THE MADDEN-JULIAN OSCILLATION IN THE CANADIAN-CLIMATE-CENTER GENERAL-CIRCULATION MODEL

The Madden-Julian oscillation (MJO) simulated by the Canadian Climate Centre general circulation model (CCC GCM) is identified by a principa l oscillation pattern (POP) analysis and compared with that observed i n the real atmosphere. The results are based upon two integrations of the CCC GCM, one with a parameterization of penetrative cumulus convec tion (EXP1) and the other with a moist convective adjustment scheme (E XP2). The signal of MJO can be detected in both integrations as the fi rst POP of the 200 hPa velocity potential along the equator. The distu rbances show a distinctive wave number one structure with the stronges t local amplitude found in the longitudes corresponding to the region of the Asian monsoon. The phase speed of the eastward wave propagation is higher in the eastern Pacific and lower in the monsoon region wher e the convective activities are strongest. These features are in good agreement with the observations. The energy spectrum of the velocity p otential peaks at the frequency corresponding to a period of about 38 days for EXP1, which is somewhat shorter compared to the observed peri ods of 40-50 days. On the other hand, two spectral peaks can be clearl y identified for EXP2, one with a period of 24 days and the other with a much longer period, somewhere near 112 days. Both peaks appear stat istically significant at 95% level. Long term data of the observed atm osphere show little indication of such spectral separation. The horizo ntal patterns identified by the POP analysis resemble to some extent t he baroclinic response of tropical flow to a heat source travelling wi th the speed of MJO. At the upper level, Rossby wave energy propagates westward with winds generally following the height contours, whereas Kelvin wave energy propagates to the east from the heat source with st rong cross-contour flow near the equator. At the lower level, the patt erns are essentially reversed. The model-generated precipitation and d iabatic heating are examined by compositing against the moving MJO. It is found in EXP2 that the composite heating distribution is coherent with the flow pattern only in a certain sector of the equator, dependi ng on whether the fast or slow mode is used to determine the reference point. The composite vertical heating profile of a slower mode tends to have a maximum found at a lower level. The sensitivity of simulated MJO to the cumulus convection scheme in the model is discussed.