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Abstract

Observations demonstrate that Madden-Julian Oscillation (MJO) propagation is affected by sea surface temperatures (SST). During low wind speed and high incoming solar radiation conditions (e.g. suppressed MJO phase), the top few meters of the ocean can form a diurnal warm layer, increasing daily SST by up to 1.0°C. Coupled ocean-atmosphere models with sufficient upper ocean resolution can simulate diurnal warm layer formation and their effects on the MJO. Analysis of Real-time Multivariate MJO (RMM) index shows that the Met Office (near) real-time coupled and atmosphere-only Numerical Weather Prediction systems are skilful in predicting the MJO within 7 lead days. The coupled model predicts faster MJO propagation by ~10% than the atmosphere-only model. Two tropical regions exhibit significant differences in MJO convection by lead day 7 between the models: central Maritime Continent (when initialized in RMM phase 1) and equatorial Indian Ocean (when initialized in RMM phase 4). The former region has warmer SST MJO anomalies in the coupled model at lead day 1, leading to increased convection within the subsequent lead days. The latter region experiences colder SST MJO anomalies in the coupled model at lead day 1, resulting in decreased convection later in the forecast. We hypothesize that the inclusion of diurnal warming of SST in the coupled model modulates MJO SST anomalies and leads to faster MJO propagation. Other potential sources for SST changes in the coupled model are explored with numerical experiments and process-based diagnostics of the upper ocean.