Abstract
Abstract. The North Atlantic Oscillation (NAO) plays an important role in the climate variability of the Northern Hemisphere, with significant consequences on long-range pollutant transport. We investigate the evolution of pollutant transport in the 21st century influenced by the NAO under a global climate change scenario. We use a free-running simulation performed by the ECHAM/MESSy Atmospheric Chemistry (EMAC) model coupled with the ocean general circulation model MPIOM, covering the period from 1950 until 2100. Similarly to other works, the model shows a future northeastward shift of the NAO centres of action and a weak positive trend of the NAO index (over 150 years). Moreover, we find that NAO trends (computed over periods shorter than 30 years) will continue to oscillate between positive and negative values in the future. To investigate the NAO effects on transport we consider carbon monoxide tracers with exponential decay and constant interannual emissions. We find that at the end of the century, the south-western Mediterranean and northern Africa will, during positive NAO phases, see higher pollutant concentrations with respect to the past, while a wider part of northern Europe will, during positive NAO phases, see lower pollutant concentrations. Such results are confirmed by the changes observed in the future for tracer concentration and vertically integrated tracer transport, differentiating the cases of “high NAO” and “low NAO” events.
Highlights
The North Atlantic Oscillation (NAO) is the most prominent recurrent pattern of atmospheric variability over middle and high latitudes in the Northern Hemisphere (NH)
A free-running simulation performed by the coupled ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC)– MPIOM model has been analysed in order to study the influence of the NAO on future pollutant transport and concentration changes
The simulation takes into account the GHG increment during the 21st century according to the ACCMIP (Lamarque et al, 2013) and RCP 6.0 scenario (Fujino et al, 2006) and uses a monthly aerosol climatology
Summary
The North Atlantic Oscillation (NAO) is the most prominent recurrent pattern of atmospheric variability over middle and high latitudes in the Northern Hemisphere (NH). When the Icelandic Low and Azores High are relatively stronger, the pressure difference is higher than average (positive NAO phase) and the north–south pressure gradient produces surface westerlies stronger than average across the middle latitudes of the Atlantic towards northern Europe. On the other hand, when the low and high surface pressures are relatively weaker (negative NAO phase), the flow has a reduced zonal component. These meridional oscillations produce large changes in the mean wind speed and direction, heat and moisture transport, surface temperature and intensity of precipitation, especially during boreal winter (Hurrell et al, 2003, and references therein). Several studies (Hurrell, 1995; Visbeck et al, 2001; Hurrell et al, 2003) have associated the westerly flow during positive NAO with warm and moist maritime air and enhanced precipitation over north-western Europe, and colder and drier conditions over the Mediterranean
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