The Crucial Role of the Initial State in MJO prediction

Tropical intraseasonal oscillations represent key sources of sub-seasonal to seasonal predictive skill in the mid-latitudes due to tropical-extratropical teleconnections. Among these oscillations, the Maritime Continent (MC) assumes particular importance due to the interaction between the Madden Julian Oscillation (MJO), tropical convection and the midlatitude jet stream, influencing subsequent weather downstream. However, a prominent deficiency in numerical weather models often discussed in the literature is that the MC tends to act as an unrealistically strong barrier for the eastward propagation of the Madden and Julian Oscillation (MJO) whereas, in reality, some developing MJO events propagate across the MC reaching the west Pacific basin. In our study, using NOAA’s Unified Forecast System, we find that the initial condition plays a crucial role for the amplitude when advancing the MJO across the MC. Intercomparing model runs initialized from two different reanalysis products (UFS replay and GEFSv12), show a significant difference in MJO amplitude (larger in runs initialized with GEFSv12) which, remarkably, is maintained throughout the 15 day forecast period. The difference in amplitude can be traced back to colder temperatures in the initial upper tropical troposphere in the GEFSv12 initialized runs, that adjust slowly and lead to stronger zonal winds at 200hPa due to a stronger pressure gradient and enhanced convection. A clear reduction in MJO amplitude error can be seen in the GEFSv12 initialized runs after 2005 when AIRS satellite observations were included in the assimilation, pointing to the importance of assimilating more observations to create a good initial state even for subseasonal forecast timescales.