The Madden Julian Oscillation (MJO): A Climate Model Increases its "Mojo"
Subramanian, A.C., Jochum, M., Miller, A.J., Murtugudde, R., Neale, R.B. and Waliser, D.E. 2011. The Madden-Julian Oscillation in CCSM4. Journal of Climate 24: 6261-6282.
Until now, General Circulation Models (GCMs) have had difficulty in representing the MJO, and consequently, its impact on larger-scale phenomena could not be captured either. This is but one factor as to why GCMs fail, to a certain degree, in representing the large-scale correctly. The inability of the GCMs to capture the MJO is due primarily to the inadequate representation of the associated convection.
In the present analysis, Subramanian et al. (2011) used the National Center for Atmospheric Research (NCAR) Community Climate System Model version 4 (CCSM4) model to examine its ability to capture the MJO, which included an upgrade to the convective parameterization scheme. This upgrade "improves the correlation between intraseasonal convective heating and intraseasonal temperature, which is critical for the buildup of available potential energy."
The authors used a 500-year simulation of the CCSM4 using pre-1850 conditions as a control run. They then extracted two ten-year periods from these data, each representing the strongest and weakest ENSO variability in the 500 year period. They compared this data set to observed Outgoing Longwave Radiation (OLR), which is a commonly used proxy for convection, as well as tropical precipitation, and zonal winds at 850 and 200 hPa taken from the NCAR/NCEP reanalyses. Lastly, the authors examined the relationship between the MJO and such general circulation features as ENSO and the monsoons in both the model and observations.
In doing so the authors found that the CCSM4 produced a feature that propagated eastward in the "intraseasonal zonal winds and OLR in the tropical Indian and Pacific Oceans that are generally consistent with MJO characteristics." Thus, the model performed well overall in capturing the MJO (Fig. 1), but there were still some differences. Whereas the observations produced a strong (weaker) wave number one (two and three) in the data, the model MJO showed more coherency among the wave numbers one-three. This suggests stronger Kelvin wave activity at the higher wave numbers in the model.
Figure 1. A Hovemöller plot of the zonal winds at 850 hPa filtered to retain the 20-100 day signal for the a) NCAR/NCEP reanalyses from 1997 and b) year 3 of the CCSM4 run. The arrow is added to demonstrate the eastward propagation of weaker 850 hPa winds which represents the MJO.
When examining the relationship to other phenomenon, the authors found that MJO activity was enhanced (weaker) during El Niño (La Niña) years. Also, the MJO was preferred in the Indian Ocean monsoon region during negative shear regimes (shear defined by the zonal wind at 850 hPa minus the meridional [north-south] wind at 200 hPa). The MJO is also preferred when the Hadley circulation in the tropics is weaker as well. All of these phenomena are interrelated and thus as the authors state; "MJO could thereby be simultaneously affected in multiple ways when these type of large-scale climate mode interactions occur and possibly feed back onto the entire coupled system."
The outcome of this work demonstrates how difficult it is to represent the current state of the climate, as well as the interannual variability in the climate system. Models continue to improve through the increase in resolution and the improvement in subgrid-scale physical processes. But even though this model showed a distinct improvement in representing the MJO, there were still some differences between the modeled and observed MJO. The MJO is one critical link between the small-scale atmospheric motions and the general circulation. Thus, it is important to continue improving the representation of the MJO in model. It is also critical to improve the model MJO in order to project any changes in the future climate.