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Prof Jennifer Francis gave last Friday’s Colloquium talk at UW,
discussing possible physical connections between Arctic Warming and
mid-latitude extreme weather: Hurricane Sandy, and all that.
Presently at Rutgers University, Francis earned her PhD at UW in 1995
[I think], working with Cliff Mass. To my embarrassment, she
remembered me, though I .. alas .. did not remember her [*]. She’s
slim, elegant, and competent. Not in that order.
Fact: the arctic is warming faster than the rest of the planet. A
consequence of this is that the temperature difference between the
pole and subtropical lower latitudes has lessened, particularly in
late summer and early fall. How does this affect the general
circulation?
First, a brief tutelage:
Francis and her colleague S.J. Vavrus of the University of Alaska,
Fairbanks, have defined and explored global and regional trends of an
interesting ‘index of wacky weather’ and meridional circulation [that
is, the North-South-ness of the winds], defined as:
MCI(x,y,t) = {|v| + v) / [v^2 + u^2]^1/2
= ‘Meridional Component Index’.
Here, ‘v’ is the northern component of the vector winds at ‘500mb’,
[ruffly, the pressure-altitude of the ‘jet stream’], and ‘u’ is the
eastern component of the same winds. Note that MCI varies over space
and time between -1 and +1, with the latter number indicating winds
blowing due north and the former due south.
Plots of the northern hemisphere winds show sinusoidal waves,
generally .. but not always .. traveling eastwards around the globe,
often with conspicuous triple maxima over the western coasts of
North America and Europe, and the eastern coast of Asia. The MCI
index quantifies the north-south components of the spatial and
temporal behavior of these waves in the jet stream.
End of tutelage.
Francis and her colleague have examined MCI indices in various
ways, and in particular Francis has globally averaged the yearly
cycles, plotted those averages over the last 25 years [as I recall],
and has detected .. faintly above the noise at present .. both a
positive displacement [the waves are shifting northwards] and a
positive trend [their amplitudes are increasing].
That is, the jet-stream is getting ‘more wiggly’, swinging both
further north and [slightly] further south, especially in late summer
and early fall, when Arctic ice-coverage is increasingly lessening.
Said in a different way, the positive lobe of the late fall ‘mode-3
Rossby wave’, normally centered a bit west of England, has on the
average moved a bit northwards, and the negative lobe, normally
centered a bit east of Florida, has moved slightly southwards.
This is exactly the pattern preceding ‘Sandy’, where a
‘blocking high’ over the northeastern Atlantic steered the
hurricane west of its traditionally eastward exit into the
central Atlantic, to the distress of eastern New Jersey and
southern New York.
Francis has provided the first ‘scientific’ explanation of Sandy’s
unusual trajectory in 2012, and the first semi-comprehensible
prediction that ‘Global Warming’ may indeed be driving the Atlantic’s
hurricane alley both later into the season and more westwards, across
the heavily populated eastern coastal US.
Caution: those plots of MCI vs year are pretty noisy and the
statistical significance of their trend is marginal. Mathematical
dynamicists and global modelers are currently exploring whether the
‘Sandy’ effect is a consequence of less arctic ice, with some
disagreement, as I understand. Still ..
Cheers,
Halstead
[*] I must be losing my .. losing my .. what is the name of that
thing we think with? [My excuse: Ms Francis never took my
splendid courses. That, and old age.]
> o__
> _,>/’_ o__,
> (*) \(*) /\/\