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T. S. M. Randriamahefasoa , C. J. C. Reason Interannual variability of rainfall characteristics over southwestern Madagascar
The interannual variability
of daily frequency of rainfall [>1 mm/day] and heavy rainfall
[>30 mm/day] is studied for the southwestern region of Madagascar,
which is relatively arid compared to the rest of the island. Attention
is focused on the summer rainy season from December to March at four
stations (Morondava, Ranohira, Toliara and Taolagnaro), whose daily
rainfall data covering the period 1970–2000 were obtained from the
Madagascar Meteorological Service. El Niño Southern Oscillation (ENSO)
was found to have a relatively strong correlation with wet day frequency
at each station and, particularly, for Toliara in the extreme
southwest. In terms of seasonal rainfall totals, most El Niño (La Niña)
summers receive below (above) average amounts. An ENSO connection with
heavy rainfall events was less clear. However, for heavy rainfall
events, the associated atmospheric circulation displays a Southern
Annular Mode-like pattern throughout the hemisphere. For ENSO years and
the neutral seasons 1979/80, 1981/82 which had large anomalies in wet
day frequency, regional atmospheric circulation patterns consisted of
strong anomalies in low-level moisture convergence and uplift over and
near southwestern Madagascar that made conditions correspondingly more
or less favourable for rainfall. Dry (wet) summers in southern
Madagascar were also associated with an equatorward (poleward)
displacement of the ITCZ in the region.
1 Introduction
Madagascar, the largest
island in the Indian Ocean, separated from the African mainland about
170 million years ago and is home to a variety of relatively fragile and
unique ecosystems. Oriented south south west to north north east, the
island extends roughly 600 km north to south and much less in the zonal
direction. Running along most of its meridional extent is a substantial
mountain range (Fig. 1)
which, due to the moisture-laden easterly trade winds from the South
Indian Ocean, leads to orographically enhanced rainfall along the east
coast. On the leeward side of the mountains (i.e., in the west and
southwest of the country), there is a relative rain shadow effect. The
highest peaks exceed 2600 m (Tsaratanana massif in the north, Ankaratra
and Andringitra massifs in the centre) with a plateau in the central
part of the island ranging in altitude from about 750–1500 m which thus
presents a significant obstacle to the low-level trade winds. These
easterly trades prevail throughout the year such that the eastern part
of the island is rainy in all months with only August and September
being somewhat dry (e.g., the station of Taolagnaro in Fig. 2c). Rainfall amounts in eastern Madagascar largely depend on the distance from the coast and the altitude (Pelleray 1954)
with several stations receiving on average over 2000 mm per year. By
contrast, southwestern Madagascar is much drier, receiving between 300
and 1000 mm annual rainfall.
Fig 1
Map of Madagascar
showing the topographic structure and the four rain-gauge stations of
Morondava (20S 44E), Ranohira (22S 45E), Toliara (23S 43E) and
Taolagnaro (25S 46E). Below the black line is the south western area, the main focus of the study
Fig. 2
Annual cycle of mean monthly rainfall amount (mm) computed for 1971–2000
The northern part of the
island experiences a monsoonal climate with the NE monsoon winds over
the western tropical Indian Ocean re-curving over the northern
Mozambique Channel to become rain-bearing northwesterlies. To its south
is the ITCZ which lies over central Madagascar in summer reaching as far
south as 17–20° S in February. The mean monthly rainfall amount is 470 mm during summer in northwestern Madagascar (Jury et al. 1995), whereas in the much drier southwest, it ranges from about 50 to 250 mm per month at this time of year (Fig. 2).
Disturbances in the easterly
trade winds, including tropical cyclones, are responsible for much of
the summer rainfall over the island. On average, the South West Indian
Ocean experiences about 9–11 tropical cyclones per season and many of
these systems impact on the eastern coast of Madagascar (Mavume et al. 2009).
