1. MJO Initiation Katherine H. Straub, Associate Professor of Earth and Environmental Sciences Susquehanna University, Selinsgrove, PA INTRODUCTION Since Madden and Julian's pioneering studies in the early 1970's (Madden and Julian 1971, 1972), the Madden-Julian Oscillation (MJO) has been analyzed via a dizzying array of unique methods, resulting in a seemingly countless number of MJO indices. The fact that new indices are still being developed 40 years later suggests that we as a community have not yet agreed on the defining characteristics of the MJO. Not knowing whether a particular convective or dynamical sequence truly represents the MJO makes identifying MJO initiation even more difficult. Perhaps for this reason, very few observational studies of MJO initiation exist. The purpose of the present study is to investigate the impact of MJO index on the identification of MJO events, focusing on the signals preceding MJO initiation. CONCLUSIONS Slowly-evolving MJO-like precursor signals precede “primary” MJO initiation, often for at least 10-20 days. These precursor signals are typically constrained to the Eastern Hemisphere, and thus do not project onto the primarily zonal wavenumber 1 RMM index. Both primary (without a prior MJO signal) and successive (with a prior MJO signal) composites show a buildup of warm, moist air over Africa, 850 (200) hPa easterlies (westerlies) over the Indian Ocean, and suppressed deep convection over the Maritime Continent and western Pacific for 10+ days prior to entering RMM phase 1. Composites of MJO initiation in other RMM phases show similar results in terms of the similarity between primary and successive events, particularly the existence of precursor dynamical and thermodynamical signals. EXAMPLE: WHEN DOES THE FIRST TOGA-COARE MJO EVENT BEGIN? Figures 1-3 are all plots of MJO activity during the TOGA-COARE period, using 3 different MJO indices. The date of MJO initiation, the strength of the MJO, and even the existence of an MJO at a given time varies from index to index. Figures 1 and 2 are identical time-longitude diagrams of 100-day highpass filtered outgoing longwave radiation (OLR), averaged from 15N-15S. In Figure 1, the overlying contours represent MJO wavenumber-frequency filtered OLR (deep convection), as in Wheeler et al. (2000). In Figure 2, the contours represent the Chen-Del Genio (2009) MJO index, a dynamical index based on upper tropospheric velocity potential. Figure 3 is the Wheeler and Hendon (2004) Realtime Multivariate MJO (RMM) index for 1 Dec 1992 – 15 Feb 1993. The RMM index is a combined measure of MJO convection and circulation. When the RMM index amplitude is > 1.0 and the index progresses in a counterclockwise direction, a significant MJO event occurs. Question: When does the TOGA-COARE MJO event begin? Fig. 1 (left). 100-day highpass filtered OLR (shading) and MJO-filtered OLR (contours, by ±10 W m -2 ) from 1 Oct 1992 – 31 Mar 1993. Fig. 2 (middle). As in Fig. 1 but contours represent the Chen-Del Genio MJO index (by ±1 SD). Fig. 3 (right). RMM index for 1 Dec 1992 – 15 Feb 1993. Each dot represents one day; every fifth day is labeled, and color-coded months are at bottom. The RMM index passes the 1.0 threshold on 22 December 1992. MJO INITIATION: DEFINITIONS The RMM index is used to define MJO initiation in 3 ways: 1.Primary initiation (Fig. 5): When the PC amplitude shows no counterclockwise propagation for at least 7 days at an average amplitude <1, the amplitude then increases to ≥1, where it remains for at least 7 days and subsequently progresses through at least 4 phases, or half of a complete MJO cycle, in the counterclockwise direction. The date of primary MJO initiation is the first date on which the PC amplitude is ≥1; the phase of initiation is defined on this same date. 2.Intensification (Fig. 6): Like primary initiation, except that the PC amplitude does show significant counterclockwise propagation prior to the amplitude increase. 3.Successive event (Fig. 7): Defined by the date on which the RMM index enters a particular phase with an amplitude >1, having propagated through at least 2 prior phases with an amplitude >1, and subsequently propagating through 2 successive phases with an amplitude >1. COMPOSITE PLOTS: FULL RMM INDEX Figures 8-13 (below) are time-longitude plots of composite primary MJO initiation events (Figs. 8-10) and successive events (Figs. 11-13) in RMM phase 1. Note the similarities not only after day 0, when the RMM index evolution should be similar, but also before day 0, particularly in the Eastern Hemisphere. Because these anomalies are not zonal wavenumber 1 in scale, they do not project strongly onto the RMM index, and thus no preexisting MJO signal is apparent. THE RMM INDEX The RMM index is derived from a multivariate empirical orthogonal function (EOF) analysis of 15N-15S averaged OLR and 850- and 200-hPa zonal winds, and thus represents a combined convection-circulation index. The first two combined EOFs represent the eastward-propagating MJO cycle, and are shown in Fig. 4. The projection of the raw data onto these EOFs is represented by the PC time series of EOFs 1 and 2 as plotted in the style of Fig. 3. Note the zonal wavenumber 1 structure of all variables, and the out-of-phase relationship between U850 and U200. Fig. 4. RMM EOFs 1 and 2 (Wheeler and Hendon 2004). Figs. 5-7. Example RMM plots for phase 1 primary initiation (left), intensification (middle), and successive events (right). Primary initiation Intensification Successive event Figs. 8-13. Composite OLR (left), U850 (middle), and U200 (right) for primary initiation (top row, based on 7 events) and successive events (bottom row, based on 47 events). ACKNOWLEDGEMENTS Many thanks to Prof. Richard Johnson and the Department of Atmospheric Science at Colorado State University, who hosted my recent sabbatical. COMPOSITE PLOTS: OLR-ONLY INDEX MJO convection is often assumed to precede the development of MJO circulation signals. If MJO events are identified using an OLR-only index, generated by projecting only the raw OLR data onto the OLR RMM EOF (solid line in Fig. 4), the propagating dynamical precursor signals are even stronger prior to day 0 than in the full RMM fields, as shown in Figs. 14-16. In individual cases, a circulation-only MJO event often develops well before the onset of significant MJO-like OLR anomalies. COMPOSITE PLOTS: MOISTURE AND TEMPERATURE In primary MJO initiation cases derived from the full RMM index, predecessor moisture and temperature fields are also very similar to those in successive events. Most striking is a buildup of warm and moist air over Africa starting on day -10, as shown in Figs. 17- 19. Successive composites are not shown, but similarities are noted. Figs. 14-16. Composite OLR (left), U850 (middle), and U200 (right) for primary initiation based on the OLR-only index. W m -2 Time Fig. 8: OLR Fig. 9: U850Fig. 10: U200 Fig. 12: U850Fig. 13: U200 Fig. 5 Fig. 6Fig. 7 Figs. 17-19. Composite precipitable water (left), 1000-hPa temperature (middle), and 1000-hPa specific humidity (right) for primary initiation cases based on the full RMM index. Fig. 17: PW Fig. 18: T1000 Fig. 19: Q1000 Fig. 11: OLR Suppressed deep convection in W. Pac Suppressed deep convection in W. Pac Easterly anomalies in IO Westerly anomalies in IO Suppressed deep convection in W. Pac Easterly/westerly U850 anomalies Westerly/easterly U200 anomalies Buildup of high PW over Africa; dry over IO, W. Pac Warm anomaly over Africa Fig. 15: U850Fig. 16: U200Fig. 14: OLR Moist anomaly over Africa Propagating cold anomaly Propagating dry anomaly Fig. 1Fig. 2 Fig. 3