Abstract
This paper presents an assessment of the Madden–Julian oscillation (MJO) simulated in five experiments using the Community Earth System Model under different model settings. The analysis focused on the effects of air–sea coupling, resolution and atmospheric physics on the basic characteristics of the MJO, including intraseasonal variance, wavenumber-frequency characteristics and eastward propagation, using outgoing longwave radiation (OLR), zonal winds at 850 hPa (U850) and at 200 hPa (U200). Five experiments are conducted for this purpose including one atmospheric model—Community Atmosphere model version 4 (CAM4), two coupled models with CAM4 or Community Atmosphere model version 5 (CAM5) as the atmospheric component at a low resolution (CLP4_2d, CPL5_2d) and two the same coupled model with a high resolution (CPL4_1d and CLP5_1d). The results show that all models have better intraseasonal characteristics in U850 than in OLR. The uncoupled model CAM4 has lower fidelity than the coupled models in characterizing MJO basic features including the temporal and spatial intraseasonal variability and the eastward propagation. With ocean feedback, the coherence of convection and circulation is improved in the coupled models. The higher resolution is helpful in improving ISV spatial distribution and eliminating low frequency bias in the frequency-wavenumber spectra although it has little improvement to MJO-band variance (power) in frequency-wavenumber spectra. The new shallow convection scheme in CAM5 improves the moisture process of the lower troposphere so that CPL5_2d and CPL5_1d have more realistic eastward propagation speed in the boreal winter and better northward propagation in the boreal summer than other models. However, the strength of the convective MJO signal in CPL5_2d and CPL5_1d are weaker than other models and observations, which is probably one of the most spurious features in CPL5_2d and CPL5_1d experiments, suggesting that the CAM5 has a weaker convection activity than its predecessor.
Highlights
The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability (ISV) in the tropics (Madden and Julian 1994)
Emphasis is placed on the impacts of model resolution, the atmosphere– ocean coupling and atmosphere physics on MJO simulation in Community Earth System Model (CESM), in particular, exploring the effects of the new shallow convection scheme of Community Atmosphere model version 5 (CAM5) on MJO simulation
We explore the capability of the latest version of CESM in MJO simulation, and the impacts of air-sea
Summary
The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability (ISV) in the tropics (Madden and Julian 1994). Emphasis is placed on the impacts of model resolution, the atmosphere– ocean coupling and atmosphere physics on MJO simulation in CESM, in particular, exploring the effects of the new shallow convection scheme of CAM5 on MJO simulation. Various observations are used to validate the MJO simulation in CESM, including OLR from the advanced very high resolution radiometer (AVHRR) estimate (Liebmann and Smith 1996), specific humidity (SH) daily data from the National Centers for Environmental Prediction (NCEP) reanalysis (Kalnay et al 1996), zonal winds in the upper (200-hPa) and lower (850-hPa) troposphere obtained from NCEP reanalysis 2 (Kanamitsu et al 2002) All of these datasets have a horizontal resolution of 2.5° × 2.5°, and we use the data time period from 1981 to 2000
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