REVIEW OF THE 2010/2011 CYCLONE SEASONS
Report of RSMC La Réunion
(RSMC La Réunion – Tropical Cyclone Centre, France)
The 2010-2011 cyclone season in the South-West Indian Ocean will go down in history for
its very low activity, almost unprecedented for more than fifty years, as it was ranked as the
second-least active season over that period. The reasons for the lack of activity are likely mostly
related to the combined powerful anomalies of the oceanic and atmospheric circulations which
ruled over the tropical Indian Ocean during the warm season and which resulted, on the oceanic
level, in a very notable deficiency of the oceanic heat content, and, on the atmospheric level, in a
large-scale subsidence over the whole western part of the tropical Indian Ocean.
To the much reduced cyclone activity has corresponded a season that was also quieter
than usual for the populations of the area, but not totally deprived of all impact though: if a single
storm has hit one of the inhabited lands (cyclone BINGIZA), it all the same had harsh and painful
consequences for the inhabitants of the North-East of Madagascar.
Only three meteors have been granted a naming during the 2010-2011 season (cyclones
ABELE in December and BINGIZA in February, plus moderate tropical storm CHERONO – named
on 17 March, which was the latest birth-date for the third tropical storm of any season in the basin).
Since 1960, when the tropical systems began to be named in the South-West Indian Ocean, no
season had ever come to an end with fewer than four named systems. As it set up that new
record-breaking value of the smallest number of named phenomena in a season, the year 20102011 would then be entitled to claim the title of the least active cyclone season of the last 50 years.
However, a more thorough evaluation compels you to realize that reality is not as simple
and forces you to relativize and even to rectify that opinion.
First of all, a fourth system could have possibly deserved to be named: subtropical
depression number 09 (in April). But because of its particular origin and structure (as well as its
location) it was eventually decided otherwise. Yet, that ambiguity about that system does not have
any consequence for the question of the record of inactivity or not, as that type of subtropical
phenomenon would certainly not have been named either in the past, in view of the naming
practices and procedures applied at the time.
On the other hand, the same procedures, different a few decades ago from what they are
today, resulted in systems of weak intensity (tropical depressions, even mere tropical disturbances)
being named in the past, whereas they would not be nowadays. Thus during the 1982-1983
cyclone season, out of the six meteors named at the time, three would not have been in the
present context, which virtually ranks the 1982-1983 season at the same level of activity as that of
the 2010-2011 season.
Lastly, the number of phenomena cannot alone characterize the activity of a cyclone
season and it is usually preferable to consider rather another metric, like the number of days of
disturbed activity, a much more representative criterion of actual cyclone activity (as it integrates
both the number of phenomena and the cumulated days of their respective life-cycles). According
to that criterion, the three named systems of the 2010-2011 season total up 13 cumulated days of
significant disturbed activity (i.e. with the presence in the basin of a system at the stage of tropical
storm or cyclone), that is to say the same number as during the 1982-1983 season. But as it is
necessary to add the 4 days of activity linked to subtropical depression n°09, the 2010-2011
season must be considered as not being equivalent to the 1982-1983 season, which therefore
remains the season that may be deemed as the really least active of the last 50 years.
An objective analysis of the archives hence leads to the conclusion that the title of the
season with the smallest number of named phenomena would – de facto – go to the 2010-2011
season, but the latter would not for all that establish a new record of low activity (which would still
belong to the 1982-1983 season), simply becoming the second-least active cyclone season in
50 years.
The decreasing trend of the disturbed activity which has been observed for the last three
years, has therefore been going on this year, so much so that it led to an almost historic low level.
The following question must then naturally be asked: “what are the reasons that could explain that
exceptional weakness of the cyclone activity in the South-West Indian Ocean?”
