The ENSO effects on tropical clouds and top‐of‐atmosphere cloud radiative effects in CMIP5 models

Abstract

AbstractThe El Niño–Southern Oscillation (ENSO) effects on tropical clouds and top‐of‐atmosphere (TOA) cloud radiative effects (CREs) in Coupled Model Intercomparison Project Phase 5 (CMIP5) models are evaluated using satellite‐based observations and International Satellite Cloud Climatology Project satellite simulator output. Climatologically, most CMIP5 models produce considerably less total cloud amount with higher cloud top and notably larger reflectivity than observations in tropical Indo‐Pacific (60°E‐200°E; 10°S‐10°N). During ENSO, most CMIP5 models strongly underestimate TOA CRE and cloud changes over western tropical Pacific. Over central tropical Pacific, while the multi‐model mean resembles observations in TOA CRE and cloud amount anomalies, it notably overestimates cloud top pressure (CTP) decreases; there are also substantial inter‐model variations. The relative effects of changes in cloud properties, temperature, and humidity on TOA CRE anomalies during ENSO in the CMIP5 models are assessed using cloud radiative kernels. The CMIP5 models agree with observations in that their TOA shortwave CRE anomalies are primarily contributed by total cloud amount changes, and their TOA longwave CRE anomalies are mostly contributed by changes in both total cloud amount and CTP. The model biases in TOA CRE anomalies particularly the strong underestimations over western tropical Pacific are, however, mainly explained by model biases in CTP and cloud optical thickness (τ) changes. Despite the distinct model climatological cloud biases particularly in τ regime, the TOA CRE anomalies from total cloud amount changes are comparable between the CMIP5 models and observations, because of the strong compensations between model underestimation of TOA CRE anomalies from thin clouds and overestimation from medium and thick clouds.