Contrasting Conditions in the U.K. Winter of 2015/16 as a Result of Remote Tropical Influences

Abstract

AbstractWinter 2015/16 exhibited contrasting weather patterns in western Europe. Early winter saw a succession of high-impact storms, and December was the wettest calendar month recorded in the United Kingdom. February experienced a shift to relatively cooler conditions. Overall, the winter was well forecast by GloSea5, the Met Office?s seasonal prediction system, capturing the positive North Atlantic Oscillation (NAO) in early winter and the northwesterly pattern of flow in February. In this paper, we use a series of atmospheric relaxation experiments to investigate tropical drivers of these signals and the shift in pressure patterns over the course of the winter. While December?s positive NAO is highly consistent with the strong El Niño event in the tropical Pacific Ocean, this pattern is shown to be modified by wavelike activity from the tropical Atlantic Ocean. The result is a shift in the direction of the prevailing flow from westerly to southwesterly, helping to explain the unusually high U.K. precipitation. In February, the surface pressure pattern is characteristic of a strong (rather than a moderate) El Niño, driven by the eastward extension of the Walker circulation. The relaxation experiments indicate that both the Pacific and Atlantic basins play a role in explaining the phenomena of winter 2015/16. To correctly predict the extratropical response over northern Europe, an accurate representation of tropical forcings in both basins is required.