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Keywords:

  • climate;
  • Australia

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

[1] Monthly values of the latitude and intensity (mean maximum pressure) of the subtropical ridge (STR), averaged across Australian longitudes (105–155°E), have been related to variations and trends in rainfall over southern Australia (south of 30°S), for 1958–2005. There is an abrupt shift of latitude of the subtropical ridge over Australia at the onset of winter. In the cooler part of the year the intensity of the STR is more closely related to rainfall variations (and correlations between latitude and rainfall are a by-product of the relationship of intensity with both latitude and rainfall), whereas in summer the latitude of the STR is more closely related to rainfall. The decline in southern Australian rainfall in recent decades (which has occurred principally in autumn) appears related to a trend towards a more intense STR, rather than a trend in the latitude of the STR.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

[2] The temporal variations of the mean latitude of the anticyclones that are a feature of the Australian region have been examined in a number of studies following Pittock [1973], and Gibson [1992], most recently by Thresher [2002], Drosdowsky [2005], Larsen [2008], and Williams and Stone [2008]) all of whom calculated the latitude of the anticyclones (hereafter called the subtropical ridge or STR) and searched for trends over time. Drosdowsky [2005] used daily (National Centers for Environmental Prediction, NCEP) reanalysis data and, separately, monthly mean data at stations, to construct a time series of the STR latitude over the east coast of Australia, whereas Larsen [2008] used the Hadley Centre high-quality global gridded mean sea level pressure data set, HadSLP2r, to construct a time series of the winter STR latitude over the longitudes 105–180°E. Neither study found strong evidence of a trend in the STR latitude. Williams and Stone [2008] found different trends (rarely statistically significant) in different months. They also reported statistically significant relationships with rainfall over parts of Australia, including southern Australia. This study extends these previous studies by:

[3] 1. Developing a time series of monthly values of the latitude of the STR, averaged across Australian longitudes (105–155°E).

[4] 2. Preparing a time series of the maximum pressure (hereafter referred to as “intensity”) of the mean STR averaged across these longitudes.

[5] 3. Examining both variables for seasonally varying relationships with rainfall across southern Australia (south of 30°S) and for seasonally varying trends.

[6] The focus of this paper is on recent variations in southern Australian rainfall because annual rainfall has declined in this region over the recent decades [Gallant et al., 2007; Taschetto and England, 2008; Indian Ocean Climate Initiative, 2002; Murphy and Timbal, 2008; P. Hope et al., Associations between rainfall variability in the southwest and southeast of Australia and their evolution through time, submitted to International Journal of Climatology, 2009]. In this paper we examine the variations in the rainfall averaged across the country south of 30°S, and the relationships of this zonally-averaged rainfall with zonally averaged pressure variations (STR latitude and/or intensity). It seems more appropriate to compare zonally averaged rainfall variations with variations in the zonally averaged pressure indices. For this reason we have calculated the latitude and intensity of the subtropical ridge across Australian longitudes, rather than relate the behaviour of the STR at a single longitude (near the east coast of the continent), as has been the case in earlier studies.

2. Data

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

[7] All data used in this study are available in auxiliary material. Also available in the auxiliary material are scatter diagrams, by month, relating all variables discussed below (year, latitude and intensity of the STR, and rainfall). The scatter diagrams also show the linear correlation coefficients between each variable. Except where stated, all calculations and figures were prepared using R (http://CRAN.R-project.org).

2.1. STR Latitude and Intensity

[8] The global gridded (5° latitude/longitude resolution) mean sea level pressure (MSLP) data set, HadSLP2r [Allan and Ansell, 2006] was obtained from the UK Meteorological Office (http://hadobs.metoffice.com/hadslp2). For each month from January 1958–December 2007, zonal MSLP were calculated for each latitude from 5–45°S, for the longitudinal range 110–155°E, which spans continental Australia. Using the natural cubic spline algorithm from the Gnuplot plotting software (http://www.gnuplot.info) a smooth curve was fitted to the zonal means for each month, and the maximum MSLP (“intensity”) and the latitude at which it occurred were determined. As a check, this method was tested with synthetic data and it was found to be capable of reproducing the location and maximum of a broad peak typical of the sub-tropical ridge. The HadSLP2r data set is updated from 2004 to near real time using monthly NCEP/NCAR data, however some monthly means of the latitude and intensity for the years 2006 and 2007 were extreme outliers relative to other values raising suspicions about the quality of the most recent data, so the study was restricted to the period 1958–2005.