The south west of the island is rarely impacted by tropical cyclones
although there are cases like TC Favio in 2007 that tracked close to the
south and southwestern coasts (Klinman and Reason 2008) and TC Dera in 2001 (Reason 2007)
that tracked southwards through the Mozambique Channel and passed quite
close to the southwestern coast of Madagascar. A few tropical cyclones
have tracked across the island influencing rainfall (e.g. Eline in 2000,
Reason and Keibel 2004).
Tropical-extratropical cloudbands, which are large convective systems
linking a tropical disturbance over the southern African tropics or the
Mozambique Channel to a frontal system passing south of South Africa
(e.g. Fauchereau et al. 2009; Manhique et al. 2011; Hart et al. 2013), are also important contributors to seasonal rainfall. As shown by Hart et al. (2013),
cloudbands frequently occur over Madagascar in the summer months with a
small peak in March before declining—in the 32 seasons analysed, none
occur between June and the beginning of October. In addition to
cloudbands, the southern part of the island also sometimes experiences
cutoff lows (Singleton and Reason 2007; Hart et al. 2013).
Lying to the southwest of
southwestern Madagascar on average is the South Indian Convergence Zone
(SICZ), a diagonal band of enhanced rainfall extending southeastwards
from southeastern Africa out over the midlatitude South West Indian
Ocean during summer (e.g. Streten 1973; Cook 2000) that is largely made up of the aggregation of all the cloud band events. Cook (2001)
argued that during El Niño events, the SICZ shifts northeastward as a
result of an El Niño Southern Oscillation (ENSO)-induced upper level
convergence maximum over the equatorial Indian Ocean. Such a shift in
the SICZ can also be interpreted as the seasonal aggregation of changes
in cloud band intensity and frequency occurring during El Niño
(Fauchereau et al. 2009; Hart et al. 2013). Cook (2000, 2001)
also suggested that a weakening of the western portion of the South
Indian Ocean anticyclone is part of the El Niño-induced impact in the
Madagascan region and leads to increased rainfall at least over the
central and northern part of the island. This weakening of the
anticyclone and development of a cyclonic anomaly southeast of
Madagascar during El Niño may be related to ENSO-induced changes in the
midlatitude stationary waves in the Southern Hemisphere (Lyon and Mason 2009).
However, examination of NCEP re-analyses, satellite rainfall data as
well as HadAM3 AGCM simulations forced with observed SSTs to assess the
ENSO response over southern Africa/Madagascar (Reason and Jagadheesha 2005)
suggests that ENSO impacts over this island are complex and that
interpreting these merely as a shift in the SICZ or as a weakening in
the South Indian Ocean anticyclone may be inaccurate.
Although there are numerous
papers on the rainfall variability of the southern African mainland,
rather little work has been done on that of Madagascar itself. Jury et
al. (1995) investigated the variability of summer rainfall over central Madagascar, and Nassor et al. (1997)
considered extreme rainfall events in the northwest of the island.
Almost no work has been done on the relatively dry southwestern region,
thus motivating the need for this study. Despite the passage of a few
tropical cyclones reaching southern Madagascar which have brought an
excess of rain and sometimes caused flooding, water shortages remain a
permanent problem here. This region is bordered by the Mozambique
Channel to its west and, to its east, the mountainous massif of Ivakoany
and the Anosy and Vohimena ranges which reach altitudes of 1000–1500 m
(Fig. 1).
Given the importance of rain-fed subsistence agriculture to the island,
interannual variability in the frequency of wet days during the summer
rainy season, heavy rain days and also the associated circulation
patterns are studied. These characteristics are chosen because wet or
dry spell frequencies are often of more interest and relevance for
subsistence farmers of crops like rice and maize than seasonal rainfall
totals (Usman and Reason 2004). The importance of wet day frequency for tropical regions has also been emphasised by Moron et al. (2007). In addition, monthly rainfall depth expectations and return periods of rainfall are calculated.
