A rather surprising question perhaps, but the answer is, ‘it depends’. There is no climatic reason to define a year from January to December, but that is what is generally done. But, is this the best definition?
What is ‘best’ probably depends on the purpose. For example, El Nino events tend to run roughly from May to May. An annual mean defined by Jan-Dec will not therefore capture the essence of the event, and will likely include cancelling anomalies. The figure below shows monthly Nino3.4 SSTs (from the HadISST dataset) in grey, and the annual means in black, using Jan-Dec (top) and June-May (bottom). Note the large difference in the variance of the annual means as labelled.
Why does the choice of definition matter? Some possible reasons include:
- Choosing appropriate metrics of inter-annual variability?
- Assessing predictability?
- For detection & attribution of recent climate trends?
For this last reason, the least variance definition may be best, so as to increase the signal-to-noise. For the other reasons the maximum variance may be ‘best’. It is also possible to see that it is not just SSTs which demonstrate such a different in variance depending on the annual definition – it can also be seen in temperatures over land (not shown).