In a recent issue of Climatic Change, Whetton and Rutherfurd (1994) (henceforth. 'WR') presented the results of research which analysed proxy rainfall data sets.
Correspondence HISTORICAL ENSO TELECONNECTIONS IN THE EASTERN HEMISPHERE: COMPARISON WITH LATEST EL NII~O SERIES OF QUINN
Introduction In a recent issue of Climatic Change, Whetton and Rutherfurd (1994) (henceforth 'WR') presented the results of research which analysed proxy rainfall data sets going back to AD 1525 in an attempt to extend the record of climatic extremes affecting the eastern hemisphere related to E1 Nifio-Southem Oscillation (ENSO). The analysis in WR included, as an important step, tests for relationships between the eastern hemisphere proxy rainfall series under consideration, and the E1 Nifio chronology of Quinn and Neal (1992). The E1 Nifio chronology is based primarily on historical records of coastal flooding and heavy rain in Peru (although some additional evidence is drawn from Chile and Brazil). For this note, revision of some of the analysis of WR has been undertaken using the revised E1 Nifio chronology of Quinn (1992). The results of these calculations, which are summarised in this note, suggest some conclusions significantly different from those drawn in WR, and also have implications for the correct interpretation of the Quinn (1992) E1 Nifio chronology. The data used in WR included annual time series of flood height of the Nile at Cairo, an index of rainfall in northern China (which was based on historical records) and tree ring widths based on teak growing in Java. In addition, WR used two extreme event chronologies: a drought and famine chronology for India, and the E1 Nifio chronology prepared by Quinn and Neal (1992). All of these series began at least as early as 1525 (the first year analysed), although there were some long breaks in the Nile record prior to 1700. The four eastern hemisphere series are proxies for rainfall variations in regions which demonstrate a rainfall deficit in E1 Nifio years in the modem record. For more information of the data sets, including their sources, see WR. An important step in the methodology adopted in WR was the use of statistical techniques to see whether, or over what periods, the data sets were interrelated in a pattern which suggested ENSO-forcing (i.e. whether the data demonstrated ENSO-related 'teleconnections'). Particular emphasis was placed on how well the Climatic Change 32: 103-109, 1996. (~ 1996 Kluwer Academic Publishers. Printed in the Netherlands.
104
CORRESPONDENCE
selected data series correlated with the Quinn and Neal (1992) chronology. WR found that ENSO-related teleconnections were present in the data back to around 1750, and prepared a chronology of 'teleconnection years characteristic of ENSO' for the period from the beginning of the eighteenth century onward. The chronology was not begun any earlier because prior to that time there was no clear evidence of the existence of an ENSO signal in the series used. WR attributed this absence of signal to poorer quality data in the earlier period of record, rather than a change in ENSO behaviour. The E1 Nifio chronology published by Quinn (1992) represents a significant revision of the chronology which appeared in Quinn and Neal (1992). The 'new Quinn chronology', as we will henceforth call it, contains a total of 32 years of changed status (a previously non-E1 Nifio year becoming an E1 Nifio year, or vice-versa). Many of these revisions can be viewed as adjustments to the existing events, such as a shift by one year in the timing of an event, or extending or reducing the duration of an event by one year. However, some events are completely deleted and some new events added. All but two of these changes occur between 1525, the beginning of the chronology, and 1786. In addition to these changes in event occurrence, the new chronology also contains changes in event strength and confidence rating. Significant changes in the E1 Nifio chronology would be expected over time as the historical research of Quinn and his colleagues has progressed, and new information sources have been discovered. Indeed, the chronology of Quinn (1992) is a revision and extension of Quinn et al. (1987) which in turn extended the original chronology of Quinn et al. (1978). Quinn (1992) says the modifications leading to the most recent E1 Nifio chronology are "as a result of further study, additional information obtained through this research, and identification with large scale ENSO developments". In Quinn (1992), 'large scale ENSO developments' would refer to the chronology of ENSO-related events in other parts of the world which are also discussed in the paper. Indeed, in addition to the regional El Nlfio chronology, Quinn (1992) presents a broadscale ENSO event chronology based on data from other parts of the world as well as the regional E1 Nifio record. For the period prior to 1824, these additional data are Nile flood height and some historical records of droughts in India (see references in Quinn, 1992). The fact that the regional E1 Nifio chronology, and not just ENSO event chronology, 0f Quinn (1992) has been produced with reference to events beyond the South American region is important, and we will return to this point in discussing the implications of the results presented below.
