The influence of El Nio on MJO over the equatorial pacific

In this paper, the influence of El Nio event on the Madden-Julian Oscillation (MJO) over the equatorial Pacific is studied by using reanalysis data and relevant numerical simulation results. It is clearly shown that El Nio can reduce the intensity of MJO. The kinetic energy of MJO over the equatorial Pacific is stronger before the occurrence of the El Nio event, but it is reduced rapidly after El Nio event outbreak, and the weakened MJO even can continue to the next summer. The convection over the central-western Pacific is weakened in El Nio winter. The positive anomalous OLR over the central-western Pacific has opposite variation in El Nio winter comparing to the non-ENSO cases. The vertical structure of MJO also affected by El Nio event, so the opposite direction features of the geopotential height and the zonal wind in upper and lower level troposphere for the MJO are not remarkable in the El Nio winter and tend to be barotropic features. El Nio event also has an influence on the eastward propagation of the MJO too. During El Nio winter, the eastward propagation of the MJO is not so regular and unanimous and there exists some eastward propagation, which is faster than that in non-ENSO case. Dynamic analyses suggest that positive SSTA (El Nio case) affects the atmospheric thickness over the equatorial Pacific and then the excited atmospheric wave-CISK mode is weakened, so that the intensity of MJO is reduced; the combining of the barotropic unstable mode in the atmosphere excited by external forcing (SSTA) and the original MJO may be an important reason for the MJO vertical structure tending to be barotropic during the El Nio.