It is about large temperature increases on a regional and local level of
maximum summer heat waves that are 6, 8, 10 degrees above your previous highs.
I.e. solar activity was high in most of the 20th centiry and then peaked in about 1985, together with a 20 - 30 year
heat lag (since it remained high until 1996 as well), and oceans take a few decades to equilbrate, (the same as
summer takes about 6 weeks to reach
maximum temperature after the
summer solstice, and every day it takes a few hours after noon to reach
maximum temperature), so the earth has taken a few decades to reach
maximum temperature after the long high in solar activity during the 20th century, and will now go down in temperature over the next few decades, with now both a negative PDO, and reduced solar activity.