Improved representation of tropical Pacific Ocean-atmosphere dynamics in an intermediate complexity climate model

A new anomaly coupling technique is introduced into a coarse-resolution dynamic climate model [the Liège Ocean Carbon Heteronomous model (LOCH)-Vegetation Continuous Description model (VECODE)-Earth System Models of Intermediate Complexity Climate deBilt (ECBILT)-Coupled Large-Scale Ice-Ocean model (CLIO)-Antarctic andGreenland Ice SheetModel (AGISM) ensemble (LOVECLIM)], improving themodel's representation of eastern equatorial Pacific surface temperature variability. The anomaly coupling amplifies the surface diabatic atmospheric forcing within a Gaussian-shaped patch applied in the tropical Pacific Ocean. It is implemented with an improved predictive cloud scheme based on empirical relationships between cloud cover and key state variables. Results are presented from a perturbed physics ensemble systematically varying the parameters controlling the anomaly coupling patch size, location, and amplitude. The model's optimal parameter combination is chosen through calibration against the observed power spectrum of monthly-mean surface temperature anomalies in the Niño-3 region. The calibrated model exhibits substantial improvement in equatorial Pacific interannual surface temperature variability and robustly reproduces El Niño-SouthernOscillation (ENSO)-like variability. The authors diagnose some of the key atmospheric and oceanic feedbacks in themodel important for simulating ENSO-like variability, such as the positive Bjerknes feedback and the negative heat flux feedback, and analyze the recharge-discharge of the equatorial Pacific ocean heat content. They findLOVECLIM robustly captures important ocean dynamics related to thermocline adjustment and equatorial Kelvin waves. The calibrated model demonstrates some improvement in simulating atmospheric feedbacks, but the coupling between ocean and atmosphere is relatively weak. Because of the tractability of LOVECLIM and its consequent utility in exploring long-term climate variability and large ensemble perturbed physics experiments, improved representation of tropical Pacific ocean-atmosphere dynamics in the model may more readily allow for the investigation of the role of tropical Pacific ocean-atmosphere dynamics in past climate changes.