characteristics of summertime daily rainfall variability ...dennis/todd_map_saprecip_2003.pdf ·...

Meteorol Atmos Phys 83, 89–108 (2003)DOI 10.1007/s00703-002-0563-9

1 Department of Geography, University College London, UK2 School of Geography and the Environment, University of Oxford, UK

Characteristics of summertime daily rainfall variabilityover South America and the South AtlanticConvergence Zone

M. C. Todd1, R. Washington2, and T. James2

With 11 Figures

Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002Published online: February 20, 2003 # Springer-Verlag 2003

Summary

This paper presents an objective analysis of the structure ofdaily rainfall variability over the South American=SouthAtlantic region (15�–60� W and 0�–40� S) during individualaustral summer months of November to March. From EOFanalysis of satellite derived daily rainfall we find that theleading mode of variability is represented by a highlycoherent meridional dipole structure, organised into 2extensive bands, oriented northwest to southeast across thecontinent and Atlantic Ocean. We argue that this dipolestructure represents variability in the meridional position ofthe South Atlantic Convergence Zone (SACZ). During earlyand later summer, in the positive (negative) phase of thedipole, enhanced (suppressed) rainfall over eastern tropicalBrazil links with that over the subtropical and extra-tropicalAtlantic and is associated with suppressed (enhanced)rainfall over the sub-tropical plains and adjacent AtlanticOcean. This structure is indicative of interaction between thetropical, subtropical and temperate zones. Composite fieldsfrom NCEP reanalysis products (associated with the majorpositive and negative events) show that in early and latesummer the position of the SACZ is associated withvariability in: (a) the midlatitude wave structure, (b) theposition of the continental low, and (c) the zonal position ofthe South Atlantic Subtropical High. Harmonic analysis ofthe 200 hPa geopotential anomaly structure in the mid-latitudes indicates that reversals in the rainfall dipolestructure are associated primarily with variability in zonalwave 4. There is evidence of a wave train extendingthroughout the midlatitudes from the western Pacific into theSACZ 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 theleading edge of the low-level trough extending from thetropics into temperate latitudes. High summer events differfrom those in early and late summer in that the rainfalldipole is primarily associated with variability in the phase ofzonal wave 3, and that tropical-temperate link is not clearlyevident in positive events.

1. Introduction

One of the principle features of the SouthernHemisphere circulation is the existence oflarge-scale convergence zones, which spawnextensive bands of cloud and rain connecting trop-ical convection with extra-tropical circulationfeatures. The two primary locations for such trop-ical-temperate interaction are the South PacificConvergence Zone (SPCZ) (for a review seeVincent et al, 1994) and the South Atlantic Con-vergence Zone (SACZ) (e.g., Streten, 1973;Yasunari, 1977; Kodama, 1992; 1993; Lentersand Cook, 1995; Liebmann et al, 1999). Boththese are quasi-permanent features. Another zoneof tropical-temperate interaction exists oversouthern Africa and the Southwest Indian Ocean

during the austral summer only (Streten, 1973;Harrison, 1986; Todd and Washington, 1999a;Cook, 2000). The SACZ is associated with anorthwest–southeast oriented cloud band, extend-ing from tropical Amazonia to the subtropicaland extra-tropical South Atlantic Ocean. It isapparent throughout the year but is strongest dur-ing the austral summer (Streten, 1973, Kodama,1992; 1993). The SACZ, has a diagonal orienta-tion, extending from the Amazonian locus ofconvective 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 maximumcross-sectional dimension of around 30� longi-tude (Fig. 1).

Both empirical studies (Kodama, 1992; 1993)and model simulations (Figueroa et al, 1995;Lenters and Cook, 1995) suggest that the SACZresults primarily from the effects of continental-ity. Convection over the Amazon basin (near60� W, 10� S) provides a large source of latentheating resulting in a continental heat low andcontributing to the upper level Bolivian high tothe southwest. The resulting low-level pressure

gradient between the continental heat low andSouth Atlantic Subtropical High (SASH) resultin large-scale moisture advection and conver-gence along the SACZ. The effects of topogra-phy are also thought to be important indetermining the mean position of the SACZ(Figueroa et al, 1995). In addition, Liebmannet al (1999) suggest that the mean position ofthe SACZ is associated with a Rossby waveguide from the midlatitudes of the SouthernHemisphere, itself a function of the basic stateof the tropical atmosphere.

