19.2 ENSO Low Nile Flood Proxy Data
329
19.2 ENSO Low Nile Flood Proxy Data
There have been several approaches to dating El Niño events, perhaps most notable
among these, William Quinn’s pioneering work in compiling anecdotal evidence
of El Niños from written accounts dating back to the Spanish conquest of South
America [5]. David Enfield [6] points out that these are limited because they only
record local phenomena and do not necessarily reflect the global scale ENSO system. Enfield is equally critical of the use of “surrogate” or “proxy” records from tree
rings and ice cores for the opposite reason: because these do not relate directly to
El Niño events but to teleconnections, that is, to “very large spatial scale. . . remote
interannual climate perturbations beyond the Pacific region of intense interaction”
(p. 95). The value and the limitations of proxy records are bound together. “These
‘teleconnections’ are one of the primary ways in which El Niño-like fluctuations
during historical and prehistorical epochs have been preserved in surrogate records
by the biosphere and geological processes” (p. 95). Enfield notes the potential for
many of the surrogate records to “provide annual resolution (or better) over periods
ranging from several centuries to a millennium or more” (p. 103) but he points to the
problem that ENSO episodes have such widely varying characteristics and vary so
much from the “canonical pattern” that “the teleconnections that produce surrogate
records are not perfectly consistent from one event to another” (p. 103).
The same caveats apply, of course, to radiocarbon dating, for which calibration
usually depends on dendrochronology (study of tree rings) which can be independently affected by the many factors determining tree growth. And for radiocarbon
dating, as for climate periods, there are differential calibration estimates according
to hemisphere.
In 1992 William Quinn sought to extend his investigation back in time by undertaking an historical study of ENSO through analysis of Nile River low flood data
from AD 622 to AD 1522 [7]. The continuity and consistency of the data made it
a natural choice, as did the fact that summer monsoonal rainfall in the Ethiopian
mountains which feeds the rivers that feed the Nile, has a recognized teleconnection
to ENSO. To further validate his use of Nile River flood data for studying ENSO
in the past, Quinn provided a table for the years 1824–1973 comparing ENSO data,
including the dating of east monsoon drought in Australia, deficient Indian summer monsoon and weak Nile flood, as a measure of deficient summer monsoon in
the mountains of Ethiopia. Quinn demonstrated a correlation strong enough to suggest that strong ENSO effects will be represented by low monsoonal rains in the
Ethiopian mountains and so in deficient Nile floods (though local weather anomalies may reduce or augment the deficiencies). There also seems to be a correlation,
sometimes delayed, between the strength of extreme ENSO events and the level of
Nile deficiency. A good example is the 1876–1878 extreme El Niño reflected in the
most extreme Nile flood deficiency in 54 years, though the other two equally low
Nile levels of 1913 and 1972 are correlated with very strong (as opposed to extreme)
El Niño events.
The most deficient Nile floods (levels 3, 4, 5, 5+) seem to record strong El Niño
events reliably. Occasionally it does, however, seem possible that deficiencies will
329
19.2 ENSO Low Nile Flood Proxy Data
There have been several approaches to dating El Niño events, perhaps most notable
among these, William Quinn’s pioneering work in compiling anecdotal evidence
of El Niños from written accounts dating back to the Spanish conquest of South
America [5]. David Enfield [6] points out that these are limited because they only
record local phenomena and do not necessarily reflect the global scale ENSO system. Enfield is equally critical of the use of “surrogate” or “proxy” records from tree
rings and ice cores for the opposite reason: because these do not relate directly to
El Niño events but to teleconnections, that is, to “very large spatial scale. . . remote
interannual climate perturbations beyond the Pacific region of intense interaction”
(p. 95). The value and the limitations of proxy records are bound together. “These
‘teleconnections’ are one of the primary ways in which El Niño-like fluctuations
during historical and prehistorical epochs have been preserved in surrogate records
by the biosphere and geological processes” (p. 95). Enfield notes the potential for
many of the surrogate records to “provide annual resolution (or better) over periods
ranging from several centuries to a millennium or more” (p. 103) but he points to the
problem that ENSO episodes have such widely varying characteristics and vary so
much from the “canonical pattern” that “the teleconnections that produce surrogate
records are not perfectly consistent from one event to another” (p. 103).
The same caveats apply, of course, to radiocarbon dating, for which calibration
usually depends on dendrochronology (study of tree rings) which can be independently affected by the many factors determining tree growth. And for radiocarbon
dating, as for climate periods, there are differential calibration estimates according
to hemisphere.
In 1992 William Quinn sought to extend his investigation back in time by undertaking an historical study of ENSO through analysis of Nile River low flood data
from AD 622 to AD 1522 [7]. The continuity and consistency of the data made it
a natural choice, as did the fact that summer monsoonal rainfall in the Ethiopian
mountains which feeds the rivers that feed the Nile, has a recognized teleconnection
to ENSO. To further validate his use of Nile River flood data for studying ENSO
in the past, Quinn provided a table for the years 1824–1973 comparing ENSO data,
including the dating of east monsoon drought in Australia, deficient Indian summer monsoon and weak Nile flood, as a measure of deficient summer monsoon in
the mountains of Ethiopia. Quinn demonstrated a correlation strong enough to suggest that strong ENSO effects will be represented by low monsoonal rains in the
Ethiopian mountains and so in deficient Nile floods (though local weather anomalies may reduce or augment the deficiencies). There also seems to be a correlation,
sometimes delayed, between the strength of extreme ENSO events and the level of
Nile deficiency. A good example is the 1876–1878 extreme El Niño reflected in the
most extreme Nile flood deficiency in 54 years, though the other two equally low
Nile levels of 1913 and 1972 are correlated with very strong (as opposed to extreme)
El Niño events.
The most deficient Nile floods (levels 3, 4, 5, 5+) seem to record strong El Niño
events reliably. Occasionally it does, however, seem possible that deficiencies will