Meteorol Atmos Phys 83, 89–108 (2003)
DOI 10.1007/s00703-002-0563-9
1
2
Department of Geography, University College London, UK
School of Geography and the Environment, University of Oxford, UK
Characteristics of summertime daily rainfall variability
over South America and the South Atlantic
Convergence Zone
M. C. Todd1 , R. Washington2 , and T. James2
With 11 Figures
Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002
Published online: February 20, 2003 # Springer-Verlag 2003
Summary
This paper presents an objective analysis of the structure of
daily rainfall variability over the South American=South
Atlantic region (15 –60 W and 0 –40 S) during individual
austral summer months of November to March. From EOF
analysis of satellite derived daily rainfall we find that the
leading mode of variability is represented by a highly
coherent meridional dipole structure, organised into 2
extensive bands, oriented northwest to southeast across the
continent and Atlantic Ocean. We argue that this dipole
structure represents variability in the meridional position of
the South Atlantic Convergence Zone (SACZ). During early
and later summer, in the positive (negative) phase of the
dipole, enhanced (suppressed) rainfall over eastern tropical
Brazil links with that over the subtropical and extra-tropical
Atlantic and is associated with suppressed (enhanced)
rainfall over the sub-tropical plains and adjacent Atlantic
Ocean. This structure is indicative of interaction between the
tropical, subtropical and temperate zones. Composite fields
from NCEP reanalysis products (associated with the major
positive and negative events) show that in early and late
summer the position of the SACZ is associated with
variability in: (a) the midlatitude wave structure, (b) the
position of the continental low, and (c) the zonal position of
the South Atlantic Subtropical High. Harmonic analysis of
the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the rainfall dipole
structure are associated primarily with variability in zonal
wave 4. There is evidence of a wave train extending
throughout the midlatitudes from the western Pacific into the
SACZ region. During positive (negative) events the largest
anomalous moisture advection occurs within westerlies
(easterlies) primarily from Amazonia (the South Atlantic).
In both phases a convergent poleward flow results along the
leading edge of the low-level trough extending from the
tropics into temperate latitudes. High summer events differ
from those in early and late summer in that the rainfall
dipole is primarily associated with variability in the phase of
zonal wave 3, and that tropical-temperate link is not clearly
evident in positive events.
1. Introduction
One of the principle features of the Southern
Hemisphere circulation is the existence of
large-scale convergence zones, which spawn
extensive bands of cloud and rain connecting tropical convection with extra-tropical circulation
features. The two primary locations for such tropical-temperate interaction are the South Pacific
Convergence Zone (SPCZ) (for a review see
Vincent et al, 1994) and the South Atlantic Convergence Zone (SACZ) (e.g., Streten, 1973;
Yasunari, 1977; Kodama, 1992; 1993; Lenters
and Cook, 1995; Liebmann et al, 1999). Both
these are quasi-permanent features. Another zone
of tropical-temperate interaction exists over
southern Africa and the Southwest Indian Ocean
90
M. C. Todd et al
during the austral summer only (Streten, 1973;
Harrison, 1986; Todd and Washington, 1999a;
Cook, 2000). The SACZ is associated with a
northwest–southeast oriented cloud band, extending from tropical Amazonia to the subtropical
and extra-tropical South Atlantic Ocean. It is
apparent throughout the year but is strongest during the austral summer (Streten, 1973, Kodama,
1992; 1993). The SACZ, has a diagonal orientation, extending from the Amazonian locus of
convective activity (centred on 60 W, 10 S)
over the South Atlantic to around 20 W, 50 S,
and is a very broad feature with a maximum
cross-sectional dimension of around 30 longitude (Fig. 1).
Both empirical studies (Kodama, 1992; 1993)
and model simulations (Figueroa et al, 1995;
Lenters and Cook, 1995) suggest that the SACZ
results primarily from the effects of continentality. Convection over the Amazon basin (near
60 W, 10 S) provides a large source of latent
heating resulting in a continental heat low and
contributing to the upper level Bolivian high to
the southwest. The resulting low-level pressure
Fig. 1. Climatological mean rainrate (mm day 1) for summer months November–March (1979–1998) from the
merged satellite, rain gauge and NWP product of Xie and
Arkin (1997). Contours range from 1–9 mm day 1 with an
interval of 1 mm day 1
gradient between the continental heat low and
South Atlantic Subtropical High (SASH) result
in large-scale moisture advection and convergence along the SACZ. The effects of topography are also thought to be important in
determining the mean position of the SACZ
(Figueroa et al, 1995). In addition, Liebmann
et al (1999) suggest that the mean position of
the SACZ is associated with a Rossby wave
guide from the midlatitudes of the Southern
Hemisphere, itself a function of the basic state
of the tropical atmosphere.
