Intrinsic Predictability Limits arising from Indian Ocean MJO Heating: Effects on tropical and extratropical teleconnections

Abstract. Since the Madden-Julian Oscillation (MJO) is a major source for tropical and extratropical variability on weekly to monthly timescales, the intrinsic predictability of its global teleconnections is of great interest. As the tropical diabatic heating associated with the MJO ultimately drives these teleconnections, the effect of the variability of heating among various episodes of the same MJO phase will limit this predictability. In order to assess this limitation, a suite of 60-day ensemble reforecasts has been carried out with the ECMWF forecast model, spanning 13 starting dates from 01 Nov and 01 Jan for different years. The initial dates were chosen so that phases 2 and 3 of the MJO (with anomalous tropical heating in the Indian Ocean sector) were present in the observed initial conditions. The 51 members of an individual ensemble use identical initial conditions for the atmosphere and ocean. Stochastic perturbations to the tendencies produced by the atmospheric physics parameterizations are applied only over the Indian Ocean region. This guarantees that the spread between reforecasts within an ensemble is due to perturbations in heat sources only in the Indian Ocean sector. The point-wise spread in the intra-ensemble (or error) variance of vertically integrated tropical heating Q is larger than the average ensemble mean signal even at early forecast times; however the planetary wave component of Q (zonal waves 1–3) is predictable for 24 days for 01 Nov starts and 28 days for 01 Jan starts. The predictability times, measured by the time at which the error variance reaches 0.5 of its saturation value, decreases to 18–20 days for zonal waves 4–10, and 14 days for waves 11–21. In contrast, the planetary wave component of the 200 hPa Rossby wave source, which is responsible for propagating the influence of tropical heating to the extratropics, is only predictable for 14 to 19 days, very close to the predictability times for the 200 hPa vorticity in the 40° N–50° N latitude belt. In terms of geographical distribution, substantial ensemble spread of heating and 200 hPa vorticity propagates from the tropics to the Northern Hemisphere storm-track regions by days 15–16. Following the growth of upper tropospheric spread in planetary wave heat flux, the stratosphere provides a feedback in enhancing the error via downward propagation towards the end of both Nov. and Jan. reforecasts.