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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.
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