The SACZ exhibits variability at a range of
timescales. Given that tropical and subtropical
South America receives most rainfall in summer,
variability in the SACZ has important practical
implications for economic and social systems in
the region. Model simulations have sought to
understand the behaviour of the SACZ in relation
to tropical heating anomalies at intra-seasonal
and interannual time scales (e.g., Silva Dias
et al, 1983; Grimm and Silva Dias, 1995). From
observational data Liebmann et al (1999) note
that intraseasonal variability within the SASA
region is highest over the SACZ and is concentrated in the 2–30 day high frequency band.
Numerous studies have indicated a relationship
between intraseasonal variability in the SACZ
activity and the 30–60 day (Madden-Julian) oscillation (Kousky and Casarin, 1986; Kousky and
Kayano; 1994; Nogues-Paegle and Mo, 1997).
A dipolar structure in rainfall anomalies is
often observed with centres of activity in the
SACZ and over the subtropical plains of South
America (e.g., Nogues-Paegle and Mo, 1997).
Such a structure is indicative of an equatorward
propagating Rossby wave train, which may be
related to convective activity in the SPCZ region
(Grimm and Silva Dias, 1995; Nogues-Paegle
and Mo, 1997; Liebmann et al, 1999). Garreaud
and Wallace (1998) noted that synoptic systems
that propagate from extra-tropical latitudes into
the tropics can stimulate activity in the SACZ at
higher frequencies. Indeed, tropical-temperate
interaction is a distinctive feature of the synoptic
climatology of the region (Ratisbona, 1976;
Kousky and Calvacanti, 1997; Marengo et al,
1997).
Despite this extensive work on the mean state
and variability of the SACZ, to date, no study has
objectively defined the nature of space=time
Characteristics of summertime daily rainfall variability
variability in rainfall over the entire SASA region
at the daily time scale representative of synoptic
scale systems. Thus, the aims of this present
study are twofold. First, to characterise objectively the variability of rainfall at synoptic time
scales, over an extensive SASA region, encompassing land and ocean regions of the tropics,
subtropics and the temperate latitudes. Second,
to identify the associated circulation patterns
characteristic of the dominant rainfall modes.
91
been assessed has by NCAR=NCEP (e.g.,
http:==wesley.wwb.noaa.gov=paobs=) on both
monthly and synoptic scales. Errors are most
pronounced in the near surface fields of the
Southern Hemisphere oceans south of 40 S.
Nevertheless, it is likely that the data adequately
represents the broad scale structure of synoptic
events for this study, focused on rainfall events
north of 40 S.
2.2 Methods
2. Data and methodology
2.1 Data
Daily rainfall estimates on a 2.5 grid for the
austral summer months of November through
February (listed in Table 1), covering the period
1986–94 inclusive were obtained from the
Reconstructed GOES Precipitation Index (RGPI)
(Todd and Washington, 1999b). RGPI estimates
of rainfall exhibit minimal bias with respect to
the GPI, but it is thought that the GPI may overestimate land-based convective rainfall totals
(Adler et al, 2001). In addition, towards the latitudinal limit of RGPI products at 40 N=S the
estimates may be contaminated by non-raining
cirrus cloud associated with extra-tropical
weather systems. Although daily rainfall products from merged sources (from 1997 onwards)
have been recently released (Huffman et al,
2001), the RGPI represents the most extensive
set of global tropical and subtropical rainfall products with high temporal sampling (3 hourly) most
suitable for analysis of synoptic scale climate
variability.
In order to study the structure of the atmosphere, 12-hourly global analyses were obtained
for these months on a 2.5 grid from the NCEP–
NCAR reanalysis project (Kalnay et al, 1996).
Data from 3 levels (850, 500 and 200 hPa) were
used in this study to represent low, mid and upper
tropospheric conditions. An evaluation of forecast products and moisture transport in the NCEP
model can be found in Mo and Higgins (1996),
and Trenberth and Guillemot (1998).
An important caveat in the use of the NCEP
reanalysis data for Southern Hemisphere studies
involves the incorrectly assimilated (shifted 180
longitude) Australian surface pressure bogus data
between 1979 and 1992, the impact of which has
The intention of this study is to determine objectively the spatial and temporal characteristics of
leading modes of daily rainfall variability in the
region. To this end, Empirical Orthogonal Functions (EOFs) (Joliffe, 1987; Richman, 1986;
White et al, 1991) of daily rainfall (with space
as variables and time as observations) were calculated for individual months of November to
March. Since we wish to assess modes of variability that may be connected across regions of
quite different rainfall climatologies, it is reasonable to use the correlation rather the covariance
matrix as an input to the EOF analysis. This
should also minimise the effect of systematic
bias in RGPI estimates across the domain.
