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 greenhouse warming. 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 during the late 20th century that leads to a ratio of Central to Eastern Pacific events that is unusual in a multi-century context. Compared to the past four centuries, the most recent 30-year period includes fewer, but more intense Eastern Pacific El Niño events. Canonical eastern Pacific (EP) El Niño events exhibit their largest sea surface temperature anomalies (SSTA) in the far eastern tropical Pacific near the Peruvian coast 1. Over recent decades, peak warming during several El Niño events has been displaced approximately 11,000 km, or 100°longitude, westwards into the central equatorial Pacific. These El Niño events are described as Central Pacific (CP) El Niño, warm-pool El Niño 2 , El Niño Modoki 3 , or Dateline El Niño 4. The displacement of maximum SSTA towards the central Pacific drives substantial shifts in atmospheric convection and circulation responses 5-7 , which alter the location and intensity of temperature and precipitation impacts associated with El Niño around the globe 3;4;8-11. Evidence is emerging that changes in El Niño Southern Oscillation (ENSO) behaviour have occurred during the instrumental period 12-15. Following the climate regime shift in 1976/1977, zonal SSTA propagation during El Niño changed from westward to eastward 16. Coincident with the shift to a positive phase of the Interdecadal Pacific Oscillation (IPO) 16-18 in 1999/2000, Pacific trade winds have strengthened 19;20. Observations indicate increasing El Niño event amplitude 21 , strong decadal variations in event frequency 22 , changes in maximum SSTA propagation direction 12;23 and delays in the onset of El Niño events 24. Since the late 1990s there has been a higher number of CP events relative to EP events, unprecedented in instrumental records 2;21;22. It is unclear whether this recent increase is part of natural climate variability 25 or a consequence of anthropogenic climate change 22. A precise picture of El Niño diversity is a challenge due to model def...

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