Route Climatology — Page 454, Lesson 426

I want to walk you through the route climatology for the Singapore–Darwin–Sydney–Auckland corridor, starting with surface winds. Let’s begin with the northeast monsoon. This wind blows from the South China Sea toward Singapore. But here’s the key: it continues right across the Equator. As it crosses, the Coriolis effect changes its direction, so it becomes the northwest monsoon. That northwest monsoon persists all the way until it reaches the Intertropical Convergence Zone, or ITCZ. In late January, the ITCZ sits just south of Darwin. Once you go beyond the ITCZ, on the other side, you encounter the southeast trade winds. However, when those southeast trades blow over land, they get modified around thermal low pressure centres — those are areas of low pressure created by intense surface heating. Now let’s look at Sydney. The southeast trade winds there can give way to two different wind types. First, a strong east/northeast sea breeze. Second, southerly winds that arrive after the passage of a polar front cold front. Those southerly winds have a local name: they’re called Southerly Busters. I’ll come back to those when we talk about cloud and precipitation. What about cyclones? Cyclones that come from the Coral Sea, passing near Brisbane, occasionally continue southeastwards over the Tasman Sea. When they do, they produce very deep lows with strong, variable winds. But apart from those cyclone events, the winds between Sydney and Auckland are generally westerly. Let’s move to visibility. Over Darwin and the area to its north, visibility is good — except when there’s precipitation from cumulonimbus clouds or thunderstorms, which we abbreviate as CB/TS. Between Darwin and Sydney, the clockwise rotation around a continental low pressure system carries dust toward the centre and south of the route. That means occasional dust storms occur, and in the northwest they’re sometimes known as “Willy-Willies”. Industrial haze near cities can reduce visibility down to 1 to 2 kilometres. Over the Tasman Sea, visibility is good except in precipitation. Now for cloud and precipitation. The Singapore and Indonesia region is one of the most active daily thunderstorm areas in the world. Why? Three factors combine: high ambient temperature, strong overland insolation — that’s heating from the sun — possibly coupled with orographic uplift, which is air forced upward by terrain, and high humidity from the abundant supply of sea water. At no time of the year is this region free from daily convective cloud. But when the ITCZ is present, it enhances instability even further. So in the southern summer, thunderstorms may be present all the way from Singapore to Darwin. The ITCZ reinforces them near Singapore in November and December and again in March, and near Darwin in January and February. South of the ITCZ, the Australian interior is mainly arid. As you head toward Sydney, the weather is mainly subtropical, except when cyclones affect it. But occasional cold troughs or cold fronts bring squally, wet weather. When those fronts pass, they cause a marked drop in temperature, cumulus and cumulonimbus clouds, squalls, and a sharp back in the wind to the south. In Sydney, that wind shift is known as a Southerly Buster. In fact, Sydney’s weather can be worst in summer. Finally, from Sydney to Auckland, eastward-travelling high-pressure cells — which sit in the subtropical high-pressure belt — are interspersed with troughs and their associated cold fronts.