Intraseasonal variability sensitivity tests in an aqua-plant general circulation model

The simulation of the Madden Julian Oscillation (MJO) in an aqua-planet general circulation model (GCM) with prescribed sea surface temperature (SST) was studied by Maloney et al (2010, hereafter M10). Starting from an SST distribution that resembled the boreal Spring mean from observations, the meridional gradient of SST was reduced to one quarter of its observed value poleward of 10 degrees, which dramatically improved the MJO simulation. Mechanism denial experiments showed that wind-evaporation feedback was necessary to destabilize the MJO mode in this model. This present work extends the aquaplanet sensitivity experiments of M10 to examine the impact of other SST basic states on intraseasonal variability, and also conducts an experiment in which the impacts of cloud radiative feedbacks are examined. The National Center for Atmospheric Research (NCAR) Community Atmospheric model version 3 (CAM3) with relaxed Arakawa Schubert scheme is used in the study. The SST distribution in the control run is the same as quarter meridional gradient zonally asymmetric simulation (QMZA) of M10. A zonally symmetric SST distribution created by using SSTs from the QMZA simulation along 150^oE was conducted to determine whether increases in off-equatorial SST could produce a similarly robust MJO simulation in a zonally-symmetric model, and whether the presence of zonal asymmetries such as mean low-level westerlies are essential for producing a realistic MJO. A simulation with the QMZA simulation SST distribution but longwave radiation prescribed to be the sum of 80% of climatology plus 20% interactive radiation was conducted to understand the effect of cloud-radiative and moisture-radiative feedbacks on the MJO simulation (hereafter called the fixed radiation model). The results show that decreasing the meridional SST gradient increases the intraseasonal power in the zonally symmetric model, although the period of the intraseasonal mode in the model becomes substantially shorter (30 days versus 50 days for the QMZA simulation). The intraseasonal mode in the zonally symmetric model resembles the WISHE mode of linear theory, with wind speed and latent heat flux anomaly maxima occurring in surface easterly anomalies to the east of maximum convection, unlike both zonally asymmetric models in which latent heat fluxes maximizes near and to the west of convection. The fixed radiation model has reduced MJO intensity, although with no change in the dominant 50-day timescale from the QMZA simulation. This experiment suggests that both cloud-radiative and wind-evaporation feedbacks may help to destabilize the MJO mode in the model.