Something unusual is showing up on the weather maps these days, which I think is worth recording along with a commentary. We are seeing fairly strong jetstream wind activity emerging from both hemispheres on regular pathways that are now out of place, after being shifted all the way down to positions near the Equator, out in the middle of the Pacific Ocean. There the two jet streams flatten out as they come together and then proceed side by side for several thousand miles before separating.
The only way to explain this phenomenon is by examining the map that shows how air pressure gradients are differentiated in the upper level of the atmosphere. Here we can see how a certain gradient of medium strength has expanded into a very broad band that is common to both hemispheres over a wide three-dimensional area, as seen shaded in a medium tone of red. This particular gradient is normally maintained in narrow bands that remain well-separated in each hemisphere. These bands are special in that they regularly serve as pathways for the outermost one of the four major jet-strength wind streams that circulate in each hemisphere, as described in a number of earlier letters. These jet winds are as active as ever today in spite of being far out of position.
The pathways would only come together like this if something else of an unusual nature were going on that could cause the major distortion we now see in the pattern of air pressure gradients at this altitude. The best place to look for such a cause would be surface-level temperatures that are low enough to cause a contraction of air density between the surface and the upper level of change. While the imagery is not a perfect fit, there is at least a suggestion of how this could happen by checking out the anomaly in sea surface temperatures in the Pacific that is the result of a prolonged period of La Nina winds blowing steadily from east to west:
Whatever the real cause may be, the jet-strength winds that are in place look like the type that could be having an effect of weather patterns, and I am curious to see if that prospect might include atmospheric river (AR) development. If these winds are in fact passing over a region of the ocean that is warm enough to have a high and steady evaporation rate they would almost surely be capturing large quantities of water vapor that normally would be rising upward to a high level but not encountering anything of this sort that would carry them off in river-like fashion. Let’s take a quick look at the sea surface temperature map, to see if there is an appropriate body of warm water that has not been cooled too much by the La Nina anomaly in the last map:
Sure enough, the potential for plenty of vapor supply looks good in the region that extends along the north side of the Equator, positioned exactly where the jet winds are active—a situation that is always favorable to AR formation. Getting evidence only requires a look at the precipitable water (PW) map for today, where I think the best view can be obtained by switching to the full global map:
Crossing over mountain ranges is always a challenge for an AR, as some of its PW will normally be sidetracked, but much of it will keep moving forward, and that is what I think we are seeing here. The result is a reinforcement of PW supplies being introduced at the same time over on the Atlantic side, creating the massive AR stream that is visible starting out from the top of the Gulf of Mexico all the way down to the low end of Central America, with its ultimate destination being a broad section of the Arctic region. The next image is interesting because it appears that whenever vapor from the Pacific piles up on top of copious amounts of vapor formation on the Atlantic side the immediate result is an overload of surplus that quickly condenses and gets rained out, right along the shoreline.
Carl