A new study has recently been published by Mandy Freund et al entitled ‘Higher frequency of Central Pacific El Niño events in recent decades relative to past centuries‘. The research is based upon a 400 year coral record gathered from various regions in the equatorial Pacific. The authors conclude that Central Pacific (CP) El Nino’s (alternatively named Modoki events or ‘warm pool’ events) have become quite dramatically more frequent in recent times and that the number of CP events compared to Eastern Pacific (EP) events – canonical El Nino’s – shows a large spike in the late 20th century, unprecedented in the 400 year record. The authors speculate at the Conversation that this may be due to global warming and that the ratio of CP to EP may continue to increase. The headline reads:
‘El Niño has rapidly become stronger and stranger, according to coral records’
WUWT has a post on this study here. Apparently, using corals to elicit seasonal changes in water temperature and thereby reveal the different ‘flavours’ of El Nino which have occurred over the past has never been done before and some researchers thought it could not be done:
This extraordinary result was teased out of information about past climate from coral cores spanning the Pacific Ocean, as part of Dr Freund’s PhD research at the University of Melbourne and the Centre of Excellence for Climate Extremes. It was made possible because coral cores – like tree rings – have centuries-long growth patterns and contain isotopes that can tell us a lot about the climate of the past. However, until now, they had not been used to detect the different types of El Niño events.
This meant El Niño researchers were constrained by what they could say about El Niño behaviour because the instrumental record was too short and it was hard to judge whether recent decadal changes were exceptional.
“By understanding the past, we are better equipped to understand the future, especially in the context of climate change,” said Dr Freund.
“Prior to this research, we did not know how frequently different types of El Niño occurred in past centuries. Now we do,” said co-author from the Centre of Excellence for Climate Extremes Dr Ben Henley.
The key to unlocking the El Niño record was the understanding that coral records contained enough information to identify seasonal changes in the tropical Pacific Ocean. However, using coral records to reconstruct El Niño history at a seasonal timescale had never been done before and many people working in the field considered it impossible.
Along with identifying a sudden increase in the relative frequency of CP events, Freund et al also recorded a contemporaneous increase in intensity of canonical El Ninos, which the authors describe as ‘unprecedented’ in the 400 year record:
Although the number of EP events has decreased since the 1980s, the intensity of EP events since then has been unusually high Fig. 4d). Despite the potential limitations of coral-based reconstructions at capturing the precise maximum SSTA amplitudes in the Niño3 and Niño4 regions during El Niño events, the observed intensity of recent EP events is substantially higher than that in the full length of the instrumental (post-1920) and multi-century period. CP events appear not to have increased in intensity. We note that the coral reconstruction underestimates the intensity of the 1982/1983 and 1997/1998 El Niño events, but does capture the intensity of other El Niño events identified in the instrumental record (discussed in Supplementary Section 5.2.5). Notwithstanding this caveat, our reconstructions offer a long-term context for seasonal El Niño variability that demonstrates the unusual recent increase in CP frequency and EP amplitude. Assuming that the reconstructions are representative of past amplitudes, the three most recent EP events (1982, 1997 and 2015) are of unprecedented amplitude relative to the past four centuries, and have seasonal instrumental SSTA in the tropical Pacific exceeding 2.4 °C (Supplementary Fig. 18)
Note that this appears to contradict the findings of Hughes et al who blame the increased intensity and frequency of (mainly) canonical El Ninos, in combination with general ocean warming, for increasingly frequent and increasingly severe episodes of mass coral bleaching. Freunde et al tell us that although EP El Ninos are becoming more intense, they’re also decreasing in frequency, whereas CP El Ninos are correspondingly becoming more frequent, but not more intense.
I question Freunde et al’s conclusion that the intensity of EP El Nino’s is unprecedented in the last 400 years. The El Nino of 1877/78, which I wrote about earlier, was comparable in many respects to the 2015/16 event. So even if Freunde at al’s dataset is useful for counting and distinguishing between EP and CP events, it does not appear to be that useful for measuring the intensity of those events.
Which brings us to the question of what really distinguishes a Modoki El Nino from a Canonical El Nino? Distribution of SST anomalies in the equatorial Pacific obviously, but is there some fundamental difference between the two ‘flavours’ in terms of the ocean/atmospheric dynamics which cause them? Can this fundamental difference, if it exists, explain why global warming might cause more Modokis to form, whilst increasing the intensity of canonical El Ninos? There are no straightforward answers to this question because the atmospheric and oceanic dynamics driving ENSO variability are extremely complicated and not very well understood and whilst it is realised that the ENSO cycle is a largely unpredictable internally driven oscillation, it may respond to external climate forcings too, in ways which are currently still the subject of ongoing research. But we can get some clues about El Nino ‘flavours’ and variability from the following 2013 study:
Following the recent discovery of the “Modoki” El Niño, a proliferation of studies and debates has ensued concerning whether Modoki is dynamically distinct from “Canonical” El Niño, how Modoki impacts and teleconnections differ, and whether Modoki events have been increasing in frequency or amplitude. Three decades of reliable, high temporal‐resolution observations of coupled ocean‐atmosphere variability in the equatorial Pacific reveal a rich diversity of El Niños. Although central and eastern Pacific sea surface temperature (SST) anomalies appear mechanistically separable in terms of local and remote forcing, their frequent overlap precludes robust classifications. All observed El Niños appear to be a mixture of locally (central Pacific) and remotely forced (eastern Pacific) SST anomalies. Submonthly resolution appears essential for this insight and for the proper dynamical diagnosis of El Niño evolution; thus, the use of long‐term monthly reconstructions for classification and trend analysis is strongly cautioned against.
