¥The data are monthly mean SLP for winter, from 1948 to 2000, on a 2.5¡ x 2.5¡ grid
for the NH north of 20¡N from the
NCEP/NCAR reanalysis
¥Some preprocessing has taken place – removing annual cycle - and area
weighting based on the square root of the cosine
of latitude has been used, but these details need not
concern us here