1. Analysis and Results WR presented a diagram showing a specialised cumulative anomaly plot for each of the three annual series (Nile, northern China and Javan tree rings) for the period
CORRESPONDENCE
105
1525 to the end of each of these three records (Figure 5 in WR). In producing the curve for each series, standardised anomalies were accumulated only for the years contained in the E1 Nifio chronology; anomalies for other years were not used. The cumulative curves from WR are presented again here compared with corresponding cumulative anomaly curves using the new Quinn chronology (Figure 1). Where a series in this plot shows a downward trend, there is a tendency for negative anomalies in the series to be associated with E1 Nifio occurrence, and thus can indicate changes over time in the strength of any relationship between E1 Nifio and the regional series. Note also that high pass filtering has been applied to the continuous data sets before this analysis (see WR) to ensure that any marked trends are not simply due to long period fluctuations in the data sets. For the Nile (Figure 1a), the original analysis showed a consistent decline in the cumulative anomaly series from 1830 onwards, thus indicating an E1 Nifio-Nile relationship for that period. However, during the eighteenth century, although the Nile data are largely complete, there is no tendency for Nile floods to be lower during E1 Nifio years. By contrast, the anomaly curve based on the new Quinn chronology shows the presence of such a trend in eighteenth century. In fact, except for a period around 1740--1755, the Nile series indicates a relationship between low floods and E1 Nifio years from around 1700 to the end of the record. Prior to 1700, little can be said about Nile behaviour as the record is largely missing. In the original analysis for north China (Figure lb), there was good evidence of association between low rainfall and E1Nifio years from 1770 onwards, except for a marked reversal in the relationship around 1915-1935. Prior to 1770, there was only a slight tendency for negative anomalies. Using the new Quinn chronology, prior to 1770 there is now a more marked relationship between low rainfall and E1 Nifio events. Indeed the curve can be viewed as showing such a tendency from the beginning of the record (1525) except for three periods when the relationship reverses (1605-1620, 1710-1760, and 1915-1935). The most recent reversal is strongest. The changes to the E1 Nifio series has also improved the relationship of E1 Nifio to Javan tree rings (Figure lc). Previously, the expected relationship of narrow rings in E1 Nifio years was present from around 1770-1890. The results using the new E1 Nifio chronology suggest that this relationship may be active from as early as 1670. The curious existence of an opposite tendency in the earliest part of the record is still present, although it is now of shorter duration. In WR, the statistical significance of the relationships suggested in Figure 1 was tested with a Mann-Whitney U-test, by comparing values during E1 Nifio years with the values in the full series. The test was applied to three subdivisions of the data (1525-1769, 1770-1879, 1880-1984), and WR considered any significance at better than the 10% level worthy of note. The results for both the old Quinn series (originally presented in Table V in WR) and the new Quinn series are presented in Table I. Marked changes in the results are apparent for the two earlier periods (1525-1769; 1770-1879). For the Nile, the relationship for the period
106
CORRESPONDENCE
(a) 5 ~ Nile oj ~
,....
a.1
-5
":.,.;:..~ -
~ -10
k ---
N -15
old
......
-20
- - i 1400
1500
"k.
:4...
new
''-
i
i
I
i
i
1600
1700
1800
1900
2000
2100
year
(b) Northern China .._m,f~j
0
o~
-2 -4
..:..- ........ _......
9r '~
-6 old -8
"i9; .~'; :,',:" -"
. . . . . . Rew
-10 1400
I
~
I
I
I
I
1500
1600
1700
1800
1900
2000
21 O0
year
Cc)
Tree Rings _~
3 84
o
2-
...;
?-"-~.:-"-: ,~ ~..
"6
--~,~t ~-
0 -1 1400
----
old
......
new
~,
l
I
I
~
I
1500
1600
1700
1800
1900
2000
year
Fig. 1. Cumulative standardised anomalies from the mean for: (a) Nile floods; (b) North China rainfall; and (c) Javanese tree rings for E1 Nifio years according to the 'old' and the 'new' Quinn chronologies. In each curve, anomalies are accumulated only for the years contained in the E1 Nifio chronology; anomalies for other years are not used.