A number of experiments were nevertheless
undertaken so that the EOF solutions presented
are not contingent upon a priori decisions. Unrotated and Varimax rotated eigenvectors of the
rainfall grid box correlation matrix were calculated over 3 spatial domains (15–60 W, 0–40 S;
5–75 W, 0–40 S; 5–80 W, 10 N–40 S). We
present only the unrotated eigenvectors of the
correlation matrix for the domain 15 W–60 W
and 0 S–40 S for two primary reasons. First, the
leading unrotated EOF emerged as a dipole pattern rather than centred, positive, uniform loadings suggested by Buell (1975). Second, EOFs
calculated over the two larger domains were very
similar. We prefer the smallest domain as the
ratio of observations to variables is maximised.
We will argue that these leading unrotated EOFs
have a physical meaning (Sect. 3).
The results of this EOF analysis for November
through to March were used to identify extreme
events characteristic of the resulting EOF 1 loading patterns. From the EOF 1 time coefficients
(given by the product of the eigenvectors and the
standardised daily rainfall time series), the major
26–29
4
–
1992
1993
1994
–
12–14,
17, 26
22
24–26, 29
2–4, 10–12,
15–17, 19–23
1991
4–6,
29, 30
1989
–
4, 9,
14, 16
–
1987
1–6, 9,
11, 19–23,
25, 26
1990
29
1986
–
11, 12
5, 28
2–4
1–7,
16–20
–
Positive
Positive
Negative
December
November
–
4, 5,
16, 29
24, 26
3, 18, 19
30
–
2, 7, 14,
15, 17, 18
Negative
–
11–17, 20–31
5, 12–17
4
3, 4, 8
6
Positive
January
–
1, 2, 8,
16, 17, 27–29,
31
1, 2, 7
13, 14, 24, 25
20
6, 13, 15
17, 18
Negative
–
1–5, 7,
8, 11–13
14
13–16,
24, 25
5
–
13, 18
Positive
February
Table 1. Period of data used in study and dates of daily data used to form composites of positive and negative events
–
3, 4, 6–8
16, 17,
19, 20
4, 22, 23
16
–
1, 22–26,
28
Negative
3–7,
13–16
31
1
14–18,
22–24, 28
8–11,
13–16
–
7, 8
Positive
March
1
7, 8
4–6, 22–26,
28, 29
6, 10
6, 10–12,
27, 30
26, 30, 31
–
11, 18
Negative
92
M. C. Todd et al
Characteristics of summertime daily rainfall variability
positive (negative) events typical of the leading
EOF in each month are identified objectively by
extracting days with coefficients above (below)
one standard deviation from the mean. These
events form the basis of composites of positive
and negative episodes for which the mean
anomalies (and associated statistical significance) of RGPI rainfall and of NCEP reanalysis
atmospheric fields were calculated.
Vertically integrated zonal (Qu) and meridional (Qv) moisture flux have been calculated by
integrating between 850 hPa and 200 hPa using
the trapezoidal rule from
ð
1 200
qu dp;
ð1Þ
Qu ¼
g 850
ð
1 200
qv dp;
ð2Þ
Qv ¼
g 850
where q is specific humidity (g g 1) and u and v
and the zonal and meridional wind components,
respectively.
3. The structure of austral summer
daily rainfall over South America
and the South Atlantic
The eigenvalue proportions of the first five unrotated EOFs of the correlation matrix of daily
rainfall in each austral summer month are shown
in Table 2. All are distinct as determined by the
North test (North et al, 1982). The leading mode
of daily rainfall variability during each of the
austral summer months of November to March
in the SASA domain is represented by a pronounced meridional dipole structure (Fig. 2).
The dipole has centres of high positive and
negative loadings organised in 2 bands oriented
northwest to southeast across South America and
the South Atlantic. The loading pattern of the
first EOF of a combined (November to March)
93
dataset has a very similar dipole structure (not
shown). Both bands fall within the broad extent
of the SACZ (Fig. 1) and taken together represent
its full extent.
Loadings are higher over the northernmost
band (shown as positive loadings in Fig. 2) compared to the southern band (shown as negative
loadings). The two phases of the dipole represented by these loading patterns are hereafter
referred to as the positive and negative phases.
Whilst the pattern is very similar between
months, there is evidence of the seasonal cycle
in the loading pattern. During the peak summer
months of January and February the positive
loadings are highest over the continental land
mass centred on 15 S, 45 W, whilst in the early
and late summer months (November, December
and March) positive loadings are highest over the
South Atlantic. In January, the bands of uniform
loadings extend less far into the temperate
regions compared to other months. In February
there is a slight discontinuity in the magnitude of
loadings along the positive axis, which may
represent 2 types of rainfall system, namely
land-based convection and transient systems
located over the ocean. Negative loadings in
February are relatively small and ill defined. It
is likely that these features reflect the greater
spatial coherence of land-based convection during the high summer months, which tends to
dominate rainfall variability.
