Combined Effects of the North Atlantic Oscillation and the Arctic Oscillation on Sea Surface Temperature in the Alborán Sea

José C Báez,Francisco Ferri-Yáñez,Luis Gimeno,Moncho Gómez-Gesteira,Raimundo Real,Inés Álvarez

doi:10.1371/journal.pone.0062201

Abstract

We explored the possible effects of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) on interannual sea surface temperature (SST) variations in the Alborán Sea, both separately and combined. The probability of observing mean annual SST values higher than average was related to NAO and AO values of the previous year. The effect of NAO on SST was negative, while that of AO was positive. The pure effects of NAO and AO on SST are obscuring each other, due to the positive correlation between them. When decomposing SST, NAO and AO in seasonal values, we found that variation in mean annual SST and mean winter SST was significantly related to the mean autumn NAO of the previous year, while mean summer SST was related to mean autumn AO of the previous year. The one year delay in the effect of the NAO and AO on the SST could be partially related to the amount of accumulated snow, as we found a significant correlation between the total snow in the North Alborán watershed for a year with the annual average SST of the subsequent year. A positive AO implies a colder atmosphere in the Polar Regions, which could favour occasional cold waves over the Iberian Peninsula which, when coupled with precipitations favoured by a negative NAO, may result in snow precipitation. This snow may be accumulated in the high peaks and melt down in spring-summer of the following year, which consequently increases the runoff of freshwater to the sea, which in turn causes a diminution of sea surface salinity and density, and blocks the local upwelling of colder water, resulting in a higher SST.

Highlights

The most important mechanism responsible for interannual climate variability in South-West Europe is the North Atlantic Oscillation (NAO), in winter [1,2,3,4,5,6,7,8]
When we analysed the effect of NAO and Arctic Oscillation (AO) together on sea surface temperature (SST) we found that the probability of observing mean annual SST values higher than average was significantly (x2 = 11.694, df = 2, p = 0.003) related to NAO and AO values of the previous year
The variation in mean annual NAO was explained by variation in the mean autumn NAO, mean winter NAO, and mean spring NAO, while variation in annual AO was explained by the mean autumn AO, mean winter AO, mean spring AO and mean summer AO (Table 1).We found that variation in mean annual SST and mean winter SST could be significantly related to the mean autumn NAO of the previous year (Table 1), while mean summer SST was related to mean autumn AO of the previous year (Table 1)

Summary

Introduction

The most important mechanism responsible for interannual climate variability in South-West Europe is the North Atlantic Oscillation (NAO), in winter [1,2,3,4,5,6,7,8]. The NAO reflects fluctuations in atmospheric pressure at sea-level between the Icelandic Low and the High of Azores. It is widely known that the positive phases of NAO induce higher than average westerly winds across northern mid-latitudes with a dry climate on the Iberian Peninsula, while the negative phases of NAO induce major precipitation in southern Europe 1. Thompson and Wallace [9] suggested that the AO is characterized by a meridional dipole in sea level atmospheric pressure between Polar Regions and mid-latitudes, and could be interpreted as the surface signature of modulations in the strength of the polar vortex aloft [9]. When the AO index is positive (characterized by a strengthening of the polar vortex), surface pressure is low in the polar region, and the opposite occurs when the index is negative

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