How much colder, on average, is the North Pole (geographically) than the near equator area in the Northern Hemisphere? What about in the Southern Hemisphere? And how would people expect these to be changing over time? What is the seasonality of these things?
Well, one must take the data I will present with a grain of salt: they come from the NCAR-NCEP reanalysis daily data (t2m, downloaded from KNMI), which uses a weather prediction model to get complete fields based on regular re-initializations with observed data from around the world. I have to wonder how much of any trends is simply a consequence of changing data or biases in model behavior…or alternatively if those factors are hiding trends rather than creating them. But anyway, I found the results a little surprising. Well, some of the results.
On average, the annual average difference in the Northern Hemisphere between the lowest ~1 degree latitude band and the highest is almost 44 degrees over the whole reanalysis period. In the Southern Hemisphere, it’s almost 73 degrees (note: this is about a degree further from the Pole, I think, since . That’s not terribly surprising, I think: high elevation of the South Pole probably makes it much colder than the North Pole-well, that’s my first guess why, anyway. I am also not terribly surprised that, over the period, there has been a reduction in the difference in both Hemispheres on an annually averaged basis:
What is a bit surprising is that the Northern Hemisphere difference appears stable until the late eighties, whereas the Southern Hemisphere difference is a bit more continuous, but mostly . I expected a larger decline earlier in the record for the Southern Hemisphere, based on Antarctic station based data I’ve seen, with not much if any in the last twenty years, and a continuous decline since the seventies in the Northern Hemisphere (keep in mind that the Reanalysis begins in the late forties, after the warmer temperatures of the thirties in the Arctic). At any rate, such a reduction in Equator to Pole Temperature differences is suggestive of enhanced heat transport and, at least in the Northern Hemisphere, the ice-albedo effect. This next finding is also not surprising, at least given the above: annual maximums in the Equator to Pole temperature differences has declined too:
Please note that neither of these trends can plausibly be related to the ice-albedo feedback effect, since they are associated with the winter season, during the six months of basically continuous darkness at the Poles. This next result may be surprising for some readers:
Now, since I looked at the breakdown of annual maximum and minimum of daily temperatures in the Arctic before, this wasn’t terribly surprising to me, but are readers surprised by the fact that, according to this data, the annual minimum of the Equator to Pole temperature differences are increasing? In the Northern Hemisphere, this seems to be occurring because, in the Arctic, air temperatures in the reanalysis seem to hit a ceiling every year that’s near the triple point of water (coincidence?) which they basically can’t seem to exceed: I looked at that and there is basically no trend in annual maximum daily temperatures in the deep Arctic. While there isn’t an obvious summer “ceiling” nor is there any connection to being close to the triple point of water the Southern Hemisphere, there isn’t any tendency of the annual maximum to warming there at all it, appears. These changes seem to indicate that there are seasonally dependent trends in the meridional transport of heat. Such a thing could be brought about by significantly spatially and seasonally heterogeneous forcing. This information cannot confirm that definitively, nor can it identify the source of such a forcing. But spatial variations in cloud coverage would be my first guess for a natural factor. Also note that, as I previously stated, one cannot really test “global mean” sensitivity on the basis of spatially heterogeneous forcing. If recent climate change is due in significant part to very spatially (and seasonally!) heterogeneous forcing, studies of “global mean” sensitivity will almost inevitably give misleading results.