Thursday, October 1, 2015
4:08 p.m.
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| "It ain't the heat, it's the humility" - the late, great Yogi Berra |
At Sea-Tac, the last time we had a month that was cooler
than average for that month was February 2014. Since then, 20 months have
passed, with 19 consecutive months being warmer than normal. That's right...
this September was the first colder than normal month since February 2014.
We've broken some impressive records in that time. Our
records to Sea-Tac go back to January 1948, but within the span of these
months, we've broken two all-time records for hottest months (March 2015 and
June 2015). In addition, December 2014 and June and July 2015 broke records for
all-time highest maximum temperature, and October 2014 and February, March,
June, and July 2015 broke record for all-time highest minimum temperature. In
other words, these 19 months weren't just warm, they were scorching!
Why so warm?
Two answers: a massive and persistent ridge of high
pressure, and a larger-than-life blob of boiling-hot (OK, maybe not that hot,
2-3 degrees warmer than normal) water in the Northeast Pacific. With this
ridge, instead of getting cool, northwesterly flow from the Gulf of Alaska and
places to the Northwest, we got warmer flow originating from our south, and our
surface winds were often offshore, further warming us due to downslope
adiabatic warming effects off the Cascades (that's fancy terminology for the
phenomenon of air warming as it sinks). With the warm water, onshore flow off
the Pacific into our region was often at least 2 degrees higher than it would
have been due to the long time it spent over the Blob of warm water.
The picture above shows our sea-level-pressure compared to
normal values, and you can clearly see that we had higher than normal pressure
this summer. As you can see, we clearly had higher than normal pressure over
the Northeast Pacific this year.
This ridge of high pressure was associated with relatively
little mixing of the ocean, so the surface got nice and warm instead of having
this warmth being mixed down deep by storms and waves. Also, these ridges are
associated with sunny skies, so the sun heated the ocean.
I was unable to find out how to get a similar chart to this
for sea surface temperature (SST), but as you can see from the February-March
2015 SST anomaly chart below, the "Blob" is located in the same area
that the higher-than-normal pressure is located. This is not a coincidence! The
Blob is a direct result of this higher-than-normal pressure.
As you can see below, the Blob wasn't just present in 2015.
It was there in 2014 too; in fact, that is when it was first named. It was a
bit further west, and a bit more "blob-like" then (at least in my
opinion).
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| Credit: NOAA Earth Systems Research Laboratory: Physical Science Division |
So, why did this persistent ridge of high pressure (and the
resulting Blob) occur? There's been a lot of study concerning it, particularly
at the University of Washington, where Professor Nick Bond (great guy, had him
for my senior capstone forecasting course spring quarter) first came up with
the "Blob" name. It turns out that the high pressure looks to have
originated from atmospheric waves originating from the western Pacific. On a
larger time scale, the Pacific Decadal Oscillation, an oscillation in Pacific Ocean
temperatures on 20-30 year time scales that has atmospheric implications, may
be a factor. Finally, some studies like those by Diffenbaugh et al. (2015) have
hypothesized that we will see more of these "Ridiculously Resilient
Ridges" and therefore Blobs in a warming climate. This is likely due to a
weakening meridional temperature gradient, as the poles (particularly the North
Pole) are warming faster than the tropics and midlatitudes. This creates a
weaker "polar vortex" (yes, the infamous vortex you hear about on
CNN) and allows strong ridges of high pressure to occur in
some regions while other regions have large troughs and arctic outbreaks. When
the vortex is all wavy like this, it is weak and in a "highly amplified"
pattern. I know amplified = more powerful most of the time, but in this
context, it refers to a lot of ridging and troughing. A strong, non-amplified
polar vortex is said to be "zonal." And yes, our record breaking heat
and the eastern U.S.'s record breaking cold as of late are directly related...
we've had the ridge, and they've had the trough.
Thanks to the strong, potentially record-breaking El Nino,
the Blob is pretty much toast, and our winter won't be as warm as last winter.
I don't think we'll have 19 consecutive months of above average temperatures
soon, but as we go further into the 21st century and really start to feel the
effects of global warming, we'll shatter more and more heat records. Someday,
air conditioning may be a standard feature for Seattle homes. That’ll probably have to do more with our rapidly growing economy than rapidly warming climate though.
On that happy note, enjoy some cooler weather. Sunday should
be beautiful, but we should have typical early October weather for this time of
the year this coming week. Come late October, things will start to feel a lot different. The
transition to storm season is a lot more abrupt than the transition out of it!
Thanks for reading,
Charlie
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