The NAO - natural variability vs forcing factors

Since we know that the NAO exerts a big influence on the wintertime climate over the Euro-Atlantic part of the hemisphere, one might think that winter warming in recent times is due to this natural mode of variability and that we’ve overestimated the effects of anthropogenic greenhouse gasses (GHGs) in the atmosphere. In my last post I talked about the pronounced positive trend in the NAO index and questioned whether this is part of its natural variability or if the NAO might be influenced by external forcing.

I’ve read a paper by Stephenson et al. (2006) that is trying to answer this question by looking at the response of wintertime NAO to increasing concentrations of atmospheric carbon dioxide. In the study they’ve used 18 global coupled general circulation models (GCMs) with a 1% per year increase in concentrations of CO₂. 15 of the models were able to simulate the NAO pressure dipole, which is one of its main features, but none of the models were able to reproduce a decadal trend as strong as that observed in its later time series. 14 out of those 15 models simulated an increasing trend in the NAO index with increasing CO₂ concentrations, but the magnitude of the response was generally small and highly model-dependent. Despite their different NAO responses, all the models showed a similar increasing NAO-like pattern in temperature and precipitation trends with increasing CO₂ concentrations, like warming and increasing precipitation over Northern Europe (which is fitting with my memories of “bad” Norwegian winters).

                   Norway; what I would call a "good" winter....................and a "bad" winter

Although the models in the study do suggest that the NAO show a weak positive response to increasing amounts of CO₂, the authors note that with the large amounts of model uncertainty as they found in this study “one has to be exceedingly careful about making inferences concerning future climate change”. Since the models simulate differences in NAO response, but similar responses in temperature and precipitation over Europe, the authors suggest that NAO is not the key determining factor for such changes.

So it seems like we cannot blame warming over Europe solely on the NAO's natural variability. There are so many factors interacting in the climate system and it is hard to know what is responsible for what. But it looks like there are external forcing mechanisms, including anthropogenic greenhouse gasses, contributing to the extreme winter climate over recent decades.

Various studies have investigated NAO trends and ended up with similar results. I would like to quote one of them; “[…]most authors agree that greenhouse gases are likely to be at least partly responsible for the long-term trend in the boreal winter NAO index”, Gillett et al. 2003. It is clear though, that a lot more research needs to be done on this subject to understand to what extent anthropogenic forcing from greenhouse gasses influence wintertime climate over Europe.

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!

The North Atlantic Oscillation

I thought I would dedicate this second blog post to the North Atlantic Oscillation (NAO)– one of the leading modes of natural variability in the Northern Hemisphere (NH). It is important to get to know this phenomenon a bit better when looking into what is causing the extreme weather and climate around the North Atlantic basin, and that because the NAO has shown to exert a strong influence on the climate over large parts of the Northern Hemisphere.

The NAO is characterized by a fluctuation in sea level pressure (SLP) between the Arctic basin and the mid-latitudes. The differences in SLP drive a westerly flow that moves through the high- and low pressure systems, creating a zonal planetary-scale wave pattern across the Northern Hemisphere. Therefore the NAO is also characterized by an out-of-phase relation in the strength of this zonal flow along ~55° and 35°. The westerly flow is strongest during winter when the largest anomalies in SLP occur, as you can see in the figure below (from Hurrell et. al). Dark (light) shading indicate negative (positive) departures from the mean . So the characters of the NAO are most pronounced during the Northern Hemisphere winter months (boreal winter).

Where these high- and low-pressure systems are located, and how strong they are, determine the phase or the index of the NAO. All results are based on the winter months when the variability is largest. A high index polarity is defined as anomalous strong subpolar westerlies, with a deeper than normal low pressure over the polar region and a higher than normal subtropical high pressure . A low index polarity is defined as anomalous weak westerlies and pressure systems. A consequence of these contrasting polarities is that anomalies in climate on seasonal time scales typically occur over large geographical regions.  These anomalies in climate include surface air temperature, SST, changes in storminess and precipitation, ocean heat content, ocean currents and their related heat transport, and sea ice cover.

High index conditions are characterized by a northeastward shift and an increased intensity in the stormtrack across the North Atlantic, from northeastern North America to northern Europe, which tend to give wet and mild conditions in these areas. Stronger northerly winds over Greenland and northeastern Canada carry cold air southward and decrease land temperatures and SST over the northwest Atlantic. Warming over North America associated with the stronger clockwise flow around the subtropical Atlantic high-pressure center is also notable. Evaporation exceeds precipitation over much of Greenland and the Canadian Arctic during high NAO index winters. Drier conditions also occur over much of central and southern Europe, the Mediterranean and parts of the Middle East, whereas more precipitation than normal falls from Iceland through Scandinavia. During low index periods the weaker (and fewer) winter storms crossing the Atlantic on a more west-easterly path bring moist air into the areas surrounding the Mediterranean. Northern Europe and the eastern part of the US experiences cold air outbreaks and hence snowy weather conditions. Below you can see a (much generalized) picture of the climate patterns created by the NAO in its different phases (found on Google).

Phu, I know that was a lot in one post, but it covers the most essential information about the NAO, which is important to know in order to be able to look at how anthropogenic activity is influencing our weather versus these natural modes of variability. In my next post I will look into if/ how anthropogenic activity is influencing the NAO and what consequences that may have/are having on our weather and climate.

If you're hungry for more or want a more detailed overview of the NAO, I suggest you read the paper I already referenced to; An Owerview of the North Atlantic Oscillation.

My very first post!

Welcome to my blog! Here I will be posting news stories, scientific research and personal thoughts about the topic of the blog. I am going to discuss whether or not anthropogenic activity has got anything to do with the extreme weather events that have occurred in Europe and North America over the past years, like wet summers, flooding, heat waves, droughts, extreme winters, snow chaos and storms, or if it is solely due to the natural modes of variability in the climate system.

For example, this summer has been a season of record braking extreme weather. It has been the wettest summer in the UK for 100 years, and it has generally been an unusually wet summer across northern and central Europe. In the Black Sea region of Russia at least 103 people were killed by intense flooding caused by sudden heavy rainfall. In the US nearly two thirds of the nation experienced some level of drought, with 39 percent of the nation suffering from severe to extreme drought, destroying farmers’ crops and livestock. Colorado experienced its worst wildfire season in a decade, with half a dozen lives lost. In June Florida was hit by tropical storm Debby, which caused extensive flooding, several tornadoes, and high winds.

What is up with this weather? Can we blame it on ourselves and our emissions? How are the predictions for the future? Is it only going to get worse? These are questions I’ll be discussing further on this blog, and I hope you will find it interesting.

I will leave you with this video from the Global Climate News channel on YouTube that is trying to explain one mechanism behind the extreme climate of 2012. Although focusing on America, it will give a taste of what this blog will be about.