MJO

Salah satu fenomena global yang mempengaruhi cuaca dan iklim Indonesia adalah Madden Julian Oscillation (MJO) (Evana et al. 2008). MJO merupakan fenomena dominan di kawasan ekuator dengan waktu periode osilasi berkisar antara 30 hingga 70 harian akibat pengaruh awan-awan konveksi yang terbentuk di atas Samudera Hindia (sebelah barat Indonesia) kemudian bergerak ke arah timur di sepanjang garis ekuator (Hermawan 2010). Menurut Seto (2002) MJO berperan penting sebagai variasi intra musim (intraseasonal variations) yang berpengaruh terhadap variasi cuaca di daerah tropis.

ln Part I (Storch and Xu 1990) the principal oscillation pattern (POP) analysis of 200mb equatorial velocity potential leads to the definition of a bivariate (POP-) index of the tropical 30- to 60-day oscillation. Using the POP pred iction scheme this index is predictable for a few days in advance. In Part II, the prediction of the equatorial velocity potential field, made by the POP method and made by two GCMs, is investigated. The POP index forecast can incorporate skillful forecasts of the equatorial velocity potential (X) field. Its ensemble correlation skill score passes the 0.50 level at 7 days, whereas persistence passes after 3 days. lf there is a strong 30- to 60-day oscillation signal in the initial state, useful forecasts of more tlnan 20 days are sometimes possible: if the initial signal is weak, the POP forecast fail s. Also. the forecast skill of two GCMs is considered. The NCAR T31 GCM appears to be quite sklllful in predicting the equatorial x-field, and in particular the 30- to 60-day oscillation. Its skill, however, is less than that of the POP scheme. The CNRM T4 GCM seems not to be able to predict the regular development associated with the tropical 30- to 60-day oscillation. The power of the POP index in explaining the equatorial x-field is a measure or the strength and dominance of the 30- to 60-day oscillation. This measure a t day 0 is an a priori indicator of the NCAR T31 GCM's skill in predicting the equatorial velocity potential field.

ABSTRACT A series of 46-day ensemble integrations starting on the 15th of each month from 1989 to 2008 has been completed with the European Centre for Medium-Range Weather Forecasts (ECMWF) forecast system. The Madden– Julian Oscillation (MJO) simulated by the hindcasts is diagnosed using an index based on combined empirical orthogonal functions (EOFs) of zonal winds at 200 and 850 hPa and outgoing long-wave radiation. Results indicate that the dynamical model is able to maintain the amplitude of the MJO during the 46 days of integrations and the model displays skill for up to about 20 days to predict the evolution of the MJO. However, the MJO simulated by the model has a too slow eastward propagation and has difficulties crossing the Maritime Continent.The MJO teleconnections simulated by the ECMWF forecast system have been compared to reanalyses. In the Tropics, the impact of the MJO on precipitation is generally consistent with reanalysis. In the Northern Extratropics, the MJO simulated by the model has an impact on North Atlantic weather regimes, but with a smaller amplitude than in reanalysis which can be partly explained by the too slow eastward propagation of the simulated MJO events. The impact of the MJO on the probabilistic skill scores has been assessed. Results indicate that the MJO simulated by the model has a statistically significant impact on weekly mean probabilistic skill scores in the Northern Extratropics, particularly at the time range 19– 25 days. At this time range, the reliability of the probabilistic forecasts over Europe depends strongly on the presence of an MJO event in the initial conditions. This result confirms that the MJO is a major source of predictability in the Extratropics in the sub-seasonal time-scale. Copyright © 2010 Royal Meteorological Society