Section 1.61.5.1 January
Strong westerlies dominate the Northern Hemisphere winter. Three speed maxima are observed; one over Japan (Fig. 2.12), one over North Africa (Fig. 2.13), and one over the East Coast of North America (Fig. 2.13). These maxima are located where the polar and subtropical jet streams typically merge. Semi-permanent anticyclones cover much of the Southern Hemispheric tropics. Over the western Pacific and Indian Ocean, the outflow from these anticyclones crosses the equator ( Fig. 2.12). This outflow serves as the upper-level branch of the compensating circulation for the low-level flow from north to south. The Southern Hemispheric TUTT (Tropical Upper Tropospheric Trough) is also apparent, extending from the region around 30°S and 110°W to the equator (0° latitude, 180° longitude).
1.5.2 July
Two prominent upper-level anticyclones exist in the Northern Hemisphere summer; one over the Tibetan Plateau ( Fig. 2.14) and the other over the Northern America (Fig 2.15). The Tibetan Plateau anticyclone is massive in size and is related to the Indian summer monsoon. TUTT phenomena exist in both the Pacific and Atlantic. The north Pacific TUTT starts in the Gulf of Alaska and stretches southwest and then west into the Philippine Sea (Fig. 2.14). In the north Atlantic, the TUTT extends from west of the United Kingdom into the Gulf of Mexico (Fig. 2.15). At times this north Atlantic TUTT stretches across Mexico and into the Pacific Ocean.
The cross-equatorial flow is entirely from the Northern to Southern Hemisphere. This upper-level flow pattern is consistent with the positions of the surface monsoon troughs and ITCZ, which all lie north of the equator at this time of year. Strong wintertime westerlies dominate the Southern Hemisphere south of 20°S with three areas of strong wind speed: one over the west of South America (Fig 2.15), one over Africa (Fig. 2.15), and one over Australia (Fig. 2.14).
(Revision date: 4 March 1999)
Chapter 2
