Loss of Significance and Multidecadal Variability of the Madden-Julian Oscillation

Change in significance and multidecadal variability of the Northern Hemispheric winter MJO during 1948-2006 is examined using NCEP-NCAR reanalysis data. Variation of the MJO power relative to a red background is estimated by isolating the MJO signal through frequency-wavenumber spectral analysis using a 10-yr sliding window. It is shown that during the period of study, the rate of increase of background power has been larger than the rate of increase of the MJO power, leading to a decreasing trend of significant MJO power. It is also found that a multidecadal variation rides on the decreasing trend of significant power of the MJO. Another finding is that the zonal mean component of the zonal wind at 200 hPa on a MJO time scale has a significant increasing trend. Both of the above trends are statistically significant at the 95% confidence level. Energetics calculations in the wavenumber domain were carried out to understand why the significant MJO power is not increasing as fast as the red background. It is shown that long waves (wavenumbers 1-3, i.e., the MJO scale) lose energy to the zonal mean flow and the rate of kinetic energy gain by the zonal mean flow from the long waves has a linear increasing trend. Thus, while the MJO is also being energized by a warming ocean, it is losing increasingly more energy to the zonal mean flow, making the zonal mean more energetic while losing its own significance at the same time. It is found that the observed multidecadal variability of the significant MJO power has no relationship with other well-known multidecadal variability. However, the authors find that the multidecadal variability of the MJO and the rate of kinetic energy exchange between the zonal mean flow and long waves are closely linked, indicating that the observed multidecadal variability of the MJO is internally driven.