Differences in Convection Following Initiation

Fig. 3: As in Fig. 2, but at ten days after convective initiation.

Preferred ENSO Phase within Each Composite

Fig. 1: Histogram of ONI (°C) for (a) real MJOs and (b) failed MJOs. The sample size, mean, and standard deviation of each subset are provided in the top left corner of each histogram.

Role of Equatorial Waves in Evolution of Convection

Fig. 4: Longitude-time plot of OLR anomalies (W m-2; shaded) , MJO filtered OLR anomalies (W m-2; black contours), Kelvin wave filtered OLR anomalies (W m-2; red contours), and Equatorial Rossby wave filtered OLR anomalies (W m-2; blue contours) for composites of (a) real MJOs and (b) failed MJOs relative to the time of convective initiation. Filtered anomaly contours are plotted at 2 W m-2 intervals beginning at 2 W m-2. The OLR anomalies are meridionally averaged from 7.5°S and 7.5°N.

A Comparison of MJO and Non-MJO Convective Initiation EventsBenjamin A. Schenkel (benschenkel@gmail.com), John G. Dwyer, Robert B. Seigel, Kyle MacRitchie

2012 NCAR Advanced Study Program Summer Colloquium

IntroductionOf the approximately four strong convective initiation events that occur annually within the Indian Ocean, nearly half of the events are associated with the Madden-Julian Oscillation. (MJO). Which factors determine whether convection remains as disorganized clusters or develops into an MJO remain undetermined. Understanding whether tropical convection will organize into an MJO is crucial towards furthering our ability to model the general circulation given the influence of the MJO on a broad range of spatiotemporal scales including the modulation of the El Niño-Southern Oscillation (ENSO; e.g., Keen 1982), generation of mid-latitude Rossby wave trains (e.g., Sardeshmukh and Hoskins 1988), and creation of a more favorable environment for tropical cyclogenesis (e.g., Liebmann 1994). The following study seeks to provide greater insight into which processes are important in dictating whether tropical convection within the Indian Ocean develops into a strong MJO event.

Data and MethodsIn this study we compare MJO-triggered and non-MJO-triggered strong convective events in the Indian Ocean to gain insight into MJO genesis. Using NOAA Outgoing Longwave Radiation (OLR) data (Liebmann and Smith 1996) as a proxy for deep convection, we separate strong convective initiation events in the Indian Ocean from October to March during 1982 to 2009 into real MJO events (N = 52 events) and failed MJO events (N = 49 events). We define a real MJO event as when normalized OLR anomalies filtered in the MJO wavenumber-frequency spectra (e.g., Wheeler and Kiladis 1999) are anomalously strong (OLR ≤ -1σ) for ten consecutive days. Strong non-MJO convective initiation events occur when normalized OLR anomalies switch from anomalously weak convection (OLR 1σ) to anomalously strong convection for ten consecutive days. The previously identified MJO events are then removed from the list of dates such that the times only consist of non-MJO events. In the following study, we define convective initiation as when OLR anomalies first drop below -1σ. Using the convective events identified from OLR data, composites of raw and normalized anomalies are constructed relative to the time of convective initiation for MJO and non-MJO events from the NCEP Climate Forecast System Reanalysis (CFSR; Saha et al. 2010). Anomalies are computed relative to an evolving 6-hr climatology from 1982 to 2009. Lastly, the Oceanic Niño Index (ONI; available at http://www.cpc.ncep.noaa.gov), which provides a measure of ENSO-3.4 SST anomalies, is used to quantify the impact of ENSO on the composites.

Large Scale Similarities at Convective Initiation

Fig. 2: Plan view of normalized (contoured every 0.25σ from -1.0σ to 1.0σ excluding 0.0σ) and raw OLR anomalies (W m-2; shaded) two days after convective initiation for (a) real MJOs and (b) failed MJOs.

Refining the Role of Prior MJOs within the Composites

Fig. 5: Longitude-time plot of 850 hPa MJO filtered (gpm; contoured at same values as the shaded anomalies) and raw geopotential height anomalies (gpm; shaded) for composites of (a) real MJOs and (b) failed MJOs relative to the time of convective initiation. The geopotential height anomalies are meridionally averaged from 7.5°S and 7.5°N.

