1. A history of ENSO behaviour and its implications
Chris Brierley (UCL Geography)
With:
Maryam Ilyas & Serge Guillas (in UCL’s Dept of Statistical Science)
Dan Lunt & Claire Loptson (in Geographical Sciences at University of Bristol)

2. Outline El Nino-Southern Oscillation in the geologic past
Different continental configurations
The “permanent El Nino” of the early Pliocene
Impact of the ice-ages
Holocene
The instrumental record
Teleconnected evidence
Nino3.4 SST anomalies
Early ambiguity
Summary of ENSO history
Lessons for the future

3. ENSO properties are not constant
El Niño-Southern Oscillation not regular
Period varies between 2-7 years
Average statistical properties also vary on centennial timescales
Need long time series to detect changes
Wittenberg, 2009 GRL

4. We have at most 150 years… Can a long-term perspective help?
IPCC AR5 Box 2.5, Fig 1

5. ENSO in Geologic History

6. Detecting ENSO in the palaeoclimate record
ENSO is the dominant global mode of inter-annual variability
Need record with annual resolution
Lake varves
Tree rings
Speleothems
Preferably demonstrate in teleconnected region
Mingram, 1998 Palaeo3
Huber & Caballero, 2003 Science

7. Data evidence
Evidence of ENSO taken to be ~3 year peak in time series spectrum
Observationally detected from 88 Ma
Sporadically seen since (due to data record)
No evidence of lack of ENSO
Records often not able to detect changes in ENSO properties

8. ENSO is fundamental to large equatorial ocean
All palaeo-simulations seem to show a kind of ENSO
Properties & location vary
Lunt et al. 2016
Clim. Past

9. Introduction to Pliocene (5-3 Ma)
Land pretty-much present-day
Sea level higher and Panama already closed
CO2 around 400 ppm
Describe the axes. UPDATE THIS FIGURE
Fedorov et al Science

10. “Permanent El Niño-like” Pliocene
Mean equatorial temperature gradient minimal
ENSO still extant
Brierley 2013 PAGES News

11. Cold Worlds Last glacial maximum (22ka) most studied
ENSO little change, but maybe damped
Inferred temperatures
Individual foraminifera
Quantile-quantile slope flatter than 1:1
Models equivocal
Ford et al 2015 Science

12. Abrupt Events Simulated ENSO responds to strong climate shocks
“Hosing” experiments replicate ice-sheet water release
Alter the Atlantic Meridional Overturning
Alter interhemispheric gradients
Tropical circulation
Liu et al 2014 Nature
IPCC AR5 WG1, Fig 5.10

13. Holocene Models focused at 6000 years ago
Seasonal cycle of solar radiation different
Palaeo-observations
More plentiful
Indicate less activity
Associated with annual cycle changes
Often short records vs centennial ENSO var.
Emile-Geay et al Nature Geosci.

14. ENSO in written History

15. Last Millennium Uses many tree-rings and prescribed teleconnection
Corals give similar result
Amplitude increase in C20th
C20th has low decadal variability
Li et al Nature Clim. Ch.

16. Instrumental Record Global weather data exist since ~1850
Spatial distribution is sparse at first
Improves quickly, but visits to Niño regions rare until 1950s
Can interpolate into region
Novel technique here

17. Probabilistic ENSO
HadCRUT4 (and interpolation) come with uncertainties
Need to revise ENSO definitions to cope:
Last “ambiguous” year is 1949, often occur before 1920

18. Instrumental record slows slight increase in ENSO amplitude
IPCC AR5 Box 2.5, Fig 1
Instrumental record slows slight increase in ENSO amplitude
Depends on dataset (interpolation), so treat with caution
May be natural fluctuation in ENSO statistics

19. Changes in “flavour” Diversity of El Niños
Labelled by location of peak warming
Have different global impacts
Several indices/names
Central Pacific type more often recently
unclear whether this is forced
Central Pacific El Niño (1994)
Eastern Pacific El Niño (1997)
DJF HadCRUT4 median via Climate Explorer

20. Summary
Found something ENSO-like whenever we can look for it in geologic past
going back 100 million years and more
Haven’t seen any massive changes
Amplitude dependence more from insolation than global temperatures
Little evidence of periodicity changes (hard to look for)
Hints of response over past 150 years
Increasing amplitude
Shift in nature
Hard to extract from uncertainties

21. Implications for future
ENSO will not disappear
Fundamental property of Pacific
Survived past changes in ocean thermal structure
Amplitude change could happen
Uncertain in sign, depends on changes in annual cycle
Frequency change will be small if it does happen
High internal variability
Changes hard to attribute, even if observed