Origins of the Excessive Westward Extension of ENSO SST Simulated in CMIP5 and CMIP6 Models
- 1 April 2021
- journal article
- research article
- Published by American Meteorological Society in Journal of Climate
- Vol. 34 (8), 2839-2851
- https://doi.org/10.1175/jcli-d-20-0551.1
Abstract
An excessive westward extension of the simulated ENSO-related sea surface temperature (ENSO SST) variability in the CMIP5 and CMIP6 models is the most apparent ENSO SST pattern bias and dominates the intermodel spread in ENSO SST variability among the models. The ENSO SST bias lowers the models’ skill in ENSO-related simulations and induces large intermodel uncertainty in ENSO-related projections. The present study investigates the origins of the excessive westward extension of ENSO SST in 25 CMIP5 and 25 CMIP6 models. Based on the intermodel spread of ENSO SST variability simulated in the 50 models, we reveal that this ENSO SST bias among the models largely depends on the simulated cold tongue strength in the equatorial western Pacific (EWP). Models simulating a stronger cold tongue tend to simulate a larger mean zonal SST gradient in the EWP and then a larger zonal advection feedback in the EWP, favoring a more westward extension of the ENSO SST pattern. In addition, with the overall improvement in the EWP cold tongue from CMIP5 to CMIP6, the excessive westward extension bias of ENSO SST in CMIP6 models is also reduced relative to those in CMIP5 models. The results suggest that the bias and intermodel disagreement in the mean-state SST have been improved, which improves ENSO simulation. An excessive westward extension of the simulated ENSO-related sea surface temperature (ENSO SST) variability in the CMIP5 and CMIP6 models is the most apparent ENSO SST pattern bias and dominates the intermodel spread in ENSO SST variability among the models. The ENSO SST bias lowers the models’ skill in ENSO-related simulations and induces large intermodel uncertainty in ENSO-related projections. The present study investigates the origins of the excessive westward extension of ENSO SST in 25 CMIP5 and 25 CMIP6 models. Based on the intermodel spread of ENSO SST variability simulated in the 50 models, we reveal that this ENSO SST bias among the models largely depends on the simulated cold tongue strength in the equatorial western Pacific (EWP). Models simulating a stronger cold tongue tend to simulate a larger mean zonal SST gradient in the EWP and then a larger zonal advection feedback in the EWP, favoring a more westward extension of the ENSO SST pattern. In addition, with the overall improvement in the EWP cold tongue from CMIP5 to CMIP6, the excessive westward extension bias of ENSO SST in CMIP6 models is also reduced relative to those in CMIP5 models. The results suggest that the bias and intermodel disagreement in the mean-state SST have been improved, which improves ENSO simulation.Keywords
This publication has 49 references indexed in Scilit:
- Improved simulation of two types of El Niño in CMIP5 modelsEnvironmental Research Letters, 2012
- The two types of ENSO in CMIP5 modelsGeophysical Research Letters, 2012
- Changes in the sea surface temperature threshold for tropical convectionNature Geoscience, 2010
- The impact of global warming on the tropical Pacific Ocean and El NiñoNature Geoscience, 2010
- Convection Parameterization, Tropical Pacific Double ITCZ, and Upper-Ocean Biases in the NCAR CCSM3. Part II: Coupled Feedback and the Role of Ocean Heat TransportJournal of Climate, 2010
- El Niño/Southern Oscillation response to global warmingProceedings of the National Academy of Sciences of the United States of America, 2009
- Validating and understanding the ENSO simulation in two coupled climate modelsTellus A: Dynamic Meteorology and Oceanography, 2007
- A coupled‐stability index for ENSOGeophysical Research Letters, 2006
- On the Radiative and Dynamical Feedbacks over the Equatorial Pacific Cold TongueJournal of Climate, 2003
- Global and Regional Scale Precipitation Patterns Associated with the El Niño/Southern OscillationMonthly Weather Review, 1987