Friday, February 5, 2010
Why was January so warm? A detective story.
As I mentioned in an earlier blog, January 2010 was the warmest on record in Seattle (back to 1891) and one of the warmest years at virtually every western Washington site with long records. This is a major record. The big question is why? What was so extraordinarily unusual last month?
Yes, it was an El Nino January and they tend to be warm...but why should this be the warmest of ALL of them?
Global warming? No way, the East Coast is having a cold year! And in any case, the West Coast is the place where global warming is weakest and most delayed in the U.S. (reason--our proximity to the slow to warm eastern Pacific).
No it is something else...the unusual alignment of several factors the provides the explanation. And the answer also explains why yesterday got to 57 and today 58F!
This is an interesting detective story. We start with a year with general El Nino conditions--one in which cold air is pushed way to our east. But it took something else--in fact two something elses.
Look at the temperature and precipitation records below you notice a few things. First, the temperatures often were higher than than the average maximum, and we NEVER dropped to the average minimum. The first half of the month was fairly warm and very wet. Clouds held in, preventing infrared nighttime cooling. Southwesterly flow brought in warm air (yes, sometimes we had pineapple express conditions). Hawaii came to you.
But then we dried out and then the temperatures got even WARMER. And is was this spike to April temperatures between January 15-21th that really threw us over. It was also during this time that low pressure records were set over much of the west. The jet stream was way south of us...over CA and northern Mexico. Normally, we are cold when the jet is that far south. What could have done it?
The answer is revealed in the next two weather charts. They are both for 4 PM on January 17th (0000 GMT January 18). The first is for a level about 5000 ft above the surface (850 mb) and the second shows sea level pressure at the same time. The pattern they show was very persistent during period of the warmest temperature. You will notice a low to the southwest of Washington and strongest flow going into California. Wind flows roughly parallel to the solid lines, with southwesterly flow going into CA and southern Oregon, and southeasterly flow over us.
That was the secret! Warm air was moving into off the ocean, circling around to our south and east and then moving into western WA from the SE. As the air descended the Cascades, the air further warmed by compression. If the low had shifted south or north we wouldn't have the perfect combination of southerly flow that sank over us and warmed us further.
And as long as we are talking about heat, January 2010 was one of the warmest on record worldwide based on satellite sensors. Check out the figure:
Why so warm? A major factor is the moderate El Nino we are in. Plus, the earth is relatively warm from the global warming of the past several decades.
And another major item: the sunspots are back on the sun. There are now 22 of them. There was a lot of concern when sunspots didn't return on schedule, but belatedly the sunspot cycle has started.
Posted by Cliff Mass at 8:52 PM