It looks like this is a good time to finish our look at the different weather patterns that have been affecting our weather this winter, because the weather pattern that has been dominating for most of the fall and winter appears to be undergoing a shift.
In the last article we continued our look at the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) and tried to picture how these two features can influence our weather. We left off by looking at how, if the NAO is positive, then it would seem that the AO would also be positive, but this is not always the case. If we have a positive NAO, that means both the Icelandic Low and the Azores High are stronger than usual. The AO is said to be positive when the upper low, or Arctic vortex, over the North Pole region is stronger than usual and at the same time the Azores High is stronger than usual.
So, if we have a positive NAO, then the Azores High is stronger than usual, but at the same time the Arctic vortex could be weak, which would result in a negative AO. Now this is when we have to start hurting our brains thinking about all the different combinations that can occur with just these two different circulation patterns and how each of these combinations can affect the weather.
As we have pointed out several times over the last month or so, both the NAO and AO have been very strongly positive for most of the fall and winter. We know that for both of these features, a positive index usually results in warmer weather for our part of the world. Since both were strongly positive, it’s not surprising that we had such a warm winter. Before we go on to why forecasters missed this when they created this winter’s long-range forecast, let’s examine some other combinations that could occur.
Let’s say that the NAO was positive but the AO was negative. One would be pushing our weather toward warmer-than-average conditions while the other would be pushing us toward colder than average. What would the end result be? That’s hard to say. The easy answer would be average weather conditions, and in the long run that might be correct. It’s possible that under these conditions, the weather might oscillate between periods of extreme warmth and cold. Much of it would depend on just how strong one of these features was compared to the other. So far we have been talking as though these features are either strongly positive or negative, but just like everything else that has do with weather, they are in a constant state of flux. That is, the strength of each pattern changes from day to day and week to week. So in reality, there are numerous different combinations of pressures that can occur, each of which can result in different overall weather patterns. It’s only when they stay in one type of pattern for an extended time, and that pattern is stronger than usual, that their influence on our weather becomes — if I dare use the term — predictable.
It’s the lack of predictability of these two patterns that makes using them for long-term weather forecasting difficult. With our current state of understanding of these circulation patterns, we can at best only forecast changes about two weeks in advance. So while we know they can possibly have a big impact on our weather, we just can’t reliably predict them far enough into the future to be of any use.
This is why forecasters tend to hang their forecasting hats on the atmospheric and oceanic circulation known as El Niño and La Niña. I have written about these features several times over the years and I don’t have time to go into any depth about them this week. The big reason forecasters put so much confidence in these features is the fact that they are a slowly changing circulation pattern we have now gotten pretty good at predicting even several months in advance.
When forecasters were trying to figure out what the long-range forecast was going to be for this winter, the Pacific was experiencing a moderate to strong La Niña and the long-range forecast for this feature showed it remaining in the moderate range for most of the winter and possibly into the spring. Now, as we are starting to enter spring, La Niña is exactly as the models predicted. So it’s understandable that forecasters would use this as their main controlling factor for our winter’s weather. Typically, when there is a moderate to strong La Niña, our part of the world sees colder-than-average conditions along with more snow, and this is what was forecast. What forecasters didn’t count on was the prolonged strong positive phase of both the NAO and AO, which basically overrode the effects of La Niña.
What will be interesting is to see is how the weather will play out over the next month or so. It appears the NAO and AO are now either negative or around neutral and have been in this state for at least the last two to four weeks. The two-week forecast for both of these continues to show them remaining neutral. With La Niña still in place, I wonder if we will finally see some of that cold, snowy weather we were supposed to be seeing all winter long.