Diversity of ENSO Events Unified by Convective Threshold Sea Surface Temperature: A Nonlinear ENSO Index
The El Niño – Southern Oscillation (ENSO) is a natural variation of ocean temperature in the tropical Pacific and a major driver of global climate variability, including precipitation extremes and tropical cyclone activity. ENSO has a diversity of spatial patterns that can alter its teleconnections. We show that the well-known failure of any single index to capture the diversity and extremes of El Niño-Southern Oscillation (ENSO) results from the inability of existing indices to uniquely characterize the average longitude of deep convection in the Walker Circulation. We present a simple sea surface temperature (SST)-based index of this longitude that compactly characterizes the different spatial patterns, or flavors of observed and projected ENSO events. It recovers the familiar global responses of temperature, precipitation, and tropical cyclones to ENSO and identifies historical extreme El Niño events. Despite its simplicity, the new longitude index describes the nonlinear relationship between the first two principal components of SST, and unlike previous indices, accounts for background SST changes associated with the seasonal cycle and climate change. The index reveals that extreme El Niño, El Niño Modoki, and La Niña events are projected to become more frequent in the future at the expense of neutral ENSO conditions.
- Williams, I. N., & Patricola, C. M. (2018). Diversity of ENSO Events Unified by Convective Threshold Sea Surface Temperature: A Nonlinear ENSO Index, Geophysical Research Letters, 45, 9236-9244.
This research was supported by the U.S. Department of Energy Office of Science (BER, RGMA Program and ASR Program).
For observed ENSO Longitude Index data, contact Christina Patricola at cmp28@iastate.edu.
Future Projections of the El Niño – Southern Oscillation and Tropical Pacific Mean State in CMIP6
The El Niño—Southern Oscillation (ENSO) is an important mode of tropical Pacific atmosphere-ocean variability that drives teleconnections with weather and climate globally. However, prior studies using state-of-the-art climate models lack consensus regarding future ENSO projections and are often impacted by tropical Pacific sea-surface temperature (SST) biases. We used 173 simulations from 29 climate models participating in the Coupled Model Intercomparison Project, version 6 (CMIP6) to analyze model biases and future ENSO projections. We analyzed two ENSO indices, namely the ENSO Longitude Index (ELI), which measures zonal shifts in tropical Pacific deep convection and accounts for changes in background SST, and the Niño 3.4 index, which measures SST anomalies in the central-eastern equatorial Pacific. We found that the warm eastern tropical-subtropical Pacific SST bias typical of previous generations of climate models persists into many of the CMIP6 models. Future projections of ENSO shift toward more El Niño-like conditions based on ELI in 48% of simulations and 55% of models, in association with a future weakening of the zonal equatorial Pacific SST gradient. On the other hand, none of the models project a significant shift toward La Niña-like conditions. The standard deviation of the Niño 3.4 index indicates a lack of consensus on whether an increase or decrease in ENSO variability is expected in the future. Finally, we found a possible relationship between historical SST and low-level cloud cover biases in the ENSO region and future changes in ELI; however, this result may be impacted by limitations in data availability.
- Erickson, N. E., & Patricola, C. M. (2023). Future Projections of the El Niño – Southern Oscillation and Tropical Pacific Mean State in CMIP6. Journal of Geophysical Research: Atmospheres, 128, e2022JD037563.
This research was supported by Iowa State University, College of Liberal Arts and Sciences and by the U.S. Department of Energy Office of Science (BER, RGMA program). High-performance computing resources provided by the National Energy Research Scientific Computing Center (NERSC).
updated 3/13/2024