%AWu, Xian [a Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado]%AOkumura, Yuko [b Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas]%ADiNezio, Pedro [c Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado]%AYeager, Stephen [a Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado]%ADeser, Clara [a Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado]%BJournal Name: Journal of Climate; Journal Volume: 35; Journal Issue: 11; Related Information: CHORUS Timestamp: 2022-05-16 02:40:46 %D2022%IAmerican Meteorological Society; None %JJournal Name: Journal of Climate; Journal Volume: 35; Journal Issue: 11; Related Information: CHORUS Timestamp: 2022-05-16 02:40:46 %K %MOSTI ID: 10367202 %PMedium: X %TThe Equatorial Pacific Cold Tongue Bias in CESM1 and Its Influence on ENSO Forecasts %XAbstract

The mean-state bias and the associated forecast errors of the El Niño–Southern Oscillation (ENSO) are investigated in a suite of 2-yr-lead retrospective forecasts conducted with the Community Earth System Model, version 1, for 1954–2015. The equatorial Pacific cold tongue in the forecasts is too strong and extends excessively westward due to a combination of the model’s inherent climatological bias, initialization imbalance, and errors in initial ocean data. The forecasts show a stronger cold tongue bias in the first year than that inherent to the model due to the imbalance between initial subsurface oceanic states and model dynamics. The cold tongue bias affects not only the pattern and amplitude but also the duration of ENSO in the forecasts by altering ocean–atmosphere feedbacks. The predicted sea surface temperature anomalies related to ENSO extend to the far western equatorial Pacific during boreal summer when the cold tongue bias is strong, and the predicted ENSO anomalies are too weak in the central-eastern equatorial Pacific. The forecast errors of pattern and amplitude subsequently lead to errors in ENSO phase transition by affecting the amplitude of the negative thermocline feedback in the equatorial Pacific and tropical interbasin adjustments during the mature phase of ENSO. These ENSO forecast errors further degrade the predictions of wintertime atmospheric teleconnections, land surface air temperature, and rainfall anomalies over the Northern Hemisphere. These mean-state and ENSO forecast biases are more pronounced in forecasts initialized in boreal spring–summer than other seasons due to the seasonal intensification of the Bjerknes feedback.

%0Journal Article