Generally speaking, to break records, there must be exceptional circumstances, or a
combination or a series of facts notably departing from the norm. In that case, if we cannot be
absolutely sure about the reasons for that exceptional listlessness of the cyclone activity over the
South-West Indian Ocean basin, the conjunction of two factors may nevertheless be suspected: a
global context and a more particular factor peculiar to this year, namely the very special climate
conditions prevailing over the whole Indo-Pacific zone, during the 2010-2011 warm season in the
southern hemisphere.
The global context is that of the quite significant decrease in the cyclone activity observed
for 4-5 years world-wide. If the decline of the activity which has been taking place during the last
few years in the North Pacific had a prevailing influence on that decrease, it is in fact a rather
general tendency, the Atlantic basin being a real exception. Thus it is the third consecutive season
that the South-West Indian Ocean has experienced a low level of activity. That global deficiency
has brought back the world-wide cyclone activity to a level that had never been known for about
thirty years and since the preceding major slack period of inactivity at the end of the seventies.
Whether it is a simple decennial or multi-decennial natural cycle, or the sign of a deeper and
lasting change, the reasons for that current bout of fainting fit of the global cyclone activity remain
unknown at present.
To that acknowledged fact of a global decreasing trend observed over several years, have
been affixed specific circumstances peculiar to the 2010-2011 warm season, and related to the
unusual situation which prevailed over the whole Indo-Pacific area, where there was an important
anomaly of the atmospheric circulation. The latter resulted in a modification in the zoning and
intensity of the upward and subsident branches of the cells associated to the Walker circulation. As
the main consequence, it generated abnormally moist conditions in the eastern tropical Indian
Ocean as far as the South-West Pacific, hence excessive rainfall over Indonesia and Australia (in
Australia the period from September 2010 to March 2011 was the wettest on record – 100% above
average) and induced widespread and disastrous flooding (Brisbane in southeast Queensland had
its worst floods since 1974), and a sustained cyclone activity around the latter (but eventually
simply in accordance with the climatologic norm); whereas, on the contrary, abnormally dry
conditions and quiet conditions in terms of disturbed cyclonic activity, prevailed over the western
part of the Indian Ocean and the central Pacific. Eastern Africa paid a heavy toll for the extreme
drought that resulted (the town of Wajir, in northeastern Kenya, only received 73 mm of rain
between October 2010 and September 2011, its driest 12-month period in the post-1950 era). This
drought, rated as one of the three most severe droughts of the last 60 years over eastern and
northern Kenya, was to be blamed for the awful famine that wreaked havoc in the African Horn.
But it is worth noticing that at the same time most of southern Africa benefited from surplus rainfall
(with some flooding occurring on several occasions as a counterpart). Thus, rainfall from JanuaryMarch 2011 was two to four times the average over many parts of a region encompassing parts of
South Africa, Zimbabwe, Botswana, Namibia and Angola.
That aforementioned atmospheric anomaly was coupled with an oceanic anomaly. Both
were under the influence of a particularly powerful La Niña event (one of the most intense of the
last six decades) which had rapidly developed during the 2010 austral winter (and had thus
followed the El Niño conditions of the 2009-2010 summer). Of course the La Niña primarily rules
over the equatorial and tropical Pacific, but it also contributed to affect and modify the atmospheric
circulation far beyond, in particular over the Indian Ocean, whereas on the oceanic level, the
prominent cooling of the central and eastern Pacific waters associated with La Niña, came along
with a warming up of the waters in the Australian region.
At the same time, the end of that austral winter witnessed the development of a negative
phase of the Indian Ocean Dipole (IOD), which then reached its climax in October, before rapidly
weakening (in a rather classic way). Manifesting through a warming up of the waters of the eastern
Indian Ocean, in the vicinity of Indonesia, that negative phase of the IOD combined with the effects
of La Niña to generate strong warm anomalies over the whole oceanic region of the South-East
Indian Ocean (anomalies frequently exceeding +2°C between Indonesia and Australia for the sea
surface temperatures).
