This has been an unusually warm summer here in the Pacific Northwest. But why?
Several people have suggested the cause is a pool of unusually warm water over the northeast Pacific, one my colleague Nick Bond has amusingly termed "the blob." How did this blob develop and what are the implications?
As noted above, the blob is a region of unusually warm water off the Northwest coast. Here is the difference from normal sea surface temperature (the SST anomaly) for this summer (courtesy of NOAA). Can you see the orange area off our coast, that is the "blob". It represents an area of water about 2-3 C warmer than normal.
The blob developed last fall and early winter, as illustrated by the sea surface temperature anomaly maps for September 26, 2013 and January 23, 2014, shown below.
The warm blob developed last winter as a result of the unusual persistence of high pressure over the eastern Pacific. Here is difference from normal for sea level pressure for Sept to December 2013. WOW! A huge anomaly with the pressure over the northeast Pacific being over 6 millibars (aka hPa) above normal. Folks, that is a big anomaly. And it was associated with our dry fall and early winter last year.
So now you know the origin of the blob: the impact of unusual fall high pressure. But how does that warm the air over us?
During the summer, the East Pacific High builds northward in the eastern Pacific and the air that reaches us on the typical summer day comes off the Pacific. To prove this, here is the trajectory of air ending over Seattle on August 29th. It was over the blob a few days earlier!
The temperature of the low-level air is greatly influenced by the sea surface temperature and thus warmer water tends to cause the air reaching us to warm. The temperature anomaly of the blog is 2-3C (roughly 4-5F) , which is quite similar to the degree of warming above normal this summer over much of the Northwest (see graphic)
A number of you have complained about the humidity being unusually high this summer. Blame the blob. Warmer air can acquire more water vapor than cooler air and the amount of water vapor air can hold increases VERY rapidly with temperature.
There is, of course, a lot of interest in whether the warm temperatures this summer were associated with global warming. It does not appear so. The warming was caused by anomalous high pressure over the eastern Pacific last fall and there is no reason to believe that has anything to do with warming the planet from the greenhouse gases mankind emits.
One final note: the persistent warm air was a bonanza for extended range weather forecasters. With the warm water holding in place, our long-range computer models, such as the Climate Forecast System (CFS) accurately predicted the warm summer months earlier. Don't believe me? Here is the proof: the June-July-August surface temperature anomaly forecast made in March! The reds are 1-2 C warmer than normal. Impressive skill.