The SACZ exhibits variability at a range oftimescales. Given that tropical and subtropicalSouth America receives most rainfall in summer,variability in the SACZ has important practicalimplications for economic and social systems inthe region. Model simulations have sought tounderstand the behaviour of the SACZ in relationto tropical heating anomalies at intra-seasonaland interannual time scales (e.g., Silva Diaset al, 1983; Grimm and Silva Dias, 1995). Fromobservational data Liebmann et al (1999) notethat intraseasonal variability within the SASAregion is highest over the SACZ and is concen-trated in the 2–30 day high frequency band.Numerous studies have indicated a relationshipbetween intraseasonal variability in the SACZactivity and the 30–60 day (Madden-Julian) os-cillation (Kousky and Casarin, 1986; Kousky andKayano; 1994; Nogues-Paegle and Mo, 1997).

A dipolar structure in rainfall anomalies isoften observed with centres of activity in theSACZ and over the subtropical plains of SouthAmerica (e.g., Nogues-Paegle and Mo, 1997).Such a structure is indicative of an equatorwardpropagating Rossby wave train, which may berelated to convective activity in the SPCZ region(Grimm and Silva Dias, 1995; Nogues-Paegleand Mo, 1997; Liebmann et al, 1999). Garreaudand Wallace (1998) noted that synoptic systemsthat propagate from extra-tropical latitudes intothe tropics can stimulate activity in the SACZ athigher frequencies. Indeed, tropical-temperateinteraction is a distinctive feature of the synopticclimatology of the region (Ratisbona, 1976;Kousky and Calvacanti, 1997; Marengo et al,1997).

Despite this extensive work on the mean stateand variability of the SACZ, to date, no study hasobjectively defined the nature of space=time

Fig. 1. Climatological mean rainrate (mm day� 1) for sum-mer months November–March (1979–1998) from themerged satellite, rain gauge and NWP product of Xie andArkin (1997). Contours range from 1–9 mm day� 1 with aninterval of 1 mm day� 1

90 M. C. Todd et al

variability in rainfall over the entire SASA regionat the daily time scale representative of synopticscale systems. Thus, the aims of this presentstudy are twofold. First, to characterise objec-tively the variability of rainfall at synoptic timescales, over an extensive SASA region, encom-passing land and ocean regions of the tropics,subtropics and the temperate latitudes. Second,to identify the associated circulation patternscharacteristic of the dominant rainfall modes.

2. Data and methodology

2.1 Data

Daily rainfall estimates on a 2.5� grid for theaustral summer months of November throughFebruary (listed in Table 1), covering the period1986–94 inclusive were obtained from theReconstructed GOES Precipitation Index (RGPI)(Todd and Washington, 1999b). RGPI estimatesof rainfall exhibit minimal bias with respect tothe GPI, but it is thought that the GPI may over-estimate land-based convective rainfall totals(Adler et al, 2001). In addition, towards the lati-tudinal limit of RGPI products at 40� N=S theestimates may be contaminated by non-rainingcirrus cloud associated with extra-tropicalweather systems. Although daily rainfall prod-ucts from merged sources (from 1997 onwards)have been recently released (Huffman et al,2001), the RGPI represents the most extensiveset of global tropical and subtropical rainfall prod-ucts with high temporal sampling (3 hourly) mostsuitable for analysis of synoptic scale climatevariability.

In order to study the structure of the atmo-sphere, 12-hourly global analyses were obtainedfor 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) wereused in this study to represent low, mid and uppertropospheric conditions. An evaluation of fore-cast products and moisture transport in the NCEPmodel can be found in Mo and Higgins (1996),and Trenberth and Guillemot (1998).

An important caveat in the use of the NCEPreanalysis data for Southern Hemisphere studiesinvolves the incorrectly assimilated (shifted 180�

longitude) Australian surface pressure bogus databetween 1979 and 1992, the impact of which has

been assessed has by NCAR=NCEP (e.g.,http:==wesley.wwb.noaa.gov=paobs=) on bothmonthly and synoptic scales. Errors are mostpronounced in the near surface fields of theSouthern Hemisphere oceans south of 40� S.Nevertheless, it is likely that the data adequatelyrepresents the broad scale structure of synopticevents for this study, focused on rainfall eventsnorth of 40� S.

2.2 Methods

The intention of this study is to determine objec-tively the spatial and temporal characteristics ofleading modes of daily rainfall variability in theregion. To this end, Empirical Orthogonal Func-tions (EOFs) (Joliffe, 1987; Richman, 1986;White et al, 1991) of daily rainfall (with spaceas variables and time as observations) were cal-culated for individual months of November toMarch. Since we wish to assess modes of vari-ability that may be connected across regions ofquite different rainfall climatologies, it is reason-able to use the correlation rather the covariancematrix as an input to the EOF analysis. Thisshould also minimise the effect of systematicbias in RGPI estimates across the domain.