2 Data and methods
Daily rainfall data for
1971–2000 for three stations in the southwest (Morondava 20° 17′ S, 44°
18′ E, Ranohira 22° 33′ S, 45° 24′ E and Toliara 23° 22′ S, 43° 39′ E)
were obtained from the Madagascan Meteorological Service. For
comparison, data for Taolagnaro (25° 1′ S, 46° 59′ E) were also
analysed—this station lies near the southernmost point of the east
coast. Consequently, it is far wetter than the other three stations. All
of these stations are located at the coast except for Ranohira (Fig. 1)
which lies at an altitude of about 825 m and is located some distance
on the leeward side of the southern mountain ranges. Data records were
complete from 1971 to 2000 for the four stations. Note that these are
the only stations in the region that have sufficient daily data for climate variability studies.
Two categories of wet days
were considered. Firstly, a day was classified as wet if more than 1 mm
of rain was recorded for that day. If more than 30 mm of rainfall was
received, then that day was classified as a heavy rainfall day. Such
threshold definitions for wet days and heavy rainfall days have been
previously used in southern Africa as well as elsewhere in the world
(e.g. Weldon and Reason 2014).
Examination of the daily data for each station indicated that 30 mm of
precipitation recorded in a single day is appropriate to define this day
as one of heavy rainfall. Standardised anomalies of the frequency of
wet days and in heavy rainfall days were then computed for each summer
and correlated with circulation and SST fields. Time series are plotted
with the year corresponding to the January of that summer (e.g. the
1979/80 summer is plotted as 1980).
To consider relationships
with ENSO, correlations between these standardised anomalies and the
Niño 3 and 3.4 indices were performed using NOAA Extended Reconstructed
SST (Xue et al. 2003; Smith et al. 2008).
Relationships with circulation fields such as 850 hPa geopotential
height and omega at 500 hPa were investigated using NCEP re-analysis
data (Kalnay et al. 1996).
Moisture fluxes were computed at 850 hPa height as the product of wind
and specific humidity at this level. Seasons with anomalous numbers of
wet or heavy rainfall days were those whose departures exceeded 1
standard deviation from the mean.
3 Results and discussion
To assess rainfall seasonality, Fig. 2
shows the mean monthly rainfall amounts at the four stations. All three
of the southwestern stations show a summer peak in rainfall with
Toliara, located the furthest south, being the driest. The station near
the southernmost tip of the east coast, Taolagnaro, is considerably
wetter than the other three stations and has a much less obvious
seasonality. Only September is relatively dry at this station, similar
to elsewhere on the eastern coast (Pelleray 1954).
Figures 3, 4, 5 and 6
show the fitted rainfall probability plots for each summer month for
each station calculated from monthly data for 1960–2010. In some cases,
the rainfall amounts on the horizontal axis have been normalised so that
the graphical fit is close to linear. The vertical axis shows the
probability of exceedance of the corresponding amounts with Table 1
summarising the associated rainfall amounts for exceedances of 25, 50
and 75 %. For example, for Morondava in December of any year, Table 1
indicates that there is a 75 % probability that the rainfall amount for
that month will be 52 mm or greater, a 50 % probability that it will be
at least 124 mm and a 25 % probability that this station will receive
225 mm or more. By definition, the return period is the reciprocal of
the probability expressed as a fraction of the corresponding dependable
rainfall; thus, the return period of 225 mm occurring at Morondava in
December is 1/0.25 or 4 years. The return periods of larger rainfall
amounts can be estimated from the corresponding panel in Figs. 3, 4, 5 and 6.
When comparing the monthly rainfall expectations between the four
stations, the patterns suggested by the monthly averages plotted in Fig.
2
of Morondava and Ranohira showing strongest seasonality (maximum in
January with a strong decrease to the March values), Toliara showing the
same behaviour but much reduced amounts and Taolognaro showing the
least seasonality but a stronger March than February value are also
apparent in Table 1.
