Multi-annual droughts in the English Lowlands: a review of their characteristics and climate drivers in the winter half-year

摘要

The English Lowlands is a relatively dry, densely populated region in thesouth-east of the UK in which water is used intensively. Consequently, partsof the region are water-stressed and face growing water resource pressures.The region is heavily dependent on groundwater and particularly vulnerable tolong, multi-annual droughts primarily associated with dry winters. Despitethis vulnerability, the atmospheric drivers of multi-annual droughts in theregion are poorly understood, an obstacle to developing appropriate droughtmanagement strategies, including monitoring and early warning systems. Toadvance our understanding, we assess known key climate drivers in the winterhalf-year (October–March) and their likely relationships with multi-annualdroughts in the region. We characterise historic multi-annual droughtepisodes back to 1910 for the English Lowlands using various meteorologicaland hydrological data sets. Multi-annual droughts are identified using agridded precipitation series for the entire region, and refined using theStandardized Precipitation Index (SPI), Standardized Streamflow Index (SSI)and Standardized Groundwater level Index (SGI) applied to regional-scaleriver flow and groundwater time series. We explore linkages between a rangeof potential climatic driving factors and precipitation, river flow andgroundwater level indicators in the English Lowlands for the winterhalf-year. The drivers or forcings include El Ni?o–Southern Oscillation(ENSO), the North Atlantic tripole sea surface temperature (SST) pattern, theQuasi-Biennial Oscillation (QBO), solar and volcanic forcing and the AtlanticMulti-decadal Oscillation (AMO). As expected, no single driver convincinglyexplains the occurrence of any multi-annual drought in the historical record.However, we demonstrate, for the first time, an association between LaNi?a episodes and winter rainfall deficits in some major multi-annualdrought episodes in the English Lowlands. We also show significant (albeitrelatively weak) links between ENSO and drought indicators applied to riverflow and groundwater levels. We also show that some of the other driverslisted above are likely to influence English Lowlands rainfall. We concludeby signposting a direction for this future research effort.