87th AMS Annual Meeting

Monday, 15 January 2007
The Role of MJO in Ensemble Forecasts of the 1997/98 El Niņo in the POAMA1 System
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Li Shi, BMRC, Melbourne, Vic., Australia; and O. Alves, H. H. Hendon, and G. Wang
To shed light on the possible role of the MJO on generating the spread of ensemble forecast, an ensemble of ninety 9-month forecasts for the 1997/98 El Niņo has been made starting on 1 December 1996 by using the current operational coupled model seasonal forecast system (POAMA1).

Each hindcast starts with the same ocean and atmospheric initial conditions except a very small sea surface temperature (SST) perturbation added to perturb the chaotic component of the atmosphere. The atmospheric initial conditions are taken from an ensemble mean of 36-member AMIP style atmospheric states, and hence have no stochastic component. And then, the ninety ensemble forecasts are separated into terciles of strong, neutral and weak group according to the intensity of monthly Nino3.4 index for the four months.

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 of 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 generate a relatively stronger downwelling eastward Kelvin wave that propagates into the centra-eastern Pacific and contributes to perturb a stronger warming SST over there.

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