Climate Letter #2076

My “Sunday special” climate letter has some amazing map imagery related to many of the operations of atmospheric rivers (ARs). This kind of information is not to be missed—just scroll down now and take a good look.  The greenhouse effect of the precipitable water (PW) the ARs carry is captured vividly in the form of extreme hot and cold temperature anomalies, ranging from about +22C (40F) to -22C at latitudes not far from the boundary of the Arctic Circle.  The warmest anomaly, in southern Greenland, is being impacted by overhead passage of a major AR that was born in the central Atlantic.  The coldest, in southern Alaska, is positioned entirely away from the path of another major AR, the one that was born in the Pacific and made landfall on the coast of the Pacific Northwest. This part of Alaska quite possibly held on to an average amount of humidity in the air close to the surface, capable of providing a small amount of warming—we have no good reason to think otherwise.  On the other hand, it more than likely had close to the lowest possible amount of humidity in the upper part of the troposphere, the part where ARs are active, which in a normal year would could contain at least some of the PW offshoots of an overhead river, or even the full impact of a major AR like the one that we now seeing over Greenland. 

In other words, the combined difference between having no AR at all up high in the sky and having a big one pass directly overhead is, shall we say, a total of around 44C or 80F at this latitude at this time of year. With just average AR and its PW content in the sky above both of these places would have reported little more than average temperatures for the day instead of the extreme anomalies.  One must always keep in mind the established fact that the material bodies of ARs are composed entirely of PW, from whence cometh precipitation. Temperature effects from ARs have yet to be established as factual, a matter that is about due for correction. I believe there is one other fact about ARs that has been established but does not get the attention it deserves concerning its material density. The density of an AR is a reflection of the density of the material it is made of, all of which is PW. The density of PW in the atmosphere at altitudes where ARs are known to exist must depend on some special kind of creation process, which is almost certainly unrelated to the density of PW’s creation at levels close to the surface, where there are no ARs. We might consider it likely that the processes involved in AR creation have governance over the creation of PW concentrations that we find in this part of the atmosphere and perhaps also the way these concentrations behave. Everything about it differs from PW volumes and behavior close to the surface. As such, PW concentrations in the heart of an AR, measured in terms of pure weight, are often likely to be much greater than the concentrations of PW down below, in spite of the fact that the air itself is much thinner at AR altitudes. 

We commonly make reference to the total weight of all the PW in a vertical column of air from the surface to the top of the atmosphere, over an area of  one square meter.  Let’s suppose we could actually measure the PW weight within each cubic meter of that column, and add them all up to get the total.  We’ll have to use our imaginations to grope for an answer, and hope to be having a good day.  What I visualize is a gradual trend of lower weights per cubic meter for the first two or three miles of ascension, which may be interrupted at times by surface winds laden with water vapor that may be swirling around or rising up some distance and heading off in some direction or another in the fashion of all ARS.  Beyond three miles or so things should change, as a whole new wind system takes over and ARs become larger and more sustainable.  If no ARs are present in the column of air at this higher altitude the decreasing weight of PW per cubic meter will stay on trend, approaching microscopic levels upon entry into the stratosphere.  If, however, an AR is present and active, which is usually the case to some extent, PW weights per cubic meter would start to balloon higher, reaching levels of density perhaps on the order of a hundred times greater than the density in the air just below the dividing line, or at least a few times greater than the density within the cubic meter at the bottom of the column. The upper level density would rise to a peak within the first mile or so of AR elevation, then trend downward again with only a few interruptions on a modest scale appearing thereafter.

Here is something else to think about.  On yesterday’s map we see cold anomalies of more than 20C in both Alaska and northern Greenland on the same day, both of them associated with very low PW values, and we also see numerous cold anomalies of 10C or more in other locations where the PW decline below normal is not so great. Let’s suppose that for some reason there is a complete stoppage of all AR activity, everywhere.  What would prevent surface temperatures all across the mid to upper latitudes from falling to levels represented by the lowest of cold anomalies?  We saw something of this sort happen last winter as far south as Texas, and we often see it in central Asia, always for the same reason. What place would be exempt? On other occasions, the total PW content at some locations is currently being measured at 8 or 10 times greater than the average number, and temperature anomalies are recorded correspondingly high extremes, again on a logarithmic scale of influence.  This tells me we don’t want the sky to be full of ARs everywhere, just like we don’t want any of the existing ARs to go away. We just want the current balance to continue about like it is today. (That goes for precipitation as well as temperatures.) Do we really know how delicate the balance is, or how it is controlled?

Carl