I noticed this graphic of global temperatures for the last winter [DJF] that NASA posted in their Twitter feed and thought it looked overly warm for IONA when anomalies this winter had been negative particularly in the north. I generated my own anomaly chart using reanalysis data which I also download from NASA. The difference in the anomalies is that I use 1981-2010 long term average to calculate them with, whilst NASA, for some unknown reason only know to themselves, use the 1951-1980 averages. By doing this it has the effect of shrinking any cooler areas quite dramatically, in fact it would give you the distinct impression that this winter as a whole had been mild across IONA when it wasn’t. I think the guidance from the WMO about long term averages – not that anyone listens to them – is that for climatological purposes the period 1961-1990 should be used. At the moment of the seven global temperature datasets two use the period 1951-1980, one the 1971-2000, three the 1981-2010, and two use the 1991-2020. You would think that between them they could agree on the same period that they all could use. With regard to this particular chart the climate skeptic could rightly complain that NASA does this to exacerbate the warming that’s gone on since the relatively cold thirty year period between 1951 and 1980 and now, and I for one wouldn’t blame them.
The note at the bottom of the NASA graphic is curious. They say that ocean data is not used over land which you would assume is how it should be. They also add that ocean data is not used within a 100 km of a reporting land station. They may do that to help in the kriging of the observational data, but I assume this means that where prevailing winds are off the ocean, a strip of sea 100 km wide will have values that are interpolated with regard to land observations rather than the sea surface temperature. I just hope that this is not how they calculate their gridded reanalysis 2 metre temperatures.