A Relative Sea Surface Temperature Index for Classifying ENSO Events in a Changing Climate

Author:

L’Heureux Michelle L.1,Tippett Michael K.2,Wheeler Matthew C.3,Nguyen Hanh3,Narsey Sugata3,Johnson Nathaniel4,Hu Zeng-Zhen1,Watkins Andrew B.3,Lucas Chris3,Ganter Catherine3,Becker Emily5,Wang Wanqiu1,Di Liberto Tom6

Affiliation:

1. a NOAA/NWS/NCEP/Climate Prediction Center, College Park, Maryland

2. b Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York

3. c Bureau of Meteorology, Melbourne, Victoria, Australia

4. d NOAA/OAR/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey

5. e University of Miami/Cooperative Institute for Marine and Atmospheric Studies, Miami, Florida

6. f NOAA Office of Communications, Washington, D. C.

Abstract

Abstract El Niño–Southern Oscillation (ENSO) is often characterized through the use of sea surface temperature (SST) departures from their climatological values, as in the Niño-3.4 index. However, this approach is problematic in a changing climate when the climatology itself is varying. To address this issue, van Oldenborgh et al. proposed a relative Niño-3.4 SST index, which subtracts the tropical mean SST anomaly from the Niño-3.4 index and multiplies by a scaling factor. We extend their work by providing a simplified calculation procedure for the scaling factor, and confirm that the relative index demonstrates reduced sensitivity to climate change and multidecadal variability. In particular, we show in three observational SST datasets that the relative index provides a more consistent classification of historical El Niño and La Niña oceanic conditions that is more robust across climatological periods compared to the nonrelative index. Forecast skill of the relative Niño-3.4 index in the North American Multimodel Ensemble (NMME) and ACCESS-S2 is slightly reduced for targets during the first half of the year because subtracting the tropical mean removes a source of additional skill. For targets in the second half of the year, the relative and nonrelative indices are equally skillful. Observed ENSO teleconnections in 200-hPa geopotential height and precipitation during key seasons are sharper and explain more variability over Australia and the contiguous United States when computed with the relative index. Overall, the relative Niño-3.4 index provides a more robust option for real-time monitoring and forecasting ENSO in a changing climate. Significance Statement The goal of this study is to further explore a relative sea surface temperature index for monitoring and prediction of El Niño–Southern Oscillation. Sea surface temperature indices are typically computed as a difference from a 30-yr climatological average, and El Niño and La Niña events occur when values exceed a certain threshold. This method is suitable when the climate is stationary. However, because of climate change and other lower-frequency variations, historical El Niño and La Niña events are reclassified depending on which climatological period is selected. A relative index is investigated to ameliorate this problem.

Publisher

American Meteorological Society

Subject

Atmospheric Science

Reference37 articles.

1. On the tropical atmospheric signature of El Niño;Ádames, A. F.,2017

2. On the relationship between SST gradients, boundary layer winds, and convergence over the tropical oceans;Back, L. E.,2009

3. Documentation of a highly ENSO-related SST region in the equatorial Pacific: Research note;Barnston, A. G.,1997

4. A decade of the North American Multimodel Ensemble (NMME): Research, application, and future directions;Becker, E. J.,2022

5. Atmospheric teleconnections from the equatorial Pacific;Bjerknes, J.,1969

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