A two-way street across three oceans: observed Madden Julian Oscillation – oceanic Kelvin wave coupling in boreal winter

Abstract. Air–sea coupling associated with the Madden–Julian Oscillation (MJO) plays a central role in subseasonal variability and prediction, yet its two-way interaction with oceanic Kelvin waves (KWs) across the tropical basins remains insufficiently constrained. Here we assess both the impact of KWs on MJO evolution through sea surface temperature (SST) changes, and the characteristics of MJO events that lead to KW generation and propagation in the Indian, Pacific and Atlantic oceans during boreal winter. In a novel way, we combine MJO index with independent KW indices for each basin, identifying regimes of enhanced and suppressed KW activity and quantify the MJO-associated forcing and the strength of air–sea feedbacks. 

The capacity of the MJO to force KWs varies markedly across basins, ranging from a minimum of 38 % of MJO events generating KWs in the Pacific, to 16.8 % in the Indian Ocean and to 9.4 % in the Atlantic. In all three basins, KW generation is favored when MJO events display strong, coherent equatorial low-level zonal wind anomalies and organized eastward-propagating convection, which force a continuous sea surface height (SSH) signal with a clear KW structure. In the Pacific, the resulting thermocline displacements propagate coherently across the basin and reach the surface in the far east, where they drive SST changes primarily through vertical advection and meridional geostrophic advection. In the Indian ocean, wind anomalies over the central basin force an SSH signal that intensifies toward the east and evolves into a coherent equatorially trapped structure, a process that appears to be modulated by a preceding oceanic Rossby wave. In the Atlantic, KW generation requires coherent westerly wind stress anomalies along the equator near the South American coast and is further favored by an atmospheric KW originating over the Indo-Pacific that promotes convection in the western basin. Conversely, events that fail to generate KWs share a common signature across basins: weaker or off-equatorial wind forcing and SSH anomalies that lose coherence before reaching the eastern side of each basin. In the Atlantic, this decoupled regime leaves convective anomalies concentrated over the eastern basin and the Guinea Dome, largely detached from the equatorial ocean response. 

From the oceanic perspective, a substantial fraction of KWs in each basin is associated with the MJO, and their presence systematically strengthens ocean–atmosphere coupling. Under enhanced KW activity, ocean dynamics govern SST tendencies in both the Pacific and the Atlantic. In the Pacific, subsurface adjustments sustain SST anomalies and support the eastward advance of the MJO even where atmospheric forcing weakens. In the Indian Ocean, by contrast, surface heat fluxes dominate SST variability. In periods of suppressed KW activity, surface fluxes dominate SST variability across all three basins, except in the eastern Pacific, where local processes such as surface current convergence dominate instead. These results establish the MJO–KW relationship as a genuinely two-way interaction, with implications for subseasonal prediction in all three tropical basins. It is necessary to study the interaction with the MJO in each basin separately or in greater detail, given the unique characteristics of each basin and its modes of variability.