Climate Letter #1893

Something to celebrate—the Windy website has returned, with improvements, after an absence of several months.  This is a tremendous source of information about almost every component of weather in the lower part of the atmosphere that you could ever think of, a perfect complement to the Weather Maps when you want more details about surface winds, current temperatures everywhere, cloud cover layers and much more.  You will want to explore it all and bookmark the link:  https://www.windy.com/

Continuing with thoughts about the high-altitude air pressure map, one thing we know for sure is that the pattern we see inside the blue zone is always closely matched up with the perimeters of regions that have very cold surface temperatures, meaning averages near the freezing point or below.  The green zone then picks up all the territory surrounding the coldest regions, where averages are staying within bounds set by around another ten degrees.  Beyond these we arrive at the red zone, shaded from lighter to darker red as average temperatures get warmer.  Actual surface air temperatures are the one critical benchmark..Temperature anomalies that occur within the zones make little difference, and neither do variations in surface air pressure, which is entirely gravity-driven. By contrast, the high-level air pressure pattern is only determined in part by gravitational weight, that being a specific fraction of the total atmosphere, like one-half (equal to 500 hPa), constantly applied in all locations. The other part is determined by the effects of variations in the upward pressure generated throughout columns of air directly above the surface, largely dependent on temperatures of that air.  The density of warm air expands, which raises its upward pressure, while cold air keeps contracting as it gets colder.

It is these variations in upward pressure that cause the actual variations that we see marked out in the imagery on the map, separated into zones of different colors in order to earmark significant points of differential due to the pressure balances that result from this meeting of downward and upward.  This new pattern of differentials fully takes shape at about three miles above sea level.  It will to a large extent, though not total, replace the pattern that exists lower down, and thus a completely new system of wind speeds and direction of movement will take over, with jetstream activity leading the way.  As a matter of interest, the remaining space above the three-mile level and still below the stratosphere, does not appear to undergo further changes in air pressure configuration.  This means jetstream activity and the full wind system at any one level in the upper troposphere will stay consistent with similar activity at other levels.

All winds that qualify as jetstreams because of their capability for reaching high velocities follow pathways that are set up at certain points within the overall spread of pressure differentials, which evolve naturally. It so happens that the two possibly most important pathways are located along the outer perimeters of the blue zone and green zone respectively. A third important pathway lies within the red zone, where the pressure differential is marked out, just not as clearly as the other two, while jets on a fourth pathway are occasionally visible deeper yet within the red zone.The two innermost pathways, both majors, have the greatest interest from the standpoint of how they could have a meaningful effect on the course of future climate change. These two pathways contain winds that have the capability to hold back the progress of precipitable water (PW) concentrations that are naturally headed in the direction of the polar region. When they do make progress their inherent greenhouse powers are magnified by a leveraging effect when added to steadily diminishing amounts of surface level PW toward the pole.

Think of this: last summer, in the Northern Hemisphere, below-zero (C) surface temperatures almost completely disappeared at times, causing the blue zone to be reduced to nothing more than few skimpy fragments. Its normal jetstream pathway came down with it, virtually eliminated. The green zone, in turn, saw its pathway jets weakened and fragmented. Overhead PW movement in the far north was freed up as a consequence, and record high temperatures were set in some places by wide margins. A substantial decline in Arctic Ocean sea ice that has continued for decades helped to bring on the warming behind this development The additional greenhouse effect introduced last summer would have generated long-lasting subsurface temperature increases along with immediately higher air temperatures, making it easier for the overall trend to be given a boost in the early part of this past winter .

Carl