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The Active and the Tranquil Equatorial and Southwest Indian Ocean

In this part of the world offers many weather considerations…
From cold fronts to tropical cyclones; the weather can be volatile. However, serenity and tranquility are also words not unknown to this region, and this is common in many areas, often lasting for extended periods of time.

By David Cannon of Weather Routing, Inc.

Seychelles to the Chagos Archipelago
For the most part, this tends to be one of the quieter areas that we will be looking at, especially farther north. Squally conditions will typically be the main issue, and during certain times of year, we are thinking a little bit about the tropics (though not as much as we’ll find farther south…more on this later). For the most part, our seas will tend to have less wind chop to them, and more swells associated with them; a treat to vacationers and surfers alike who are looking for the perfect destination.

Squall clusters are typically found within a broad west to east oriented trough of low pressure, the northern extent of which during the summer season (December into March) is generally found across the Equatorial Indian Ocean, between 05N and 05S. Little overall motion of the trough can be expected, only northward and southward “oscillations”, as ridging from high pressure farther south also shows “subtle” changes in location and coverage/strength.

Squall areas within the trough generally move from east to west, and tend to be disorganized, bringing periodic localized higher winds and seas, mainly confined to areas within or very close to squalls. Effects from organized tropical cyclones are rare, especially farther west, in and near the Seychelles. Any effects at all are generally which cyclones are in their formative stages of development, and cyclones with time will turn more toward the south and southwest, as they encounter “weaknesses” in ridging farther west, on the northwest and west sides of highs.

Otherwise, the weather during summer is quiet. Wind directions are variable, and are dependent on the exact location of the axis of the aforementioned trough. There is a tendency toward more easterlies and southerlies (on the south side of the trough), with wind speeds at their strongest only from force 3-4 (outside of squalls). Seas across the region will tend to consist mainly of long-period E-SE swells, a byproduct of the persistent ridging off to the south. Heights will generally be of no more than 5-7 feet, with the largest and shortest-period seas often found farther south, including exposed waters on the southern extent of the Seychelles and Chagos Archipelago.

As we go on in time, into autumn and early winter (later March into June) the trough begins to show a general northward migration, north of the Equator, and by winter, equatorial regions are more on the south side of the trough. Interaction between the trough and the high pressure ridge to the south will bring somewhat higher E-SE trades to the area, but generally of no more than force 5-6. Lighter winds will be found the farther north one travels, where trough/ridge interaction is less. Likewise, seas will tend to be longer-period and to a lesser extent lower farther north, though as a whole, combined seas will tend E-SE, averaging from 4-7 feet, the largest of which occurring where “surges” of higher southeast winds are found, which often mix in with long-period southerly and southwesterly sets, well north of any cold fronts. These conditions are more prevalent during the winter season as well (June into September), as trough/ridge interaction becomes most prominent across the region.

During the remainder of the year, the trough gradually makes its return south of the Equator. This means the higher wind surges that are found during much of the autumn and winter are less frequent and less prolonged, and larger long-period swells are less likely to impact the region. General wind directions are E-SE, speeds averaging from force 3-5, tending lower closer to the trough axis (light and variable winds are also common along or very near the trough axis itself). Combined seas will generally consist of long-period E-SE swells, generally of no more than 5-6 feet, tending highest in southernmost waters, including the exposed and open waters near the southern Seychelles and the southernmost Chagos Archipelago.

Currents across the region generally run from west to east, within the Equatorial Countercurrent, which is actually (in part) a return flow from the North Equatorial Current, found just off to the north in the northern Indian Ocean. Speeds within the Equatorial Current average close to 1.0 knots, though faster speeds are often noted, especially during the summer season, when speeds run from 1.0-1.5 knots.

Seychelles to Mauritius / Reunion and Madagascar / Mozambique Channel
This area is not without its dangers, and we have a little more to talk about in this region. The weather tends to be more volatile than areas farther north, and the effects from tropical cyclones and (to a somewhat lesser extent) cold fronts are always to be considered when traveling in this part of the world.

Paramount in importance is the tropics. Tropical cyclone frequency increases during the late spring and early summer (November/December 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 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 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).

