It has been a strange winter so far, a lot of it due to La Nina. This blog will discuss the mechanics of La Nina, how it has impacted our weather in Alabama, and what we expect for the rest of Winter.
After a terrible, sudden onset drought kept us almost completely without rain for 3 months in the autumn, until late November, we have finally started to get some rainier weather the past few weeks, and the rain events last weekend, where most of us received 3-5" of rain, helped to lessen the drought significantly. Much of Alabama that was in stage 3 drought last week is now down to stage 2.
The drought is improving any way you look at it. In mid-November our 30-day and 60-day running tallies of rainfall were at zero, and our 90-day running rainfall got down to 0.69" on November 27. Now, our 60-day is 10.92", so our rain over the past 2 months has been about normal, and a month or two more of normal rainfall should get us out of the drought. Rivers are already flowing more normally, lakes are filling back up, and soil moisture is getting better.
2. Warm temperatures
It has also been much warmer than normal for winter here in Alabama. Some of that has been due to the lack of rain, rainy days, and water in the ground to absorb solar energy, but we have not seen the big cold fronts as often as we normally do, and the surges of warm air have been much more frequent than normal. Since Nov. 1, our average temperature has been 53.9 degrees in Birmingham, about 6 degrees above normal. That despite an ice and snow event on January 6, followed by lows of 17 and 13 degrees and a high temperature on 1/7 of only 31 degrees. Since November 1, we have had 33 days with high temperatures of 70 or higher, compared to 14 normally for that period, and the third most on record for Nov-Jan. We have dropped below freezing 17 nights, compared to a normal of 29 for the winter through Jan 25. To kill bugs and help tree health in the Summer, we need some more cold weather this winter.
3. La Nina
Our warm, somewhat dry winter has been largely due to La Nina. It is the opposite of El Nino. In La Nina, the easterly trade winds over the Pacific blow faster than normal (see snow in Hawaii, etc.), upwelling cold water. Since last summer, sea surface temperatures have been below normal in the equatorial waters in the central and eastern Pacific (white area in upper-right corner is Mexico).
The stronger than normal east to west trade winds are moving in a direction opposite that of the earth's rotation (west to east), so this lessens the atmosphere's angular momentum. The earth tries to drag the atmosphere with it, so one way that the atmosphere can balance this loss of angular momentum during a La Nina is by creating stronger than normal jet streams at midlatitudes (like in North America). This is exactly what has happened, as shown below by the anomaly (or departure from normal) in westerly winds aloft (300 mb, or about 30,000 feet). Notice the large negative anomaly (indicating abnormally easterly winds) in the tropics, and the large positive anomaly (indicating abnormally large westerly jet stream winds) over the US.
The jet stream generally separates cold air from warm air, so with a stronger than normal jet stream, there have been bitterly cold intrusions of air into the northern and western U.S., but the jet stream has prevented a lot of the cold air from making it into the southern and eastern U.S. One can see the temperature anomalies for the winter so far. Temperatures have averaged about 4 C (8 F) below normal in the northern and western US, and 2-3 C (4-6 F) above normal over the southeastern U.S. This has caused large temperature gradients over the country, therefore a strong jet stream, and helps offset the lack of angular momentum in the atmosphere caused by the strong trade winds in the Pacific.
4. What is coming?
It appears that the La Nina is weakening, with eastern Pacific waters already warming and trade winds subsiding. We can look at the temperature anomalies in the upper 1000 feet of the ocean in the eastern Pacific to look for what is coming. And that shows the La Nina should end in the next month or so, as upper-ocean cold water anomalies have decreased rapidly.
If La Nina is ending, it may mean the jet stream will let up somewhat over North America, especially by late February. and allow Arctic air to plunge farther south more often, and more rain-producing systems. But, the days are already getting longer and the sun is getting stronger over North America now, so opportunities for very cold air and snow start to dwindle in late February and March.
In the short-term, it is getting colder over the next few days, with below normal temperatures over the southeast U.S. and above normal temperatures to the north. This is indicative of a weaker jet stream. For now anyway.
But, in 2 weeks or so, we see the same pattern showing up in computer models of extreme cold to our north yet mild air here, indicating that the strong La Nina jet stream may not be letting go yet.
Expect colder temperatures the next few days, then a warm up next week. As far as the rest of Winter goes, we don't know for sure yet, but the weather pattern may be headed toward something more normal over the US, maybe in time for a late season Arctic blast or two here in Alabama. Late season Arctic blasts are generally not as cold as those in December and January, so that could be a good thing! (Or, could we get an April snow like we did in 1987? Probably not.)
Dr. Tim Coleman