A number of experiments were neverthelessundertaken so that the EOF solutions presentedare not contingent upon a priori decisions. Unro-tated and Varimax rotated eigenvectors of therainfall grid box correlation matrix were calcu-lated over 3 spatial domains (15–60� W, 0–40� S;5–75� W, 0–40� S; 5–80� W, 10� N–40� S). Wepresent only the unrotated eigenvectors of thecorrelation matrix for the domain 15� W–60� Wand 0� S–40� S for two primary reasons. First, theleading unrotated EOF emerged as a dipole pat-tern rather than centred, positive, uniform load-ings suggested by Buell (1975). Second, EOFscalculated over the two larger domains were verysimilar. We prefer the smallest domain as theratio of observations to variables is maximised.We will argue that these leading unrotated EOFshave a physical meaning (Sect. 3).

The results of this EOF analysis for Novemberthrough to March were used to identify extremeevents characteristic of the resulting EOF 1 load-ing patterns. From the EOF 1 time coefficients(given by the product of the eigenvectors and thestandardised daily rainfall time series), the major

Characteristics of summertime daily rainfall variability 91

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92 M. C. Todd et al

positive (negative) events typical of the leadingEOF in each month are identified objectively byextracting days with coefficients above (below)one standard deviation from the mean. Theseevents form the basis of composites of positiveand negative episodes for which the meananomalies (and associated statistical signifi-cance) of RGPI rainfall and of NCEP reanalysisatmospheric fields were calculated.

Vertically integrated zonal (Qu) and meridio-nal (Qv) moisture flux have been calculated byintegrating between 850 hPa and 200 hPa usingthe trapezoidal rule from

Qu ¼1

g

ð200

850

qu dp; ð1Þ

Qv ¼1

g

ð200

850

qv dp; ð2Þ

where q is specific humidity (g g� 1) and u and vand the zonal and meridional wind components,respectively.

3. The structure of austral summerdaily rainfall over South Americaand the South Atlantic

The eigenvalue proportions of the first five unro-tated EOFs of the correlation matrix of dailyrainfall in each austral summer month are shownin Table 2. All are distinct as determined by theNorth test (North et al, 1982). The leading modeof daily rainfall variability during each of theaustral summer months of November to Marchin the SASA domain is represented by a pro-nounced meridional dipole structure (Fig. 2).The dipole has centres of high positive andnegative loadings organised in 2 bands orientednorthwest to southeast across South America andthe South Atlantic. The loading pattern of thefirst EOF of a combined (November to March)

dataset has a very similar dipole structure (notshown). Both bands fall within the broad extentof the SACZ (Fig. 1) and taken together representits full extent.

Loadings are higher over the northernmostband (shown as positive loadings in Fig. 2) com-pared to the southern band (shown as negativeloadings). The two phases of the dipole repre-sented by these loading patterns are hereafterreferred to as the positive and negative phases.Whilst the pattern is very similar betweenmonths, there is evidence of the seasonal cyclein the loading pattern. During the peak summermonths of January and February the positiveloadings are highest over the continental landmass centred on 15� S, 45� W, whilst in the earlyand late summer months (November, Decemberand March) positive loadings are highest over theSouth Atlantic. In January, the bands of uniformloadings extend less far into the temperateregions compared to other months. In Februarythere is a slight discontinuity in the magnitude ofloadings along the positive axis, which mayrepresent 2 types of rainfall system, namelyland-based convection and transient systemslocated over the ocean. Negative loadings inFebruary are relatively small and ill defined. Itis likely that these features reflect the greaterspatial coherence of land-based convection dur-ing the high summer months, which tends todominate rainfall variability.

