Route Climatology — Page 454, Lesson 425

I want to walk you through the route climatology for the Singapore–Australia sector, starting with the winter picture in July. Let’s look at the upper winds first. Above Singapore, the upper equatorial easterlies will blow until 10° south. That means from the equator down to about ten degrees south latitude, the wind at high altitude comes from the east. After you cross that line—south of 10°S—the winds will increase from the west. So the wind direction flips from easterly to westerly once you’re past that latitude. Now, in the southern winter, tropical North Australia remains hot, whereas the south is comparatively cool. That temperature difference over the intervening subtropical and continental high—the high-pressure area that sits between the hot north and the cool south—will produce the westerly subtropical jet stream at the 200 mb level. The 200 mb level is very high in the atmosphere, roughly around 36,000 to 40,000 feet. This jet stream runs across the centre of the continent around 25° south. Speeds may reach 100 knots. So on a winter flight from Singapore down to, say, Sydney, you could encounter a strong westerly jet across central Australia. From Sydney to Auckland, the westerly wind will moderate to 60–70 knots. So the wind is still from the west, but it drops from a possible 100 knots down to a more moderate 60 to 70 knots on that leg. Icing is not a special problem on this route. So you don’t need to plan for significant icing conditions on the Singapore–Australia sector. Now let’s move to the summer picture in January. I want to walk you through the pressure systems and weather. First, the ITCZ—the Intertropical Convergence Zone. Over Singapore, the ITCZ is southbound in November and December, and northbound in March. Its southerly extreme—the farthest south it reaches—is just south of Darwin at the end of January. So the ITCZ affects the Singapore–Darwin section of the route from November to March. That means during the southern summer, you’ll be flying through or near the ITCZ on that first leg. Next, the continental low. The Australian subtropical high belt of winter has moved south with the sun. Over Australia itself, intense insolation—that’s strong solar heating—brings low pressure to the interior. So instead of the high pressure you see in winter, the centre of Australia becomes a low-pressure area in summer. Now, cyclones. Tropical cyclones and associated weather form adjacent to the ITCZ over the Coral and Timor Seas. These are the seas to the north and northeast of Australia. They move at 10 to 15 knots in one of two general directions. First, they can move westwards, close to the North Australian coast. Second, they can curve to the left from the Coral Sea around the South Pacific High to affect the Australian east coast at Brisbane. Occasionally they travel further southeastwards, degrading to a deep depression as they cross the Tasman Sea to New Zealand’s North Island. So a tropical cyclone can weaken into a deep depression—still a significant low-pressure system—by the time it reaches New Zealand. Let me bring up the figures that show these winds. shows the upper winds for July, the winter picture. shows the upper winds for January, the summer picture. And shows the January surface weather and wind velocity, with upper wind velocity in yellow. These will help you see the patterns I’ve just described.