The Great Southern Route : 2013 - 2014
GSR | 253 will normally last one to two days at a time, and will occur in the form of lighter N-NE-E winds, generally from force 3-5. Combined seas in turn will abate and become longer-period, generally easterly in direction, generally from 4-7 feet. Now we move onward through winter (June through August), and the weather during this time can be summed up in one word: active. Cold fronts are more frequent and stronger during this time, often extending as far north as 25S. Ridging from high pressure tends to be weaker and somewhat farther east as well, allowing fronts to maintain their strength longer as they move toward/through the southern portion of the region, although portions of persistent ridging in the South Atlantic will break away and move into the Indian Ocean in the wake of fronts, eventually merging with the Indian Ocean ridge. The passage of fronts and approach of following high pressure ridges west of fronts will often bring surges of higher SE-S winds. Gale force winds and large SE-S seas, often as high as 10-15 feet, are common during these times, especially farther south, in and near Mozambique Channel. These larger seas are found farther to the north and east as well, though farther north, the seas will tend to be longer-period and somewhat lower than those farther south. As transitory ridges move farther east and merging of highs occurs, winds veer, becoming E-SE. Easing of winds and abatement of combined seas occurs farther south, in and near Mozambique Channel, as veering occurs, and as fetch becomes limited. Further easing in winds occurs throughout the area as winds veer further, becoming northerly and easterly in direction as the next cold front approaches from the west (Figure 2). a somewhat weakened state, as compared to winter months. The axis of the ridge during the period is generally found between 32S and 35S, with ridging as far north as 15S-20S. Periodic weakening of the ridge will occur as fronts move into the Indian Ocean, though the ridge later becomes "reinforced" and builds farther north/ west as transitory highs/ridges from the South Atlantic move in and merge with this ridge. Again, east to southeast trade winds are most commonly found, speeds generally from force 4-6. However, surges of higher SE winds, generally as high as force 6-7 are found farther south (south of 15S), as ridging from high pressure intensifies. This is also common in southeast to south winds, especially in Mozambique Channel, where local "channeling" or funneling" of winds exists, after fronts pass near/south of the region. Weakening of the ridge ahead of fronts will induce a veering of winds, generally N-NE-E in direction, with speeds generally from force 3-5. Combined seas will tend to be from the east to southeast, heights generally from 4-8 feet, though note that the passage of cold fronts and resultant higher wind surges can in fact bring more SE-S seas. These can often reach as high as 8-10 feet, especially toward late April/May, when fronts tend stronger. In fact, southerly seas again the faster north to south currents in and near Mozambique Channel can induce even larger sets, often well in excess of 10 feet when stronger fronts and following transitory highs moveing south of the area. Any southeast to southerly seas, will tend to be longer-period the farther north you travel. Again, breaks in E-SE trade winds will occur as fronts pass off to the south. These breaks and much of January), reaching its "peak period during late January and February. Cyclones will normally begin their life cycle as disorganized or poorly organized squall clusters on the south side of the Equatorial Trough mentioned previously, and are generally found in the Indian Ocean waters between 08S and 15S during their formative stages of development. With time, these squalls will initially track southwest to westward, and in many cases become better organized, developing into full fledged tropical cyclones. As they encounter a "weakness" in the ridging to the south, tropical cyclones will tend to turn south to southwestward, and eventually, they will weaken and become non-tropical entities as they encounter the cooler waters and more hostile environment aloft south of 25S. Given the typical south to southwest turn of tropical systems, one must always be mindful of the possible requirement of evasive action to avoid any potential impacts from such systems. Fortunately, in our advanced satellite age, and with the wealth of real-time data that we Meteorologists have at our disposal, these systems almost never come without warning, and with proper planning and guidance, the chances of any direct impacts is slim to none. Aside from the tropics, trade winds dominate dAring the summer season. Cold fronts remain well to the south (generally south of 35S) and weaken readily as they move into the extreme southwest Indian Ocean. Persistent high pressure covers much of the Indian Ocean along and south of 15S during December/January, and is about as large and strong as one might expect at any point in the year, showing only the slightest and most subtle changes in coverage and strength, governed mainly by the location of the Equatorial trough and the approach of any weakening fronts farther west (See Figure 1). East to southeast winds are most commonly found during summer, with speeds generally from force 4-6, and E-SE combined seas averaging from 4-7 feet. Higher winds (more like force 5-6) are found where there is greater interaction between persistent ridging (high pressure) across the area, and the Equatorial Trough to the north. This will generally be confined to waters south of 10S-15S. Breaks in the E-SE trade wind regime develop when ridging weakens, normally occurs as cold fronts move into the southwest Indian Ocean and weaken. As this occurs, winds veer some, becoming NE-E, with speeds generally from force 3-5, and longer-period Easterly combined seas generally from 4-7 feet. Note that protected areas (in immediate coastal waters of western Madagascar for example) tend to see somewhat lower seas (toward the lower end of the combined sea ranges), as "fetch" becomes more limited. Later in the year (March through May), we talk less about the tropics to be sure, as cyclone frequency decreases. In fact, by late April/May, we are hardly talking about the tropics at all, but cold fronts do begin to enter the weather picture. Fronts extend farther north than their summertime counterparts, and by May are found as far north as 30S, "emerging" into the southwest Indian Ocean about every three to four days, and weakening as they encounter the persistent semi-permanent ridge off to the east. Speaking of the semi-permanent ridge, it is still a dominant feature this time of year, though at Figure 1 -- Typical summer weather patterns across the Southwest Indian Oceans. The directions of tropical cyclones are denoted by the arrows pointing west to southwestward.
2010 - 2011