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)

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

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

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

ENSO in Geologic History

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

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

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

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. 2013 Science

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

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

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

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. 2016 Nature Geosci.

ENSO in written History

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. 2013 Nature Clim. Ch.

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

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

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

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

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

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