Now, the NAO

In a recent post, we discussed the factors that may be driving the North Atlantic Oscillation up or down, as opposite  trends have been put forward to explain droughts in the Mediterranean and the unusual behaviour of hurricane Sandy . What kind of simulation could provide an answer ?

First of all, some lengthy, but hopefully interesting background- you may need to read our previous post to better get the context. The North Atlantic Oscillation index summarizes a picture the winter-to-winter climate variability in the North Atlantic European sector, ranging from the intensity of the westerly winds, through temperature in Scandinavia and precipitation in the Mediterranean. A more positive index in a single month or winter season indicates that westerly winds have been stronger and Mediterranean precipitation has been less than usual. Basically, the NAO describes the strength of the Azores high pressure system.

So far, climate simulations so far tend to show that the NAO should turn more positive due to greenhouse gas forcing. More recently, a few studies (see here for a popular-scientific story)  suggested that the retreat of sea-ice in the Arctic in autumn would induce anomalous heat fluxed from the ocean that in turn would push the NAO to a more negative state. So we have two opposing pulls both originated in the increase of atmospheric concentrations of greenhouse gases. Let us have a look first at the NAO index observed since 1820.

 Perhaps the best description of this figure is that it is hard to say that something spectacular is happening recently. There is an upward trend from 1900 until 1960 followed by a downward trend thereafter. The interannual variations are very strong and we find extreme positive and negative values distributed along the whole period. The upward trend encompass a period of global rising temperatures from 1900 until 1945 and a period of cooling tin 1960-1975 and warming temperatures from 1975 onwards. So at first sight it is difficult to ascribe the NAO variations in October to global temperature variations and thus to diminishing sea-ice. It is also noteworthy that the negative trend in the last 30 years, which is linked to increasing precipitation in the Mediterranean is not compatible with the future projections derived from climate models included in the IPCC 2007 report. This has been highlighted in a recent paper by one of our students, Armineh Barkhordarian. Essentially, all models predict decreasing Mediterranean precipitation in autumn, whereas observartions indicate an increase in the last 30 years.
A view of the NAO index simulated in a simulation with the model ECHAM4-HOPE (ECHO) over the past millennium and in the this century under the most pessimistic scenario of greenhouse emissions neatly illustrates this:

We see again very large interannual variations but a certain tendency for the NAO to be in a more positive state in periods with stronger external forcing, such as the Mediaeval Warm Period and the 20^th century, and even more clearly in the 21^st century. This behaviour is quite typical of the models included in the IPCC 2007 report. It was too much work for this post to calculate the NAO index for all models for just the month of October, but the winter mean index has been analysed before

by others:

Not all models display a clear trend in the future, but if a trend is present, it is positive. I have tested that for a few models that exhibit a trend, the behaviour of the simulated October NAO index is consistent with the simulated whole-winter NAO. In particular, the model ECHO-G and the model ECHAM5-OM, both being sequential versions of the same model ECHAM, display the same NAO trend.

Finally, let us consider the model ECHAM6-OM, also known as model MPI-ESM-LR, which is already part of the suite of models included the forthcoming IPCC 2013 report. This result turned to be quite surprising. The figure below shows the October NAO index obtained in a scenario simulation driven by the most pessimistic scenario RCP 8.5, which  is roughly comparable to the previous SRES scenario A2:

We have three simulations with the MPI-ESM model driven by the same scenario. The difference among the simulations are the initial conditions in 1^st January 2006. This small ensemble would roughly represent the uncertainty due to internal climate variability, distinct and independent  in simulation, as opposed to the common signal generated by the external forcing.

The model ECHAM6 shows a quite different behaviour. The NAO index in October shows almost no trend until about 2080, and a negative trend thereafter, when the greenhouse gas forcing becomes really strong It seems thus that the NAO in this model is less sensitive to the external forcing, and that the NAO reaction is opposed to the one simulated by ECHAM5 and ECHAM4. This is not the first case of a flip in the NAO trend when a new version of a climate model is introduced. In an somewhat older paper we already noticed that the British model HadCM2 and the German model ECHAM3 model produced opposite NAO trends when driven by increasing concentrations of greenhouse gases, with ECHAM3 later agreeing with the  subsequent version of the  British model HadCM3.

Even more intriguing is now the fact that the winter NAO index, i.e. the mean over October to March, and not only picking the October month,   does not display any discernible  trend in the MPI-ESM simulations:

And now the question for you, interested reader: what type of simulation should be conducted to disentangle the influence of the 'direct' greenhouse gas forcing and of the indirect influence of diminishing Arctic sea-ice cover ?