Use of Scale Interactions as a Framework for the Maintenance of the Madden Julian Oscillation

In this study, a scale interactions and energetics formulation is utilized to examine the kinetic (KE) and available potential energy (APE) maintenance of the Madden Julian Oscillation (MJO). An analysis that computes the energy exchange among frequencies of atmospheric phenomena, with the MJO as the centerpiece, is the primary focus of this study. The rotational and divergent components of the wind have been examined here, as these components have been found to elicit significant information regarding the overall atmospheric circulation, globally, in the tropics and in both hemispheric middle-latitude regions. The interactions among frequencies can occur both in-scale (quasi-nonlinear quadratic) and between scales (nonlinear triad interactions). A third examined component of the energy equations is the APE to KE, baroclinic exchange over specified frequencies. This is also of the in-scale type of exchanges. An analysis of 22 years of global, full-atmosphere (ECMWF) Re-Analysis data is undertaken, where the energetics of the MJO are computed for the global, tropics and hemispheric middle-latitudes. Specified frequencies with the time period of 3-7 days (synoptic), 30-60 days (MJO), 180 days (Semi-Annual), 365 days (Annual), 3-7 years (El Nino) and decadal (10 years and beyond) were the primary indices of MJO maintenance used within this study. These energetics were computed for the 300 hPa and 850 hPa levels of the atmosphere, for the entire 22 years, the 30 hPa maximum amplitude region of the Quasi-Biennial Oscillation, and for selected time periods during the ENSO episode lasting from 1997 to early 2001. The salient findings of this research are that the MJO is maintained within the upper and lower troposphere primarily by the baroclinic conversion of APE to KE through in-scale exchanges. The MJO is secondarily perpetuated, by the nonlinear scale interactions occurring among a triad of scales, that act to contribute positively to the MJO time-scale. Finally, the time mean-time transient buildup of KE on the MJO time-scale ranks third in the overall maintenance of the MJO. In the context of the KE exchanges, the rotational components have been illustrated to dominate those of the divergent form, which agrees with previous research indicating that the rotational component of the wind accounts for nearly 95 percent of the total horizontal wind. For the APE exchanges, the converse to the KE exchanges has been found, where the divergent components of the transfers typically is the prominent component among it and the rotational component. This is explained to occur as the divergent component of the wind typically has a stronger meridional component than the rotational wind, thus an increase in the horizontal temperature advection that comprises the APE equations is larger for the divergent component. Results in this study indicate that at 30 hPa, the QBO does not transfer KE to the MJO time-scale via triad interactions. Among the ENSO energetics computations, the maintenance of the MJO within the tropics during the neutral phase is dominated by the high frequency rotational component, whereas in the El Nino phase, the weak MJOs are losing energy most prominently in the tropics through the semi-annual time-scale. This result indicates that the nonlinear interactions of a triad of scales can offer one scale as the primary contributor of the MJO maintenance, while in a weak MJO phase, another unique scale can be leading the removal of KE from the MJO. For the ENSO event, geographic distributions of the triad exchanges of KE and APE components displayed strong positive contributions in latitudes poleward of 10 degrees, largely due to the stronger wind-speeds in these regions advecting the vorticity (rotational) and divergence (divergent) at comparatively larger magnitudes. The geographic distributions of KE and APE also made clear the strong spatial variability occurring among all time-scales.