1.7 Mean Tropospheric Zonal Winds

Figure 2.17 shows the latitude-altitude cross section of mean zonal wind speeds for the four seasons. The positive and negative values indicate westerly and easterly wind components, respectively. Some prominent features are:: (a). Subtropical Jet Streams. The tropospheric (below 100-hPa level) westerly wind speed maxima, located near the 200-hPa level, have seasonal variations in terms of magnitude and location.; (b). Easterly Wind Regime. The mean easterlies in the topics have two maxima in the vertical, one is located near the earth's surface, the other in the upper stratosphere.; (c). Vertical Shear. The tropospheric mean zonal wind speeds generally increase with altitude; thus, vertical wind shear is positive. This vertical shear is stronger in the middle and higher latitudes than that in the tropics.; (d). The strong wind speeds in the upper stratosphere near 65° latitude have seasonal variations. These strong speeds are commonly called the polar night jet.

Some seasonal characteristics of mean zonal wind speed in the troposphere are:

1.7.1 December - February: a. The Northern Hemisphere mid-latitude jet reaches its seasonal maximum intensity with a mean speed 70 knots or 35 m/s around 30°N.; b. The surface easterly maximum (10 knots or 5 m/s) at 10°N <0 and shaded area) in near-equatorial latitudes and descending motion (W is about two-and half times as strong as the one near 24°S.; c. In the tropics, there are mean westerlies in the upper troposphere except in the equatorial zone from 3°N to 10°S.
1.7.2 March - May: a. The tropical easterlies are nearly symmetrical with respect to the equator.; b. The intensity of the Northern Hemisphere jet decreases from the previous period.
1.7.3 June - August: a. The Northern Hemisphere subtropical jet reaches its seasonal minimum intensity around 45°N.; b. The Southern Hemisphere subtropical jet reaches its seasonal maximum intensity.; c. The tropical easterlies becomes more intense than in the last season, and occupy a wider and deeper zone. The mean speeds increase with height. The surface maximum (6 kt or 3 m/s) is located at 15°S.
1.7.4 September - November: a. The Northern Hemisphere jet has increased in intensity.; b. The tropical easterlies have decreased in the middle and upper troposphere from the previous season.

Section 1.6 Section 1.8