During World War II, American pilots, while flying westward in the vicinity of Japan and other islands in the Pacific theater, reported ground speeds dramatically lower than the aircraft's indicated air speed. Flying at very slow speeds relative to the ground could have meant only one thing - one lollapalooza of a headwind! Check out the image on the left, which shows the long-term-average wind speeds and directions at 200 mb over Asia and the western Pacific Ocean during meteorological winter (December, January and February). The narrow ribbon of fast 200-mb winds near latitude 30 degrees marks the mean position of the subtropical jet stream ("STJ", for short). Although pilots could make little headway on some of their missions, they had made a serendipitous discovery! Indeed, the subtropical jet was one of the last major tropospheric features to be discovered by direct human observation.
As the image above suggests, the subtropical jet stream is stronger over the western Pacific region, on average, than any other place in the world. That's primarily because the Himalayan and Tibetan high ground interrupt and divert the generally westerly flow of air in the upper troposphere. Farther east, diverted branches of air flow back together, creating a confluence zone near Japan, which, as you remember from Meteo 101, often houses jet streaks. But the overall mechanism for maintaining the subtropical jet stream near 30 degrees latitude is the overall tendency for air parcels to conserve their angular momentum in the upper branches of the Hadley Cells.
I emphasize the word "tendency" here. Let me explain. Theoretically, air starting from rest (relative to the earth's surface) high over the equator will reach latitude 30 degrees with an eastward speed of 134 meters per second (262 knots, convert), assuming that it conserves its angular momentum (the air parcel's angular velocity increases as its distance from the earth's axis of rotation decreases).
But the subtropical jet does not reach such breakneck speeds. That's because parcels do not completely conserve their angular momentum. Tall mountains and towering cumulonimbus, for example, exert some drag on air parcels moving poleward in the upper branches of the Hadley Cells. Regardless of these and other impediments to the conservation of angular momentum, it is fair to say that air parcels tend to conserve angular momentum as they spiral inward toward the earth's axis of rotation, throwing their angular momentum "into the mix" we call the subtropical jet stream.
As I mentioned earlier, some of the speeds we observe from the winter climatology of 200-mb winds over Asia (revisit the image at the top of this page) is likely the result of the effects of the Himilayan Plateau (more details will be forthcoming in Lesson 5). For the most part, however, what you see is, fundamentally, a consequence of the conservation of angular momentum. With the idea of conservation in mind, I'll add that the earth's rate of rotation (which, in part, governs the magnitude of the Coriolis force) largely determines the poleward extent of the STJ.
Unlike the polar jet stream, whose grassroots can be traced to large horizontal temperature gradients in the lower troposphere, the roots of the subtropical jet only extend down to about 400 mb. I again show you the cross section of the earth's general circulation (below) and draw your attention to the subtropical front, which is a boundary between upper-tropospheric air in the middle latitudes and mid-tropospheric air in the Tropics. In the final analysis, wind speeds associated with the STJ increase with increasing altitude from 400 mb until they reach a maximum at 200 mb (or sometimes 150 mb). Note that the STJ lies on the equatorward side of the subtropical front near the break in the tropical convective tropopause.
It turns out that the subtropical jet is stronger during winter than summer, despite the greater poleward extent of the upper branch of the summer hemisphere's Hadley circulation. Whoa Nellie! Come again? Grenci, I thought you just got done saying that air parcels tend to conserve their angular momentum. Assuming that lofty air parcels travel farther poleward in summer, wouldn't the subtropical jet accelerate as parcels spiral closer to the earth's axis of rotation? What's up with that?
To begin, check out the summer climatology of 200-mb winds over North America and adjacent oceans (below). You can see two relatively weak streaks of 200-mb winds associated with the mean position of the summer subtropical jet stream... one stretches from Hawaii toward the Southwest U.S. and the other heads from the mid-Atlantic Ocean toward northwest Africa. These "streaks" of 200-mb winds pale in comparison to the robust winter STJ, wouldn't you agree?
As it turns out, intense solar heating over the land masses in the northern hemisphere's subtropical region upsets the apple cart of the Hadley circulation. In a nutshell, it basically gets much hotter at latitudes near 30 degrees north (mostly over land) than over equatorial regions, thereby reversing the typical north-south temperature gradient. To confirm this observation, check out the long-term average of temperatures over the tropics and subtropics for June, July and August. Given that our prototype model of the Hadley Cell is rooted in the assumption that the belt of maximum heating occurs over equatorial regions, it should come as no surprise that when this belt shifts poleward to the subtropics, our model of the idealized Hadley circulation breaks down. Concomitantly, the strength of the subtropical jet stream takes a hit. So the STJ does not play as important a role in the overall weather pattern during summer.
During winter, however, the robust subtropical jet stream can contribute to major winter storms over the middle latitudes. As I pointed out earlier, the subtropical jet stream is a semi-permanent feature whose poleward extent is largely fixed by the rate of rotation of the earth (remember that the Coriolis force is a function of the earth's rotation rate). By and large, the northernmost reach of the subtropical jet corresponds to the southernmost extent of the more nomadic polar jet stream. So it's safe to assume that the two jet streams sometimes interact. One such interaction resulted in a surprise, memorable snowstorm in Washington, D.C., and other parts of the Middle Atlantic and Southeast States. The Presidents' Day Storm of 1979 (satellite image, SLP map) developed rapidly off the Middle Atlantic Coast. For all you tropical buffs out there, that is not an "eye" (in the hurricane sense of the word) at the center of the storm. Rather, it is a seclusion, which we talked about in Lesson 1. One of the ingredients for the Presidents' Day storm of 1979 was a jet streak traveling in the subtropical jet stream. The 200-mb reanalysis from the Climate Diagnostics Center for 18Z on February 17, 1979, shows that the subtropical jet stream had already penetrated far northward over the Middle Atlantic Seaboard. Also note the 200-mb jet streak packing winds in excess of 75 meters per second (convert) over the Tennessee Valley at the time.
As the schematic upper-air chart above indicates, intense, upper-level cyclones traveling over the middle latitudes during winter sometimes draw the subtropical jet stream northward from its mean position. When the STJ carries a formidable jet streak over the eastern half of the nation, the stage can be set for rapid cyclogenesis, particularly over the Atlantic Seaboard, where the natural land-sea temperature contrasts provide favorable breeding grounds for storms (as you discovered in the case of the Presidents' Day snowstorm of 1979). On the schematic, note that the subtropical jet stream takes an eventual anticyclonic turn southward as it starts to return toward its mean position. Check out the infrared satellite image at 12Z on February 19, 1979. It clearly shows the anticyclonic turn of the subtropical jet that surged northward to act as a catalyst for the Presidents' Day storm of 1979.
There is one facet of the general circulation that I haven't yet addressed -
the high-altitude summer easterlies that steer hurricanes and tropical storms
westward across the Atlantic Ocean toward the contiguous states.
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