Madden–Julian Oscillation Propagation Speed Simulated in CMIP6 Models

Faithful simulation of the Madden–Julian oscillation (MJO) can significantly improve extended-range prediction, but numerical models have difficulty in simulating the MJO. A primary challenge is simulating the MJO propagation speed optimally. In this study, we examine the propagation speed of MJO during boreal winter in CMIP6 models and reveal the key factors controlling the MJO propagation speed in these models. The result shows that most of the CMIP6 models can well simulate the MJO’s eastward propagation over the Indo-Pacific warm pool, but the simulated MJOs in different models have diverse propagation speeds. Evidently, the Kelvin- and Rossby-wave responses to MJO heating are the key circulation factors affecting the simulated MJO propagation speed, with stronger Kelvin-wave and weaker Rossby-wave responses corresponding to faster propagation. The variation in the MJO circulation structure among models is attributed to the diverse background sea surface temperature (SST). Models simulating faster MJO typically have a higher SST over the central Pacific (CP) and western North Pacific (WNP). Warming over CP affects the MJO speed in two ways. First, this zonal expansion of the warm pool can increase the horizontal scale of the MJO, leading to a stronger Kelvin-wave response that favors faster MJO propagation. Second, it increases the moisture content over the CP, which weakens the zonal moisture gradient over the West Pacific, resulting in the acceleration of the MJO by enhancing the zonal moisture advection. In contrast to previous studies, we find that the SST warming over WNP also affects the MJO speed. The SST warming over the WNP yields a more symmetrical SST distribution about the equator, which enhances the moisture content over the Maritime Continent, leading to a more equatorial symmetric distribution of the background-specific humidity during boreal winter. This symmetric distribution of moisture is conducive to the development of the Kelvin-wave response. The results of this study not only have implications for understanding MJO dynamics but also indicate that realistic simulation of MJO requires fidelity in the simulation of background states.