From this we argue that in early (November–
December) and late (March) summer the patterns of positive (negative) loadings are likely to
represent modes of tropical (subtropical) convection that link with transient disturbances in the
temperate latitudes. During high summer
(January–February) the positive (negative)
loading pattern is dominated by land-based convection over the tropics (subtropics), and the contribution of temperate transient systems is less
Table 2. Variance of monthly EOFs and sampling errors based on North test (North et al, 1982); = ¼ pass x ¼ fail
EOF
Nov
n ¼ 180
Dec
n ¼ 186
Jan
n ¼ 217
Feb
n ¼ 169
Mar.
n ¼ 217
1
2
3
4
5
Var.
10.05
6.78
5.84
5.11
4.67
North
0.31=
0.21=
0.28=
0.26=
Var.
11.4
7.93
5.46
5.00
4.80
North
0.34=
0.32=
0.27=
0.25 x
Var.
10.0
8.8
6.85
6.13
4.9
North
0.33=
0.34=
0.27=
0.25=
Var.
8.31
6.52
5.42
4.52
4.10
North
0.34=
0.30=
0.28=
0.25=
Var.
8.94
6.75
5.1
4.45
3.9
North
0.34=
0.30=
0.28=
0.25=
94
M. C. Todd et al: Characteristics of summertime daily rainfall variability
Fig. 2. EOF 1 weights of daily rainfall for (a) November, (b) December,
(c) January, (d) February, (e) March, shown as correlation coefficients
( 100) between the daily rainfall time series at each of the 2.5 2.5
grid boxes and the EOF time coefficients (scores)
Fig. 3. Correlation coefficients of the December leading EOF time coefficients and the time series of lagged rainfall (at lags
of 3 to þ3 days)
96
M. C. Todd et al
Fig. 4. Time coefficients of EOF 1 for (a) November, (b) December, (c) January, (d) February, (e) March
Characteristics of summertime daily rainfall variability
97
Fig. 5. Composite mean rainfall anomalies (mm day 1) for (a) March positive events, (b) March negative events, (c) January
positive events, and (d) January negative events
significant. The EOF loading patterns discussed
above suggests that rainfall patterns in early=late
summer (November, December and March) may
be distinct from those in high summer (January
and February). To avoid repetition, throughout
the rest of the paper we use data from March
and January as representatives of early=late and
high summer, respectively.
The spatial pattern of correlation coefficients
between the leading EOF time coefficients and
daily rainfall, at time lags ranging from 3 to
þ3 days, remains remarkably stable (Fig. 3).
This strongly suggests that the modes represented by the leading EOF loadings are, at the
time scale of a few days, predominantly quasistationary, rather than propagating systems.
Thus, the meridional dipole pattern appears not
to be simply a statistical artefact of synoptic systems that propagate from southwest to northeast
over the entire study region.
By extracting days with coefficients above
(below) one standard deviation from the mean
the major positive (negative) typical of the leading EOF in each month are objectively identified
(Fig. 4, Table 1). In all months, positive events
are associated with an extensive zone of substantial positive rainfall anomalies of extending
along the SACZ (Fig. 5). The proportional contribution of these relatively few positive events to
total rainfall is highest over the subtropical
Atlantic peaking at between 50–70% (Fig. 6).
This suggests that the positive events here capture a large proportion of the activity of the
SACZ, and that this feature probably represents
the dominant rainfall system(s) over the Atlantic
region between 10 S and 35 S. In March rainfall
anomalies extend from Amazonia to the midlatitude and in such periods the cloud bands are of
the order of 3000–4000 km in length.
Negative events are associated with slightly
smaller positive (negative) rainfall anomalies in
most months over the subtropical plains of South
America and the adjacent Atlantic at the southernmost extreme of the zone occupied by the
SACZ (central Brazil and the northern SACZ)
(Fig. 5). Enhanced rainfall during negative events
extends northward along the central Andes,
including the Bolivian Altiplano region. The
98
M. C. Todd et al
Fig. 6. Proportional contribution (%) to total monthly rainfall of (a) March positive events, (b) March negative events, (c)
January positive events, and (d) January negative events
Table 3. Amplitude and variance of first 10 harmonics (most important 3 in bold) of composite mean 200 hPa geopotential
anomalies (gpm) for positive minus negative events
1
Nov
amplitude
Variance
Dec
amplitude
Variance
Jan
amplitude
Variance
Feb
amplitude
Variance
Mar
Amplitude
Variance
2
3
4
5
6
7
8
9
10
32.5
24.2
6.0
0.8
17.6
7.1
37.5
32.1
30.5
21.3
20.3
9.5
6.5
1.0
10.9
2.7
3.3
0.2
5.3
0.6
28.4
9.0
26.4
7.7
25.3
7.1
70.6
55.5
22.2
5.4
15.2
2.6
27.2
8.2
15.8
2.8
7.9
0.7
7.1
0.5
35.3
20.0
21.4
7.3
52.8
44.9
33.2
17.8
7.3
0.8
5.1
0.4
11.7
2.2
11.4
2.1
10.0
1.6
11.9
2.3
58.9
28.9
42.8
15.2
31.6
8.3
43.5
15.8
57.9
27.9
15.6
2.0
2.6
0.1
11.6
1.1
3.6
0.1
6.6
.04
42.2
27.6
12.5
2.4
4.6
0.3
39.9
24.6
38.4
22.9
28.6
12.7
15.8
3.9
9.1
1.3
14.9
3.4
3.9
0.2
Characteristics of summertime daily rainfall variability
99
Fig. 7. Composite mean anomalies of 850 hPa geopotential
height (gpm) (contours) and 850 hPa wind (ms 1) (vectors)
for (a) March positive events, (b) January positive events, (c)
December negative. For contour plot shaded areas are statistically significant at 95% level. For vector plot only those
anomalies significant at the 0.05 level are shown
proportional contribution of negative events is
smaller too, with maximum of 30% most months
(Fig. 6).