Fig. 3
Monthly rainfall probability plot for Morondava (blue crosses are rainfall amounts and the dashed line is the fitted probability distribution)
Fig. 4
Monthly rainfall probability plot for Ranohira (blue crosses are rainfall amounts and the dashed line is the fitted probability distribution)
Fig. 5
Monthly rainfall probability plot for Taolagnaro (blue crosses are rainfall amounts and the dashed line is the fitted probability distribution)
Fig. 6
Monthly rainfall probability plot for Toliara (blue crosses are rainfall amounts and the dashed line
is the fitted probability distribution). For March, 14 years received
less than 10 mm, which constitute 30 % of the total data. These years
were discarded and the fitted (lognormal) distribution is relative to
the rest of the data (70 %)
Table 1
Monthly rainfall depth expectations for December, January, February and March for each station
Station
Probability of exceedance (%)
December
January
February
March
Morondava
75
52
175
104
44
50
124
258
198
61
25
225
345
292
115
Ranohira
75
86
83
120
65
50
190
207
170
100
25
270
295
228
137
Taolagnaro
75
56
88
59
86
50
151
155
122
149
25
219
243
185
225
Toliara
75
14
32
26
0
50
53
71
48
28
25
117
125
113
49
Figure 7
plots the monthly distribution of the frequency of wet days at each
station with Toliara and Taolagnaro again showing a much reduced and
much increased value, respectively. For Morondava, Ranohira and Toliara,
the number of wet days increases significantly in either November or
December with peak values in January. During February, there is a small
decrease in wet day frequency followed by a sizeable decline in March,
the last summer month. From April to October, there are rather few wet
days on average. However, for Taolognara, all months show relatively
large numbers of wet days except September and there is a small relative
peak in January and February and then in April.
Fig. 7
Wet day frequency (number of days per month) computed for 1971–2000
A time series of standardised summer rainfall anomalies for each station is given in Fig. 8
with El Niño (La Niña) summers denoted as red (blue) columns and
neutral as unshaded. It is clear that there are several neutral summers
at each station that are either substantially wet or dry. However, of
the 7 El Niño summers, 5 (Taolagnaro and Toliara) or 6 (Morondava and
Ranohira) summers received below average rainfall. For La Niña, 4 out of
6 summers received average or above average rainfall. Thus, Fig. 8
suggests that there may be a link between ENSO and rainfall amount in
southwestern Madagascar. Further west, note that a relatively strong
ENSO rainfall linkage is found over the southern African mainland
(Reason et al. 2000; Reason and Jagadheesha 2005; Ratnam et al. 2014).
Fig. 8
Time series of standardised anomaly in summer rainfall amount for each station. In each case, red corresponds to El Niño and blue to La Niña events
Figure 9 shows the number of wet days and the number of heavy rainfall days at each station through the record, whereas Fig. 10
plots the standardised anomalies in each parameter. It is clear from
the former that seasons with high numbers of wet days do not necessarily
have high numbers of heavy rain days and vice versa. From Fig. 10,
it is evident that most El Niño (La Niña) summers have below (above or
near-) average numbers of wet days and that there are also some neutral
seasons with large anomalies such as 1979/80 (dry) and 1981/82 (wet).
Correlations of the standardised wet day frequency anomaly at each
station with various ENSO indicators (SOI and the Niño 3, 3.4 and 4
indices) are shown in Table 2
(only coefficients at above 95 % statistical significance are shown).
The strongest correlations are between wet day frequency at Toliara and
any of the ENSO indices (r ranges between −0.50 and −0.61 for the SST indices and r = 0.60
for the SOI, all significant at 99 % or better). For the other
stations, the correlation coefficients with the various ENSO indices are
in the range −0.30 to −0.45.
Fig. 9
Number of days per summer season that are wet (blue part of each column) or receive heavy rainfall (red part of each column)
Fig. 10
Standardised anomalies in wet day frequency. In each case, red corresponds to El Niño and blue to La Niña events
Table 2
Correlation coefficients between the standardised anomaly of the frequency of wet days of each station and various ENSO indices
Index
Station
SOI
Nino 1.2
Nino 3
Nino 3.4
Nino 4
Morondava
0.37
−0.44
Ranohira
0.35
−0.33
−0.38
Taolagnaro
0.34
−0.33
Toliara
0.60
−0.51
−0.58
−0.61
−0.50
Only those correlation coefficients that are significant at the 95 % confidence level are shown
Although there are
relatively strong ENSO relationships for wet day frequency, this is not
the case for heavy rain day numbers (Fig. 11). Anomalously high frequency in heavy rain days can occur during both El Niño and La Niña events. Figure 11
shows that 1981/82 was a season with an anomalously large number of
heavy rainfall days at all stations whereas 1979/80 experienced very few
heavy rain days at all stations.
