Recent trend in the NAO index

Below you can see the winter station-based index of the NAO, based on the difference of normalized sea level pressure (SLP) between Lisbon, Portugal and Reykjavik, Iceland. Red bars indicate positive index and blue bars negative index. (Figure found here).

As you can see from this figure, over recent decades the winter index has exhibited a pronounced trend toward a strong positive index, especially prominent around year 1990. As described in my last blog post, this positive trend corresponds to lowered surface pressure over the Arctic and increased surface pressure over the subtropical North Atlantic, with anomalously strong westerlies. I also talked about the related climatic conditions and weather patterns.

I am originally from Norway, a country thought of being a freezing cold, snow-covered place during winter. But as I look back, I have a hard time remembering those really snowy, proper winters where we could go skiing all winter long. I mostly remember grey skies, drizzling rain, and "slush" snow in the streets, which obviously is a consequence of the recent positive trend of the NAO, which typically give northern Europe mild and wet winter conditions.

So it is pretty clear that the NAO controls, in large parts, climate conditions during winter around the North-Atlantic basin. But don’t you find this recent positive index phase a bit striking? Yes, the NAO is a natural mode of climate variability, but why does it now exhibit this upward trend, prominent in its time series? To me this seems a bit “unnatural”. Are there external factors, such as anthropogenic forcing, influencing the NAO and causing this positive trend? These are questions I will be discussing in my next blog post, so stay tuned!