During the remainder of the year (September through October), the weather is certainly active, though cold fronts tend to be somewhat weaker and less frequent than during winter. Generally speaking, one can expect a frontal passage to occur about every three to four days, with fronts extending as far north as 25S-30S, moving well into the southwest Indian Ocean before encountering ridging from high pressure farther east and slowing/weakening later on.

As for our semi-permanent high pressure ridge, that particular feature is still prevalent, and during this period generally extends as far north as 15S. As with winter months, the ridge will be reinforced and intensify some as transitory high pressure ridges farther west move into the Indian Ocean and eventually merge with the semi-permanent ridge, again, inducing surges of higher E-SE trade winds across the region.

However, the greatest weather dangers occur as fronts approach and pass farther to the south, particularly in and near Mozambique Channel and much of Madagascar. Post frontal passage S-SW winds can still reach as high as gale force, especially within Mozambique Channel, partly due to local funneling of winds in the area, and as stronger fronts pass. The faster north to south currents also aid in bringing larger post-frontal passage S-SW seas, with such seas reaching as high as 10-12 feet as S’ly sets run against the faster currents across the area.

Outside of Mozambique Channel, currents across the area generally run from east to west, and northeast to southwest on the north and northwest sides of the South Equatorial Current. By and large, slower currents are found, averaging anywhere from 0.4-0.7 knots outside of the faster current areas mentioned previously.

South of Madagascar / Mozambique
Channel to South Africa
We’re looking farther south, and as the more seasoned veterans of the sea can attest to, we’re looking at an area that is not without its weather dangers and bouts of adverse conditions. The worst of the conditions occurs during the winter season, as stronger gales/storms pass off to the south, and their associated strong cold fronts approach and pass from the west. With the approach of fronts, winds veer, becoming north to northwest to west, increasing and often approaching gale force. With the passage of fronts, winds shift, becoming S-SW, and periods of gale to storm force winds often occur, especially along and near the southern African coast. These winds often last about 24-36 hours before high pressure moves in from the west and winds ease.

Post-frontal passage seas are typically from the south, southwest, and west, and can be quite large, often reaching as high as 12-15 feet. Especially large seas will be found within the fast east to west moving Agulhas Current along the South African coast, and in southwest to westerly sets. Larger seas typically last for about 1-2 days at a time, abating as lighter winds (from following high pressure ridges) develop and persist.
High pressure ridges moving into and across the region between cold fronts bring fairly significant breaks in the weather as winds veer becoming more easterly in direction over time. Winds can be as low as force 3-4 very near the centers or axes of these high pressure areas, and seas then turn abate, leaving mainly residual S-SW swells, which become long-period as time wears on and lighter winds continue to persist.

Later in the year, during the Spring and much of the Summer, the weather becomes increasingly less volatile, in that the effects from cold fronts, while still evident, are not to the degree of winter months. Fronts only pass through the region about every four days or so during late spring and summer, often remaining south of the South African coast during the period as well.

Pre and post frontal passage winds are typically below gale force, especially during late Spring and Summer, when winds usually max out at force 6-7, and are mainly confined to coastal waters of South Africa, and follow the strongest of frontal passages. Combined seas behind fronts likewise are not as high during this period as during Winter, though S-SW sets can reach as high as 10 feet along and near coastal South Africa. Similar Westerly seas can be found in larger sets within the Agulhas Current.

Transitory high pressure ridges still make their presence known well into the spring, again, bringing continued veering in winds toward more easterlies, and easing conditions as well, speeds again as low as 3-4, generally for one to two day periods before the next front approaches from the west, and winds veer further to more northerlies and increase some. Long-period south to southwest seas become longer-period and mix with and/or give way to more E-SE sets, often as low as 3-6 feet, tending higher the farther south one travels.
Late in the summer and during autumn, cold fronts return and become stronger as the weeks and months wear on. Frontal passages occur along the South African coast about every three to four days, bringing northerly and westerly winds that during much of the autumn reach as high as force 6-7 across much of the region. Localized areas of gale force (force 8) winds occur along the South African coast as well. Combined seas become northerly and westerly as well, building and often reaching as high as 7-8 feet.

