The impact of MJO on the onset of the 1997/98 El Niño in a coupled model system

An ensemble of ninety 9-month forecasts of the onset of the 1997/98 El Niño was used to shed light on the possible role of the MJO during the onset phase. The ensemble forecasts all started on 1 December 1996 and were made using the current operational coupled model seasonal forecast system (POAMA1) of the Australian Bureau of Meteorology. The atmospheric initial condition for the ensemble forecasts was taken from an ensemble mean of 36-member AMIP style atmospheric states, and therefore does not contain stochastic noise. The oceanic initial conditions were taken from an ocean data assimulation system. Each forecast of the ensemble starts with the same ocean and atmospheric initial conditions, except for a very small sea surface temperature (SST) perturbation added to perturb the chaotic component of the atmosphere. These ninety ensemble forecasts are separated into terciles of strong, neutral and weak according to the intensity of monthly Nino3.4 index at four months lead. After compared the difference behaviour of the composited first MJO event between the strong group and the weak group, it is shown that the MJO-related convective activity in the strong group, which can be traced back to the eastern Indian Ocean, propagates further east into the centra-eastern Pacific, compares to that in the weak group during the first four months. Further analyses show that the relatively stronger and further eastward expanding westerly surface stress, associated with the MJO activity in the strong group, definitely produces a relatively stronger downwelling eastward Kelvin wave that propagates into the centra-eastern Pacific and contributes to perturb a stronger warming SST over there.