Climate Letter #1894

A general conclusion to be drawn from yesterday’s letter is that when areas of below-freezing surface temperatures in a hemisphere are diminished, which is a regular event each spring, the “blue zone” of air pressure differentials in that hemisphere will shrink back in size as a direct result. The jetstream wind pathways tied to the blue zone should react accordingly, to some extent by shrinking, maybe also weakening, and possibly fragmenting. If one were to suppose that freezing temperatures might effectively be altogether eliminated by mid-summer, it follows that the blue zone would simply disappear, taking down all of its natural jetstream pathway activity at the same time. This is not a far-fetched idea. It came close to happening that way in the Arctic region just last summer.

In the Southern Hemisphere the whole situation is different.  A large part of the surface under the blue zone is highly elevated and covered with ice, which seldom allows temperatures to rise above freezing.  Another large portion is marked by either seasonal sea ice or freezing cold water, both of which inhibit any significant amount of temperature warmup.  The blue zone and its wind effects thus tend to stay in place from season to season.  By comparison, the NH does have Greenland’s ice sheet and a fair amount of summer sea ice in place to help protect the blue zone from annihilation but in both cases there are issues concerning durability. While I think of it, a recent study found evidence that Greenland’s ice completely vanished at least once within the past million years, under conditions less conducive than those now being created.  The authors have described their work in a separate story for public readership, published by The Conversation.  Here is the link: https://theconversation.com/ancient-leaves-preserved-under-a-mile-of-greenlands-ice-and-lost-in-a-freezer-for-years-hold-lessons-about-climate-change-157105.

The principal jetstream pathway controlled by the blue zone is the one that tracks along the thin blue line at its outer perimeter. It is one of the two majors that function effectively as means of holding off efforts on the part of precipitable water (PW) concentrations to spread across the inner polar region and make it warmer. Ideally, the two major pathways are positioned in close proximity, allowing them to reinforce each other with accelerated wind speeds. Should one of the two disappear, the other, meaning the one produced by the green zone, which is naturally the stronger of the two, left working alone, can only become much less effective. We saw this happen last summer, resulting in broad and lengthy Arctic heatwaves.

Now, making liberal use of the imagination, let’s suppose that over time the green zone suffers the same fate as the blue zone and simply disappears. Average temperatures everywhere would need to rise by an improbable ten degrees above freezing for this to happen. Extreme, yes, but not unthinkable. Things must have been that way 50-some million years ago in the Eocene era, when both poles had near-tropical climates in place. With nothing but a red zone from pole to pole, PW formed from high rates of evaporation would probably rise and quickly fall out in much the same way as it does today in the tropics. Aside from anything that remote, what can we say about everything that might develop during the period in between, and sooner rather than later? Further deterioration of blue zone jet winds, to an extent leaving the green zone jets mostly on their own, carries with it a suggestion that high-altitude PW concentrations, even if they do not grow in volume, will have more freedom of movement and thus more likelihood for penetrating the polar regions on a scale not seen for a very long time. This by itself would further chip away at the durability of those ten degrees of temperature needed to hold any resemblance of an effective green zone in place. It’s not an attractive prospect.

Carl