On the predictability of the interannual behaviour of the Madden-Julian Oscillation and its relationship with El Nino

The Madden-Julian Oscillation (MJO) is the dominant mode of tropical variab ility at intraseasonal timescales. It displays substantial interannual vari ability in intensity which may have important implications for the predicta bility of the coupled system. The reasons for this interannual variability are not understood. The aim of this paper is to investigate whether the int erannual behaviour of the MJO is related to tropical sea surface temperatur e (SST) anomalies, particularly El Nino, and hence whether it is predictabl e. The interannual behaviour of the MJO has been diagnosed initially in the 40 -year National Centers for Environmental Prediction/National Center for Atm ospheric Research (NCEP/NCAR) Reanalysis. The results suggest that prior to the mid-1970s the activity of the MJO was consistently lower than during t he latter part of the record. This may be related either to inadequacies in the data coverage, particularly over the tropical Indian Ocean prior to th e introduction of satellite observations, or to the real effects of a decad al time-scale warming in the tropical SSTs. The teleconnection patterns bet ween interannual variations in MJO activity and SST show only a weak, barel y significant, influence of El Nino 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 integra tions with the Hadley Centre climate model (HadAM2a), forced by observed SS Ts for 1949-93, has been used to investigate the relationship between MJO a ctivity and SST. HadAM2a is known to give a reasonable simulation of the MJ O, 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 tha t, for the uncoupled system, with the atmosphere being driven by imposed SS Ts, there is no reproducibility of the activity of the MJO from year-to-yea r. The interannual behaviour of the MJO is not controlled by the phase of E l Nino and would appear to be mainly chaotic in character. However, the mod el 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, suggestin g that the MJO may become more active as tropical SSTs increase. This resul t may have implications for the effects of global warming on the coupled tr opical atmosphere-ocean system. Since the observed and simulated MJOs display clear seasonality in their oc currence, the relationship with interannual changes in the mean seasonal cy cle of the tropical circulation has also been investigated. In contrast to the MJO, the interannual variability in the mean seasonal cycle is reproduc ible and influenced by the phase of El Nino. The implications of these resu lts for the predictability of the tropical atmosphere-ocean system are disc ussed, particularly with reference to the strong El Nino event of 1997 whic h developed in association with a period of intense MJO activity.