Fig. 11
Standardised anomalies in heavy rain day frequency. In each case, red corresponds to El Niño and blue to La Niña events
Correlations of the standardised anomalies in wet day frequency with global SST (Fig. 12)
show an ENSO-like signal for the three southwestern stations,
particularly Toliara. In the Indian Ocean, the correlation pattern is
also reminiscent of the mature phase of ENSO which typically shows a
dipole pattern of one sign in the tropical Indian Ocean and the opposite
sign in the southern midlatitudes (Reason et al. 2000).
These patterns suggest that local SST does not play an important role
in directly modulating the number of wet days; instead, ENSO may change
the atmospheric circulation patterns near southwestern Madagascar and
thereby influence rainfall. For example, assessment of a number of
individual ENSO events in NCEP re-analyses and an atmospheric GCM forced
with observed global SST anomalies indicated that changes in the
tradewinds near southern Madagascar and associated low-level convergence
could be linked to the observed rainfall amounts (Reason and
Jagadheesha 2005).
For Taolognaro, the ENSO signal is weaker and there is a region of
strong negative correlation in the southwest Indian Ocean. This signal
is in the opposite sense to what would be expected if local SST were
increasing the number of wet days, for example, by increasing the
moisture content and instability of the lower atmosphere in the region.
Instead, it implies that the changes in wet day frequency at Taolognaro
could be caused by regional atmospheric anomalies that also modify the
local SST. For heavy rain day frequency (Fig. 13),
only Toliara indicates a clear ENSO signal and only Taolagnaro has a
correlation signal (−0.2 to −0.3) in the southwest Indian Ocean. For
Morondava and Ranohira, the spatial correlations show little in the
Indian Ocean or in the tropical Pacific.
Fig. 12
Correlation between wet day frequency anomalies and SST. In each case, coloured areas are statistically significant at 95 %
Fig. 13
Correlation between heavy rain day frequency anomalies and SST. In each case, coloured areas are statistically significant at 95 %
To assess regional atmospheric patterns, the wet day time series was correlated with 850 hPa geopotential height (Fig. 14).
Each station shows the general increase (decrease) in geopotential
height in the tropics/subtropics across the western Pacific and Indian
Oceans associated with El Niño (La Niña) (Reason et al. 2000)
which, again, is strongest for Toliara. In addition, there is an area
of relative strong negative correlation (implying cyclonic conditions)
in the Mozambique Channel/Madagascan region. Such a regional circulation
pattern would be favourable for both tropical extratropical cloudbands
and tropical storms/cyclones and hence increased wet days and rainfall
over southern Madagascar. As implied by Fig. 15
which shows the mean summer moisture flux and its divergence at the
850-hPa level, the cyclonic pattern in the Mozambique Channel/Madagascan
region in Fig. 14
effectively reinforces the climatology, thereby indicating a wetter
rainy season than average. Correlations between wet day frequency and
omega at the 500-hPa level were also performed (Fig. 16).
Omega indicates relative uplift or subsidence, and it was found that
there was a relatively strong negative correlation over the Mozambique
Channel and southern Madagascar. Thus, as expected, there is relative
uplift in the midtroposphere for seasons with increased wet day
frequency.
