Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 740 – 754, April 2010 Part A A dynamical ocean feedback mechanism for the Madden–Julian Oscillation Benjamin G. M. Webber a *, Adrian J. Matthews a,b and Karen J. Heywood a a School of Environmental Sciences, University of East Anglia, Norwich, UK b School of Mathematics, University of East Anglia, Norwich, UK *Correspondence to: Benjamin G. M. Webber, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK. E-mail: b.webber@uea.ac.uk Composite analysis is applied to study the dynamical ocean response to Madden–Julian (MJ) events, measured by anomalies in sea surface height from the merged TOPEX/Poseidon–European Remote Sensing satellite altimetry dataset. In each of the tropical ocean basins, significant equatorial waves are forced, which are shown to modulate the sea surface temperature (SST) by 0.2–0.3 ◦ C in the absence of strong surface heat fluxes. In the Indian Ocean there is a clear dynamical response which may play a significant role in generating later MJ events. Surface westerly winds, associated with the active phase of the Madden–Julian Oscillation (MJO), force an eastward-propagating oceanic downwelling equatorial Kelvin wave, which, on reaching the eastern boundary at Sumatra, forces reflected downwelling equatorial Rossby waves and coastal Kelvin waves. The coastal Kelvin waves propagate southwards towards northern Australia and northwards into the Bay of Bengal, and will be important for local physical, chemical and biological processes. The equatorial Rossby waves propagate westward across the Indian Ocean, arriving in the western Indian Ocean approximately 80–100 days after the initial Kelvin wave was generated. The arrival of these waves generates positive SST anomalies which leads to convection and may trigger the next-but-one MJ event, or amplify the low-frequency tail of the MJO. This constitutes a coupled feedback mechanism from the ocean dynamics onto the MJO, somewhat similar to the delayed oscillator mechanism for the El Ni˜ no Southern Oscillation. Copyright c  2010 Royal Meteorological Society Key Words: MJO generation mechanism; equatorial Kelvin waves; equatorial Rossby waves; Indian Ocean dynamics; intraseasonal variability; delayed oscillator Received 24 August 2009; Revised 16 December 2009; Accepted 10 February 2010; Published online in Wiley InterScience 21 April 2010 Citation: Webber BGM, Matthews AJ, Heywood KJ. 2010. A dynamical ocean feedback mechanism for the Madden–Julian Oscillation. Q. J. R. Meteorol. Soc. 136: 740 – 754. DOI:10.1002/qj.604 1. Introduction At intraseasonal time-scales, the Madden–Julian Oscillation (MJO) is the primary source of tropical atmospheric variability. It consists of eastward-propagating regions of enhanced and suppressed convection and rainfall, associated with baroclinic wave structures evident in the wind and pressure fields (Madden and Julian, 1971, 1972). The MJO is a quasi-periodic occurrence with a broad spectral signal. It is typically defined by a period of ∼30–60 d but is associated with convective variability over a broader spectral range (Salby and Hendon, 1994), including scale interactions with high-frequency convection (Majda and Biello, 2004; Batstone et al., 2005). The MJO is most active in the warm pool of the Indian Ocean and the western Pacific, but it modulates atmospheric variability globally (Matthews et al., Copyright c  2010 Royal Meteorological Society