The warm Summer and our warm and dry Autumn has been largely a result of the
persistent La Niña across the Pacific Ocean. This La Niña has been quite
unique because instead of fading away during the transition season
(Autumn/Spring) it actually strengthened the cold SST ( sea surface temperature)
signal across the central Pacific to the coldest levels in a May period since the 1950s!
So what is going on? Does Mother Nature have a La Nina Hangover that she
just can't seem to shake off. First of all, what is La Niña?
Check out my video explainer on La Niña.
In a nutshell it is cooler than normal seas across the Eastern
Equatorial Pacific. In addition, seas are warmer than normal further
West. Trade winds across the equator that blow from East to West are
stronger than normal. These subtle changes have a big effect on global
weather. I like to think of La Niña as a big high pressure machine that
fuel injects passing highs and also causes their tracks around the globe
move further South (or North in the Northern Hemisphere) compared to their
mean position.
So what's the hold-up? Why won't La Niña just fade away?
Wave motions in the Pacific Ocean as well as motions in the atmosphere
above are keeping the La Niña machine going (for the time being) and it
has even lingered longer than some of the seasonal climate models
had predicted months ago. The current projections are still a bit
mixed - with some models saying the La Niña will fade to neutral (neither
La Niña or El Nino) but others are keeping La Niña in place for perhaps
another 6 months or so.
Another warm Winter for New Zealand ? Yes, quite possibly, especially with
the lingering effects of the marine heat wave in the surrounding seas. This
warmer than normal air will help to temper any of the Antarctic blasts
that head our way in the short term. One thing worth pointing out with
the lingering La Niña is to watch out for a few surprises from the Southern
Ocean......
The squeeze zone where the bigger anticyclones meet the seasonal depressions from the South can at times bring some active spells of rain, gales and even snow.
It can't be warm all the time!