DiscussionOur two sets of composites initially exhibit some similarities with enhanced convection over the Indian Ocean and suppressed convection over the Western Pacific (Fig. 2). The discrepancies between the composites become more pronounced as convection gradually dissipates from west to east along the equator (Fig. 3b) in the failed MJOs while the real MJOs exhibit an expansion in the areal extent of convection (Fig. 3a). Filtering the OLR anomalies reveals that convection is likely triggered by a prior MJO event in the real MJO composite (Fig. 4a) . In contrast, convection in the failed MJOs appears to be initiated by the juxtaposition of a convectively active Kelvin wave and Equatorial Rossby wave (Fig. 4b) which may partially explain why the life cycle of the failed MJO is shorter. Further insight into the evolution of convection is provided in the lower troposphere geopotential heights which more distinctly show the propagation of a convectively suppressed MJO into the Indian Ocean at the time of convective dissipation for failed MJOs (Fig. 5b). Although the suppressed MJO is relatively weak, it appears to be partially responsible, along with El Niño (Fig. 1b), for creating a hostile environment for convection for failed MJOs due to anomalous subsidence over the Indian Ocean and Western Pacific. The arrival of the suppressed MJO in the failed composites may explain why convection dissipates in failed MJOs from west to east in the Indian Ocean (Fig. 6). In their totality, these results may suggest that without a prior convectively active MJO, strong convective events that occur during El Niño may be unable to organize into an convectively active MJO event due to anomalous subsidence to the west from a convectively suppressed MJO and to the east from El Niño (Fig. 7b).

Acknowledgmentsand References

This research began at the 2012 NCAR ASP Summer Colloquium with continued funding from NCAR to present this work at AGU. Part of our travel is also sponsored by the FSU Congress of Graduate Students and Robert Hart (FSU). The authors would like to thank Mitch Moncrieff (NCAR), George Kiladis (ESRL), Lance Bosart (SUNY-Albany), Chidong Zhang (RSMAS), and Klaus Weickmann (ESRL) for fruitful discussions. We also acknowledge the FSU HPC for contributions to results presented in this paper.------------------------------------------------------------------------------------------------------------------

Liebmann, B., H. Hendon, and J. Glick, 1994: The Relationship Between Tropical Cyclones of the Western Pacific and Indian Oceans and the MJO. J. Meteor. Soc. Japan, 72, 401-412.Liebmann, B., and C.A. Smith, 1996: Description of a Complete (Interpolated) OLR Dataset. BAMS, 77, 1275-1277.Keen, R., 1982: The Role of Cross-Equatorial Tropical Cyclone Pairs in the Southern Oscillation. MWR, 110, 1405-1415.Saha, S. and Coauthors, 2010: The NCEP Climate Forecast System Reanalysis. Bull. Amer. Meteor. Soc., 91, 1015-1057.Sardeshmukh, P. and B. Hoskins, 1988: The Generation of Global Rotational Flow by Steady Idealized Tropical Divergence. J. Atmos. Sci., 45, 1228-1251.Wheeler, M. and G. Kiladis, 1999: Convectively Coupled Equatorial Waves: Analysis of Clouds and Temperature in the Wavenumber-Frequency Domain. J. Atmos. Sci., 56, 374-399.

Demise of Failed MJOs due to Prior MJO?

Fig. 6: Plan view of 850 hPa normalized (contoured every 0.25σ from -1.0σ to 1.0σ excluding 0.0σ) and raw 850 hPa geopotential height anomalies (gpm; shaded) for failed MJO composites (a) five days prior, (b) five days after, and (c) seven days after convective initiation.

Conceptual Model of Real and Failed MJOs

Fig. 7: Hypothesized conceptual model of (a) real MJOs and (b) failed MJOs as the convective envelope approaches the Maritime Continent synthesized from the composites. Green shading denotes anomalously moist areas while orange shading denotes anomalously dry regions.

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OLR Hovmöller for Real MJOs OLR Hovmöller for Failed MJOsa)

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