Abstract/Summary

The diurnal warm layer in the upper ocean develops during low surface winds and high incoming solar radiation conditions, often increasing sea-surface temperatures (SSTs) by up to 1°C. The suppressed phase of the Madden–Julian Oscillation (MJO) favours the formation of such a layer. Here, we analyse the coupled ocean–atmosphere and atmosphere-only numerical weather prediction systems of the UK Met Office to reveal that important differences arise from the representation of the diurnal warm layer in the coupled model. Though both models are skilful in predicting the MJO to at least a 7-day lead time, the coupled model predicts approximately 12% faster MJO RMM phase speed propagation than the atmosphere-only model due to the ability to resolve diurnal warming in the upper ocean that rectifies onto MJO-associated SST anomalies. The diurnal warming of SST (dSST) in the coupled model leads to an increase in daily mean SST compared with the atmosphere-only model persisted foundation SST. The strength of the dSST in the coupled model is modulated by MJO conditions. During suppressed MJO conditions on lead day 1, the dSST is enhanced, leading to 0.2°C warmer daily mean MJO-associated SST anomalies and increased convection in the coupled model by lead day 7. During active MJO convection, the dSST is suppressed, leading to 0.1°C colder MJO-associated SST anomalies in the coupled model and reduced convection by lead day 7. This variability in dSST further amplifies the MJO propagation speed, underlining the importance of the two-way feedback between the MJO and the diurnal cycle of SST and the need to accurately represent this process in coupled models.