LOS GALOS SAQUEAN ROMA

Meteorological droughts are driven by precipitation and available energy. They are initiated by atmospheric circulation and weather systems that cause lower precipitation and/or higher evaporation than normal; such as high-pressure, anticyclonic weather systems. As such, meteorological droughts are part of the natural variability of climate and weather as well as part of anthropogenically driven changes in climate. The occurrence of drought usually coincides with altered seasonal timing, location and persistence of weather patterns for longer than normal, which may arise from short atmospheric anomalies or decadal scale oceanic temperature anomalies. As a result, both small scale (local and regional) seasonal and daily weather patterns and large scale (continental and global) multi-year ocean-atmosphere processes are drivers of meteorological drought. Mid- latitude locations can experience meteorological drought at any time throughout a year because of the high variability of both climate and weather (Sheffield and Wood 2011). Multi-decadal and inter-decadal large-scale modes of climate variability and sea surface temperatures (SSTs) have been shown to cause precipitation variability and drought in Europe (Cassou et al. 2005; Dai 2011). Modes of variability that drive European climate and weather include: the Atlantic Multi-decadal Oscillation (AMO) (Sutton and Hodson 2005), the North Atlantic Oscillation (NAO) (Hurrell and Van Loon 1997), and the Arctic Oscillation (AO), with the El Niño Southern Oscillation (ENSO) a well-known driver of global climate on

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inter-annual timescales (Ropelewski and Halpert 1987); although, this is associated more as a driver of drought in North and South America, Africa, Asia and Australia (Salinger 2005; Herweijer et al. 2007; Dai 2011).

Atlantic Multi-decadal Oscillation (AMO)

The AMO is a mode of variability occurring in the North Atlantic Ocean and is related to temperature and precipitation fluctuations over North America, Europe, West Africa and east South America, with oscillations on a multi-decadal timescale (60-80 years, NCAR 2014). Figure 2.3 demonstrates how regions are affected by positive and negative modes of the AMO. The negative mode of the AMO is related to dry conditions over western northern Europe. Fluctuations of the AMO since 1870 are shown in Figure 2.4 and a proxy record from 1567, reconstructed from tree-ring chronologies by Gray et al. (2004), is presented in Figure 2.5. Associations between the AMO and droughts in the USA (McCabe et al., 2008) and Central America and Mexico (Mendez et al., 2010) have been examined and found to explain a large degree of drought variability with interaction with the El-Niño Southern Oscillation (ENSO). The role of the AMO as a driver in drought causation in Europe, and specifically the UK, has not previously been investigated.

Page | 24 Figure 2.3: Regions impacted by the AMO in different modes. Source Sheffield and Wood (2011)

Figure 2.4: Index of the AMO, Source: NCAR (2014). SST anomalies averaged over the North Atlantic (0 to 60oN, to 0 to 80oW) for 1870-2011, relative to 1901 to 1970 (0C). The heavy line with fill from the low-pass filter depicts the revised AMO by Trenberth and Shea (2006)

Page | 25 Figure 2.5: Reconstructed proxy AMO (1567-1990) time series from tree ring chronologies. Source Gray et al. (2004)

The North Atlantic Oscillation (NAO)

The NAO is an inter-annual mode of climate variability that manifests in the pressure difference between the Icelandic low and the Azores High pressure regions (Hurrell et al. 2003). It has a greater influence on northern hemisphere weather in winter, but has been shown to have some influence during summer (Folland et al. 2009; Blade et al. 2011). The NAO is part of the Arctic Oscillation (AO), which is a wider mode of variability affecting the northern hemisphere. The AO manifests as anomalies between the Arctic and mid-latitudes (40oN). Different modes of these oscillations influence the position of the Jet stream, moving it either further north or south in the Atlantic Ocean, which determines the strength of westerly winds over North America and Europe. In general, positive modes bring warmer and wetter weather to northern Europe and cooler, drier weather to southern Europe. The converse is generally the case in negative modes of the NAO and AO (Figure 2.6) (Zhou et al. 2001; Hurrell et al. 2003). For example, López-Moreno and Vicente- Serrano (2008) in an analysis of European droughts (1901-2000), showed that during positive (negative) modes of the NAO, negative (positive) SPI values (drought), were

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observed in southern Europe, with an opposite pattern observed for northern Europe. Figures 2.7 and 2.8 show the variability in modes of the NAO index and AO index.

Figure 2.6: Regions impacted by the NAO in different modes. Source Sheffield and Wood (2011)

Figure 2.7: Time series of the winter (DJFM) NAO index from 1823 updated from Jones et al. (1997b) to end of 2012/2013. The smooth line is a 10-year Gaussian filter, Source: CRU (2014)

Page | 27 Figure 2.8: Time series of the winter (Nov-Mar) index from 1950. Source: NOAA (2014a) Historical perspectives

Brooks and Glasspoole (1928) investigated the causes of droughts occurring during the nineteenth and early twentieth century in Britain, with particular focus on the dry year of 1887 and the drought of 1921. They identified that two distributions of pressure are associated with dry years and (long) droughts in the UK: anticyclonic and east-wind.

“…the…cause of a long period of settled fine weather in England is a north-easterly extension of the Azores anticyclone towards or over the British Isles. At the same time the Icelandic low retreats far to the north-east, towards the Arctic Ocean; the main tracks of depressions run well to the north, and only the north-west of Scotland comes to any extent under their influence”. (Brooks and Glasspoole, 1928, pg 109)

Here, they describe anticyclonic (high) pressure systems associated with the negative mode of the NAO and suggest these are a more frequent cause of prolonged dry weather and drought in the UK, and attribute a persistent anticyclone as the cause of the drought of 1921 as well as:

Page | 28 “Of the nine chief droughts of the past seventy years [1858-1928]…, namely, those of 1864, 1868, 1880, 1887, 1893, 1896, and 1921, were of this anticyclonic type. The second type of drought [east wind] occurred in 1895 and 1911”. (Brooks and Glasspoole, 1928, pg 112) The east-wind pressure distribution (reversal of normal orographic pressure distribution) is described as:

“…pressure is higher than usual to the north of the British Isles and lower than usual to the south, so that the normal fall of pressure from south to north is greatly weakened or even reversed. The orographic type of rainy season is associated with unusually low pressure to the north-west, in the direction of Iceland, but in this type of drought, the pressure is relatively highest…to the north-east, over Scandinavia, than towards Iceland…if the differences in the pressure are large…pressure is actually higher to the north than to the south of this country, and in place of the usual south-west winds we have a period of easterly winds. This type of drought is generally felt over the whole country… [and] is not usually prolonged or intense. It can, however be very aggravating… owing to its origin over the continent of Europe.” (Brooks and Glasspoole, 1928, pg 112-113)

Although not a direct cause of droughts, land-atmosphere (soil moisture feedback and presence of snow) interactions have been identified as playing a role in prolonging droughts (Zampieri and D'Andrea 2009). Both observational (Koster and Suarez 2004) and modelling (Wu et al. 2005) studies have found that soil moisture anomalies have an impact on seasonal precipitation and contribute to the maintenance of drought conditions through recycling of precipitation. The effects of precipitation recycling can be quantified in terms of a ratio, that is, the ratio of regional precipitation derived from land evaporation to the total precipitation (from land and ocean evaporation). During drought, dry soils can reduce evaporation and disrupt this mechanism causing a reduction in recycled precipitation, and prolonging drought conditions in the same region or initiating them elsewhere (Dominguez et al. 2009).

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