The Equatorial Deep Ocean Structure Associated With the Madden‐Julian Oscillation From an Ocean Reanalysis

Abstract The Madden‐Julian Oscillation (MJO) is the dominant weather system in the tropics on week‐to‐week time scales. Here, we exploit almost two decades of Argo float observations, assimilated into an ocean reanalysis, to examine the deep ocean response to the MJO. A coherent MJO signal in potential density and zonal velocity anomalies is observed down to at least 2,000 m (the typical maximum depth of Argo observations) in the Pacific and Indian Ocean basins, but not in the Atlantic. The signal is consistent with equatorial Kelvin wave structures. Below the thermocline, the anomalies are characterized by a vertical tilt, downward and to the east. Two theoretical frameworks are used to interpret the signal. The vertical mode framework is reasonably successful in representing the eastward propagation of equatorial Kelvin waves in the thermocline. The first internal mode is efficiently forced by the MJO surface winds, and energy is transferred to the second, third, and higher internal modes through nonlinear processes. However, these vertical modes are not useful for interpreting the deeper structure. Instead, ray paths are constructed using a vertically propagating wave framework. Rays forced by the low‐frequency component of the MJO (with periods near 60 days) are consistent with the vertical tilt of the observed response. These ray paths explain the observed “shadow zones,” where no coherent signal is found within 6,000 km of the western boundary of each ocean basin at 2,000 m depth.