From this we argue that in early (November–December) and late (March) summer the pat-terns of positive (negative) loadings are likely torepresent modes of tropical (subtropical) convec-tion that link with transient disturbances in thetemperate latitudes. During high summer(January–February) the positive (negative)loading pattern is dominated by land-based con-vection over the tropics (subtropics), and the con-tribution 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 x¼ fail

EOF Nov n¼ 180 Dec n¼ 186 Jan n¼ 217 Feb n¼ 169 Mar. n¼ 217

Var. North Var. North Var. North Var. North Var. North1 10.05 0.31= 11.4 0.34= 10.0 0.33= 8.31 0.34= 8.94 0.34=2 6.78 0.21= 7.93 0.32= 8.8 0.34= 6.52 0.30= 6.75 0.30=3 5.84 0.28= 5.46 0.27= 6.85 0.27= 5.42 0.28= 5.1 0.28=4 5.11 0.26= 5.00 0.25 x 6.13 0.25= 4.52 0.25= 4.45 0.25=5 4.67 4.80 4.9 4.10 3.9


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)

94 M. C. Todd et al: Characteristics of summertime daily rainfall variability

Fig. 3. Correlation coefficients of the December leading EOF time coefficients and the time series of lagged rainfall (at lagsof �3 to þ3 days)

Fig. 4. Time coefficients of EOF 1 for (a) November, (b) December, (c) January, (d) February, (e) March

96 M. C. Todd et al

significant. The EOF loading patterns discussedabove suggests that rainfall patterns in early=latesummer (November, December and March) maybe distinct from those in high summer (Januaryand February). To avoid repetition, throughoutthe rest of the paper we use data from Marchand January as representatives of early=late andhigh summer, respectively.

The spatial pattern of correlation coefficientsbetween the leading EOF time coefficients anddaily rainfall, at time lags ranging from �3 toþ3 days, remains remarkably stable (Fig. 3).This strongly suggests that the modes repre-sented by the leading EOF loadings are, at thetime scale of a few days, predominantly quasi-stationary, rather than propagating systems.Thus, the meridional dipole pattern appears notto be simply a statistical artefact of synoptic sys-tems that propagate from southwest to northeastover the entire study region.

By extracting days with coefficients above(below) one standard deviation from the meanthe major positive (negative) typical of the lead-ing EOF in each month are objectively identified

(Fig. 4, Table 1). In all months, positive eventsare associated with an extensive zone of substan-tial positive rainfall anomalies of extendingalong the SACZ (Fig. 5). The proportional con-tribution of these relatively few positive events tototal rainfall is highest over the subtropicalAtlantic peaking at between 50–70% (Fig. 6).This suggests that the positive events here cap-ture a large proportion of the activity of theSACZ, and that this feature probably representsthe dominant rainfall system(s) over the Atlanticregion between 10� S and 35� S. In March rainfallanomalies extend from Amazonia to the midlati-tude and in such periods the cloud bands are ofthe order of 3000–4000 km in length.

Negative events are associated with slightlysmaller positive (negative) rainfall anomalies inmost months over the subtropical plains of SouthAmerica and the adjacent Atlantic at the south-ernmost extreme of the zone occupied by theSACZ (central Brazil and the northern SACZ)(Fig. 5). Enhanced rainfall during negative eventsextends northward along the central Andes,including the Bolivian Altiplano region. The

Fig. 5. Composite mean rainfall anomalies (mm day� 1) for (a) March positive events, (b) March negative events, (c) Januarypositive events, and (d) January negative events


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 geopotentialanomalies (gpm) for positive minus negative events

1 2 3 4 5 6 7 8 9 10

Novamplitude 32.5 6.0 17.6 37.5 30.5 20.3 6.5 10.9 3.3 5.3Variance 24.2 0.8 7.1 32.1 21.3 9.5 1.0 2.7 0.2 0.6Decamplitude 28.4 26.4 25.3 70.6 22.2 15.2 27.2 15.8 7.9 7.1Variance 9.0 7.7 7.1 55.5 5.4 2.6 8.2 2.8 0.7 0.5Janamplitude 35.3 21.4 52.8 33.2 7.3 5.1 11.7 11.4 10.0 11.9Variance 20.0 7.3 44.9 17.8 0.8 0.4 2.2 2.1 1.6 2.3Febamplitude 58.9 42.8 31.6 43.5 57.9 15.6 2.6 11.6 3.6 6.6Variance 28.9 15.2 8.3 15.8 27.9 2.0 0.1 1.1 0.1 .04MarAmplitude 42.2 12.5 4.6 39.9 38.4 28.6 15.8 9.1 14.9 3.9Variance 27.6 2.4 0.3 24.6 22.9 12.7 3.9 1.3 3.4 0.2

98 M. C. Todd et al

proportional contribution of negative events issmaller too, with maximum of 30% most months(Fig. 6).

4. Atmospheric structure

Circulation anomalies for early=late summer willbe considered separately from high summer, andwe use data from March and January, respec-tively, as representatives.

