLS. Overall, this work shows that NAO circulation patterns are an important governing factor for European wintertime composition and air pollution.

Highlights

Atmospheric circulation can play an important role in the transport and accumulation of air pollutants from and over source regions (e.g. Pope et al, 2014; Stohl, 2006; Quinn et al, 2007)
Ozone Monitoring Instrument (OMI) tropospheric column NO2 (TCNO2) was sampled under the wintertime (November– February) North Atlantic Oscillation (NAO)-high (Fig. 2a) and NAO-low (Fig. 2b) for 2005–2015
Statistically significant signals are found on both sides of the North Atlantic as a result of changes in the westerly circulation during the two NAO phases

Summary

Introduction

Atmospheric circulation can play an important role in the transport and accumulation of air pollutants from and over source regions (e.g. Pope et al, 2014; Stohl, 2006; Quinn et al, 2007). Over North America, the North Atlantic and Europe, the wintertime North Atlantic Oscillation (NAO) is one of the most prominent and frequent modes of atmospheric variability. It represents the redistribution of atmospheric mass between the Arctic and subtropical North Atlantic (Hurrell and Deser, 2010) controlling pressure gradients, wind flows, storm tracks and moisture budgets (Hurrell, 1995; Osborn, 2006). During the NAO positive phase (NAO-high), the climatological Icelandic low and Azores high-pressure systems both intensify, leading to enhanced westerly circulation (storm tracks) across the Atlantic and into north-western Europe. The NAO negative phase (NAO-low) results in a weakening of this meridional pressure gradient leading to reduced westerly winds and a reorientation of the storm tracks over southern continental Europe

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