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1. Introduction

Winter 2009/10 and 2010/11 were exceptional, with a strongly negative North Atlantic Oscillation (NAO) index and severe cold temperatures in Eurasia and North America. The Met Office seasonal forecast system GloSea4 successfully forecast the broad details of both winters. Several mechanisms have been proposed that suggest potential predictability in winter. Here we use GloSea4 to explore which of these mechanisms were important in 2009/10 and 2010/11.

2. Winter 2009/10

The negative NAO was forecast by the then-operational “low top” version of GloSea4 (fig 1). We looked at possible reasons for the forecast by correlating different November indices (Eurasian snow cover, Arctic geopotential height, Arctic sea ice) with the DJF mean NAO index for GloSea4 runs for 1989-2002. However, weak correlations suggest none of these factors played a big role in the forecast. A further possible mechanism exists, however.

4. Winter 2010/11

December 2010 also showed a strong negative NAO and cold temperatures across northern Europe. This was forecast by the high top GloSea4 (fig 5) but different forcing factors to 2009/10 may have been important. The GloSea4 hindcasts were examined to see which forcing factors the model responds to.

7. Conclusions

Our results suggest GloSea4 correctly predicted the extreme winters of 2009/10 and 2010/11 by simulating two different mechanisms. The winters could be linked since the North Atlantic SST pattern of winter 2010/11 may have been created by the extreme negative NAO of winter 2009/10. These results show promise for improved forecasts of extreme winters, with clear social and economic benefits.

References:

D.R. Fereday, A. Maidens, A. Arribas, A.A. Scaife, J.R. Knight, Seasonal forecasts of northern hemisphere winter 2009/10, Environ. Res. Lett. 7 (2012), doi: 10.1088/1748-9326/7/3/034031

A. Maidens, A. Arribas, A.A. Scaife, C. MacLachlan, D. Peterson, J.R. Knight Predictability of the North Atlantic Oscillation in early winter 2010-2011 (submitted to Mon. Wea. Rev.)

3. Sudden stratospheric warmings (SSWs)

In a SSW the stratospheric polar vortex breaks down and generates a downward propagating signal. This leads to reduced westerly winds and cold conditions at the surface a few weeks later. Two SSWs occurred in 2009/10. We used a “high top” version of GloSea4 that resolves the stratosphere to examine this mechanism. Results are closer to observations, with higher Arctic mean MSLP in February than the low top GloSea4 (fig 2).

The high top model produces more SSWs in El Niño years (fig 3) consistent with previous work, and also reproduces the observed downward propagating signal in the zonal wind field (fig 4).

Low top GloSea4 High top GloSea4 Observed

Figure 1: Winter (DJF) mean sea level pressure (MSLP) for 2009/10.

Figure 2: Top row - high top minus low top MSLP. Bottom row – distribution of Arctic mean DJF MSLP anomaly for high top (black) and low top (red) runs with observed value (blue spots).

Figure 3: December Nino3.4 vs December QBO for 2009/10 and preceding years. Pie charts show proportion of members with/without SSWs for each year in red/blue.

Figure 4: Observed and model zonal mean zonal winds for 2009/10 at60°N as a function of height.

Observed Closest high top ensemble member

Figure 5: Observed and GloSea4 MSLP and temperature for Dec 2010.

MSLP Temperature

Ob

serv

edG

loS

ea4

5. Hindcast analysis

Composites of hindcasts with extremes in proposed forcing fields were made to assess their effect in GloSea4. The GloSea4 La Niña composite shows blocking for northern Europe (fig 6), while North Atlantic SST composites show a negative NAO pattern (fig 7).

6. Atmosphere only runs with relaxed SST

To see which SST regions are important, an ensemble was forced with observed SST and sea ice for winter 2010. Further ensembles had either SST in the tropical Pacific, SST in the North Atlantic or sea ice relaxed to climatology. Results show North Atlantic SSTs had most influence on the December 2010 negative NAO (figs 8,9).

Figure 6: Composite December MSLP for La Niña years forobservations (left) and GloSea4 (right).

Figure 7: Composite initial Nov SST, GloSea4 Dec MSLP and 1.5mtemperature for outer quintiles of North Atlantic SST tripole index.

SST MSLP Temperature

Po

siti

veN

egat

ive

Figure 8: December MSLP anomalies for atmosphere only runs.

Figure 9: December temperature anomalies for atmosphere only runs.

Control Tropical Pacific relaxed

North Atlantic relaxed Sea ice relaxed

Control Tropical Pacific relaxed

North Atlantic relaxed Sea ice relaxed

Seasonal forecasts of the exceptional boreal wintersof 2009/10 and 2010/11David Fereday, Anna Maidens, Jeff Knight, Adam Scaife, Alberto Arribas, Craig MacLachlan, Drew Peterson