The role of ocean variability in simulating the MJO

The impact of ocean high frequency variability on the MJO formation is investigated in sensitivity-type numerical experiments. The control experiment uses super-parameterized version of the Community Climate System Model, version 3 (SP-CCSM3). Second, an uncoupled experiment is conducted with the atmospheric component of the SP-CCSM3, forced by prescribed, daily SST from the control simulation. The high-frequency variability of the SST in the uncoupled simulation (referred to as SP-CAM5d) is removed by applying a 5-day running mean to the daily SST simulated by the coupled model. The experimental design ensures that the mean climates of the two simulations are very close and the resulting differences are due to lack of SST variability shorter than 5 days.

The spatial structure of the intraseasonal variability during boreal winter in the uncoupled simulation is significantly different than in the SP-CCSM3. In SP-CAM5d, the 850-mb zonal wind and OLR variability is reduced in the Indian Ocean and Western Pacific. The spectrum of the uncoupled simulation shows a redistribution of power in the Indian Ocean region with the spectral peak at around 50 days vs. 30-40 days produced by the coupled simulation.

The MJO signal in the control experiment exhibits a realistic lifecycle. SP-CCSM3 successfully simulates the observed phase relationship of precipitation and low-level winds, as well as their coherent propagation from the Indian Ocean into the western Pacific Ocean. The MJO-like variability simulated by the uncoupled simulations is less organized and the phase relationship between circulation and convection is altered.

Lag correlations between convection and surface fields show that in SP-CAM5d the correlation between convection and SST becomes weaker, and not always a positive SST anomaly is followed by a maximum in convection.

The response of the free-tropospheric moisture to the changes in the SST is a weakening of the vertical moisture transport associated with the MJO activity. Analysis of the moist static energy (MSE) budget of the two simulations shows that in the uncoupled model the build up of column MSE occurs simultaneously with MJO deep convection. The discharge of MSE is dominated by the surface fluxes.