The colder than normal, if not
harsh winter in South Asia can be attributed to the combined effect of La Niña
and the Indian Ocean Dipole (IOD), oceanic processes in the Central Pacific and
Indian Ocean, respectively. Both factors tend to draw the cold air of higher
latitudes to the tropical and sub-tropical Asia.
The La Niña appeared to have
peaked this month with NIÑO3.4 values at 0.8 deg C and Southern Oscillation Index
(SOI) value at +12.8. The La Niña has started to markedly weaken with most
models suggesting an end of the event during the coming autumn season. But as
the late John Daly, a great among climatologists, observed, the Southern
Oscillation is the primary driver of year-to-year global temperature, with a 6
to 9 month lag time.
Accordingly, while the La Niña
has peaked and weakening, its impact on global temperature is just starting to
kick in. We are now seeing the La Niña effect beginning to be the prime driver
of winter in the Northern Hemisphere. Winter had been mild within the mid-latitudes
till mid January because Arctic Oscillation (AO) and its cousin North Atlantic
Oscillation (NAO) had been in their positive phases. But a pattern change has
taken place with winter intensifying despite the AO and NAO remaining
largely positive as the La Niña is beginning to swamp their effect.
Joe D'Aleo reports: "The
Northern Hemisphere came hard out of the box early with well above normal snow.
Then a strng [polar vortex] took over and preventing the snow from expanding
very far into the lower 48 and west into Europe. A breakdown of that vortex is
underway and snow has started to fall in the northern US and in Europe. Here is
the current snowpack."
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