Why can you ride powder with Christmas one winter, but is it as green as grass the other? That's an interesting question and the key to the answer can be found in the distribution of pressure across the Atlantic. There basically are two dominant pressure formations that regulate the weather. These are the famous high pressure are above the Azores (the H in the map below) and the low pressure area near Iceland (the L in the map below). Does this sound familiar to you?
High pressure above the Azores versus low pressure close to Iceland
But in reality, the Iceland low pressure area (L) and the Azores high pressure area (H) are not as static on a fixed location like in the map above. In addition, the pressure sometimes differ. Sometimes there's a huge pressure difference, sometimes there's a minimal pressure difference, but it's always in motion.
This fluctuation in the pressure differences is called the North Atlantic Oscillation (NAO). Meteorologists talk about the NAO index. The index that indicates continuous fluctuating pressure differences. Below you can see the NAO index for the past 100 years. Orange means that there was a positive NAO index, while blue indicates a negatiev index. Blue? Orange? Positive? Negative? And why would you want to know this? Well, it is clear that the NAO is particularly relevant for winter in central Europe.NAO index the past 100 years
Positive NAO index
A positive NAO index during winter means that the Azores high pressure area is very strong (as in: very high air pressure), and the same time the low pressure system close to Iceland is very strong as well (as in: very low air pressure). This situation results in a strong westerly current pushing (very) mild and moist air from the Atlantic towards Europe. This strong westerly jet stream brings storm after storm with lots of precipitation.
A positive NAO will bring snow to Norway
A negative NAO index during winter means that the low pressure area in Iceland and the high pressure area near the Azores aren't that powerful. The westerly jet stream is therefore less powerful resulting in less storms coming to European continent from the Atlantic Ocean. Winters with a mainly negative NAO index are often less warm, will keep moist and mild air at a distance and provide room for cold air from Russia. Looking at the cold(er) winters of the last few years, there was a negative NAO index during these winters.A negative NAO brings snow to the Alps
Alright. A very positive NAO ensures mild weather, high chances of snow in Norway and often too warm and dry weather in the Alps where the snow line is very high. A very negative NAO cuts off the supply of mild air on a regular basis and there is room for cold temperatures and snowfall deep into the valleys. This offers perspective. Right? But where can you find the current NAO index?
Follow it every day
The current NAO index and the outlook for the short and medium term can be followed daily on the website of the American NOAA, so you have a bit of an idea of what's coming.
Can we use the NAO index as a winter forecaster?
Until recently we assumed that we couldn't use to NAO index to predict what kind of winter we will get, but there is news. A study called 'Skillful long-range prediction of European and North American winters' was published in March 2014 and that shows perspective. The next couple of years will show to what extent the findings from this research will help us to calculate the NAO already for months in advance. Because if we are able to that, we have a better idea whether to ride in the Alps or search for powder in different mountain ranges.
**The NAO is an important parameter for our winter and there are studies, models and parameters available to assist us in a season forecast concerning the NAO. More about NAO in the next couple of weeks.