Using the blocking index of Tibaldi and
Molteni (1990) we can consider the frequency of DJF "blocked days" for Neutral, Warm
and Cold episodes as defined by the magnitude of the SST anomalies in the tropical Pacific. Cold
episodes are defined as [C+,C], warm episodes [W+,W], and neutral episodes as [W-,C-,N]. For each panel the
number of DJF seasons is shown in square brackets. We have also included in the middle panel on the right-hand
side a plot of the neutral episodes defined simply as [N]. This can be compared with the middle left-hand side
panel where neutral episodes have been defined as [W-,C-,N]. In each panel the DJF 51 year average has been
plotted in red for ease of comparison.