2.2. Australian Rainfall South of 30°S

[9] The monthly rainfall in each year 1958–2005, averaged across Australian land areas south of 30°S was calculated from the DIAGNOSE software [Jones et al., 2004] using gridded rainfall data prepared by the Australian Bureau of Meteorology.

3. Annual Cycle of the STR Over Australia

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

[10] Figure 1 shows box-and-whisker diagrams of the mean monthly latitude and intensity of the STR separately for each month of the year. Figure 1 illustrates the familiar annual progression of the STR to lower latitudes of about 28–30°S in winter (June–August) and higher latitudes of about 36–38°S in summer (December–February). Williams and Stone [2008] found similar annual cycles of the STR latitude in all their data sets.

image

Figure 1. Box-whisker plots showing annual cycle of monthly mean latitude and intensity of the subtropical ridge over Australia. Data from 1958–2005. Thick horizontal line is the median value; bottom and top of the boxes are the 25th and 75th percentiles; vertical dashed lines show the smaller of the range or 1.5 times the interquartile range; circles denote potential outliers (points that are more than 1.5 times the interquartile range from the median).

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[11] One aspect of the annual cycle of the STR latitude in Figure 1 that is not obvious in Williams and Stone is the large jump of about four degrees of latitude between April and May. The absence of such a jump from Williams and Stone [2008], who focussed on east Australian longitudes, suggests that the jump in Figure 1 may reflect a continental rather than a specifically east coast phenomenon. It may well be that the large jump is related to the change in ocean-land surface temperature contrast which occurs about May (when land temperatures fall below regional ocean temperatures). Radok and Grant [1957] also documented a tendency for an abrupt transition from summer mean atmospheric flow to a wintertime pattern over Australia, although they only examined a few years' data (1949–52).

[12] Figure 1 also shows the annual cycle of the intensity of the STR, illustrating that as the STR shifts equatorwards its maximum pressure increases, whereas lower pressures occur during the summer months with the STR located further polewards. There is no obvious rapid jump in pressures between April and May accompanying the rapid shift in latitude at this time of year.

4. STR Relationships With Southern Australian Rainfall

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

[13] Correlations of the STR latitude and intensity with southern Australian rainfall are listed for each month in Table 1, along with the correlations between intensity and latitude. The relationships between the variables for two illustrative months, January and July, are shown in Figures 2 and 3. Similar diagrams for all months are in the auxiliary material. Between April and October, the intensity is strongly, negatively correlated with rain, i.e., high pressures are accompanied by low rainfall (e.g., see Figure 3). Over summer the correlations between intensity and rain are weak (e.g., Figure 2). Latitude is strongly, negatively correlated with rain during summer, and strongly, positively correlated with rain through the colder months. That is, during the cooler months of the year, rainfall is lower when the STR is situated further poleward than normal. However, the magnitudes of the rain-latitude correlations during the cooler months are weaker than the rain-intensity correlations. Note that the correlation between rain and latitude jumps from −0.27 in April to 0.50 in May, i.e., at the same time of year when the STR latitude also shifts abruptly.

image

Figure 2. Scatter diagrams for January between Year, STR latitude (JanLat; °N), STR intensity (JanInt; hPa) and rainfall total (JanRain; mm). Data from 1958–2005. Correlations between variables are listed in top right hand part of figure.

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image

Figure 3. Scatter diagrams for July between Year, STR latitude (JulLat; °N), STR intensity (JulInt; hPa) and rainfall total (JulRain; mm). Data from 1958–2005. Correlations between variables are listed in top right hand part of figure.

