The Great Southern Route : 2008 - 2009
WEATHER ROUTING 034 | GSR existent due to cool ocean water and high upper level winds. In this region, typical tropical cyclones develop in the Solomon Islands to north of Fiji, and typically track southeast or south-southeastward to the dateline or 175W, as figure 7 shows. After crossing the dateline, systems typically weaken rapidly due to cooler water further south. Tropical systems rarely form east of the dateline, and never near South America, although, El Nino regimes, particularly stronger El Ninos that are associated with well-above normal sea surface temperatures can allow for cyclone development well east of the dateline, as far east as 150W! Once formation occurs, these "rare" tropical cyclones will tend to track in a general W-SW- S'ward motion, weakening as they get further south (south of 25S, in cooler waters and a more "hostile" environment in the mid to upper levels of the atmosphere). Ocean currents vary greatly throughout this region. From 10S southward to about 25S, a generally westward flowing current of 0.5-0.7 knots is found through much of the year, due to the trade winds. Further south, from 25S to about 30S is a transition zone. Here, currents tend to flow southwestward from 25S to 30S, and southward from 30S to 35S at speeds of 0.5-0.7 knots. From 35S to 40S, ocean currents tend to flow southeastward at 0.5-0.7 knots. THE WESTERN PACIFIC, WEST OF THE INTERNATIONAL DATELINE TO AUSTRALIA, SOUTH OF 10S. No matter the time of year, this part of the world has a wide range of weather. From the volatile and ever changing to tranquil and persistent, from serene to tempestous....this area has it all. Two distinct weather regimes are to be noted: winter (more frequent/stronger cold fronts, transitory high pressure ridges, and trade wind surges), and summer (weaker, less frequent cold fronts and tropical cyclones). During late autumn and winter/early spring (May through October), we find cold fronts moving off of the eastern coast of Australia about every three days or so, generally extending as far north as 20S, as shown in figure 8. Fronts then continue their trek across the Tasman and Coral Seas, finally slowing/stalling and weakening once near/ east of the International Dateline, with forward progress slowed by the presence of persistent large high pressure further east. In the wake of fronts, high pressure ridges "emerge" into the Tasman and Coral Seas, eventually merging with and reinforcing the aforementioned persistent high pressure ridge further east. These "transitory highs" are responsible for higher than average E-SE-S winds across much of the area, particularly south of 15S-20S, where winds as high as gale and near gale force and E-SE combined seas as high as 13-15 feet are not uncommon! Aside from the higher than normal E-SE-S'lys, E-SE winds generally average from force 4-6, with E-SE combined seas generally from 4-8 feet. These winds will tend lower in far northern waters within this region (closer to 10S), and near the centers of high pressure areas moving offshore (light and variable winds are found under or very near high centers). Furthermore, E-SE combined seas will tend to be lower and longer-period the further north one travels. Breaks in the trade wind regime occur when cold fronts approach and pass across or south of a given location. North of 20S, where "true" frontal passages are less likely to occur, winds will tend lighter, with NW-N-NE winds generally from force 3-5 and longer-period NE-E combined seas lowering to 3-6 feet. Further south, interacting between cold fronts (and associated gales/storms further south) and high pressure ridging further east will bring higher winds/seas. Here, winds tend to veer, becoming NE-NW-WNW, increasing to as high as force 6-7, followed by S-SW-W winds as high as gale force (force 8-9), especially near/south of 25S-30S, with large S-SW-W combined seas likely, as high as 10-15 feet, shorter-period and larger further south, somewhat closer to gales/storms, and where stronger frontal passages occur. During the period from late spring through early/ mid autumn (November through April), cold fronts become less frequent and weaker, and by December/January only extend about as far north as about 35S, reaching the southeast coast of Australia about every four to five days, as shown in figure 9. With persistent high pressure found further west (into/near the eastern Tasman Sea), fronts this time of year will slow and weaken more readily, doing so as they move into and across the southernmost Tasman Sea toward southern New Zealand. Transitory high pressure ridges behind fronts are less prevalent across this region. Rather, in between cold fronts, high pressure further east will build westward into and across the Tasman and Coral Sea, bringing "periodic" (generally lasting 2-3 days) higher E-SE trade winds, generally as high as force 5-6, with lighter winds (force 4-5) found further north, between 10S and 20S. Combined seas during these trade wind surges generally very from E-SE-S, generally as high as 8-9 feet, but tending lower and longer- period the further north one travels. It is during this period that our focus turns toward the tropics. Tropical cyclone frequency Figure 6 -- Typical conditions in the Equatorial Pacific Figure 7 -- Typical conditions in the Central and Eastern Pacific from the Dateline to South America Tropical cyclone frequency reaches its peak in January/February in the southwest Pacific, and development areas in the weeks leading up to this period expands greatly.
2010 - 2011