Every day now when we look at the daily map of temperature anomalies we get the same story: the NH will be up by an expected amount or more while the SH is down, which means the SH is cooler now than it was thirty years ago. The difference between the two hemispheres has been running by a full one degree or a bit more. The numbers at the bottom of this map are typical:
The globe as a whole is always stuck in the middle. Today’s +0.2C is typical. Such a low number may sound like good news, compared with what we are accustomed to seeing for the current global warming trend—around +0.2C per decade—but we really need to get a handle on what is happening to cause the big hemispheric discrepancy, and how long are things going to stay this way? The news is not at all good for those of us who live in the NH, and means very little for people who live anywhere in the tropics (+0.4C). Most of the really cool trend is found inside of the Antarctic zone (-1.9C) and, as you can see on the map, it extends from there all the way out past the latitude of 60S, which is mostly ocean. Something is causing that entire region not just to be cold, which is normal, but quite a bit colder now than it was on an average day thirty years ago. This is in spite of all the greenhouse energy we have been adding to the atmosphere, which spreads out over the entire surface. But does it spread out evenly?
I belong to a school of thought that says “no.” The “ordinary” greenhouse gases, all of which have long lives in the atmosphere, do spread out evenly, maybe not quite perfectly, but with gaps of not more than around 10% in most cases, or for the important ones. The one greenhouse gas that I keep writing about, water vapor, which happens to be at least as powerful as all the others put together, does not spread out the least bit evenly, except that variations within the tropical region generally stay low. Outside of the tropical belt its distribution gets more and more irregular as it approaches either of the polar zones. On many occasions the difference in overhead H2O from one short number of days to the next can be a full 100% (or -50% in reverse) and the range will at times extend up to 200% or even 400%. Those figures lead to some serious temperature differentials all the way up to a maximum of 30 degrees—in just days We can pick numbers of that magnitude right off the daily maps, in some places!
It is quite possible that the amount of water vapor (or precipitable water) in the high-altitude layer of the atmosphere south of 60S latitude is lower on most current days than it was on an average day thirty years ago, maybe quite a bit less. This could cause a steady flow of daily temperature anomalies for the entire region being well on the cold side of average. What would cause such a situation to happen? I think a wall of very strong and tightly packed jetstream winds is in place, beyond those of thirty years ago, and holding back the movement of larger amounts of vapor. An example from today:
Next we can see the consequences of being held back from the oncoming vapor’s point of view. Notice that no such difficulty exists in the north, where vapor streams can penetrate deeply the high latitudes much more easily, thanks in large part to seasonal factors, but not entirely:
There is a good reason behind the great strength and tightly packed formation of jetstream winds in the south: their pathways are being governed as always by the positioning of high-altitude air pressure differentials, as currently observed on this next map. Notice how large and solid the blue zone is, and how snugly the thin construction of the green zone fits around its border:
I can only guess at this, but I reckon that the blue zones we see today are consistently larger than those that existed on an average day thirty years ago. The shape of the blue zone basically reflects the extent of below-freezing temperatures on the surface below. According to some well-accepted research, the surface of the Southern Ocean beyond latitudes of around 60S is being cooled these days by meltwater flow moving away from the undersides of the massive ice shelves around the continent. This could be just the early stages of a long-term trend, and we might look for a continued pronounced skewing of global temperatures as a result.
Carl