4.1 Positive early and late summer mode

In late summer (March) positive events are asso-ciated with a clear southwest to northeast trough-

ridge-trough structure at 850 hPa over the SASAregion. A low-level trough extends along the axisof the SACZ cloud band from the tropics oversouthern Brazil deep into the midlatitudes overthe Atlantic. 200 hPa geopotential anomalies areof the same sign as those at 850 hPa (Fig. 8a)over the ocean, but are southwestward leaning,indicating a baroclinic atmosphere in the SACZregion. The SACZ trough is associated with awave train structure of significant 200 hPa geo-potential anomalies in a great circle around thesouthern hemisphere, with a wavelength of 90�

indicative of a wave-4 structure. Harmonic anal-ysis of the 200 hpa geopotential anomaly fields(Table 3) indicates that wave 4 dominates the

Fig. 7. Composite mean anomalies of 850 hPa geopotentialheight (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 statisti-cally significant at 95% level. For vector plot only thoseanomalies significant at the 0.05 level are shown


zonal structure at 40� S for positive minus nega-tive events in November and December and isimportant in March. An enhanced subtropicaljet (STJ) is apparent at around 25� S (not shown),related to the upper-level ridge located over thecentre of tropical convection, and the trough tothe southeast at subtropical latitudes (Fig. 8a).

Low-level wind anomalies (Fig. 7a) show thata marked cyclonic circulation occurs around thetropical-temperate system. This converges with

the anticyclonic flow around the SASH to pro-duce strong poleward flow along the SACZ. Thestructure of anomalous vertically integratedmoisture flux (Fig. 9a) is similar, suggesting thatit is the low-level wind structure that is of impor-tance rather than specific humidity differences.Moisture is advected into the SACZ along 2 prin-ciple pathways: (a) within the northwesterly flowbetween 5–15� S around the continental low, and(b) in the anticyclonic flow around the western

Fig. 8. Composite mean anomalies of 200 hPa geopotentialheight (gpm) (contours) for (a) March positive events, (b) Jan-uary positive events, (c) December negative. Positive (negative)anomalies are shown as solid (dashed) lines and the contourinterval is 20 gpm. Shaded areas are statistically significant at95% level

100 M. C. Todd et al

periphery of the SASH. The absolute magnitudeof moisture transported in these two conduits issimilar (not shown). The northwesterly flux isrelated to the large scale southeastward re-curv-ing of moist northeasterly trades at around 7� S(Fig. 9a). Convergence occurs along the SACZand is associated with the rainfall maximalocated at the leading edge of the low leveltrough (Fig. 9a). A pronounced anomalous tem-perature gradient results from advection withinthis circulation (Fig. 9a) with a cold front alignedalong the trailing edge of the rain band. Com-bined with an ample supply of moisture fromthe tropics and subtropics this creates the thermo-dynamic conditions necessary for the develop-ment of deep convection.

The evolution of these systems may be studiedby means of Hovmoeller plots of composite rain-fall anomalies. Positive rainfall anomalies pre-cede the events at latitudes 0�–10� S and 10�–20� S (Fig. 10). Convection within the formerband shows predominantly eastward propagationprior to the onset of positive events. Westwardpropagating transients in the midlatitudes areclearly visible. Those that connect with theSACZ during positive events appear around 2days prior to the event onset.

4.2 Positive high summer mode

As in early=late summer, the positive events dur-ing January (representative of high summer) are

Fig. 9. Composite mean anomalies of rainfall (mm day� 1)(shaded), 850 hPa temperature (K) (contour) and vertically in-tegrated moisture flux (100 kg m� 1 s� 1) (vectors) for (a)March positive events, (b) January positive events, (c) Decem-ber negative. For 850 hPa temperature and moisture flux onlythose anomalies significant at the 0.05 level are shown


associated with 200 hPa geopotential anomalies ina wave structure throughout the southern hemi-sphere midlatitudes. In January, however, thestructure is characteristics of a zonal wave-3 pat-

tern (Fig. 8b, Table 3) and the circulation patternsprovide much less evidence of tropical-temperateinteraction (Figs. 7b, 8b). The figures for Februaryare difficult to interpret given the poor definition

Fig. 10. Composite mean rainfall anomalies (mm day� 1) for the 10 days preceding and 5 days following onset of Marchpositive 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). Nearsurface geopotential anomalies are dominated bya strong continental low centred on 45� W, 25� S(Fig. 7b), which links with a trough extending

eastward at subtropical latitudes over the AtlanticOcean. The SASH at 20� W is anomalously strongwhich leads to an enhanced northwest–southeastpressure gradient. This results in an anomalouspoleward low level flow (Fig. 7b) and moisture

