Thursday, October 1, 2015
|“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).
|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,