NAO-BLO phase space
We use a novel framework based on the NAO and Blocking (NAO-BLO) phase space to study changes between circulation patterns associated with high-impact temperature anomalies over Europe. The NAO-BLO diagram is based on the leading Empirical Orthogonal Functions (EOFs) of daily geopotential height at 500 hPa computed for the Euro-Atlantic region. Figure 1 shows the two leading EOFs : EOF1 (Fig. 1a) shows a close resemblance to the positive phase of the NAO pattern and EOF2 (Fig. 1b), with its high centred over Scandinavia and a low to the east over the Atlantic Ocean, is clearly reminiscent of the anomalous flow during Northern European blocking events. We refer to EOF1 as NAO and EOF2 as the Blocking (BLO) pattern. These two EOFs are used to define a phase space in which the low frequency variability over the Euro-Atlantic region is characterised by the projection onto these two orthogonal patterns of NAO± and Blocking/anti-Blocking (trough over Scandinavia). This very simple view uses the same concept as the well-known MJO index of tropical variability from Weeler & Hendon (2004), where a two-dimensional phase space defined by the first two principal components (RMM1 and RMM2) of a combined fields (OLR zonal wind at 850 hPa and 200 hPa) averaged between 15°S and 15°N is used to represent the time evolution of the tropical organized convection associated with the MJO events. The NAO-BLO space explains 31% of the daily winter variability over Europe.
Link with extreme temperatures
The systematic relationship between the NAO-BLO space and the severe cold European spells is highlighted in Fig.2 where the distribution of the severe cold events, detected using the 2m temperature re-analysis, is represented in the NAO-BLO space. The re-analysis temperature data cover the November to February winter periods from 1980 to 2015. We identify severe cold events over 9 predefined regions covering the European domain (Fig.2a). A severe event is detected if daily-mean temperatures are cooler than the 10th percentile of the daily climate for at least 60% of the grid-points located over each individual region and if this criterion is satisfied for 4 consecutive days. The corresponding projection into the NAO-BLO phase space is determined from the re-analysis 500hPa geopotential field from the first day of the event. For each of the 9 regions an NAO-BLO diagram depicts all the events detected in that region. For the northern regions 2 and 3 the majority of the severe cold episodes correspond to NAO- events with rather large amplitudes. For region 1 an equal number of cold episodes are characterized by NAO- and anti-blocking type of circulation. For the Central and Southern regions, severe cold events are associated with NAO- and with blocking. The projections onto the NAO- and Blocking are generally very large indicating that this space is optimal to describe most of the severe cold episodes. For the Eastern European domain (regions 3,6,9) there are some cases with relatively smaller projections. This indicates that for these regions the NAO-BLO phase space is less optimal in representing the link with severe temperatures. However, overall Fig.2 shows that there is a strong link between large projections onto NAO- and BLO sectors and the occurrence of severe cold spell over Europe.
Forecast trajectories
An illustration of the evolution of the atmospheric flow and the forecast trajectories in the NAO-BLO space is given in Fig.3. The forecast trajectories from day 0 to day 10 of the ENS initiated on 23 February 2018 indicate a clear transition from a Scandinavian Blocking into a NAO- by day 5. Verifying analysis (black line) and the ensemble mean trajectories (red line) match quite closely and the evolution of forecast uncertainties (rather low in this case) is well depicted by the NAO-BLO phase space. Indeed, in late February early March, a spell of severe low temperatures with significant snowfall was associated with the establishment of a large pressure system over Scandinavia and later a development into a NAO- circulation.
Further reading:
Ferranti L, Magnusson L, Vitart F, Richardson DS. How far in advance can we predict changes in large-scale flow leading to severe cold conditions over Europe?
Q J R Meteorol Soc 2018;144:1788–1802. https://doi.org/10.1002/qj.3341