A common approach to studying the Madden-Julian Oscillation (MJO) relies on indices derived from the first two leading empirical orthogonal function (EOF) modes, under the implicit assumption that these modes do not change over time. However, recent changes in the MJO characteristics imply the possibility of changes in the MJO leading modes, which have not received much attention. Here, we find that the leading EOF modes of MJO convection have, in fact, undergone significant changes since 1979. The changes manifest as a weakening of MJO variability over the tropical south Indian Ocean and a westward shift of the variability center over the southwestern Pacific Ocean. These variations are closely associated with changes in the background horizontal wind and moisture, which are attributed to the La Niña-like sea surface temperature changes in the Pacific and the warming of Indian Ocean. These findings indicate that the information carried by the MJO indices, as well as the climate impact associated with the leading EOF modes, varies over time, suggesting that caution is warranted when employing EOF-based MJO indices to study the long-term trends of MJO activity and its response to climate change.

Madden-Julian Oscillation, Empirical orthogonal function, MJO index, Climate change