I have been posting on ways of dealing with cells in the HADCRUT 4 surface temperature anomaly grid which have no data. This follows a new much-discussed paper by Kevin Cowtan and Robert Way in QJRoyMetSoc. They used satellite data and kriging to extend the data, particularly into polar regions. The result that created particular interest concerned trends in a recent period, 1997-2012, which has been characterised as a "pause", partly because HADCRUT showed little trend. C&W found that the trend was significantly greater with this extra care.
I have been discussing this, in posts here and here. In the second post, I showed that simply infilling missing cells each month with the average for the latitude band, rather than implicitly by the global average (the effect of omission) also gave a substantially greater trend over this period.
Interest has been expressed in the seasonal nature of this change. The Arctic amplification is expected to be predominantly a winter effect (Serreze). So here are plots of the various infills (None, my lat av, C&W and also UAH for comparison) over the four seasons, and annually. I have taken William Connolley's suggestion of plotting against sin latitude to avoid overemphasising the polar bands (Mercator-style).
Here is the plot. It's an active plot - you can choose your season. The "HAD4 Lat" is my infill with latitude band averages, with parameter r=0.1, as described here. The other datasets were introduced and plotted here.
The background stripes indicate the latitude bands; N pole is on the right. There's a faint red vertical marking multiples of 30°. In the centre is a little figure showing the global totals, on the same scale. DJF etc indicate the seasons by initial of the months.
Here is a table of the global trends, 1997-2012:
|Season||HADCRUT 4||HAD 4 Lat Av||UAH||C&W Hybrid|
An interesting aspect is that, while the Arctic does have a marked maximum in NH winter, that is a minimum season of of global trends, and quite markedly so. There is a substantially negative trend in the adjacent N latitudes, from about 40-60°. It is interesting to speculate on whether these are related. The global trend was also negative for this season in the original HADCRUT, but the infilling, particularly with the C&W hybrid, had the greatest effect.
The Antarctic region does not have a marked seasonality in the trend, and nor does any other (they don't have big trends at all).