Fig. 11. Composite mean rainfall anomalies (mm day� 1) for periods preceding and following onset of December negativeevents averaged over latitudinal bands (a) 0�–10� S, (b) 10�–20� S, (c) 20�–30� S, (d) 30�–40� S


convergence (Fig. 9b) along the SACZ between40� W, 15� S and 30� W, 40� S. Rainfall is there-fore located to the northeast of the low centre.Westerly moisture flux anomalies from Amazoniaaround the continental low are again related to thelarge scale southeastward re-curving of moisttrade winds near 7� S (Fig. 9b) but are more pro-nounced than in early and later summer and dom-inate over the northeasterly flux around the SASH,in absolute magnitude also (not shown).

4.3 Negative mode

The structure of rainfall during negative events issimilar in all months. Here we show data forDecember as a representative sample. The large-scale circulation pattern associated with negativeevents in all summer months is dominated by amid-latitude wave pattern again with a wavelengthof about 90� (Figs. 7c and 8c). Waves 4 and 5dominate the zonal 200 hPa geopotential anoma-lies at 40� S in all months except January wherewave 3 is most important (Table 3). In contrast tothe positive mode the wave is phase shifted. Atrough of low pressure extends north from the tem-perate latitudes of around 50� S to the subtropicalplains of South America. This temperate transientsystem links with an enhanced subtropical conti-nental low near its mean position of 60� W, 20� S.The SASH has an anomalous westward extentwith centred near 15� W, 37� S. Anomalous cy-clonic low-level circulation around the temperatetrough converges with an intensified flow aroundthe SASH to create a poleward flow south of 25� Snear 40� W (Fig. 7c). Vertically integrated anom-alous moisture flux anomalies (Fig. 9c) are domi-nated by these flows and show convergence alongthe leading (eastward) edge of the temperatetrough and are associated with the positive rainfallanomalies. Moisture flux convergence is alsostrongly enhanced by a moist northerly flow fromthe tropics around the continental low. The conver-gence of cool temperate air and the warm, moistnorth and northeasterly conveyors from the tropicsand subtropics results in a pronounced tempera-ture gradient (Fig. 9c) that is likely to create theinstability necessary for intense convection.

Hovmoeller plots (Fig. 11) plots clearlyshow the eastward propagating transient between30�–40� S. This generally develops 2 days priorto the onset of events, and tends to precede con-

vection in the subtropical latitudes 20�–30� S.Rainfall in the subtropical latitudes does not ap-pear to migrate substantially with the temperatetransient system. It is interesting to note also thatconvection in the 0�–10� S and 10�–20� S lati-tude band is heavily suppressed before and dur-ing negative events.

5. Discussion

During the austral summer the leading mode ofdaily rainfall variability in the SASA region isrepresented by a highly coherent meridionaldipole structure, organised into 2 extensivebands, both oriented northwest to southeastacross the continent and Atlantic Ocean. Here,we argue that the loadings represent variabilityat the daily time scale in the meridional positionof the SACZ within the broad band of rainfallshown in the climatology. The EOF loadingsare similar to the negative (positive) phases ofthe 10–20 day intra-seasonal SASA rainfall‘‘seesaw’’ identified by Nogues-Paegle and Mo(1997), who suggests a link between the dipolepattern and the 30–60 day oscillation in tropicalrainfall. Grimm and Silva Dias (1995) suggestthat the intra-seasonal oscillation propagatesfrom the Pacific to the Atlantic regions throughthe stimulation of a wave train by enhanced rain-fall along the SPCZ. Kiladis and Weickmann(1992) show that intraseasonal (14–30 day) var-iations in SACZ activity relate to a wave train inthe tropical and subtropical upper level wester-lies. The results of Liebmann et al (1999) arebroadly concurrent in that an observed 13-dayperiodicity in SACZ activity is associated withwave activity impinging from the midlatitudes,along a preferred Rossby wave-guide in this sec-tion of the southern Hemisphere, itself a functionof the basic state of the tropical atmosphere.

However, from the time coefficients (Fig. 4) itis clear that reversals in the phase of the SACZdipole also occur on shorter time scales related tosynoptic variability. Whilst our analysis hereincludes variability at all time scales beyond1-day, we are able to resolve individual synopticevents and therefore to characterise more pre-cisely the atmospheric circulation associatedwith the dipole phases. Reversals in the dipolephase are associated with 3 primary featuresof the large-scale circulation; (a) the phase of


specific midlatitude planetary wave structures,(b) the meridional position of the continentallow, and (c) the zonal position of the SASH.

