Abstract
AbstractMotivated by the previous case study, this work shows that dynamical variations of mixed Rossby–gravity waves with tropical depression–type circulations (MRGTDs) are possible drivers of convective initiation and propagation of the Madden–Julian oscillation (MJO) by performing statistical analysis. MJO events initiated in the Indian Ocean (IO) in boreal winter are objectively identified solely using outgoing longwave radiation data. The lagged-composite analysis of detected MJO events demonstrates that MJO convection is initiated in the southwestern IO (SWIO), where strong MRGTD–convection coupling is statistically found. Further classification of MJO cases in terms of intraseasonal convection and MRGTD activities in the SWIO suggests that 26 of 47 cases are related to more enhanced MRGTDs, although they can also be secondarily affected by Kelvin waves. For those MRGTD-enhanced MJO events, MJO convective initiation is primarily triggered by low-level MRGTD circulations that develop via the enhancement of downward energy dispersion in accordance with upper-tropospheric baroclinic conversion. This is supported by the modulation of MRGTD structure associated with zonal wave contraction due to upper-tropospheric zonal convergence, and plentiful moisture advected into the western IO. Following this MRGTD-induced MJO triggering and midtropospheric premoistening in the IO contributed by MRGTD shallow circulations as well as intraseasonal winds during the MJO-suppressed phase, low-level MRGTD winds with eastward group velocity successively trigger convection to the east, which helps MJO convective propagation over the IO. The interannual atmospheric variability may affect whether the presented MRGTD-related processes are effective or not.
Highlights
The Madden–Julian oscillation (MJO) (Madden and Julian 1971, 1972) is known to be the most prominent convective disturbances in the tropics
We show that the MJO cases triggered by mixed Rossby–gravity waves with tropical depression–type circulations (MRGTDs) in the southwestern IO (SWIO) are not rare; for those cases, we clarify the robust contributions of MRGTD dynamics to dynamical and moistening processes that lead to MJO convective initiation and propagation over the Indian Ocean (IO) by applying lagged-composite and regression analyses
Motivated by a TSY19 case study, which discovered that mixed Rossby–gravity (MRG) can drive MJO convective initiation and propagation in the IO, we examined the generality of the interaction between observed MRGTDs and MJO during the boreal winters in 1982–2012
Summary
The Madden–Julian oscillation (MJO) (Madden and Julian 1971, 1972) is known to be the most prominent convective disturbances in the tropics. Yasunaga (2011) and Dias et al (2017) suggest that MRG amplitudes are not significantly higher within MJO active convective envelopes than other wave disturbances, their analyses do not focus on our target region and timing To examine this question, we computed the variance of OLR filtered for other typical waves [Kelvin wave (Kelvin), n 5 1 equatorial Rossby wave (ER), n 5 0 eastward and n 5 1 westward inertia–gravity wave (EIG and WIG)] with FFT, and compared them with the MRGTD-filtered OLR variance at the initiation phase of the MJO. As found by TSY19, the wave packet formation of lower-tropospheric MRGTDs precedes the center of moist anomalies and active convection associated with the MJO This suggests that low-level MRGTD wind variations could help drive MJO propagation over the IO through convective triggering in a sufficiently moistened environment. To evaluate the processes that can develop MRGTD dynamical fields before the MJO initiation of MRGTD-E, we conducted MRGTD-related EKE budget analysis with the following equation:
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