North Atlantic circulation indices: links with summer and winter UK temperature and precipitation and implications for seasonal forecasting

Abstract

UK seasonal mean temperature and precipitation conditions are extremely variable from one year to the next but in the last decade have featured several cool, wet summers and mild, wet winters interspersed with some notable cold winter episodes. Jet stream variability is a major determinant of these fluctuations and is often represented by the North Atlantic Oscillation (NAO) index. Recent work has shown some evidence of promising predictability in the winter NAO from 1 to 2 months ahead, while summer predictability remains very limited. Although the phase and magnitude of the NAO influences total UK rainfall, there are regional variations which it does not explain. Here we examine the relationship between UK regional summer and winter precipitation and temperature and a range of North Atlantic atmospheric circulation indices. While the NAO shows a significant relationship with temperature in both seasons and summer rainfall over most of the UK, the picture in winter is more complicated, with other circulation indices such as the East Atlantic pattern explaining rainfall anomalies in southern England. Other indices also show significant relationships with precipitation in regions where the NAO does not. Because UK weather is determined by the interplay between different circulation indices, attention should be given to developing seasonal forecasts of other circulation indices to complement the NAO forecasts. We also find that some potential drivers of jet stream variability are significantly associated with UK temperature and rainfall variability, particularly in summer. This provides further scope for producing seasonal forecasts based directly on these drivers. Improved seasonal forecasts will be useful to a range of end users in agriculture, energy supply, transport and insurance industries and can be extended to other UK weather variables such as extreme rainfall events and storm frequency, and related metrics such as wind power capacity and solar energy.

Greenland blocking index daily series 1851–2015: Analysis of changes in extremes and links with North Atlantic and UK climate variability and change

“…Our findings generally support the Woollings et al (2008) hypothesis that negative NAO arises from Greenland blocking and that positive NAO mainly represents the absence of blocking (rather than "anti-blocking" or negative GBI conditions). This is also in agreement with the results of Davini et al (2012b), who found that the first empirical orthogonal function of the 500 hPa geopotential height over Europe and the North Atlantic did not resemble the NAO for GBI cases but more closely resembled an East Atlantic circulation pattern (e.g., Hall et al, 2015;Hall and Hanna, 2018) and that North Atlantic jet variability was then no longer related to Greenland blocking changes. Also in support of our findings that high-GBI and low-NAO events do not always align, Rimbu et al (2017) found that stableisotope variations in GrIS cores are more closely related to measures of Atlantic-European/Greenland blocking than the NAO index.…”

“…This is also in agreement with the results of Davini et al . (), who found that the first empirical orthogonal function of the 500 hPa geopotential height over Europe and the North Atlantic did not resemble the NAO for GBI cases but more closely resembled an East Atlantic circulation pattern (e.g., Hall et al, ; Hall and Hanna, ) and that North Atlantic jet variability was then no longer related to Greenland blocking changes. Also in support of our findings that high‐GBI and low‐NAO events do not always align, Rimbu et al .…”

“…() suggest that cyclonic (anticyclonic) wavebreaking displays a NW‐SE (NE–SW) tilt. Under high GBI conditions, therefore, this tilt tends to deflect cyclones south (Priestley et al, ), which naturally gives rise to more negative NAO conditions, which over much of the United Kingdom are typically relatively cold and dry in winter but wet in summer (Hall and Hanna, ). Rossby wave breaking in the east Atlantic is noted to precede the main/peak change in the jet stream in that region by ~2 days, and this is in accordance with our finding that the NAO low point sometimes lags peak GBI by 1–2 days.…”

Seasonal Prediction of Northern European Winter Air Temperatures From SST Anomalies Based on Sensitivity Estimates

“…Smith et al () and Hall et al () identified, besides North Atlantic SST, several drivers of NAO variability that enable predictability, among them, stratospheric variability, sea ice concentration, snow cover, solar variability, and El Niño. In northern Europe, the NAO is a major determinant of the air temperature (Hall & Hanna, ). Yet Hall and Hanna () concluded in their study the limited value of the NAO for the predictability of UK winter temperatures.…”

“…In northern Europe, the NAO is a major determinant of the air temperature (Hall & Hanna, 2018). Yet Hall and Hanna (2018) concluded in their study the limited value of the NAO for the predictability of UK winter temperatures.…”