This does not perhaps bode well for Freund et al’s classification of EP/CP events based on seasonal sea surface temperature anomalies derived via coral proxies.
The commonly cited distinction is the location of maximum SST anomaly (Figures 1a and 1b). However, in terms of total SST, both events featured similar equatorward contractions and eastward expansions of the Indo‐Pacific warm pool, while a suppression of the eastern Pacific cold tongue occurred only in the EP El Niño (Figures 1d and 1e). It may therefore be hypothesized that EP El Niños are fundamentally related to CP El Niños except that a Bjerknes‐like thermocline feedback progresses and leads to a suppression of the cold tongue.
As regards the trend over the thirty year period 1982-2011:
It is also interesting to contrast either El Niño pattern with that of the linear trend over the period 1982–2011 (Figures 1c and 1f). The trend is characterized by a meridional expansion (but no zonal translation) of the warm pool edge and a small but significant westward extension of the cold tongue, therefore resembling neither the warm nor cold form of an EP or CP event. The response of the mean state of the tropical Pacific to the anthropogenic rise in atmospheric CO2 concentration is a subject debated with equal vigor [Collins et al., 2010]; note that the trend over the most recent 30 years—a period during which the mean annual growth rate of atmospheric CO2 (1.73 ppm/year [Tans, 2012]) was greater than at any point in modern recorded history—offers no evidence of the strong warming in the central and eastern equatorial Pacific Ocean predicted by global models.
What is immediately clear that was less obvious in Figure 2b and all but hopeless in coarser monthly observations (Figure S2) is the fact that every El Niño—even the most notoriously “EP” events (1982–1983 or 1997–1998)—involves a distinct and geographically separated SST anomaly that develops and evolves in the central Pacific as described above. Likewise, El Niños that might be judged “CP” by various objective or subjective criteria (e.g., 1994–1995) involve clear and separate SST anomaly development in the eastern Pacific. It is more often the case, and well illustrated by the 2006–2007 event, that a given El Niño is a mixture of SST anomalies developing (and likely interacting through large‐scale coupled processes) in both the central and eastern equatorial Pacific.
So it would appear that the rigid distinction between CP and EP events based on SST anomalies in the central and eastern pacific is somewhat less rigid than thought and involves both objective and subjective criteria.
To summarize, nature exhibits a rich diversity of El Niños composed of a mixture of SST anomalies initially developing in the central and eastern Pacific. The results presented herein suggest that CP and EP El Niños are, in theory, mechanistically distinct (i.e., CP SST anomalies are a local response to wind forcing, whereas EP SST anomalies emerge as a remote response). However, the obvious potential for interaction between wind forcing and SST anomalies at opposite ends of the basin explains why, upon closer examination with suitable observations, they occur simultaneously more often than not. The usual view of feedbacks involved in ENSO, summarized in Figure 5a [from Zelle et al., 2004], contains all of the fundamental dynamics to explain SST anomalies without geographic variation. Separating the local from remote processes and explicitly invoking basin‐scale atmospheric feedbacks (à la Bjerknes) enables one to envision how the known dynamics already account for simultaneous mixtures of distinctly CP and EP SST anomalies in every El Niño (Figure 5b).
Modoki El Ninos are not a new phenomenon, even though they have been recently identified. They may not even be a uniquely distinct phenomenon in terms of their difference to canonical El Ninos. Freund et al acknowledge that they are not new; their study identifies an increase in the frequency of EPs way back in the 1600s. But the impression given by the authors’ Conversation article is that the sudden increase in EPs is a new and emergent phenomenon:
The new ‘flavour’ of El Niño
A new “flavour” of El Niño is now recognised in the tropical Pacific. This type of El Niño is characterised by warm ocean temperatures in the Central Pacific, rather than the more typical warming in the far Eastern Pacific near the South American coast, some 10,000km away.
By the end of the 20th century, though, our research shows a sudden change: a sharp increase of Central Pacific El Niño events becomes evident.
Our understanding of the new Central Pacific flavour of El Niño is hindered by the fact that El Niño events happen only every 2-7 years.
At a seasonal timescale, we can see the characteristic patterns of past El Niño events in the chemistry of the corals. These patterns tell us which El Niño is which over the last 400 years. It is in this continuous picture of past El Niños obtained from coral archives that we found a clear picture of an unusual recent change in the Pacific’s El Niño flavours.
This extraordinary change in El Niño behaviour has serious implications for societies and ecosystems around the world.
Once again, language and phraseology is all important in the communication of scientific research to the general public. Traditional El Ninos affecting the central and eastern Pacific have become measurably more intense since the early 1980s, but it is dubious if this increase in intensity is unprecedented, even since the late 19th century, let alone over the last four centuries. Freund et al’s coral data may reveal an ‘unprecedented’ increase in the frequency of CP El Ninos, but variability in the frequency of such events is not unprecedented, neither are they as ‘strange’ and different from EP El Ninos as might be supposed. So, to summarise, El Ninos are not getting stronger, not getting stranger and are just as lacking in charm – in terms of their devastating effects upon weather and society – as they were probably back in 1877/78 when the last super El Nino wrought havoc.