In all months the 200 hPa height anomalyfields of positive and negative events show a welldefined and opposing trough-ridge wave patternpoleward of 30� S over the SASA region. This ispart of a characteristics wave train extendingthroughout the southern hemisphere mid andhigh latitudes. Thus, the activity and position ofthe SACZ is intimately connected to wave activ-ity emerging from the midatitudes of the easternPacific. Rainfall is located at the leading edge ofthe upper level trough such that reversals in theposition of the SACZ between positive and nega-tive events are associated with phase changes inthese wave structures.

In early=late (high) summer this 200 hPaheight field is dominated by variability in wave4 (wave 3). Nogues-Paegle and Mo (1997), andLiebmann et al (1999) however, find that an alter-nation in a trough-ridge pattern with wavelengthof 70� longitude (wave 5) is associated withintraseasonal rainfall variability over the regionduring the December–February seasons. It maybe that in early and late summer the position ofrainfall in the SACZ is influenced by transienteddies in the midlatitudes associated with eitherwave 4 or 5, whilst in high summer it is asso-ciated with variability in quasi-stationary wave 3.In high summer during positive events the tropi-cal-temperate link is less common and notimposed on the composites. This may reflectthe southward displacement of the midlatitudewesterlies in high summer.

However, upper level troughs often form to thesoutheast of the upper level highs associated withthe upper level divergence related to tropicalconvection (Kodama, 1992; Figueroa et al,1995). In support of this we find some evidencethat enhanced rainfall during the positive phase ispreceded by eastward propagating convection inthe Amazon region. In addition, the STJ is anom-alously strong at around 25� S, with rainfalllocated at the leading edge of the upper leveltrough. This is in agreement with Kodama(1993) who identify the STJ as an importantmechanism of frontogenesis along the SACZ.

In the positive phase the composites reveallow-level structures similar to those found in pre-vious studies SACZ activity in its mean position.

The east–west pressure gradient between theanomalous low over eastern Brazil and the east-ward shifted SASH results in a moist low level,poleward conveyor, with rainfall along the axisof the SACZ, extending into the midlatitudes.This supports the results of Kodama (1992;1993) who found that this poleward flow is crit-ical for maintaining convective activity along theSACZ, by the generation of low-level conver-gence and conditional instability. Moisture con-vergence and rainfall appear to be driven byinteraction of the continental low and the SASH,in accordance with the observational and ide-alised results of Kodama (1992), and Lentersand Cook (1995), respectively. Negative low-level temperature anomalies develop to thesouthwest of the rainfall maxima associated withadvection from the south Atlantic region aroundthe low-level trough, leading to the developmentof a cold front and consequent uplift.

The results here, indicating the dominance ofmoisture advection from Amazonia (particularlyin high summer), are in line with those ofLiebmann et al (1999) but differ from those ofKodama (1992), which identified easterliesaround the SASH as the dominant moisturesource. In fact, the importance of the continentallow in modulating the circulation was demon-strated by the idealised simulations of Lentersand Cook (1995) which suggest that the south-eastward recurving of the trades at around 7� S isrelated directly to the presence of the continentallow itself rather than any topographic forcing ofthe Andean Cordillera. Kodama (1993) suggeststhat the meridional position of the SACZ isdependent on the degree of connection with the‘‘monsoon westerlies’’ from Amazonia. The dis-tinctive moisture flux anomalies in the positiveand negative mode of the SACZ seems to re-affirm this, whereby the former (latter) exhibita strong (weak) contribution of moisture fromthe low-level tropical westerlies along the SACZ.In positive (negative) events convection overAmazonia is enhanced (suppressed) prior to theevents. The magnitude of the Amazonian mois-ture source is likely to be one factor explainingthe greater rainfall intensities associated withevents in the positive mode compared to thosein the negative mode.

Negative events in all summer months are as-sociated with a near reversal of this structure.


