In the last few months, we have been surprised by the great floods that hit several areas in Europe and Asia. Not infrequently this is all associated with climate change due to global warming that occurs on Earth, which causes extreme weather to occur. This, as reported by the IPCC in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR-4) in 2007, indicates a precipitation increase in the northern parts of Europe and a decrease in the Mediterranean area for a future warmer climate. The simulation shows about a 10% decline in precipitation across the region by both the middle and at the end of the century (Chenoweth et al., 2011).
The Arctic Oscillation (AO) refers to an atmospheric circulation pattern over the mid-to-high latitudes of the Northern Hemisphere. The AO is a major source of intra-seasonal variability over the United States, North Atlantic, and Europe during winter. It modulates the circulation pattern over the middle and high latitudes, thereby regulating the frequency and intensity of significant weather events (Thompson and Wallace, 1998, 2001). Several studies examined the response of the AO and the NAO to increase greenhouse gas emissions and global temperature by using climate models and found that an increase in greenhouse gases is expected to lead to an increase in AO and to a lesser extent in the NAO values.
The North Atlantic Oscillation (NAO) describes the fluctuations in the difference of atmospheric pressure at sea level between two semi-permanent centers of low and high pressure in the North Atlantic: the Icelandic Low and the Azores High. Fluctuations between these centers control the strength and direction of westerly winds and location of storm tracks across the North Atlantic. The NAO is significantly stronger in winter than in the other seasons, therefore, most studies on the NAO focus on winter months, when the influence of the NAO on surface temperature and precipitation is highest.
Indeed, the same precipitation event may or may not generate a flood, depending on soil saturation and therefore on prior precipitation. Givati A., and Rosenfeld D. (2013), examined the effect of changes in global circulation phenomena like the AO on regional climatology by using synoptic observations from reanalysis and by statistical analysis of actual rain gauges in Israel. Understanding the fundamental causes for the trends in the AO and the associated changes in precipitation provides us with some insights into possible future changes in the regional distribution of precipitation.
Apparently, a transition of the AO relationships to rainfall occurs within the Israeli study area, where it is positively associated with rainfall in the south and negatively to the north. The negative correlation continues more strongly farther north in Turkey, where Türkeş and Erlat (2003), Küçük et al. (2009) showed an extensive negative correlation between winter precipitation and a positive phase of the North Atlantic. Oscillation Index (NAO), which is highly correlated with the AO. The positive trend in the AO and NAO is manifested as migration northward of the storm tracks also in Western Europe (Hurrell, 2003). Nasr-Esfahany et al. (2011) showed that during a positive phase of the NAO, which is positively correlated with the AO, there is a less tendency to form eddies over the central Mediterranean and enhancement to the south of the eastern Mediterranean and over the northern Red Sea.
The NAO is driven by differences in atmospheric pressure between the North Pole and the Equator. The NAO is significantly stronger in winter than in the other seasons, therefore, most studies on the NAO focus on winter months, when the influence of the NAO on surface temperature and precipitation is highest. Recent, rapid changes in temperatures in the Arctic are interfering with these pressure levels and changing the track of the oscillation and storms as well. According to this study, the storms are arriving later and as a result, some river flooding happens later too.
In a new study in 2019, published in Geophysical Research Letters and recently featured in Nature Research Highlights, we have now shown that there is indeed a significant relationship between the NAO and flood losses. For example, persistently positive NAO during 2015/2016 winter contributed to wetter-than-usual conditions in Northern Europe as a cluster of unusual storms hit the UK and Ireland. This caused catastrophic flooding throughout the region and, as a result, significant damage and disturbance. The RMS study states that NAO predictions will play an important role in improving preparedness and resilience to flooding disasters. the earlier we can predict a period of lower or higher NAO, the better prepared it will be to deal with the imminent impact.
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Written by: Kurnia Wardhani M.J.