The equatorial deep ocean structure associated with the Madden-Julian Oscillation from an ocean reanalysis

Robbins, Connor, Matthews, Adrian J., Hall, Rob A., Webber, Ben G. M. and Heywood, Karen J. (2025) The equatorial deep ocean structure associated with the Madden-Julian Oscillation from an ocean reanalysis. Journal of Geophysical Research - Oceans. ISSN 2169-9275 (In Press)

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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 2000 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 characterised 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 6000 km of the western boundary of each ocean basin at 2000 m depth.

Item Type:	Article
Additional Information:	Open research: This study has been conducted using E.U. Copernicus Marine Service Information; the GLORYS12V1 ocean reanalysis data were downloaded from GLORYS12V1, E.U. Copernicus Marine Service Information (CMEMS), Marine Data Store (MDS), DOI: 10.48670/moi-00021 (accessed between 01-02-2024 and 09-02-2024). The vertical modes were calculated using the Dynmodes package, downloaded from https://www.eoas.ubc.ca/~sallen/AIMS-workshop/ modules/dynmodes.html, accessed on 20 February 2024, an implementation in Python of the original Dynmodes package by John Klinck, 1999, which can be found at https://github.com/sea-mat/dynmodes. The TAO/TRITON and RAMA mooring data were downloaded from the the NOAA/PMEL GTMBA Project Office at https://www.pmel.noaa.gov/gtmba/pmel-theme/pacific-ocean-tao Funding information: C.R. was funded, and A.J.M., R.H. and K.J.H. were partially funded, by the UK Natural Environment Research Council (NERC) TerraMaris project (grant NE/R016704/1).
Faculty \ School:	Faculty of Science > School of Environmental Sciences University of East Anglia Research Groups/Centres > Theme - ClimateUEA
UEA Research Groups:	Faculty of Science > Research Groups > Fluids & Structures Faculty of Science > Research Groups > Centre for Ocean and Atmospheric Sciences Faculty of Science > Research Groups > Collaborative Centre for Sustainable Use of the Seas Faculty of Science > Research Groups > Climatic Research Unit
Depositing User:	LivePure Connector
Date Deposited:	06 Aug 2025 17:30
Last Modified:	06 Aug 2025 17:30
URI:	https://ueaeprints.uea.ac.uk/id/eprint/100097
DOI:	10.1029/2025JC022457

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