With the passage of fronts, southerly and westerly winds often reach gale and near gale force, with higher winds found the farther south one travels. Combined seas become southerly and westerly as well, building and often in excess of 10 feet in many areas, especially along and near the South African coast. These higher winds normally last for as much as 36-48 hours at a time before further veering and easing of winds and abatement of combined seas occurs as high pressure areas approach/pass, in the wake of fronts.

South of the South African Coast
to Kerguelen and Vicinity
Well this area is about as active as it gets in the Indian Ocean, and this region is certainly not for the faint of heart. Frequent gales and storms affect this region, and adverse weather is commonplace as well. “Breaks” in the weather occur, but they tend to be more relative breaks than anything else. Finding calm winds and smooth seas is really a tall order here.

Gales and storms generally track from west to east between 45S-55S, and are generally spaced about 2-4 days apart. They are largest and strongest during the winter season, generally during the July/August timeframe, when the large semi-permanent high pressure ridge in the southern Indian Ocean is strongest. Subsequently, the worst weather in this region occurs during this particular time period. Gale to storm force winds occur as gales/storms pass through and interact with ridging off to the north.

Additionally, large seas commonly occur across this region, with heights 12feet or greater occurring about 60% of the time during the period from May through September. During the summer, the frequency of such large seas is reduced, occurring about 40% of the time during that time of year, as gales tend to be somewhat weaker, less expansive, and track a bit farther south than their winter counterparts. Winds during summer of course tend to be somewhat lighter as well, though individual gales can still bring winds approaching if not reaching gale force (force 8).

The concerns of encountering a tropical cyclone in this part of the world are nil. The waters are too cold and the environment in the upper levels of the atmosphere is far too hostile to sustain any tropical entities. However, as tropical cyclones farther north transition and become non-tropical (or extratropical), they can at times affect this region, bringing additional bouts of heavy weather with their approach and passage.
Being so far south in the Southern Hemisphere, you might expect the climate to be cold, and you would be right. The climate is cold and windy, with rain, sleet, and snow falling on the majority of the days of the year. It is not excessively cold, and the buildup of sea ice or the passage of icebergs is typically not of concern here.

In Summary: Where To Go and When
So far, we’ve taken a rather in-depth look at the weather in this part of the world, but what does it mean when it comes to finding the best weather? Well, this concluding section will endeavor to answer that question.

Taking everything into consideration, summer is your time of year if you want to maximize your travel options. During that time of year, cold fronts are weaker, less frequent, and do not extend as far north as during other times of the year, so adverse weather naturally does not cover as wide an area during summer.

Of course, there ‘s one very important drawback to consider for summer travels, and that’s the tropics. One will need to keep a close eye on the weather, especially in areas where tropical cyclones are of concern, such as across Madagascar and in the Mozambique Channel. Keep routing options open and consider stoppage or “bail out” ports where necessary to maximize berth around cyclones should they become a factor in your travel plans.

Equatorial destinations such as the Seychelles will tend to be a more favorable travel destination during much of the year for those looking to avoid bad weather on a grander scale time wise. Cold fronts are unheard of and the effects from tropical cyclones are more during the formative stages of development of such systems, as they pass off to the south of this region. Seas will tend to be long-period, consisting mainly of easterly and southerly swells, generally in the wake of cold fronts well to the south, and tropical systems farther south.

Those travelers farther south (south of 25S) during the period from late autumn through early spring will want to time their travels in between the passage of cold fronts, as centers or axes high pressure ridges approach and pass, bringing lighter winds and lower, longer-period swells. Those who are looking to travel south of the South African coast should certainly be well-prepared for what lies ahead, as adverse weather will quite often be of concern in this region during almost the entirety of the year.

So there you have it, a look at the weather of the southwestern Indian Ocean. As you can see, Mother Nature has something for everyone in this neck of the woods. From quiet, tranquil weather, to the active and tempestuous, this region has it all. Hopefully this editorial will educate as to what to look for and what to avoid in finding that perfect travel destination.