On the atmospheric level, the negative phases of the Indian Ocean Dipole are usually
associated with westerly wind anomalies in the near-equatorial zone of the Indian Ocean. That was
actually observed during the 2010 episode, with a westerly wind anomaly in low troposphere,
particularly strong over the East of the basin. But above all, that westerly wind anomaly lasted long
after the end of the negative phase of the IOD, and persisted during the whole warm season (until
April 2011). It reached a peak between 20 November 2010 and 20 January 2011, with westerly
wind anomalies reaching 20 knots over the South-East Indian Ocean on monthly average. As the
ongoing La Niña episode brought about at the same time easterly wind anomalies over the nearequatorial western Pacific, an exacerbated convergence settled on a long term basis on a large
scale in low troposphere over the whole area around Indonesia and Australia (the counterpart in
altitude being a strong divergent anomaly). The large-scale upward motion which resulted from it
brought about an increase in the cloud covering and the convective activity over a wide area
extending from southern Asia to Australia, with excess rainfall (making 2011 the Australia’s
second-wettest calendar year since national rainfall records began in 1900 – the wettest year
remaining 1974, also a La Niña year).
To that enhanced upward branch of the Walker circulation corresponded subsident
branches that were also boosted, both over the central and eastern Pacific and the western Indian
Ocean, where subsidence reached its culmination in January. That marked subsidence logically
had a hampering influence on the cloud and convective activity, and, as an indirect consequence,
on the cyclone activity, which, in fact was almost non-existent for two months between December
and early February.
This being said, if the preceding explanation clearly makes sense, it would be risky to try
and extrapolate generalities from it. Thus, attempting to try and establish a supposed link between
an ongoing La Niña episode over the Pacific and the cyclone activity over the South-West Indian
Ocean will very soon lead to a dead end. As a matter of fact, if one looks at the past, one notices
that during the La Niña years, no clear tendency emerges: anything may happen in terms of
cyclone activity over our basin, even a cyclone activity above the average, as was the case during
the 1988-1989 season, the last occurrence of a La Niña event whose magnitude can compare with
that of 2010-2011. Ditto if one focuses more specifically on the cyclone seasons associated with
La Nina episodes and preceded by a negative phase of the Indian Ocean Dipole. Since 1958 there
have been four other years of the same type: in 1964, 1971, 1974 and 1975. The last three
mentioned were followed by cyclone seasons that could be qualified as almost normal over the
South-West Indian Ocean, whereas the 1964-1965 season ranks among the most active of the last
50 years. There is no comparison with the year 2010-2011 hence. Finally, it should incidentally be
recalled that the reference year in terms of inactivity, the 1982-1983 season, which was previously
mentioned several times, was under the influence of a major El Niño event… Once again all this
highlights the extremely low predictability of the cyclone activity over our basin, even in the
presence of very strong and conspicuous climate signals, both for the austral oscillation and the
Indian Ocean Dipole.
However, an additional important element attracts our attention: during almost the whole
atypical 2010-2011 warm season, the oceanic heat content remained much below normal over
most of the tropical Indian Ocean lying between latitudes 5 and 15°South and to the west of
longitude 90°East, i.e. over the usual cyclogenesis area where the great majority of the storms of
the South-West Indian ocean basin form (including in particular the privileged cyclogenesis area
corresponding to the sector situated to the south of the Chagos Archipelago). Whether that
powerful negative anomaly of the oceanic heat content (much more spectacular than the one
exhibited by the sea surface temperature) took place in connection or not with the settling of
strongly subsident atmospheric conditions over the basin (and of equatorial westerly winds
abnormally strong over the East of the Indian Ocean), there is no doubt that it played a major part
in the notable weakness of the cyclone activity over the South-West Indian Ocean, as it certainly
accounted to a large extent for the total absence of cyclogenesis observed this season within the
wide zone to the south of the Chagos (between longitudes 65 and 85°East).
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