Influence of Air-Sea Interactions on the Madden-Julian oscillation in the Presence and Absence of Extratropical Forcing

Extratropical influences (or the tropics-extratropics interactions) have been proposed as a possible trigger for the initiation of the Madden-Julian oscillation (MJO). However, to what extent the influences from the extratropics are modulated by the local air-sea interactions are not known. In this study, we explore the role of air-sea interactions on the MJO structure and propagation in the tropical Indian Ocean using a unique GCM-based framework. In the “uncoupled” simulation, the ECHAM4 model is integrated for 20 years using the climatological monthly mean sea surface temperature (SST). In the “coupled” experiment, a slab ocean model is coupled in the tropical Indian and west Pacific Oceans (40°E-180°E, 20°S- 20°N) to explore the role of air-sea coupling on the MJO. Note that the influences from the extratropics were not disturbed in the coupled and uncoupled simulations.

In the third experiment (referred as NSEW), model prognostic variables are relaxed in the tropical Atlantic region (80°W-0°, 20°S- 20°N) and in the 20°-30° latitude zones toward the “uncoupled” climatological annual cycle to suppress the influences from the circumnavigating waves and the extratropics, respectively. The comparison between the control and NSEW reveal the relative roles of the local and external forcing without the air-sea coupling. In the fourth experiment (coupled-NSEW) a slab ocean model is added to the NSEW over the same region as “coupled” experiment. The comparison of NSEW with coupled-NSEW will expose the contribution of air-sea coupling on the MJO and its initiation in the absence of external forcing.

The results indicate that the differences are larger between NSEW and coupled-NSEW than that between uncoupled and coupled. This indicates that the local air-sea coupling could further modulate the circulation anomalies induced by the extratropical influence. The detailed structure of the MJO, and the phase relationship between different variables in the simulations will be discussed in the meeting along with the implications and limitations of the results.