Fig. 14
Correlations between wet day frequency anomaly and 850-hPa geopotential height. In each case, coloured areas are statistically significant at 95 %
Fig. 15
Mean DJFM moisture flux and divergence at the 850-hPa level
Fig. 16
Correlations between wet day frequency anomaly and 500-hPa omega (coloured areas are statistically significant at 95 %)
For heavy rain day frequency (Fig. 17),
the tropical and ENSO circulation patterns tend to be smaller by
comparison to those for wet day frequency, and now, there is a signal
reminiscent of the Southern Annular Mode (SAM) (e.g. Thompson and
Wallace 2000; Reason and Rouault 2005)
for both Morondava and Taolagnaro and, to lesser extent, at Ranohira
and Toliara. These patterns suggest that when the SAM is in its positive
phase, heavy rainfall over southwestern Madagascar is more likely to
occur. In this phase of SAM, the subtropical anticyclones shift
southward with easterly wind anomalies in the subtropics. The
high-pressure anomalies in the midlatitude region near South Africa
associated with this SAM-like pattern are located such to imply
increased easterly wind flow of moist unstable air from the South West
Indian Ocean towards southern Madagascar. This feature, together with
weak negative correlations in the southern Mozambique Channel and
southern Madagascar which imply relative cyclonicity there, are
favourable for heavy rainfall.
Fig. 17
Correlations between heavy rain day frequency anomaly and 850-hPa geopotential height. In each case, coloured areas are statistically significant at 95 %
The correlation patterns
for omega at 500 hPa (not shown) are negative over Madagascar, the
Mozambique Channel and neighbouring South West Indian Ocean, consistent
with heavy rainfall over the southern part of the island.
4 Case studies—non-ENSO anomalous years: 1979/80 and 1981/2
Figures 8 and 10
show that there are some neutral seasons that were substantially wet or
dry and with large anomalies in wet day frequency implying that ENSO is
not the only factor of importance. For improvements in the
understanding of regional climate variability and the potential of
seasonal forecasting, it is therefore useful to investigate the
circulation patterns associated with neutral seasons with large
anomalies in rainfall characteristics. As case studies, this section
considers the neutral summers of 1979/80 and 1981/82 which stand out as
ones with anomalously low and high numbers of wet days, respectively, at
the three southwestern stations as well as in heavy rain days (Fig. 11). There is also anomalously low heavy rain day frequency at Taolagnaro in 1979/80 and the reverse in 1981/82 (Fig. 11c). Figure 18
shows the 850-hPa moisture flux anomaly over the region for 1979/80
indicating a large cyclonic anomaly and moisture convergence east of
Madagascar with relative divergence over the southwest of the island.
Globally, the 850-hPa geopotential height anomaly displayed a weak El
Niño-like pattern (not shown) with weak positive height anomalies over
southern Africa and southwestern Madagascar as well as the maritime
continent region (not shown). The ITCZ was weaker over the tropical
South West Indian Ocean and southeastern Africa during this season.
Furthermore, all of western Madagascar, and particularly the south west,
shows strong relative subsidence which extends across the Mozambique
Channel and most of southern Africa (Fig. 19).
This pattern is very unfavourable for cloud band occurrence across the
region or tropical storm development in the Mozambique Channel
consistent with the much reduced number of wet days and drier conditions
in this season. Furthermore, there were large cool SST anomalies over
the southwest Indian Ocean/greater Agulhas Current system south of
Madagascar (not shown), a pattern unfavourable for summer rainfall in
southeastern Africa/southern Madagascar (Reason and Mulenga 1999; Reason 2002).
Fig. 18
Anomalies in moisture flux (vectors) and divergence (contours) for DJFM 1979/80
Fig 19
Anomalies in omega at the 500-hPa level for DJFM 1979/80. Positive (negative) values indicate relative sinking (uplift)
During the 1981/82
summer, which had anomalously high wet day and heavy rain day
frequencies, the large-scale 850-hPa geopotential height (not shown)
displayed weak negative anomalies in the maritime continent region and a
wave number 3 pattern in the Southern Hemisphere with a strong
anticyclonic anomaly to the southeast of Madagascar which would enhance
the advection of moist marine air towards the region. Further north, a
large cyclonic anomaly established itself across the southern Mozambique
Channel from eastern Zimbabwe to eastern Madagascar (Fig. 20).
The strong westerly anomalies on the eastern side of the channel led to
substantial low-level moisture convergence over southwestern Madagascar
that extended to a second cyclonic anomaly to the south east of the
island. Coupled with this pattern was a region of strong relative uplift
in the midtroposphere that extended from south east of Madagascar
across the southwest of the island towards the central part of the
channel (Fig. 21).
