The influence of large-scale westerly winds on Madden-Julian Oscillations

The influence of large-scale zonal winds on the intraseasonal variabilities (ISVs) is examined with two NCAR CCSM3 model runs. The control run is a standard CCSM3 run with Zhang-McFarlane scheme for deep convection. The convection run has the same configurations as the control run except that the parameterization of convective momentum transport (CMT) is included. As a result of including CMT, the easterly wind bias over the tropical region from the eastern Indian Ocean to the western Pacific Ocean is reduced. The improved westerly winds lead to a better simulation of tropical ISVs in the atmosphere, which look more like realistic Madden-Julian Oscillations (MJOs). The strength of ISVs in the convection run is detectably stronger than that in the control run. In the convection run, the vertical profile of intraseasonal zonal winds (obtained with a band-pass filtering from 20 – 100 days) tilts westwards with heights, which is a favorable structure for the release of the potential energy due to the baroclinic instability. However, in the control run, the profile of intraseasonal zonal winds is almost vertically uniform, thus the ISVs are weaker than those in the convection run. Significant improvement in the organization of the simulated MJOs in the convection run is more interesting. The quadrature relation between the first two principal components (PCs) of the EOF analysis is well-captured, which indicates an eastward propagation of the simulated ISVs. In contrast, in the control run, there is almost no quadrature relation between the first two PCs, indicating no propagating intraseasonal signals in the zonal direction, although there are also some enhanced ISVs. Therefore, the improved westerly winds due to including CMT help to establish the coherent evolvement between the first two EOF modes of ISVs and hence contribute to the eastward propagation of simulated MJOs. Applying the method of stationary phase and with NCEP reanalysis, the eastward propagation speed of MJOs (~ 5 m/s) is likely to be determined by the mean zonal speed, which highlights the importance of the interaction between intraseasonal MJOs and the background state. This result is supposed to be helpful to reduce the too fast simulated MJOs (equivalently shorter MJO periods in simulations), if it is properly resolved in a model.