4. Atmospheric structure
Circulation anomalies for early=late summer will
be considered separately from high summer, and
we use data from March and January, respectively, as representatives.
4.1 Positive early and late summer mode
In late summer (March) positive events are associated with a clear southwest to northeast trough-
ridge-trough structure at 850 hPa over the SASA
region. A low-level trough extends along the axis
of the SACZ cloud band from the tropics over
southern Brazil deep into the midlatitudes over
the Atlantic. 200 hPa geopotential anomalies are
of the same sign as those at 850 hPa (Fig. 8a)
over the ocean, but are southwestward leaning,
indicating a baroclinic atmosphere in the SACZ
region. The SACZ trough is associated with a
wave train structure of significant 200 hPa geopotential anomalies in a great circle around the
southern hemisphere, with a wavelength of 90
indicative of a wave-4 structure. Harmonic analysis of the 200 hpa geopotential anomaly fields
(Table 3) indicates that wave 4 dominates the
100
M. C. Todd et al
Fig. 8. Composite mean anomalies of 200 hPa geopotential
height (gpm) (contours) for (a) March positive events, (b) January positive events, (c) December negative. Positive (negative)
anomalies are shown as solid (dashed) lines and the contour
interval is 20 gpm. Shaded areas are statistically significant at
95% level
zonal structure at 40 S for positive minus negative events in November and December and is
important in March. An enhanced subtropical
jet (STJ) is apparent at around 25 S (not shown),
related to the upper-level ridge located over the
centre of tropical convection, and the trough to
the southeast at subtropical latitudes (Fig. 8a).
Low-level wind anomalies (Fig. 7a) show that
a marked cyclonic circulation occurs around the
tropical-temperate system. This converges with
the anticyclonic flow around the SASH to produce strong poleward flow along the SACZ. The
structure of anomalous vertically integrated
moisture flux (Fig. 9a) is similar, suggesting that
it is the low-level wind structure that is of importance rather than specific humidity differences.
Moisture is advected into the SACZ along 2 principle pathways: (a) within the northwesterly flow
between 5–15 S around the continental low, and
(b) in the anticyclonic flow around the western
Characteristics of summertime daily rainfall variability
101
Fig. 9. Composite mean anomalies of rainfall (mm day 1)
(shaded), 850 hPa temperature (K) (contour) and vertically integrated moisture flux (100 kg m 1 s 1) (vectors) for (a)
March positive events, (b) January positive events, (c) December negative. For 850 hPa temperature and moisture flux only
those anomalies significant at the 0.05 level are shown
periphery of the SASH. The absolute magnitude
of moisture transported in these two conduits is
similar (not shown). The northwesterly flux is
related to the large scale southeastward re-curving of moist northeasterly trades at around 7 S
(Fig. 9a). Convergence occurs along the SACZ
and is associated with the rainfall maxima
located at the leading edge of the low level
trough (Fig. 9a). A pronounced anomalous temperature gradient results from advection within
this circulation (Fig. 9a) with a cold front aligned
along the trailing edge of the rain band. Combined with an ample supply of moisture from
the tropics and subtropics this creates the thermodynamic conditions necessary for the development of deep convection.
The evolution of these systems may be studied
by means of Hovmoeller plots of composite rainfall anomalies. Positive rainfall anomalies precede the events at latitudes 0 –10 S and 10 –
20 S (Fig. 10). Convection within the former
band shows predominantly eastward propagation
prior to the onset of positive events. Westward
propagating transients in the midlatitudes are
clearly visible. Those that connect with the
SACZ during positive events appear around 2
days prior to the event onset.