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Table 1. Correlations and Partial Correlations Between Subtropical Ridge Intensity and Latitude, Southern Australian Rainfall, and Yeara
MonthCorrelationsPartial Correlations
Rain-IntensityRain-LatitudeIntensity-LatitudeYear-RainYear-IntensityYear-LatitudeRain-Intensity (Latitude)Rain-Latitude (Intensity)Year-Rain (Intensity)Year-Rain (Latitude)
  • a

    Data from 1958–2005. Correlations exceeding 0.33 (0.24) in magnitude are statistically significant at the 1% (5%) level. Note that latitude is °N, so a negative correlation with “Year” would indicate a trend away from the equator. In the section of the Table labelled “Correlations” the top row of each column indicates the variables whose correlations, for each month of the year, are listed below. In the section of the table labelled “Partial correlations” the row headings indicate the variables whose partial correlations (after removal of the effect of the variable in parentheses) are listed for each month of the year. So the column labelled “Rain-Intensity (Latitude)” lists, for each month, the partial correlations between Rain and Intensity, after removal of the relationships of Latitude with each variable. Note that the partial correlations are calculated separately for each month.

Jan0.00−.48−.35−.050.14−.03−.20−.51−.05−.07
Feb0.11−.25−.350.02−.070.020.03−.230.030.03
Mar−.03−.37−.41−.150.40−.09−.21−.42−.15−.20
Apr−.48−.27−.22−.170.36−.19−.57−.440.00−.23
May−.690.50−.63−.190.28−.18−.560.120.01−.12
Jun−.740.61−.820.080.010.14−.530.010.13−.01
Jul−.760.64−.84−.150.24−.15−.530.010.05−.07
Aug−.730.48−.75−.040.160.05−.64−.150.11−.07
Sep−.560.13−.570.04−.090.35−.60−.28−.01−.01
Oct−.40−.24−.40−.06−.010.16−.56−.48−.04−.02
Nov−.30−.47−.240.070.08−.17−.48−.590.10−.01
Dec0.13−.36−.61−.04−.05−.11−.12−.36−.03−.09

[14] Also shown in Table 1 are the partial correlations between rain and latitude (after removal of the effect of intensity) and between rain and intensity (after removal of the effect of latitude). Comparisons of the relative magnitudes of these two sets of partial correlations can indicate which of latitude and intensity is the dominant cause of rainfall fluctuations, and how their relative influence changes through the year. During the cooler months (May–September), the rain-intensity partial correlations (after removal of effect of latitude) are very much stronger than are the rain-latitude partial correlations (after removal of the effect of intensity). So, variations in the intensity of the STR appear to be the dominant, “immediate” cause of variations in the rainfall, rather than variations in the STR latitude. The strong, positive latitude-rain correlations during the cooler months appear to be a by-product of the relationship of intensity with both rain and latitude. Of course, a more fundamental factor must be the underlying cause of the changes in the STR and thus of rainfall – the relationships discussed here are only causal in a simple, immediate sense.

[15] The situation is different during the warmer months, when the partial correlations between rain and latitude (after removal of the effects of intensity) are stronger than the partial correlations between rain and intensity (after removal of the effects of latitude). So, during November–March it appears to be the variations in the latitude of the STR that are the principal factor leading to variations in rainfall. The weak rain-intensity correlations indicate that the STR intensity has little effect on rain over the warmer months. The dominant effect of latitude during the warmer months, when the STR is generally located south of the continent, is understandable. If the STR was displaced even further south, this would allow more tropical influence on the rainfall of the southern parts of the continent, compared to years when the STR was located further north (and thus situated over the southern parts of the continent).

[16] During the cooler months, the positive correlations between rain and STR latitude indicate that in years when the STR was anomalously northward this would allow more influence from mid-latitude systems, leading to increased rainfall. However, this effect is weaker than the effect from the intensity of the STR – when the STR is more intense than normal, rainfall would be low. The very strong negative correlations between intensity and latitude of the STR indicate that during the cooler months, when the STR is stronger than normal it is also shifted polewards.

[17] Throughout the year, latitude and intensity of the STR are strongly, negatively correlated. That is, when the STR is situated further southwards than normal, its intensity (i.e., maximum pressure) is above average. This relationship is opposite in sign to the relationship between intensity and latitude shown through the annual cycle where intensity increases as latitude decreases (i.e., a positive relationship).

