A re-interpretation of impact of the Icelandic Low and Azores High on winter precipitation over Iberian Peninsula
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ORIGINAL PAPER
A re-interpretation of impact of the Icelandic Low and Azores High on winter precipitation over Iberian Peninsula M. J. Iqbal 1 & Shahnaz Feroz Rashid 2
Received: 23 June 2014 / Accepted: 10 September 2015 / Published online: 22 February 2016 # Saudi Society for Geosciences 2016
Abstract Several studies show that North Atlantic Oscillation (NAO) has dominant influence on the variability of climate over Iberian Peninsula. The traditional definition of the NAO index is a very simple representation of the atmospheric state of negative correlation between the Azores High and the Icelandic Low. The NAO index does not take into account the geographically extensive nature of the high and the low pressure systems and their movements. Fluctuations in both the characteristic shapes and positions of the Azores High and the Icelandic Low influence atmospheric circulation over the north Atlantic region. We study interannual variations of winter rainfall over Iberia, taking into account variations in the Azores High and the Icelandic Low pressure systems. Analysis presented in this paper shows that the north–south migrations of the Azores High exert the greatest impact on winter precipitation over Iberian Peninsula. Using NCEP/NCAR reanalysis data, this approach allows us to understand the changes in atmospheric circulation through which latitudinal shifts of the Azores High influence regional precipitation over Iberia.
Keywords Azores High . Icelandic Low . Winter rainfall
* Shahnaz Feroz Rashid [email protected] 1
Institute of Space and Planetary Astrophysics, University of Karachi, Karachi, Pakistan
2
Department of Mathematical Sciences, University of Karachi, Karachi, Pakistan
Introduction The North Atlantic Oscillation (NAO) has been recognized for more than 60 years as one of the major patterns of atmospheric variability in the Northern Hemisphere (Walker 1924; Walker and Bliss 1932; Trigo et al. 2002; Rogers 1984; Barnston and Livezey 1987; Hurrell 1995; Hurrell and van Loon 1997). The NAO index is defined by measurement of the pressure difference between the Icelandic Low and Azores High at two fixed locations: Lisbon, Portugal and Stykkisholmur, Iceland (Hurrell 1995). When sea-level pressure is below normal over the Icelandic region and above normal over Azores, the NAO is said to be in its high index. At these time, the jet stream and storm tracks tend to be displaced pole ward of their normal positions, temperatures tend to be unseasonably mild over Eurasia and most the USA, and northern Europe experiences heavier than normal rainfall while the Mediterranean bask in sunshine (Wallace and Hobbs 2005). In contrast, episodes of abnormally high pressure over the Arctic (i.e., low NAO index) tend to be marked by relatively frequent occurrence of cold air outbreaks over Eurasia and the USA. Recently, a number of studies have shown the relevance of the NAO to the winter surface climate of the Northern Hemisphere in general and over the Atlantic/European sector in particular (Hurrell 1995; Qian
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