Today’s letter will be devoted to image comparisons, with a special focus on “Geopot” maps showing high-altitude air pressure configurations. I saved these in the archives a year ago in mid-July, so we do year-to-year comparisons for both hemispheres as well as how they stack up today. We’ll be thinking about whether or not these images mean anything. Here is the NH today:
Here is how it looked a year ago on July 16th. The blue zone had almost completely disappeared by then while the green zone was highly fragmented, and literally fading in color. This year both zones are holding up much better than last, a combination that favors a stronger and more compact pattern of jetstream wind activity.
Of course everything is relative. Let’s head down to the SH for a current comparison. The blue zone in particular is in eye-popping contrast with the one in today’s NH. In som places it hardly leaves any room for the green zone, the presence of which depends on 500hPa’s pressure response to upward pressures emanating from a relatively narrow range of surface temperatures (zero to about 10 or 12C).
Here is how the same view looked a year ago on July 20th. The biggest difference I can see is that this year’s green zone is a little larger in total area. I don’t have temperature maps handy to compare, but I know that cold daily anomalies in the Antarctic region have been making an unusually strong run this year. Today for example the figure is a very chilly minus 4.2C.
Same-day jetstream comparisons between the hemispheres, which can be clearly depicted on a single map, are always interesting. Last year’s maps were not nearly s well-constructed as they are today, which leaves us with nothing worth showing. Today the contrast between NH and SH is about what we expect, given that jet strength and positioning are always governed by the prevailing air pressure configurations at that altitude. Keep in mind that the stronger the winds are the more effective they will be at blocking the natural poleward movement of precipitable water (PW) concentrations at that level of the atmosphere.
The overall outcome for PW movement in the two hemispheres at this time is clearly revealed on this final map. There is no better explanation for the difference than the differences in local jetstream capabilities. Note how PW streams in the south consistently have trouble passing beyond about 45S degrees in latitude while similar streams can pass 75N in the north. These capabilities are always seasonally reduced between winter and summer. If they are reduced with regularity, to any extent, on a given string of calendar days from one year to the next, PW movement will be enhanced, most likely having an effect on temperature anomalies, leading toward the kind of result described in my letter two days ago.
Carl