The role of air–sea coupling in the simulation of the Madden–Julian oscillation in the Hadley Centre model†

The role of air–sea coupling in the simulation of the Madden–Julian oscillation (MJO) is explored using two configurations of the Hadley Centre atmospheric model (AGCM), GA3.0, which differ only in F, a parameter controlling convective entrainment and detrainment. Increasing F considerably improves deficient MJO-like variability in the Indian and Pacific Oceans, but variability in and propagation through the Maritime Continent remains weak. By coupling GA3.0 in the tropical Indo-Pacific to a boundary-layer ocean model, Kprofile parametrization (KPP), and employing climatological temperature corrections, well-resolved air–sea interactions are simulated with limited alterations to the mean state. At default F, when GA3.0 has a poor MJO, coupling produces a stronger MJO with some eastward propagation, although both aspects remain deficient. These results agree with previous sensitivity studies using AGCMs with poor variability. At higher F, coupling does not affect MJO amplitude but enhances propagation through the Maritime Continent, resulting in an MJO that resembles observations. A sensitivity experiment with coupling only in the Indian Ocean reverses these improvements, suggesting that coupling in the Maritime Continent and West Pacific is critical for propagation. We hypothesize that, for AGCMs with a poor MJO, coupling provides a ‘crutch’ to augment MJO-like activity artificially through high-frequency SST anomalies. In related experiments, we employ the KPP framework to analyze the impact of air–sea interactions in the fully coupled GA3.0, which at default F shows a similar MJO to the uncoupled GA3.0. This is due to compensating effects: an improvement from coupling and a degradation from mean-state errors. Future studies on the role of coupling should separate these effects carefully.