The Influence of Surface Forcings on Prediction of the North Atlantic Oscillation Regime of Winter 2010/11

Abstract

December 2010 was unusual both in the strength of the negative North Atlantic Oscillation (NAO) intense atmospheric blocking and the associated record-breaking low temperatures over much of northern Europe. The negative North Atlantic Oscillation for November-January was predicted in October by 8 out of 11 World Meteorological Organization Global Producing Centres (WMO GPCs) of long-range forecasts. This paper examines whether the unusual strength of the NAO and temperature anomaly signals in early winter 2010 are attributable to slowly varying boundary conditions [El Niño-Southern Oscillation state, North Atlantic sea surface temperature (SST) tripole, Arctic sea ice extent, autumn Eurasian snow cover], and whether these were modeled in the Met Office Global Seasonal Forecasting System version 4 (GloSea4). Results from the real-time forecasts showed that a very robust signal was evident in both the surface pressure fields and temperature fields by the beginning of November. The historical reforecast set (hindcasts), used to calibrate and bias correct the real-time forecast, showed that the seasonal forecast model reproduces at least some of the observed physical mechanisms that drive the NAO. A series of ensembles of atmosphere-only experiments was constructed, using forecast SSTs and ice concentrations from November 2010. Each potential mechanism in turn was systematically isolated and removed, leading to the conclusion that the main mechanism responsible for the successful forecast of December 2010 was anomalous ocean heat content and associated SST anomalies in the North Atlantic.

Simple Statistical Probabilistic Forecasts of the Winter NAO

“…This is consistent with the conclusions of Maidens et al (2013), who found that Atlantic SSTs were a major contributory factor to the negative NAO of this year. Interestingly though, in both models around 50% of the negative forcing comes from west Indian Ocean rainfall, which is indicative of convective activity and divergence leading to Rossby wave propagation.…”

“…In addition, N80 shows an extratropical influence from the June North Atlantic tripole, while in N93 the October AMO is selected as a predictor and solar forcing is also significant in the longer time series. The solar forcing term here is at a lead of around 3 yr, consistent with that identified from other studies (Scaife et al 2013;Gray et al 2013;Andrews et al 2015). A number of very similar correlations are found at lead times ranging from 6 months to 3 yr.…”

“…Monthly sunspot numbers are used to create a normalized index , available from the Solar Influences Data Analysis Center (http://sidc.oma.be/). Regression is also carried out using a lead of 1-5 yr of the solar cycle over the NAO as recent studies suggest that there is a lagged North Atlantic climate response to solar variability (Scaife et al 2013;Gray et al 2013)i na d d i t i o nt oa shorter time-scale response operating via changes in the stratospheric polar vortex (Ineson et al 2011).…”

Impact of a 30% reduction in Atlantic meridional overturning during 2009–2010

“…In models, the NAO-negative Here we attribute the dipole pattern of warm tropical waters and cold subtropical waters (two-thirds of the tripole pattern) to the 30 % slowdown in the AMOC. In a seasonal forecast for the winter of 2010-2011 over northwestern Europe, it was found that the distribution of upper ocean temperature anomalies in October-November 2010 was the key factor allowing severe winter conditions to be successfully forecast 3 months in advance (Maidens et al, 2013). On this basis we argue that the AMOC slowdown created an upper ocean temperature distribution that helped push the atmospheric circulation into record-low NAO negative states in both the winters of 2009-2010 and 2010-2011 with consequent effects on UK winter weather (Osborn, 2011).…”

Recent Wind-Driven Variability in Atlantic Water Mass Distribution and Meridional Overturning Circulation

“…The resultant northward heat transport within the North Atlantic affects both the long-term climatic state over northern Europe (Trenberth and Caron 2001;Johns et al 2011) and the interannual climate variability across the North Atlantic basin (Maidens et al 2013). This interannual variability can be very pronounced.…”