A dynamical ocean feedback mechanism for the Madden–Julian Oscillation

Abstract

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 Niño Southern Oscillation.

Ocean Rossby waves as a triggering mechanism for primary Madden–Julian events

“…This is consistent with the mechanism proposed by Webber et al (2010). However, it is likely that such oceanic forcing will be most important, and most apparent in observations, when other triggering mechanisms are weak or non-existent, such as is the case for primary events (Matthews, 2008).…”

“…In addition to this thermodynamic coupling mechanism, there is also the potential for the ocean dynamics to force the MJO (Han et al, 2001;Fu, 2007;Webber et al, 2010). In the Indian Ocean, the dynamical ocean response to the MJO consists of equatorial Kelvin waves which reflect into equatorial Rossby waves on reaching the coast of Sumatra (Oliver and Thompson, 2010;Webber et al, 2010).…”

“…In the Indian Ocean, the dynamical ocean response to the MJO consists of equatorial Kelvin waves which reflect into equatorial Rossby waves on reaching the coast of Sumatra (Oliver and Thompson, 2010;Webber et al, 2010). These reflected Rossby waves propagate westwards to arrive in the western Indian Ocean approximately 90 days later, where they lead to positive SST anomalies and thus trigger convection within the region where MJO events are initiated (Webber et al, 2010). The magnitude of these composite warm anomalies is similar to the flux-induced anomalies, being around 0.2…”

Large-Scale Oceanic Variability Associated with the Madden-Julian Oscillation during the CINDY/DYNAMO Field Campaign from Satellite Observations

“…These SSH anomalies could propagate westward by radiating Rossby waves from the eastern boundary, e.g., [37][38][39][40][41]. Figure 9 shows the longitude-time diagram of SSH at 4°N and 4°S.…”

“…Recent studies [41,46] suggest that reflected Rossby waves, originally generated from Kelvin waves forced by westerly winds associated with the MJO, may cause the significant SST change in the central Indian Ocean, and this anomalous SST could further act as a trigger of the initiation of convection for generating a subsequent MJO event. During March 2012, atmospheric convection was initiated in the central Indian Ocean, and it was developed to a MJO event that propagated to the western Pacific in late March [5].…”