107
CORRESPONDENCE
TABLE I Mean standardisedanomaliesof the Nile flood,Javanesetree rings, and North China rainfall series in E1 Nifioyears accordingto the 'old' and 'new' Quinn chronologies. Statisticalsignificanceis based on a Mann-WhitneyU test and is shown with asterisks: p < 0.1 (*), p < 0.05 (**), p < 0.01 (***). Results for the 'old' Quinn E1 Nifiochronologyare the same as they were in WR Series Quinn 1525-1769 1770-1879 1880-1984 Fullperiod Nile
old new
0.01 -0.06
-0.10" -0.22***
-0.21"** -0.21"**
-0.09*** -0.12"**
China
old new
-0.02 -0.04**
-0.06* -0.07**
-0.03 -0.03
-0.03* -0.05**
Java
old new
0.06*** 0.03*
-0.04** -0.04**
0.01 0.01
0.02 0.01
1770-1879 has strengthened markedly and is now significant at the 1% level. For northern China rainfall, there is now an association between E1 Nifio and low rainfall significant at the 5% level for both the 1525-1769 and 1770-1879 periods, whereas previously a weak relationship was present in 1770-1879 only. For the Javanese tree-rings, the unexpected contrary relationship in the period 1525-1769 has weakened considerably. In WR, the association between the occurrence of drought years in India and E1 Nifio was also tested using a contingency table analysis (Table VI in WR). This has also been repeated using the new Quinn E1 Nifio series, but the results are not significantly affected.
Discussion The marked improvement in the relationships between the three series considered here and E1 Nifio when the latest Quinn E1 Nifio series is used has important implications. However, the new results need to be interpreted in the knowledge that Quinn used data beyond the South American region in producing his most recent revision of the E1 Nifio series. As his most important additional source of data was the Nile, it is perhaps not surprising that the changes in event occurrence during the eighteenth century have resulted in a marked improvement in the Nile and E1 Nifio association in that period. It appears that the series are not independent. However, there is no reason to believe that the changes were in any way influenced by northern Chinese rainfall or Javanese tree rings, where there is also a marked improvement in the relationship (and neither of these series are correlated with the Nile prior to 1769 - see Table III in WR). In particular, there is now evidence of an ENSO signal in the Chinese series back to the beginning of our analysis period
108
CORRESPONDENCE
(1525), and in the Javanese tree ring series back to around 1670. Previously, WR concluded that there was little evidence of a signal prior to the mid-eighteenth century. Results of this study suggest that the northern China rainfall series and the Javanese tree ring series are likely to be significantly better indicators of past ENSO variability than was indicated in WR. As a consequence, there is considerably more confidence in the 1700-1979 chronology of ENSO and anti-ENSO teleconnection events identified in WR. It also suggests that extending the chronology to the period before 1700 would be viable, and this will be undertaken in the future in conjunction with studies incorporating proxy ENSO data from additional sources. The E1 Nifio chronologies of Quinn have been the foundation for research into ENSO variations during the pre-instrumental period. The field was pioneered by Quinn, and many subsequent researchers have used the Quinn chronologies to help assess potential ENSO indicators for the pre-instmmental period as they have become available (e.g., Lough and Fritts, 1985, 1989; Nicholls, 1988; Enfield and Cid, 1991; Enfield, 1992; Wang, 1992; Diaz and Pulwarty, 1992, 1994, and WR). Such uses of Quinn's E1 Nifio chronologies will no doubt continue, and the most recent (Quinn, 1992) is likely to be in considerable demand. It is precisely because of this wide usage, that this note sounds a word of warning. Researchers have generally assumed that the Quinn E1 Nifio chronology is a regional chronology i.e. it is based on data from the west coast of South America. Indeed, the recent studies of Diaz and Pulwarty (1992, 1994) make this assumption in their use of the Quinn (1992) chronology. Although earlier versions of the chronology appear to be regional (primarily the Peruvian region, although broader South American data are used), the Quinn (1992) chronology is not. A careful reading of Quinn (1992), and the results presented here, suggest that the record of ENSO-related climatic anomalies elsewhere in the world have been used in revising the chronology. This means that using this chronology to examine El Nifio frequency or to compare it with other regional ENSO indicators could lead to biased results. It must therefore be stressed that the Quinn (1992) chronology should not be used where a purely regional series is required. For this reason, the Quinn (1992) chronology was not used here to revise the list of ENSO and anti-ENSO teleconnection years presented in WR. (Furthermore, Hocquenghem and Ortlieb (1992) have noted that Quinn's use, even in the earlier chronologies, of data from parts of Peru and Chile which have differing modem ENSO signals may have distorted his identification of E1 Nifio events.) It should be noted that we are not criticising in any way the large scale ENSO-event chronology also presented in Quinn (1992), which remains a very useful indicator of global-scale ENSO activity. It is with sadness that the authors note the death of Bill Quinn in early 1994. However, it appears that the work he pioneered will continue. Recent studies by Hocquenghem and Ortlieb (1992), Ortlieb and Machare (1993) and Ortlieb (1994) have detailed efforts to improve and extend the Quinn chronologies.
