On the predictability of the interannual behaviour of the Madden‐Julian oscillation and its relationship with el Nin̄o

Abstract

The Madden‐Julian Oscillation (MJO) is the dominant mode of tropical variability at intraseasonal time‐scales. It displays substantial interannual variability in intensity which may have important implications for the predictability of the coupled system. the reasons for this interannual variability are not understood. the aim of this paper is to investigate whether the interannual behaviour of the MJO is related to tropical sea surface temperature (SST) anomalies, particularly El Niño, and hence whether it is predictable. The interannual behaviour of the MJO has been diagnosed initially in the 40‐year National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis. the results suggest that prior to the mid‐1970s the activity of the MJO was consistently lower than during the latter part of the record. This may be related either to inadequacies in the data coverage, particularly over the tropical Indian Ocean prior to the introduction of satellite observations. or to the real effects of a decadal time‐scale warming in the tropical SSTs. the teleconnection patterns between interannual variations in MJO activity and SST show only a weak, barely significant, influence of El Niño in which the MJO is more active during the cold phase. As well as the NCEP/NCAR Reanalysis, a 4‐member ensemble of 45‐year integrations with the Hadley Centre climate model (HadAM2a), forced by observed SSTs for 1949‐93, has been used to investigate the relationship between MJO activity and SST. HadAM2a is known to give a reasonable simulation of the MJO, and the extended record provided by this ensemble of integrations allows a more robust investigation of the predictability of MJO activity than was possible with the 40‐year NCEP/NCAR Reanalysis. the results have shown that, for the uncoupled system, with the atmosphere being driven by imposed SSTs, there is no reproducibility of the activity of the MJO from year‐to‐year. the interannual behaviour of the MJO is not controlled by the phase of El Niño and would appear to be mainly chaotic in character. However, the model results have confirmed the low‐frequency, decadal time‐scale variability of MJO activity seen in the NCEP/NCAR Reanalysis. the activity of the MJO is consistently lower in all realizations prior to the mid‐1970s, suggesting that the MJO may become more active as tropical SSTs increase. This result may have implications for the effects of global warming on the coupled tropical atmosphere‐ocean system. Since the observed and simulated MJOs display clear seasonality in their occurrence, the relationship with interannual changes in the mean seasonal cycle of the tropical circulation has also been investigated. In contrast to the MJO, the interannual variability in the mean seasonal cycle is reproducible and influenced by the phase of El Niño. the implications of these results for the predictability of the tropical atmosphere‐ocean system are discussed, particularly with reference to the strong El Niño event of 1997 which developed in association with a period of intense MJO activity.