Anyone who has had the misfortune of chatting to me since the cold winter of 2009-10 may remember me going on and on about how this was all to do with something called the North Atlantic Oscillation (NAO). Well, my North Atlantic Climate science is a bit rusty (sorry Tavi), but it seems that some proper scientists have come to my rescue. NASA have released an image which shows temperature anomalies for the North Atlantic area for the beginning of December. The measurements are from the Moderate Resolution Imaging Spectroradiometer (MODIS) which is aboard NASA’s Aqua satellite. The image is shown below.
The image shows the deviation in temperature between the 3rd-10th December 2010 from the average during the previous 8 years. It is clear that Northern Europe and the East coast of the USA have experienced much colder temperatures this year. Interestingly, Southern Greenland has experienced much warmer temperatures. The scientists at NASA attribute this to the Arctic Oscillation (AO).
Arctic Oscillation (AO) is the dominant pattern of non-seasonal sea-level pressure variations north of 20N. Its two states are described below:
- Positive Arctic Oscillation (AO+) would give lower than normal Arctic Air pressures and this would result in higher than normal temperatures in N.Europe and N. America.
- Negative AO (AO-) would give higher than normal Arctic air pressures and therefore lower temperatures over N. Europe and N. America.
The general AO has fluctuated over the past couple of centuries, but has been predominantly positive from around 1980 (1,2). NASA attribute the very cold December we have had this year to a strongly AO-.
North Atlantic Oscillation
North Atlantic Oscillation (NAO) is pretty similar to AO and some scientists believe the AO and NAO to be closely linked. The NAO describes the difference in pressures between the Azores High pressure zone and the Icelandic Low pressure zone. It also has two states:
- NAO+ this is the result of large differences so a strong Azores High and an intense Icelandic Low. A NAO+ would result in cool summers and mild wet winters over Western Europe.
- NOA- this is the result of small differences so weak Azores Highs and weak Icelandic Lows. The NAO- results in cold winters in Northern Europe as storms track further south towards S. Europe and Africa.
So what happened last year? Well the cold weather of the winter 2009-10 was also put down to a negative AO . However, it is not as simple as saying one thing was, or is, responsible for the colder winters. Solar activity was also cited as a reason for the cold winter of 2009-10 (3). Mike Lockwood of the University of Reading noticed that climate discrepancies such as extreme cold periods during periods when, overall, it was warmer than normal were more probably during periods of low solar activity. Others suggest that the cold winter was the result of synchronous El Nino and NAO- events (4) .
That’s all very nice but i suppose the question that most will ask is “are we to expect more cold harsh winters in the next few years?” This question is very difficult to answer and i am in no way qualified to do so. So, i am not going to make any predictions, that can be quite a dangerous thing to do. Richard Seager, a meteorologist with the Lamont Doherty Earth Observatory at Columbia University in the US said that having an El Nino event and a strong negative NAO was “a once in a century”(5) event. The previous time this may have happened was in 1783-84. Researchers debate whether the 1783-84 event was the result of an El Nino/-NAO or a massive volcanic eruption in Iceland. You see, volcanic eruptions pump massive amounts of particles high into the atmosphere and this can result in a cooling of global temperatures. Ahhh, have you seen where i am going with this? Can anyone remember what happened in May 2010? This is why climate science is so tricky. There are so many things that may affect climate and the weather.
Thanks to NASA for the great image. If you want to get hold of the large (25Mb tiff) then you can find it on this NASA page. I have added some below that i have used to construct this post. If you are interested in finding out more about why we have been having such cold weather they are a good place to start. However, the subject area is pretty complicated. What i always like to remember is that, glaciologically speaking, we are still technically in an ice age as we still have significant bodies of ice in both hemispheres.
NewsSecond > Falling Snow in Australia in the Summer
(1) – http://nsidc.org/arcticmet/patterns/arctic_oscillation.html
(2) – (5) Thompson, D. W. J., and J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett., 25, No. 9, 1297-1300.
(3) – M Lockwood et al 2010 Environ. Res. Lett. 5 024001 (doi: 10.1088/1748-9326/5/2/024001)
(4) – R. Seager, Y. Kushnir, J. Nakamura, M. Ting and N. Naik, 2010: Northern hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10. Geophys. Res. Lett.,Vol. 37, L14703, doi:10.1029/2010GL043830.
(5) – http://www.bbc.co.uk/news/science-environment-11152077
Footnote on how the image was made.
As the post seems to have rambled into a discussion of climate science, this foot note tries to get back to the GIS and remote sensing behind it. To produce the image shown in the post, NASA will have captured data over the landmasses surrounding the North Atlantic. The MODIS sensor on the Aqua Satellite can sample the entire Earth in 1-2 days. Data will have been captured between the 3rd-10th December in each of the study years (2002-10). Averages will have been derived for the period 2002-09 which are then used to calculate the anomalies within the 2010 data. This exercise requires a massive amount of data to produce one image, but this image is significant as it shows how temperature fluctuations vary over very large areas.