CORRESPONDENCE
109
References Diaz, H. E and Pulwarty, R. S.: 1992, 'A Comparison of Southern Oscillation and E1 Nifio Signals in the Tropics', in Diaz, H. E and Markgraf, V. (eds.), El Ni~o: Historical and Palaeoclimatic Aspects of the Southern Oscillation, CUP, Cambridge, pp. 175-192. Diaz, H. E and Pulwarty, R. S.: 1994, 'An Analysis of the Time Scales of Variability in Centuries-Long ENSO-Sensitive Records in the Last 1000 Years', Clim. Change 26, 317-342. Enfield, D. B.: 1992, 'Historical and Prehistorical Overview of the E1 Nifio/Southern Oscillation', in Diaz, H. E and Markgraf, V. (eds.), El Ni~o: Historical and Palaeoclimatic Aspects of the Southern Oscillation, CUP, Cambridge, pp. 96-117. Enfield, D. B. and Cid, S. L.: 1991, 'Low-Frequency Changes in E1 Nifio-Southern Oscillation', J. Clim. 4, 1137-1146. Hocquenghem, A. M. and Ortlieb, L.: 1992, 'Eventos E1 Nifio y Iluvias anormales en la costa del Peru: siglos XVI-XIX', Bull. Ist. Fr. Etud. Andines, Lima 21(1), 197-278. Lough, J. M. and Fritts, H. C.: 1985, 'The Southern Oscillation and Tree Rings: 1600-1961 ', J. Clim. Appl. Meteorol. 24, 952-966. Lough, J. M. and Fritts, H. C.: 1989,' Historical Aspects of E1Nifio/Southern Oscillation - Information from Tree Rings', in Glyrm, P. W. (ed.), Global Ecological Consequences of the 1982--83 EINi~o Southern Oscillation, Elsevier, Amsterdam, pp. 285-321. Nicholls, N.: 1988, 'More on Early ENSOs: Evidence from Australian Documentary Sources', Bull. Am. Meteorol. Soc. 69, 4-6. Ortlieb, L.: 1994, 'Major Historical Rainfalls in Central Chile and the Chronology of ENSO Events During the XVth-XIXth Centuries', in Villagren, C. (ed.), Revista Chilena Historia Natural, Special volume on 'Quaternary of Chile', (in press). Ortlieb, L. and Machare, J.: 1993, 'Former E1 Nifio Events: Records from Western South America', Global and Planetary Change 7, 181-202. Quinn, W. H.: 1992, 'A Study of Southern Oscillation-Related Climatic Activity for A.D. 622-1900 Incorporating Nile River Flood Data', in Diaz, H. F. and Markgraf, V. (eds.), ElNifto: Historical and Palaeoclimatic Aspects of the Southern Oscillation, CUP, Cambridge, pp. 119-149. Quinn, W. H. and Neal, V. T.: 1992, 'The Historical Record of E1Nifio Events', in Bradley, R. S. and Jones, P. D. (eds.), Climate since AD 1500, Routledge, London, pp. 623-648. Quinn, W. H., Neal, V. T., and Antunez de Mayolo, S. E.: 1987, 'El Nifio Occurrences over the Past Four and Half Centuries', J. Geophys. Res. 92, 14, 449-14, 461. Quinn, W. H., Zopf, D. O., Short, K. S., and Kuo Yang, R. T.: 1978, 'Historical Trends and Statistics of the Southern Oscillation, E1 Nifio, and Indonesian droughts', Fish. Bull. 76, 663-678. Wang, S.: 1992, 'Reconstruction of E1 Nifio Event Chronology for the Last 600 Year Period', Acta Meteorol. Sinica 6, 47-57. Whetton, P. H. and Rutheffurd, I.: 1994, 'Historical ENSO Teleconnections in the Eastern Hemisphere', Clim. Change 28, 221-253. -
*CSIRO Division of Atmospheric Research, PETER WHETf ON*, ROBERT ALLAN*, P.B. No. 1, Mordialloc, Vic 3195, Australia and IAN RUTHERFURD** **Department of Civil Engineering, Monash University, Clayton, Vic 3178, Australia
