Question Time

My recent posts have raised some interesting questions. I thought I'd tackle them as a post in itself such was their relevance.

Ok here goes, question number 1:

"Cool post Isabela. It seems that the NAO is really important in determining the formation of extra-tropical cyclones. You mentioned that anthropogenic climate change might be influencing the NAO. Despite the lack of unequivocal evidence, has a causal mechanism by which climate change influences the NAO been identified?"

Thanks for your question. A causal mechanism for NAO variability is a complex and controversial topic. These have been a number of interpretations and many have used GCM simulations under different parameterisation. Robertson et al (2000) suggest SST distribution plays a role in modulating both interannual variability and regional modes. It is certainly clear that SSTs are changing in the midst of a changing climate, too.

Others have suggested anthropogenic climate change may influence the strength of the stratospheric vortex and in turn the NAO. In both cases, though, predictive models lack skill and robustness. Modelling improvements will be key in attributing a causal mechanism.

Question 2:

"Looking at the NAO index diagram you have there also appears to be a prolonged positive phase between 1900 and 1920, were there an increased storm frequency then as well?"

Here's a reminder of exactly what we mean by a positive phase...

Firstly, an important point to note is that the term North Atlantic Oscillation was only coined in the 1920s by Sir Gilbert Walker. Walker's 1924 writings eluded to the traditional description of the NAO through correlation between pressure anomalies between the sub tropics and Iceland. Consequently uncertainty is induced through an incomplete knowledge of the atmospheric system itself, and, secondly, through limited recording of weather events. Nevertheless the period from ~1900 to 1930 did represent a period where a positive phase persisted over multiple winters. I recommend this paper by Andrade et al (2008). The authors look at historic records of storm frequency from the Azores region. This extract summarises some important conclusions from the paper:

"A number of major periods of contrasting storminess were identified. The first period (AD 1836–1870) is characterized by a distribution with two maxima and low yearly storm frequency (2–3 storms/yr). A second period extends between AD 1870 and 1920 and corresponds to an irregular distribution of storm frequency that decreases in time, maximum values reported in 1879 and 1886; the first years of this period record an abrupt peak of storminess (> 8 storms/yr) that drops to 1 storm/yr by the end of the 1911–1920 decade. A third period corresponds to the decades of AD 1920–1940 with uniform distribution and low storm frequency. After 1940 (period 4) a general trend of increasing storminess emerges (2–3 to 4–5 storms/yr on average) with maxima occurring in the middle of the 1980s."