Seasonal mean anomalies associated with interannual variability of the Madden-Julian oscillation

In order to identify controlling factors for intensity of the Madden-Julian oscillation (MJO), seasonal mean anomalies associated with MJO variation are investigated in zonal wind, sea surface temperatures (SST), outgoing longwave radiation (OLR) among others. Indexes to indicate MJO intensity are constructed, and linear regression technique are employed

It is found that strong MJOs are associated with westerly vertical wind shear (stronger westerly at higher altitudes). In DJF and MAM, the anomalous westerly shear is induced by angular momentum convergence by the strong MJO, while in JJA and SON stationary zonal wavenumber one transport angular momentum into the tropics. Critical latitudes for these waves propagating away from the tropics may explain this difference. Although statistically less clear, it is also found that positive SST anomalies are located near the dateline when the MJO is more active. The SST anomalies are associated with eastward shift of low OLR areas and lower troposphere convergence, and westerly zonal wind anomalies near surface, which may induce and/or maintain the SST anomalies by suppressing upwelling through Ekman transport. The wind shear anomalies are consistent with climatological seasonal cycle of MJO intensity and vertical wind shear in the tropics, which show stronger MJOs with stronger westerly shear in boreal winter and spring, while the SST anomalies are not particularly consistent with the seasonality.

Some discussions are made on an assumption that a stronger westerly vertical shear induces a stronger MJO. In boreal summer and winter, the interannual variation would be created by interaction between the MJO and the vertical shear. In boreal summer and winter, the interannual variability of the MJO may be induced by wind shear forcing by a stationary zonal wavenumber one which may be part of variation in the SST-OLR-wind interaction.