4.2 Positive high summer mode
As in early=late summer, the positive events during January (representative of high summer) are
102
M. C. Todd et al
Fig. 10. Composite mean rainfall anomalies (mm day 1) for the 10 days preceding and 5 days following onset of March
positive events averaged over latitudinal bands (a) 0 –10 S, (b) 10 –20 S, (c) 20 –30 S, (d) 30 –40 S
associated with 200 hPa geopotential anomalies in
a wave structure throughout the southern hemisphere midlatitudes. In January, however, the
structure is characteristics of a zonal wave-3 pat-
tern (Fig. 8b, Table 3) and the circulation patterns
provide much less evidence of tropical-temperate
interaction (Figs. 7b, 8b). The figures for February
are difficult to interpret given the poor definition
Characteristics of summertime daily rainfall variability
103
Fig. 11. Composite mean rainfall anomalies (mm day 1) for periods preceding and following onset of December negative
events averaged over latitudinal bands (a) 0 –10 S, (b) 10 –20 S, (c) 20 –30 S, (d) 30 –40 S
of the negative mode in this month. Once again,
the STJ is enhanced at 25 S (not shown). Near
surface geopotential anomalies are dominated by
a strong continental low centred on 45 W, 25 S
(Fig. 7b), which links with a trough extending
eastward at subtropical latitudes over the Atlantic
Ocean. The SASH at 20 W is anomalously strong
which leads to an enhanced northwest–southeast
pressure gradient. This results in an anomalous
poleward low level flow (Fig. 7b) and moisture
104
M. C. Todd et al
convergence (Fig. 9b) along the SACZ between
40 W, 15 S and 30 W, 40 S. Rainfall is therefore located to the northeast of the low centre.
Westerly moisture flux anomalies from Amazonia
around the continental low are again related to the
large scale southeastward re-curving of moist
trade winds near 7 S (Fig. 9b) but are more pronounced than in early and later summer and dominate over the northeasterly flux around the SASH,
in absolute magnitude also (not shown).
4.3 Negative mode
The structure of rainfall during negative events is
similar in all months. Here we show data for
December as a representative sample. The largescale circulation pattern associated with negative
events in all summer months is dominated by a
mid-latitude wave pattern again with a wavelength
of about 90 (Figs. 7c and 8c). Waves 4 and 5
dominate the zonal 200 hPa geopotential anomalies at 40 S in all months except January where
wave 3 is most important (Table 3). In contrast to
the positive mode the wave is phase shifted. A
trough of low pressure extends north from the temperate latitudes of around 50 S to the subtropical
plains of South America. This temperate transient
system links with an enhanced subtropical continental low near its mean position of 60 W, 20 S.
The SASH has an anomalous westward extent
with centred near 15 W, 37 S. Anomalous cyclonic low-level circulation around the temperate
trough converges with an intensified flow around
the SASH to create a poleward flow south of 25 S
near 40 W (Fig. 7c). Vertically integrated anomalous moisture flux anomalies (Fig. 9c) are dominated by these flows and show convergence along
the leading (eastward) edge of the temperate
trough and are associated with the positive rainfall
anomalies. Moisture flux convergence is also
strongly enhanced by a moist northerly flow from
the tropics around the continental low. The convergence of cool temperate air and the warm, moist
north and northeasterly conveyors from the tropics
and subtropics results in a pronounced temperature gradient (Fig. 9c) that is likely to create the
instability necessary for intense convection.
Hovmoeller plots (Fig. 11) plots clearly
show the eastward propagating transient between
30 –40 S. This generally develops 2 days prior
to the onset of events, and tends to precede con-
vection in the subtropical latitudes 20 –30 S.
Rainfall in the subtropical latitudes does not appear to migrate substantially with the temperate
transient system. It is interesting to note also that
convection in the 0 –10 S and 10 –20 S latitude band is heavily suppressed before and during negative events.
5. Discussion
During the austral summer the leading mode of
daily rainfall variability in the SASA region is
represented by a highly coherent meridional
dipole structure, organised into 2 extensive
bands, both oriented northwest to southeast
across the continent and Atlantic Ocean. Here,
we argue that the loadings represent variability
at the daily time scale in the meridional position
of the SACZ within the broad band of rainfall
shown in the climatology. The EOF loadings
are similar to the negative (positive) phases of
the 10–20 day intra-seasonal SASA rainfall
‘‘seesaw’’ identified by Nogues-Paegle and Mo
(1997), who suggests a link between the dipole
pattern and the 30–60 day oscillation in tropical
rainfall. Grimm and Silva Dias (1995) suggest
that the intra-seasonal oscillation propagates
from the Pacific to the Atlantic regions through
the stimulation of a wave train by enhanced rainfall along the SPCZ. Kiladis and Weickmann
(1992) show that intraseasonal (14–30 day) variations in SACZ activity relate to a wave train in
the tropical and subtropical upper level westerlies. The results of Liebmann et al (1999) are
broadly concurrent in that an observed 13-day
periodicity in SACZ activity is associated with
wave activity impinging from the midlatitudes,
along a preferred Rossby wave-guide in this section of the southern Hemisphere, itself a function
of the basic state of the tropical atmosphere.