5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

[18] Also listed in Table 1 are correlations of STR latitude and intensity with year, and also rain-year correlations. These indicate that there are moderately strong (but not statistically significant at the 5% significance level) rainfall trends in autumn (March–May) rainfall (and also in July). There are strong positive trends in STR intensity in these months, suggesting that it is these trends that are causing the autumn rainfall trend. This is confirmed by the partial correlations between rain and year (after removal of the effect of intensity). These are close to zero in April, May, and July, indicating that without the trend in STR intensity, rainfall in these months may not have exhibited a downward trend. On the other hand, removal of the effect of latitude on the rain-year correlations through calculating partial correlations does little to reduce the strength of the correlations (in fact this procedure increases the strength of the correlations in some months, notably March and April). So, any trends in latitude of the STR do not appear to be contributing to the rainfall trends. This is not surprising since the latitude exhibits a strong trend only in September (and rainfall does not exhibit a strong trend in this month).

[19] The relative strengths of the influence of latitude and intensity on the rain trends is illustrated in Figure 4 which shows the time series of the March–May total rainfall along with time series of the March–May average STR intensity and latitude. The strong trend towards increased intensity is clear, as is the negative relationship between intensity and rainfall (r = −0.46). Given this negative relationship, one should expect a decline in rainfall over the 1958–2005 period, associated with the strong trend to increased intensity. On the other hand, Figure 4 reveals little evidence of a relationship between latitude and rainfall (r = −0.06), so even the weak trend of the STR towards higher latitudes did not contribute to the rainfall decline. It must be noted, however, that this weak March–May correlation between latitude and rainfall occurs because two months of this season (March, April) exhibit statistically significant negative correlations between the variables while the other month (May) exhibits a strong positive correlation between latitude and rainfall. So combining the three months leads to the negative correlations being offset by the positive correlation in May, thus resulting in a very weak correlation over the total autumn period. Nevertheless, as Table 1 shows, rainfall trends are still evident in each of March, April, and May even if the possible effect of trends in latitude in these months is removed through partial correlation in each month separately. So, in none of the autumn months is the latitude an important factor contributing to the observed decline in rainfall.

image

Figure 4. Time series of (top) STR intensity (hPa), (middle) rainfall (mm) and (bottom) STR latitude (°N), for March–May. Dashed lines are linear trends.

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6. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

[20] This is the first study to examine the relationship of southern Australian rainfall with both the latitude and intensity of the STR. The examination, jointly, of the two indices of the STR behaviour reveals that the interactions between the STR and rainfall are rather more complex than is sometimes believed. Specifically, this study has demonstrated that:

[21] 1. There is an abrupt shift of latitude of the subtropical ridge over Australia at the onset of winter.

[22] 2. In winter the intensity of the STR appears to be the dominant factor causing southern Australian rainfall variations (and relationships between rainfall and STR latitude variations are a by-product of the relationship between latitude and intensity of the STR), whereas in summer the latitude of the STR seems to be more important in determining rainfall variations. Of course, the STR intensity and latitude variations are driven by some other factor and it will be this (at present unknown) factor that should be considered the underlying “cause” of the rainfall variations, rather than the variations in the STR. Nicholls [2009] discussed possible, more fundamental causes of southern Australian rainfall variations.

[23] 3. The decline in southern Australian rainfall in recent decades (which occurs principally in autumn) appears to be related to a trend towards a more intense STR, rather than a trend in the latitude of the STR.

