Precipitation and the North Atlantic Oscillation: a study of climatic variability in northern England

Abstract

Variations in precipitation in Yorkshire, UK, are linked to large-scale atmospheric circulation using the North Atlantic Oscillation (NAO) index, objective Lamb weather types, and long-term seasonal and annual precipitation indices at seven sites. Since the 1960s, a significant decrease in summer precipitation has been observed across the region, with increasing winter precipitation at western locations since 1970. At western sites, precipitation is highly correlated to the NAO, with positive and negative winter-NAO indices producing anomalies from the long-term winter precipitation average of 112% and 77% respectively. The recent winter half-year precipitation increase is concentrated in March and related to a large increase in monthly NAO and the increased occurrence of westerly weather types. The decrease in summer precipitation, however, is caused by both declining receipt from westerly weather types and their decreased occurrence. This is significantly correlated to a decreasing summer index for the NAO. The NAO, the frequency of weather types and precipitation totals are all intimately linked on an annual and monthly time scale. Strong connections between winter precipitation and the NAO may prove very important, particularly in western Yorkshire where most surface water supplies are located. If the NAO can be predicted using sea-surface temperature, it may allow the estimation of winter reservoir replenishment in western Yorkshire a few months in advance. This may provide an important predictor of future hydrological drought episodes and facilitate forward planning and management of water resource systems located in climatologically similar regions of the UK.

Long-term variations in orographic rainfall: analysis and implications for upland catchments

“…However, between 1971 and 2000, there was a modest increase in the frequency of these circulation types during winter months (Table 2), significant at the 95% confidence level. This increase in frequency of winter westerlies has been linked to the positive winter NAO index since the early 1970s (Fowler & Kilsby, 2002) and is broadly consistent with results reported by Gilles et al (2006) from the Rhine-Meuse basin region of Germany. This is not, however, the only long-term factor resulting in an increase in winter rainfall in the Eden Valley rainshadow region.…”

“…Statistically insignificant changes to annual rainfall may be a consequence of the relatively short data records, but the analysis indicates that winters are getting wetter, affected by higher intensity events, and are contributing a greater proportion of total annual rainfall. This is entirely consistent with expected rainfall changes under the positive phase of the North Atlantic Oscillation (NAO) during the period (Wilby et al, 1997) and with changes seen elsewhere in the UK (Kiely, 1999;Osborn et al, 2000;Fowler & Kilsby, 2002).…”

“…The first is a change in frequency of key weather patterns, such as an increase in the number of rain-bearing westerly air flows in winter months (Kiely, 1999). The second is a change in the character of the airmass, for example an increase in the amount of rain associated with a given weather pattern (Fowler & Kilsby, 2002). The LWT catalogue was used to identify changes in both the frequencies of different weather patterns and the amount of rainfall for each weather pattern.…”

Linking climate change modelling to impacts studies: recent advances in downscaling techniques for hydrological modelling

“…This assumption has shown to be questionable in the observed record (e.g. Huth, 1997;Slonosky et al, 2001;Fowler and Kilsby, 2002) and is best tested using long records or model validation on a period with different climate characteristics . Nonstationarity may be attributed to an incomplete set of predictor variables that exclude low-frequency climate behaviour, inadequate sampling or calibration period, or temporal change in climate system structures (Wilby, 1998).…”

Rural land management impacts on catchment scale flood risk

“…Hurrell (1995) found links between shifts in the NAO and changes in UK temperatures and precipitation totals. Fowler and Kilsby (2002a) found a positive correlation between the NAO and the precipitation quantities in the west of the UK and a negative correlation in the east. However, the relationship does not seem to be that simple, with Wedgbrow (2002) finding a lag between the changing NAO index and the change in UK weather.…”