However, from the time coefficients (Fig. 4) it
is clear that reversals in the phase of the SACZ
dipole also occur on shorter time scales related to
synoptic variability. Whilst our analysis here
includes variability at all time scales beyond
1-day, we are able to resolve individual synoptic
events and therefore to characterise more precisely the atmospheric circulation associated
with the dipole phases. Reversals in the dipole
phase are associated with 3 primary features
of the large-scale circulation; (a) the phase of
Characteristics of summertime daily rainfall variability
specific midlatitude planetary wave structures,
(b) the meridional position of the continental
low, and (c) the zonal position of the SASH.
In all months the 200 hPa height anomaly
fields of positive and negative events show a well
defined and opposing trough-ridge wave pattern
poleward of 30 S over the SASA region. This is
part of a characteristics wave train extending
throughout the southern hemisphere mid and
high latitudes. Thus, the activity and position of
the SACZ is intimately connected to wave activity emerging from the midatitudes of the eastern
Pacific. Rainfall is located at the leading edge of
the upper level trough such that reversals in the
position of the SACZ between positive and negative events are associated with phase changes in
these wave structures.
In early=late (high) summer this 200 hPa
height field is dominated by variability in wave
4 (wave 3). Nogues-Paegle and Mo (1997), and
Liebmann et al (1999) however, find that an alternation in a trough-ridge pattern with wavelength
of 70 longitude (wave 5) is associated with
intraseasonal rainfall variability over the region
during the December–February seasons. It may
be that in early and late summer the position of
rainfall in the SACZ is influenced by transient
eddies in the midlatitudes associated with either
wave 4 or 5, whilst in high summer it is associated with variability in quasi-stationary wave 3.
In high summer during positive events the tropical-temperate link is less common and not
imposed on the composites. This may reflect
the southward displacement of the midlatitude
westerlies in high summer.
However, upper level troughs often form to the
southeast of the upper level highs associated with
the upper level divergence related to tropical
convection (Kodama, 1992; Figueroa et al,
1995). In support of this we find some evidence
that enhanced rainfall during the positive phase is
preceded by eastward propagating convection in
the Amazon region. In addition, the STJ is anomalously strong at around 25 S, with rainfall
located at the leading edge of the upper level
trough. This is in agreement with Kodama
(1993) who identify the STJ as an important
mechanism of frontogenesis along the SACZ.
In the positive phase the composites reveal
low-level structures similar to those found in previous studies SACZ activity in its mean position.
105
The east–west pressure gradient between the
anomalous low over eastern Brazil and the eastward shifted SASH results in a moist low level,
poleward conveyor, with rainfall along the axis
of the SACZ, extending into the midlatitudes.
This supports the results of Kodama (1992;
1993) who found that this poleward flow is critical for maintaining convective activity along the
SACZ, by the generation of low-level convergence and conditional instability. Moisture convergence and rainfall appear to be driven by
interaction of the continental low and the SASH,
in accordance with the observational and idealised results of Kodama (1992), and Lenters
and Cook (1995), respectively. Negative lowlevel temperature anomalies develop to the
southwest of the rainfall maxima associated with
advection from the south Atlantic region around
the low-level trough, leading to the development
of a cold front and consequent uplift.
The results here, indicating the dominance of
moisture advection from Amazonia (particularly
in high summer), are in line with those of
Liebmann et al (1999) but differ from those of
Kodama (1992), which identified easterlies
around the SASH as the dominant moisture
source. In fact, the importance of the continental
low in modulating the circulation was demonstrated by the idealised simulations of Lenters
and Cook (1995) which suggest that the southeastward recurving of the trades at around 7 S is
related directly to the presence of the continental
low itself rather than any topographic forcing of
the Andean Cordillera. Kodama (1993) suggests
that the meridional position of the SACZ is
dependent on the degree of connection with the
‘‘monsoon westerlies’’ from Amazonia. The distinctive moisture flux anomalies in the positive
and negative mode of the SACZ seems to reaffirm this, whereby the former (latter) exhibit
a strong (weak) contribution of moisture from
the low-level tropical westerlies along the SACZ.
In positive (negative) events convection over
Amazonia is enhanced (suppressed) prior to the
events. The magnitude of the Amazonian moisture source is likely to be one factor explaining
the greater rainfall intensities associated with
events in the positive mode compared to those
in the negative mode.
Negative events in all summer months are associated with a near reversal of this structure.
