Higher frequency of Central Pacific El Niño events in recent decades relative to past centuries

Abstract

El Niño events differ substantially in their spatial pattern and intensity. Canonical Eastern Pacific El Niño events have sea surface temperature anomalies that are strongest in the far eastern equatorial Pacific, whereas peak ocean warming occurs further west during Central Pacific El Niño events. The event types differ in their impacts on the location and intensity of temperature and precipitation anomalies globally. Evidence is emerging that Central Pacific El Niño events have become more common, a trend that is projected by some studies to continue with ongoing climate change. Here we identify spatial and temporal patterns in observed sea surface temperatures that distinguish the evolution of Eastern and Central Pacific El Niño events in the tropical Pacific. We show that these patterns are recorded by a network of 27 seasonally resolved coral records, which we then use to reconstruct Central and Eastern Pacific El Niño activity for the past four centuries. We find a simultaneous increase in Central Pacific events and a decrease in Eastern Pacific events since the late twentieth century that leads to a ratio of Central to Eastern Pacific events that is unusual in a multicentury context. Compared to the past four centuries, the most recent 30 year period includes fewer, but more intense, Eastern Pacific El Niño events.

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Articles
https://doi.org/10.1038/s41561-019-0353-3
1School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia. 2ARC Centre of Excellence for Climate System Science, University of
Melbourne, Parkville, Victoria, Australia. 3Climate and Energy College, University of Melbourne, Parkville, Victoria, Australia. 4ARC Centre of Excellence
for Climate Extremes, University of Melbourne, Parkville, Victoria, Australia. 5School of Earth, Atmosphere and Environment, Monash University, Clayton,
Victoria, Australia. 6NESP Earth Systems and Climate Change Hub, CSIRO, Aspendale, Victoria, Australia. 7School of Earth, Atmospheric and Life Sciences,
University of Wollongong, Wollongong, New South Wales, Australia. 8Research School of Earth Sciences, Australian National University, Canberra,
Australian Capital Territory, Australia. 9ARC Centre of Excellence for Climate Extremes, Australian National University, Canberra, Australian Capital
Territory, Australia. 10ARC Centre of Excellence for Climate Extremes, Monash University, Clayton, Victoria, Australia. *e-mail: freundm@unimelb.edu.au
Canonical Eastern Pacific (EP) El Niño events exhibit their
largest sea surface temperature anomalies (SSTA) in the far
eastern tropical Pacific near the Peruvian coast1. Over recent
decades, peak warming during several El Niño events has been
displaced by approximately 11,000 km, or 100° longitude, west-
wards into the central equatorial Pacific. These El Niño events are
described as Central Pacific (CP) El Niño, warm-pool El Niño2, El
Niño Modoki3 or Dateline El Niño4. The displacement of maximum
SSTA towards the central Pacific drives substantial shifts in atmo-
spheric convection and circulation5–7, which alter the location and
intensity of temperature and precipitation impacts associated with
El Niño around the globe3,4,8–11.
Evidence is emerging that changes in the El Niño Southern
Oscillation (ENSO) behaviour occurred during the instrumental
period12–15. After the climate regime shift in 1976/1977, zonal SSTA
propagation during El Niño changed from westward to eastward16.
Coincident with the shift to a positive phase of the Interdecadal
Pacific Oscillation16–18 in 1999/2000, Pacific trade winds strength-
ened19,20. Observations indicate an increasing El Niño event
amplitude21, decadal variations in event frequency22, changes in
maximum SSTA propagation direction12,23 and delays in the onset
of El Niño events24.
Since the late 1990s there has been a higher number of CP events
relative to EP events, unprecedented in instrumental records2,21,22.
It is unclear whether this recent increase is part of natural climate
variability25 or a consequence of anthropogenic climate change22. A
precise picture of El Niño diversity is a challenge due to model defi-
ciencies in simulating El Niño and the short and sparse coverage of
instrumental observations across the equatorial Pacific25,26. In this
study, we extend the record of El Niño diversity into the past using
a network of coral data that spans the tropical Indo-Pacific ocean.
Spatial and temporal patterns of El Niño types
We used a network of 27 seasonally resolved coral records to recon-
struct past EP and CP El Niño events (Methods and Supplementary
Information). The network includes four Sr/Ca records (a proxy
for sea surface temperature (SST)) and 23 oxygen isotope (δ18O)
records. The δ18O signal preserved in coral banding is determined
by the source isotopic composition of the surrounding seawater
(δ18OSW) and the equilibrium isotopic fractionation between the
seawater and carbonate, which is inversely related to temperature27.
The oxygen isotopes are fractionated throughout the annual cycle
(and thus preserve the SST variations across the calendar year) and
the δ18OSW is affected by the advection of water masses with dif-
ferent isotope signatures and precipitation–evaporation changes.
Precipitation–evaporation changes also affect the sea surface salin-
ity (SSS), so δ18O is often used as an SST, SSS or SST–SSS proxy. The
relationship between salinity and δ18OSW is complex and can depend
on local conditions28. In general, if the SSS is relatively constant,
the δ18O in corals is mainly determined by SST variability and vice
versa. High variability of SSS and SST can make the interpretation
of δ18O in corals more complex. We carried out extensive testing of
our methods and explored possible sources of error, which included
testing the δ18O signal in the individual coral records and our net-
work as a whole (Supplementary Information). We found that all of
the coral δ18O records in our network have a strong link to ENSO,
with correlations to SST and SSS, parameters which in turn vary
with the spatial and temporal patterns of CP and EP El Niño events.
Higher frequency of Central Pacific El Niño events
in recent decades relative to past centuries
Mandy B. Freund 1,2,3*, Benjamin J. Henley 1,2,4,5, David J. Karoly1,2,6, Helen V. McGregor 7,
Nerilie J. Abram 8,9 and Dietmar Dommenget5,10
El Niño events differ substantially in their spatial pattern and intensity. Canonical Eastern Pacific El Niño events have sea sur-
face temperature anomalies that are strongest in the far eastern equatorial Pacific, whereas peak ocean warming occurs further
west during Central Pacific El Niño events. The event types differ in their impacts on the location and intensity of temperature
and precipitation anomalies globally. Evidence is emerging that Central Pacific El Niño events have become more common, a
trend that is projected by some studies to continue with ongoing climate change. Here we identify spatial and temporal patterns
in observed sea surface temperatures that distinguish the evolution of Eastern and Central Pacific El Niño events in the tropical
Pacific. We show that these patterns are recorded by a network of 27 seasonally resolved coral records, which we then use to
reconstruct Central and Eastern Pacific El Niño activity for the past four centuries. We find a simultaneous increase in Central
Pacific events and a decrease in Eastern Pacific events since the late twentieth century that leads to a ratio of Central to Eastern
Pacific events that is unusual in a multicentury context. Compared to the past four centuries, the most recent 30 year period
includes fewer, but more intense, Eastern Pacific El Niño events.
NATURE GEOSCIENCE | VOL 12 | JUNE 2019 | 450–455 | www.nature.com/naturegeoscience
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