What happened to La Nina 2010?
I found this interesting, especially given the wailing and gnashing of teeth coming from the PNW of late. The graphics didn't cut and paste so you'll have to go to NOAA for that or avoid the techno babble and just skip down to the conclusions paragraph below.http://www.wrh.noaa.gov/otx/cases/La...aNina_2011.php
As we near the end of January 2011, some may be asking "What happened to La Nina?". First, a couple of points to clarify about La Nina.
The La Nina of 2010/11 has reached "Moderate" strength.
There is not a 1-to-1 relationship between the strength of La Nina and it's effects. This misinformation has been going around since the emergence of La Nina this past summer.
There are other atmospheric oscillations that also affect our weather patterns. Unfortunately, these patterns are:
Not linked to sea surface temperatures (like La Nina and El Nino).
As such, they are not as persistent as La Nina and El Nino. They vary on a multi-week time scale.
Unlike La Nina and El Nino, these atmospheric oscillations are not predictable beyond 10 days.
The current La Nina is expected to persist through the spring of 2011 and into the early summer. Some computer forecasts predict that La Nina will stick around into next winter.
One of the atmospheric oscillations is known as the Arctic Oscillation, or AO. Like most oscillations, the AO has positive, negative, and neutral phases. In the positive phase, atmospheric pressure in the Arctic is lower than normal, while pressure in the mid-latitudes (i.e. the U.S. and Europe) is higher than normal. This pattern tends to keep colder air closer to the north pole and prevents cold air intrusions into the mid-latitudes. The negative phase has the pressure anomalies reversed (higher pressure in the Arctic and lower pressure in the mid-latitudes). The result is more frequent cold air intrusions.
The time series below shows the phases of the AO since 1950. As you can see, the AO phase has been strongly negative last winter and this current winter. But notice that the two recent snowy La Nina winters in the Inland Northwest (2007/08 and 2008/09) were characterized by weakly positive AO.
So, while the presence of La Nina tips the scales in favor of a colder/wetter winter for the Northwest, it doesn't guarantee it. And one of the other variables (that we can't forecast) is the Arctic Oscillation. To see what affect the combination of the La Nina and AO has, look at the figures below.
In these images, the left 3 images are for La Nina (the "C" is for Cold Phase). The middle 3 images are Neutral, and the right 3 images are Warm (El Nino). The top images show the positive phase of the AO, the middle 3 the neutral AO, and the bottom 3 are the negative AO. Since we're currently in a La Nina, we'll just focus on the left 3 images.
The first figure shows the surface temperature anomalies for the Nov/Dec/Jan time frame. Notice that all 3 left panels (La Nina with AO+, AO neutral, and AO-) have warm or normal temperatures in the Northwest for the first half of the winter. Also note that the C AO- (which is what we've had this winter) shows colder than normal for the upper plains states.
The next figure below shows the same period, but precipitation anomalies. All three left panels show wetter than normal conditions in the Northwest. And indeed, it has been wetter than normal for the first half of the winter. Also note that for the C AO- the northeast US is wetter than normal, which again corresponds well.
OK, so now we've seen that La Nina coupled with the negative AO phase bring wet but near-normal temperature conditions to the Pacific Northwest, which has borne out fairly well so far. So where's the cold weather of La Nina? Let's look at the next 3 months, February through April.
The figure below shows the temperature anomalies for the Feb-Mar-Apr period. Note how markedly different they are for the Northwest compared to the first half of the winter. All three left panels (i.e. all 3 phases of the AO) show a marked cold anomaly for the Northwest for the latter half of winter and early spring.
And the corresponding precipitation anomalies are shown below. Here, we see quite a difference between the positive AO (upper left panel) and the negative phase (lower left panel). For the positive AO coupled with La Nina, we should expect wetter-than-normal conditions to continue in the Northwest. But for a negative AO, we actually see drier than normal conditions.
So, what does all this mean? Let's summarize what we've seen:
The current La Nina winter of 2010/11 has been linked with a negative phase of the Arctic Oscillation (AO).
The previous La Nina winters of 2007/08 and 2008/09 had positive AO phase.
The "cold" weather of La Nina is typically felt more in the 2nd half of winter and spring in the Northwest.
La Nina is wetter than normal in the Northwest for the first half of the winter. But in the 2nd half, it can actually be drier than normal if the AO is negative.
Of course, there are other oscillations (North Atlantic Oscillation, Antarctic Oscillation, Pacific-North American Pattern) that we haven't even discussed which also can have an effect on our weather patterns.
So what should we expect for the next few months? The odds are that we will see colder-than-normal temperatures. In most cases, we would also see wetter-than-normal conditions. But if the AO remains negative (which we can't forecast beyond about 10 days), drier-than-normal conditions are to be expected.
