Seasonal forecasts of northern hemisphere winter 2009/10

Abstract

Northern hemisphere winter 2009/10 was exceptional for atmospheric circulation: the North Atlantic Oscillation (NAO) index was the lowest on record for over a century. This contributed to cold conditions over large areas of Eurasia and North America. Here we use two versions of the Met Office GloSea4 seasonal forecast system to investigate the predictability of this exceptional winter. The first is the then operational version of GloSea4, which uses a low top model and successfully predicted a negative NAO in forecasts produced in September, October and November 2009. The second uses a new high top model, which better simulates sudden stratospheric warmings (SSWs). This is particularly relevant for 2009/10 due to its unusual combination of a strong El Niño and an easterly quasi-biennial oscillation (QBO) phase, favouring SSW development. SSWs are shown to play an influential role in surface conditions, producing a stronger sea level pressure signal and improving predictions of the 2009/10 winter.

Influence of the Eurasian snow on the negative North Atlantic Oscillation in subseasonal forecasts of the cold winter 2009/2010

“…These cold conditions over North America and Europe coincided with one of the most extreme negative phases of the North Atlantic Oscillation (NAO) in the observational record (e.g. Fereday et al 2012). The North Atlantic jet stream also had an extremely pronounced southward displacement through most of the DecemberFebruary period (Santos et al 2013).…”

“…On the one hand, the winter-mean 500 hPa NAO pattern was better reproduced when the stratosphere was nudged as in Ouzeau et al (2011) using the MeteoFrance Arpege model. Also, the high-top model in Fereday et al (2012) which better resolves the stratosphere and better predicts stratospheric sudden warmings showed improved NAO forecasts. On the other hand, Jung et al (2011) found that nudging the ECMWF forecast model in the stratosphere did not improve NAO forecasts.…”

“…Several studies investigated the external factors that, in addition to internal atmospheric variability, could have potentially contributed to the negative NAO phase: sea surface temperature (SST) over the Atlantic or over the equatorial Pacific, late-summer Arctic sea ice extent, landatmosphere coupling involving the Eurasian snow cover, stratospheric polar vortex variability and low solar shortwave radiative forcing (Cohen et al 2010;Fereday et al …”

North Atlantic SST Anomalies and the Cold North European Weather Events of Winter 2009/10 and December 2010

“…Several mechanisms have been proposed for the anomalously low winter temperatures. These include the teleconnection pattern of the Northern Hemisphere and El Niño-Southern Oscillation (ENSO) and anomalous Eurasian snow cover (Cohen et al 2010), anomalous Arctic sea ice extent (Strey et al 2010), and sudden stratospheric warmings (Fereday et al 2012). The North Atlantic experienced anomalous SSTs during these two winters (Taws et al 2011) and these are known to influence atmospheric circulation over northern Europe (Cassou et al 2007).…”

Short-term cropland responses to temperature extreme events during late winter

“…Among all, heat waves are predicted to become more frequent, intense and longer lasting (Karl and Trenberth, 2003;Meehl and Tebaldi, 2004), especially in certain areas like central-western Europe, where the length of summer heat waves has doubled and the frequency of hot days has almost tripled in recent decades (Della Marta et al, 2007). Regarding cold waves, instead, a recent paper foresaw an increased likelihood of cold in the European region (Fereday et al, 2012).…”