The Great Southern Route : 2013 - 2014
210 | GSR pressure will result in an offshore progression and weakening of the thermal trough. As a result, southeast winds along the western Australian coast become light and sometimes variable (Beaufort Force 4 or less) along the trough axis. Swells generally lower to southwest 4-7ft with the highest sets farther south during these events. On the other hand, an inland progression and strengthening of the trough can be expected in the wake of these following high pressure areas, as the semi-permanent high pressure ridge builds farther east. This will increase southeast winds to approximately Beaufort, Force 4-6 and southwest swells of 6-10ft (the highest conditions occurring farthest south along the western coast of Australia). Cold fronts are more frequent and stronger during the winter months. These fronts extend as far north as 26S to 28S and reach the southwestern Australian coast every three days. During spring and autumn, these cold fronts are weaker and shift farther south to approximately 28S to 30S. By the summer, the fronts only reach as far north as 30S and pass over the extreme southwestern coast of Australia every four to five days. Ahead of these cold fronts, winds are generally out of the north to northwest of Beaufort Force 3-5 and swells are northwest 4-7ft during the summer. In the winter, the winds will increase out of the northwest of Beaufort Force 4-6 and swells northwest averaging 6-9ft ahead of these fronts. Behind the fronts, winds increase out of the south to southwest Beaufort Force 4-6 and swells build out of the southwest of 6-9ft during the summer. During the winter, winds will increase out of the southwest of Beaufort Force 6-8 and swells build out of the southwest 9-13ft behind cold fronts. However, sustained gale force winds and swells of 10-15ft out of the southwest can develop along the Australian coast south of 25S during exceptionally strong storms. The southeastern Indian Ocean and Australian tropical seasons begin in October and usually end in April, with peak activity in this season typically during February. The majority of tropical cyclone development usually occurs near the equatorial trough, between approximately 06S and 10S, in the southeastern Indian Ocean. Once development occurs, the most likely tracks will be toward the W-SW, along the northern periphery of the semi-permanent ridge of high pressure. Over time, tropical cyclones will usually track more south and southeastward over the southwestern Indian Ocean and weaken over cooler waters between 20S and 25S. Farther east, tropical cyclones generally develop close to and within the equatorial trough near the Arafura Sea and Gulf of Carpentaria. Once a tropical cyclone develops, the system will usually track west-southwestward and will take one of two tracks. The system can track southwestward and make landfall along the northwestern coast of Australia (or in the Gulf of Carpentaria move inland over Northern Territory). The other track is out over the southeastern Indian Ocean with an eventual turn south and southeastward, where the tropical cyclone weakens and becomes extratropical over cooler waters south of 20S. There are times that a tropical cyclone will move inland over the Cape York Peninsula then re-intensify over the Gulf of Carpentaria, before following similar tracks described. situated farther north this time of year and interacting with the equatorial trough. Closer to the equatorial trough axis (between 5S and 10S), winds are generally out of the east approximately 1-2 Beaufort Force lower, with swells 2-3ft lower. Outside of any squalls, trade winds and swells are similar across the Java and Banda Seas as described near the equatorial trough across the southeastern Indian Ocean. Winds generally funnel from Torres Strait through the Gulf of Carpentaria and Arafura Sea especially during the winter months. During the summer months, winds are out of the east to southeast Beaufort Force 4-5 (though force 6 to, at times, 7 are more likely near and in Torres Strait). Swells are usually out of the east to southeast of 6-10ft (highest just west of Torres Strait). By winter, east to southeast winds will increase 1-2 Beaufort Force higher with swells building 2-3ft higher across these regions. These surges occur when a ridge of high pressure builds across Australia and interacts with the equatorial trough. The winds are stronger in the winter with these highs shifting farther north over Australia during this time of year. Farther south, a thermal trough of low pressure is usually found along the western Australian coast south of approximately 20S. The movement and strength of the trough will affect the winds and seas along the coast. The approach and passage of any cold fronts and the eventual approach and passage of any following areas of high THE FIRST is the equatorial trough that extends from the equator southward to 10S and from 50E eastward along Indonesia and New Guinea. Showers and squalls associated with the equatorial trough generally move westward and are more numerous from January through March. This feature changes little in strength and is quasi-stationary throughout the year. However, over the southeastern Indian Ocean, the equatorial trough can shift farther south a few degrees in the summer and north in the winter. A ridge of high pressure is a semi-permanent feature across the Southeast Indian Ocean. From November through March, the west to east oriented axis of the ridge is found along 32S to 35S and from approximately 50E eastward to 105E. The ridge shifts farther north in June through October along approximately 29S to 32S. At times, portions of this ridge will move eastward across Australia during June through August or further south across the Great Australian Bight from December through February. Winds across the southeastern Indian Ocean from 10S to 20S are generally east to southeast of Beaufort Force 3-5 and swells 5-8ft during the summer months (December through February). However, these winds increase during the winter months (June through August) out of the east and southeast of Beaufort Force 4-6 and swells 6-10ft. The reason for the higher conditions in the winter is due to the ridge of high pressure Asia & Indian Ocean Weather Conditions WEATHER ROUTING From the Southeastern Indian Ocean and Indonesia to Northern and Western Australia: Two semi-permanent weather features can be found across the southeastern Indian Ocean and near Indonesia. By Amanda Delaney, Mark Neiswender & Brian Whitley of Weather Routing, Inc.
2010 - 2011