THE 2012-2013 CYCLONE SEASON

IN THE SOUTH-WEST INDIAN OCEAN

The activity of cyclone season 2012-2013 was practically in perfect accordance with the norm, whether in terms of number of days of disturbed activity or number of cyclone days. Only one element somewhat departing from the norm must be underlined however, namely the high number of cyclones that developed, since out of the ten tropical storms which formed, seven of them evolved into mature tropical cyclones (of which three became intense cyclones). A proportion that was remarkable, without being exceptional. Out of the ten meteors, four of them had an influence on the inhabited lands, but only cyclone HARUNA claimed a number of victims (in Madagascar).

So the disturbed activity was maintained at a sustained level during that season, although having been slightly reduced compared to the 2011-2012 previous season. If the final number of ten storms observed over the South-West Indian Ocean basin situates that 2012-2013 season slightly above the climatological average (which corresponds to 9 storms as a reminder), the number of days of disturbed activity, a much more representative parameter of the actual cyclone activity (as it aggregates both the number of phenomena and the cumulated days of their respective life-cycles), indicates an activity that is perfectly in keeping with the climatological average. Whether it is for the number of cumulated days with the presence over the basin of a system at the stage of tropical storm or of tropical cyclone, which was 52 (therefore almost equivalent to the climatological average set at 51 days – for a median set at 48 days) or for the number of cyclone days (i.e. with the presence over the basin of a mature tropical cyclone – with hurricane force winds), which precisely corresponded to the reference climatological average (that is 19 days), it is indeed difficult to be closer to the normal figures.

In view of the high number of cyclones that developed that season, one might have expected that average value of 19 cyclone days to be exceeded, but such was not the case, because of the duration of the tropical cyclone stages which remained limited in time (3 days in general, for a maximum of 4 for tropical cyclone CLAUDIA alone). And if the proportion of cyclones (seven out of ten tropical storms that formed) was noteworthy (remember that the norm is rather a rate of about 50% of tropical storms that reach then the stage of tropical cyclones), one is however far from a record. Thus, during the 2001-2002 season (that of tropical cyclone DINA in particular), out of the eleven tropical storms that had formed, nine had reached the stage of mature tropical cyclone (of which six became intense cyclones)…

The 2012-2013 season started in a staggering way, as it kicked off extremely early, before mid October (knowing that only five or six seasons have had their first tropical storm budding sooner in the calendar since the beginning of the satellite era – i.e. since 1967), but above all with a first cyclogenesis that resulted in a totally exceptional and unprecedented event, ANAIS becoming the first intense tropical cyclone observed in October in the South Indian Ocean. That phenomenon was all the more out-of-norm as it was eventually the most intense cyclone of the season, which was out of the ordinary for a system that developed at the very beginning of the austral spring! One other thing, more anecdotal to be sure, but also unusual: that cyclogenesis occurred over the West of the basin (i.e. to the west of the Chagos Archipelago), whereas the first phenomena of the season often form farther to the east (frequently to the southwest of Sumatra, between longitudes 80° and 100°East).

It was not until the end of November that the second mature system of the season developed (there was nothing abnormal about that on the other hand). Then the storms followed one another, without overlapping in time, which is worth noting, since there was no coexistence at any time over the basin of two systems having reached a significant intensity (only HARUNA’s pre-genesis phase coincided with the end of GINO’s life-cycle). But it is remarkable and rather incredible that none should reach the maximum of intensity at which ANAIS had peaked as early as 14 October, in spite of the development of six new cyclones, of which two others were ranked as intense…

The activity peak took place in a classic way in the heart of the season, in the months of January and February, which concentrated five of the ten phenomena of the term. Then followed a period of almost five weeks without any system forming, hence a month of March unaffected by any recorded system whatsoever, a situation that had not occurred for almost thirty years, since the similar previous case dated back to the 1983-1984 season. The season came to an end lately, around mid May, with the cyclolysis of tropical storm JAMALA; in terms of date of season’s end, the fact ranks that season after the last quintile. As it was spread all in all over more than six months, the 2012-2013 season has thus been one of the longest in contemporary history.

The cyclogeneses stemmed from two prevailing zones: the far East of the basin, with half of the systems (five therefore) that originated from an area situated between longitudes 80° and 90°East, and the (wide) zone around the Chagos Archipelago (for four of them). Only one system concerned the Mozambique Channel (HARUNA). The centre of gravity of the formation areas was therefore somewhat displaced to the eastern part of the basin, and as the zonal tracks did not prevail, especially for the mature cyclone phenomena, it resulted in the inhabited lands concentrated in the western portion of the basin, being less exposed to the cyclone hazards, and more particularly so for Madagascar.

As a matter of fact, none of the systems that formed to the east of longitude 80°East approached the lands either from near or far, whereas those coming from the area of the Chagos headed south before reaching the Great Island, and thus concerned only the Mascarene Islands and the related outposts (the Agalega and Saint-Brandon Archipelagos, and the Islet of Tromelin). The eastern coast of Madagascar, frequently impacted, was thus spared during that season, which had not happened since the 2004-2005 season (for eight years then). And the Great Island only suffered from HARUNA’s passage, the sole phenomenon that formed in the Mozambique Channel and which hit the southwestern seaboard of Madagascar at the tropical cyclone stage. Southern Africa, and the Mozambique in particular, which had been affected several times during the previous season, enjoyed an extremely quiet season this time, without being concerned by the slightest significant phenomenon.