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Climatic studies show hemispheric pressure patterns differ in a characteristic fashion depending on whether zonal (east-west) or meridional (north-south) flow dominates within the mid-latitudes. Since winds are pressure generated, a numerical ZONAL INDEX based on horizontal pressure differences between 35° and 55° latitude was developed to measure the strength of these westerly winds. The higher the index, the greater the zonal component. The lower the index, the greater the meridional component. A particular index may remain nearly stagnant for several weeks, especially in winter, or last for only a few days. Today, zonal indices are seldom computed; merely estimated. The estimate is based on the current circulation and weather patterns. Since reference is made to zonal index, you must have an understanding of what is implied by HIGH, LOW, and CHANGING zonal indices.

High Zonal Index

With a high zonal index, long-wave ridges and troughs are weaker (have less amplitude) and fewer in number. The Icelandic and Aleutian lows are well developed and are at, or slightly north of, their normal positions. Their axes and the orientation of their associated troughs are east-west. The Atlantic and Pacific subtropical highs are north of their normal positions, and their orientation is strongly east-west. The Great Basin and Siberian highs are present. There’s little westward extension to the high over the Great Basin. (The Great Basin encompasses all of Nevada, the western one-third of Utah, and portions of eastern California, southern Oregon and Idaho.) Highs are absent at higher latitudes. Fronts are well north and predominantly oriented east-west. They and their associated lows move rapidly east ward. The highs of mid-latitudes are moderately developed and also progress east at a rapid pace. There are fewer polar outbreaks; therefore, the polar regions get colder, while mid-latitude temperatures are moderate. The weather is generally fair in the mid-latitudes. The stormiest weather is along 60°N.

Low Zonal Index

With a low zonal index, long-wave ridges and troughs are very steep (have great amplitude), and there are more of them. Cutoff centers are common. Short waves have larger amplitudes than normal and are a predominant feature of the 700-mb chart. The Icelandic and Aleutian lows are weak, are split into two cells, and are oriented north-south. The Atlantic and Pacific subtropical highs are weak, split, oriented north-south, and are centered farther south than normal. The polar highs merge and are strongly developed. Frontal systems are more common, with sharp tempera-ture contrasts across them. The fronts are oriented north-south. Stormy weather is frequent in low latitudes, while high latitudes experience storm-free weather and mild temperatures.

Changing Zonal Index

Hemispheric weather patterns are, for the most part, always changing. As the pattern changes, so does the zonal index. A pattern change from a high-index situation to a low, or vice versa, is simply one of transition. It doesn’t happen overnight. A changing index is one that is either increasing or decreasing. INCREASING or DECREASING indices are also based on the circulation and weather pattern.

INCREASING ZONAL INDEX.— With an increasing zonal index, the number of long waves in the weather pattern decreases. Cutoff centers weaken. Migratory systems intensify and speed up, especially in higher latitudes, resulting in a more east-west orientation of associated frontal systems. The eastern cells of the Icelandic and Aleutian lows and subtropical highs weaken while moving eastward. The western cells of these systems move northeastward toward their normal positions. A polar high stagnates over the Great Basin.

DECREASING ZONAL INDEX.— With a decreasing zonal index, the number of long waves in the weather pattern increases. Cutoff centers are more likely to develop. Migratory systems slow down, especially in the higher latitudes. Frontal systems gradually shift from a east-west orientation to one that is more north-south. The Aleutian and Icelandic lows and subtropical highs move southward and begin to split. Polar highs intensify, and outbreaks of cold polar air into lower latitudes take place. 


Winds in the upper-level circulation pattern reach maximum speeds in narrow streams that meander in wavelike fashion above both hemi-spheres. By definition, when these streams of high-speed winds are thousands of miles long, hundreds of miles wide, and a few miles deep (vertical extent), they are termed jet streams. Jet streams are three-dimensional features. They have length, width and depth. Isotach analyses done on upper-level constant-pressure charts provide us with a view of two of the dimensions. They outline a jet stream’s axis and show its width latitudinally and length hori-zontally.

These analyses also show that jets are not continuous around the globe; they stop, start, split, merge, and can exist side by side within a few hundred miles of one another or be thousands of miles apart. The third dimension (depth) is best determined from vertical wind profiles developed from wind information obtained from radiosonde, rawinsonde, and pilot reports. How-ever, you can obtain a crude picture of a jet’s depth by stacking upper-level charts. Three-dimensional studies also reveal a core of higher speed winds existing within each jet stream. The winds in the jet core differ from those elsewhere in the stream in that they are stronger and have a constant velocity. In other words, their direction and speed are unchanging. The importance of this is related to wind shear, experienced in and around jet streams. If there is no change in wind speed and direction in the core, there can be no wind shear there. Outside of the core, however, wind shear can be significant. The shear associated with the polar-front jet stream will be discussed in depth later in this unit.

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