106
M. C. Todd et al
In contrast to positive events moisture is
transported primarily around SASH. Convection over Amazonia is suppressed during and
prior to negative events, suggesting that active
periods of the SACZ in its southern position
are independent of diabatic heating over
Amazonia. The structure of rainfall anomalies
during negative events is very similar to that
associated with synoptic systems identified by
Garreaud and Wallace (1998) who provide a
detailed evaluation of the dynamics of these
systems. However, although such systems do
often propagate northeastward the hovmoeller
plots (Fig. 11) and the lagged EOF=rainfall
correlations (Fig. 3) show that the dipole pattern over the SASA region is not an simply
artefact of such propagating systems. Liebmann
et al (1999) suggest that these ‘‘cold surges’’
lead to enhanced convection over western
Amazonia rather than the SACZ. Here, we find
that they may be an important stimulus for
activity in the SACZ in its more southerly
position. That rainfall associated with the midlatitude transient precedes that over the subtropical plains suggests that the circulation may
trigger convection over the subtropical plains.
The results are also broadly in line with the
idealised results of Lenters and Cook (1995)
who note that the transient component of the
circulation is associated with a southward displacement in the SACZ. Overall, our results
provide support for many existing hypotheses
of the mechanisms associated with activity of
the SACZ in its mean position, namely the
effect of continentality, Amazonian convection
and the midlatitude planetary wave structure.
This suggests that a combination of these factors are involved in determining periods of
active SACZ convection. However, we have
been able to infer the influence of distinctive
components of the large-scale circulation associated with changes in the meridional position
of the SACZ at the daily time scale. Our
results also highlight important distinctions
within the summer season.
The principle limitation of this study is the
relatively short record of rainfall data. It should
be noted, however, that the RGPI represents the
most extensive set of quasi-global rainfall data
with the high temporal resolution necessary to
accurately derive daily rainfall. Nevertheless,
some caution needs to be attached to the interpretation of these results owing to the relatively
short duration of the data set.
6. Conclusions
The austral summer rainfall climatologies of the
SASA region show the SACZ as a broad band of
high rainfall up to 30 in zonal extent, oriented
northwest to southeast. The SACZ is often considered to occupy the central part of this band.
Previous work has studied the mean state of the
SACZ (Kodama, 1992; 1993) and submonthly
variability (Liebmann et al, 1999) as well as rainfall variability over South America at intraseasonal scales (Nogues-Paegle and Mo, 1997)
and at daily scales over the subtropical plains
(Garreaud and Wallace, 1998). Using recently
released satellite-derived rainfall products, the
present study provides the first objective analysis
of daily rainfall variability over the SASA
region. This is novel in that it provides the temporal and spatial resolution adequate to resolve
specific synoptic conditions and the associated
atmospheric circulation anomalies.
The results presented in this work suggest that
the SACZ is a complex phenomenon, which
exhibits distinct patterns of spatial variability at
daily time scales, dominated by a meridional
dipole structure. From reanalysis data the
mechanisms responsible have been alluded to,
and we have highlighted important differences
related to the position of the SACZ, the season
and, quite likely, the time scale of variability. In
all cases identified in this study, the meridional
location and activity of the SACZ is related to the
phase of the zonal wave structure associated largely with wave 4 in early=late summer and wave 3
in high summer. In addition, variability is associated with the position and intensity of the
South American continental low and the South
Atlantic subtropical high, which together drive
a convergent, poleward flux of moisture resulting
in the extensive cloud and rainfall observed. In
early=late summer the SACZ is associated with
transient disturbances originating in the midlatitudes such that the synoptic scale systems represent coupled tropical-temperate systems. When
the SACZ occupies its primary position (positive
mode) during high summer the role of anomalous
moist northwesterlies is particularly pronounced,
Characteristics of summertime daily rainfall variability
confirming the importance of Amazonia as a
source of moisture. Given that the principle
moisture conveyor associated with active periods
of the SACZ in its primary position links with the
Atlantic trade winds, it is possible that interannual variability in the strength of the trade wind
circulation (Aceituno, 1988) may impact on the
activity of the SACZ at such time scales.
In describing the patterns of atmospheric circulation, moisture flux, and rainfall, we have
shown the significance of both tropical and
extra-tropical dynamics in the development of
SACZ cloud bands. The results suggest that a
complete understanding of rainfall variability
should account for both tropical and temperate
dynamics. Over Southern Africa tropical-temperate trough systems have been shown to be of
profound significance in the meridional transfer
of energy and momentum (Harrison, 1986). A
full consideration of the momentum and oceanatmosphere energy fluxes associated with the
synoptic systems identified here over the SASA
region would be valuable. Finally, given that
variability in the activity of the SACZ occurs at
a range of scales, future work should consider
variability at longer inter-annual time scales.
Acknowledgements
The authors are grateful for the University of Oxford for
support. NCEP reanalysis data were obtained from the
National Centre for Atmospheric Research.
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Author’s address: Prof. C. Todd, Department of Geography, University College London, 26 Bedford Way, London,
WC1H 0AP UK (E-mail: [email protected])