Role of air-sea interaction on the Madden–Julian Oscillation using a coupled AGCM–slab ocean model

Characteristics of the Madden-Julian Oscillation (MJO) were assessed in a modified version of the atmosphere general circulation model of the Australian Bureau of Meteorology Research Centre (BAM) using a CAPE closure mass flux convection scheme. Uncoupled circulation model simulations using BAM showed similar-to-observed variability at MJO scales, however rather than showing a broad spectral peak of period 30-80 days as observed, BAM showed two modes of “MJO-like” eastward propagation at intraseasonal scales with similar characteristics to the observed MJO. Originating over the Indian Ocean in southern summer, the MJO in BAM propagated eastward with a period of 60-100 days (slow mode of propagation) before speeding up over the Indo-Pacific warm pool region and propagating with a 30-50 day period (fast mode) across the western and central Pacific, decaying just east of the dateline.

Slab-ocean coupling improved the MJO in BAM over the Indo-Pacific warm pool by reducing its period of propagation from 60-100 days to 45-70 days, showing better agreement with the 30-80 day observed oscillation. Air-sea coupling produced an MJO that propagated faster than its uncoupled counterpart due to a feedback between SST and convection, where convection was organised by intraseasonally varying SST. While the wave-CISK mechanism supported the generation of MJO-like activity in the model, wind-evaporation feedback operated in conjunction with low-level moisture convergence in a weak background state to amplify an increase in moisture flux ahead of the MJO convection over a region of intraseasonally-varying positive SST anomalies. The Indo-Pacific warm pool was conducive to such a mechanism, while the strong easterly background state of the cooler central Pacific did not support such a mechanism where the fast mode of propagation in BAM dominated. Referred to as enhanced moisture convergence-evaporation feedback (EMCEF), this mechanism resulted in a significant increase in convective precipitation east of the convective centre, which acted to speed up the eastward propagating convective anomaly in the model. The results showed that the MJO is primarily an atmospheric phenomenon, with air-sea interaction improving upon, but not critical for, the existence of the MJO.