These circulation anomalies were favourable for the development of
cloud bands and tropical storms across the southern part of the channel
and southwestern Madagascar leading to increased rainfall and wet day
frequency during this summer. SST anomalies during 1981/82 summer showed
a positive subtropical South Indian Ocean dipole pattern, favourable
for increased rainfall over southeastern Africa and southwestern
Madagascar (Behera and Yamagata 2001; Reason 2002).
Fig 20
Anomalies in moisture flux (vectors) and divergence (contours) for DJFM 1981/82
Fig 21
Anomalies in omega at the 500-hPa level for DJFM 1981/82. Positive (negative) values indicate relative sinking (uplift)
It is of interest to note
that the dry 1979/80 and wet 1981/82 summers over southwestern
Madagascar had, respectively, more and less cloudbands over the southern
African mainland than average (Hart et al. 2013)
and were also, respectively, wetter and drier over large parts of
Namibia, northern South Africa, Botswana and Zimbabwe. To our knowledge,
these types of opposing rainfall signals between subtropical southern
Africa and southwestern Madagascar have not been noted before.
Furthermore, there were less than average numbers of tropical storms in
the Mozambique Channel in 1979/80 but an average number in 1981/82
(Mavume et al. 2009) consistent with the former season being drier over southwestern Madagascar with less rain days and the opposite for 1981/82.
5 Conclusion
The climate variability of
Madagascar has not been much studied, especially the southwest. This
region of the island is much drier than the eastern and northern parts
as it lies in a rain shadow due to its location on the leeward side of
the mountains that run along the island and which present an obstacle to
the prevailing easterly trade wind flow.
Given that much of the
population of the island depends on rain-fed agriculture, focus was
placed on the interannual variability of the number of wet days and
heavy rain days in the summer (December to March) rainy season. In
subsistence agriculture, a rainy season that experiences regular
rainfall (high wet day frequency) is more beneficial than one with a
large seasonal rainfall total but infrequent and irregular rainfall
(high heavy rain day frequency but relatively low wet day frequency)
(Usman and Reason 2004).
Four rainfall stations were considered, three in the southwest of the
island and the other at the southern extremity of the east coast for
comparative purposes. These were the only stations in southwestern
Madagascar that had sufficient daily data for climate variability
analysis. A wet day was defined as one with at least 1 mm of recorded
rainfall whereas a heavy rainfall day was one experiencing at least
30 mm of rainfall. The rainy season begins in December and ends in March
for the three southwestern stations (Morondawa, Ranohira and Toliara),
whereas almost the entire year, except perhaps September, is wet at
Taolagnaro as it lies on the windward side of the mountains and is
impacted by moist easterly trade winds throughout the year. Another
topographic effect is seen at Ranohira which, although lying polewards
of Morondawa, is in fact wetter since the mountains upstream of this
station are lower and narrower in zonal extent than for Morondawa.
It was found that most
summers with an anomalously high (low) number of wet days occurred
during La Niña (El Niño) events whereas there was a less obvious link
between heavy rainfall days and ENSO. In general, summer rainfall
amounts tend to be more (less) during La Niña (El Niño) years. The
strongest correlations with ENSO at wet day frequency were found for
Toliara (|r|∼0.5–0.6) although the
correlations at the other stations are still significant at the 95 %
level. However, there were also some notable neutral summers such as
1979/80 and 1981/82 that experienced well below (above) average numbers
of wet days and reduced (increased) rainfall amount. Although there were
no obvious ENSO linkages with heavy rain day frequency, a Southern
Annular Mode (SAM)-type pattern was clearly present for two of the
stations with the other two stations showing these patterns to a weaker
extent.
In all cases, the
circulation patterns associated with summers with anomalous wet day
frequency consisted of strong anomalies in regional low-level moisture
convergence and uplift that made conditions correspondingly more or less
favourable for rainfall. Dry (wet) summers in southwestern Madagascar
were also associated with an equatorward (poleward) displacement of the
ITCZ in the region.
Acknowledgments
The first author is grateful to Carnegie WIO-RISE for the funding of her MSc research from which this manuscript is written.
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