In contrast to positive events moisture istransported primarily around SASH. Convec-tion over Amazonia is suppressed during andprior to negative events, suggesting that activeperiods of the SACZ in its southern positionare independent of diabatic heating overAmazonia. The structure of rainfall anomaliesduring negative events is very similar to thatassociated with synoptic systems identified byGarreaud and Wallace (1998) who provide adetailed evaluation of the dynamics of thesesystems. However, although such systems dooften propagate northeastward the hovmoellerplots (Fig. 11) and the lagged EOF=rainfallcorrelations (Fig. 3) show that the dipole pat-tern over the SASA region is not an simplyartefact of such propagating systems. Liebmannet al (1999) suggest that these ‘‘cold surges’’lead to enhanced convection over westernAmazonia rather than the SACZ. Here, we findthat they may be an important stimulus foractivity in the SACZ in its more southerlyposition. That rainfall associated with the mid-latitude transient precedes that over the subtrop-ical plains suggests that the circulation maytrigger convection over the subtropical plains.The results are also broadly in line with theidealised results of Lenters and Cook (1995)who note that the transient component of thecirculation is associated with a southward dis-placement in the SACZ. Overall, our resultsprovide support for many existing hypothesesof the mechanisms associated with activity ofthe SACZ in its mean position, namely theeffect of continentality, Amazonian convectionand the midlatitude planetary wave structure.This suggests that a combination of these fac-tors are involved in determining periods ofactive SACZ convection. However, we havebeen able to infer the influence of distinctivecomponents of the large-scale circulation asso-ciated with changes in the meridional positionof the SACZ at the daily time scale. Ourresults also highlight important distinctionswithin the summer season.

The principle limitation of this study is therelatively short record of rainfall data. It shouldbe noted, however, that the RGPI represents themost extensive set of quasi-global rainfall datawith the high temporal resolution necessary toaccurately derive daily rainfall. Nevertheless,

some caution needs to be attached to the inter-pretation of these results owing to the relativelyshort duration of the data set.

6. Conclusions

The austral summer rainfall climatologies of theSASA region show the SACZ as a broad band ofhigh rainfall up to 30� in zonal extent, orientednorthwest to southeast. The SACZ is often con-sidered to occupy the central part of this band.Previous work has studied the mean state of theSACZ (Kodama, 1992; 1993) and submonthlyvariability (Liebmann et al, 1999) as well as rain-fall variability over South America at intra-seasonal scales (Nogues-Paegle and Mo, 1997)and at daily scales over the subtropical plains(Garreaud and Wallace, 1998). Using recentlyreleased satellite-derived rainfall products, thepresent study provides the first objective analysisof daily rainfall variability over the SASAregion. This is novel in that it provides the tem-poral and spatial resolution adequate to resolvespecific synoptic conditions and the associatedatmospheric circulation anomalies.

The results presented in this work suggest thatthe SACZ is a complex phenomenon, whichexhibits distinct patterns of spatial variability atdaily time scales, dominated by a meridionaldipole structure. From reanalysis data themechanisms responsible have been alluded to,and we have highlighted important differencesrelated to the position of the SACZ, the seasonand, quite likely, the time scale of variability. Inall cases identified in this study, the meridionallocation and activity of the SACZ is related to thephase of the zonal wave structure associated large-ly with wave 4 in early=late summer and wave 3in high summer. In addition, variability is asso-ciated with the position and intensity of theSouth American continental low and the SouthAtlantic subtropical high, which together drivea convergent, poleward flux of moisture resultingin the extensive cloud and rainfall observed. Inearly=late summer the SACZ is associated withtransient disturbances originating in the midlati-tudes such that the synoptic scale systems repre-sent coupled tropical-temperate systems. Whenthe SACZ occupies its primary position (positivemode) during high summer the role of anomalousmoist northwesterlies is particularly pronounced,


confirming the importance of Amazonia as asource of moisture. Given that the principlemoisture conveyor associated with active periodsof the SACZ in its primary position links with theAtlantic trade winds, it is possible that interann-ual variability in the strength of the trade windcirculation (Aceituno, 1988) may impact on theactivity of the SACZ at such time scales.

In describing the patterns of atmospheric cir-culation, moisture flux, and rainfall, we haveshown the significance of both tropical andextra-tropical dynamics in the development ofSACZ cloud bands. The results suggest that acomplete understanding of rainfall variabilityshould account for both tropical and temperatedynamics. Over Southern Africa tropical-temper-ate trough systems have been shown to be ofprofound significance in the meridional transferof energy and momentum (Harrison, 1986). Afull consideration of the momentum and ocean-atmosphere energy fluxes associated with thesynoptic systems identified here over the SASAregion would be valuable. Finally, given thatvariability in the activity of the SACZ occurs ata range of scales, future work should considervariability at longer inter-annual time scales.

Acknowledgements

The authors are grateful for the University of Oxford forsupport. NCEP reanalysis data were obtained from theNational Centre for Atmospheric Research.

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Author’s address: Prof. C. Todd, Department of Geogra-phy, University College London, 26 Bedford Way, London,WC1H 0AP UK (E-mail: [email protected])

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