A Secular Shift of the Madden-Julian Oscillation and Its Relation to Western Pacific Ocean Warming

In the context of global warming, understanding changes in the Madden-Julian Oscillation (MJO) is of great importance, because of its worldwide impact on surface weather and climate. By analyzing the temporal evolution of the phase relationship between Kelvin waves and the MJO, we find a secular phase shift of the MJO after year 2000, at which the MJO exhibits an eastward extension over the Maritime Continent and the Western Pacific Ocean. Further, we examine the influence of sea surface temperature (SST) variations on the anomalous convection and Kelvin waves, and demonstrate that the recent warming trend of the Western Pacific SST is a factor causing the changes of the MJO over this region. The results deepen our understanding of recent modeling and observational studies about the changes of the MJO and confirm the enhanced activity of the MJO over the past few decades. Plain Language Summary The Madden-Julian Oscillation (MJO) is a propagating wave system which travels eastward from the Indian Ocean to the Pacific. The MJO activity exerts substantial influence on weather and climate over the tropics and extra-tropics. Here we find a secular change in the MJO: The MJO activity increased over the Maritime Continent over the past few decades, which can be observed from the eastward expansions of both convective activity and Kelvin waves over this region. Further analysis suggests that the recent Western Pacific Ocean warming plays a role in the secular shift of the MJO. Our results show that this is an imprint of the effects of global warming on the atmospheric circulation. Key Points The phase differences between Kelvin waves and the Madden-Julian Oscillation (MJO) started to increase in 2000 The MJO-related circulation over the Western Pacific experienced an eastward extension after 2000 The recent warming trend of Western Pacific SST likely caused the MJO shift