[24] The strong, positive correlations (typically around 0.5) evident between STR latitude and rainfall during the cooler months (Table 1) might lead an investigator to conclude that the latitude of the STR was an important factor influencing southern Australian rainfall variations. The investigator might then suppose that it would be important to investigate the possible causes of variations in the STR latitude, if we are to uncover the causes of variations and trends in southern Australian cool-season rainfall. However, the analysis described herein demonstrates that this latitude-rainfall relationship is in fact a by-product of the stronger relationships of the intensity of the STR with rainfall and with STR latitude. The strong positive rainfall-latitude relationship is reduced to near zero, if the influence of the STR intensity on rainfall and STR latitude is removed. So, if we are to understand the fundamental causes of variations and trends in southern Australian cool season rainfall, the primary focus should be on the causes of trends in the intensity of the STR, and not its latitude.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information
  • Allan, R. J., and T. J. Ansell (2006), A new globally complete monthly historical mean sea level pressure data set (HadSLP2), 1850–2004, J. Clim., 19, 58165842.
  • Drosdowsky, W. (2005), The latitude of the subtropical ridge over eastern Australia: The L index revisited, Int. J. Climatol., 25, 12911299.
  • Gallant, A. J. E., K. J. Hennessy, and J. Risbey (2007), Trends in rainfall indices for six Australian regions: 1910–2005, Aust. Meteorol. Mag., 56, 223239.
  • Gibson, T. T. (1992), An observed polewards shift of the southern hemisphere subtropical wind maximum—A greenhouse symptom? Int. J. Climatol., 12, 637640.
  • Indian Ocean Climate Initiative (2002), Climate Variability and Change in South West Western Australia, 34 pp., Dep. of Environ. Water and Catchment Prot., East Perth, West. Aust., Australia.
  • Jones, D., D. Collins, N. Nicholls, J. Phan, and P. Della-Marta (2004), A new tool for tracking Australia's climate variability and change, Bull. Aust. Meteorol. Oceanogr. Soc., 17, 6569.
  • Larsen, S. H. (2008), Australian winter anticyclonicity, 1850–2006, J. Geophys. Res., 113, D06105, doi:10.1029/2007JD008873.
  • Murphy, B. F., and B. Timbal (2008), A review of recent climate variability and climate change in southeastern Australia, Int. J. Climatol., 28, 859879, doi:10.1002/joc.1627.
  • Nicholls, N. (2009), Local and remote causes of the southern Australian autumn-winter rainfall decline, 1958–2007, Clim. Dyn., doi:10.1007/s00382-009-0527-6.
  • Pittock, A. B. (1973), Global meridional interactions in stratosphere and troposphere, Q. J. R. Meteorol. Soc., 99, 424437.
  • Radok, U., and A. M. Grant (1957), Variations in the high tropospheric mean flow over Australia and New Zealand, J. Meteorol., 14, 141149.
  • Taschetto, A. S., and M. H. England (2008), An analysis of late twentieth Century trends in Australian rainfall, Int. J. Climatol., doi:10.1002/joc.1736.
  • Thresher, R. E. (2002), Solar correlates of Southern Hemisphere mid-latitude climate variability, Int. J. Climatol., 22, 901915.
  • Williams, A. A. J., and R. C. Stone (2008), An assessment of relationships between the Australian subtropical ridge, rainfall variability, and high-latitude circulation patterns, Int. J. Climatol., doi:10.1002/joc.1732.

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data
  5. 3. Annual Cycle of the STR Over Australia
  6. 4. STR Relationships With Southern Australian Rainfall
  7. 5. Trends in the STR and Their Relationship to Southern Australian Rainfall Trends
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

Auxiliary material for this article contains a data set and twelve figures.

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FilenameFormatSizeDescription
grl25855-sup-0001-readme.txtplain text document3Kreadme.txt
grl25855-sup-0002-ds01.txtplain text document13KData Set S1. Monthly mean data for latitude and maximum intensity of the subtropical ridge.
grl25855-sup-0003-fs01.epsPS document814KFigure S1. Scatter diagram of January values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0004-fs02.epsPS document853KFigure S2. Scatter diagram of February values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0005-fs03.epsPS document791KFigure S3. Scatter diagram of March values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0006-fs04.epsPS document877KFigure S4. Scatter diagram of April values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0007-fs05.epsPS document876KFigure S5. Scatter diagram of May values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0008-fs06.epsPS document865KFigure S6. Scatter diagram of June values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0009-fs07.epsPS document838KFigure S7. Scatter diagram of July values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0010-fs08.epsPS document959KFigure S8. Scatter diagram of August values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0011-fs09.epsPS document902KFigure S9. Scatter diagram of September values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0012-fs10.epsPS document922KFigure S10. Scatter diagram of October values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0013-fs11.epsPS document833KFigure S11. Scatter diagram of November values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0014-fs12.epsPS document893KFigure S12. Scatter diagram of December values of year, latitude, and intensity of subtropical ridge, and rainfall south of 30°S.
grl25855-sup-0015-t01.txtplain text document2KTab-delimited Table 1.

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