9:15 AM - 9:30 AM
[AAS03-02] Horizontal scale and propagation speed selection of the MJO-like disturbances in a zonally asymmetric aquaplanet
★Invited Papers
Keywords:Madden–Julian oscillation, Resolution dependency
The Madden–Julian oscillation (MJO) is observed as the slow (3–8 m/s) eastward propagation of O(103 km)-scale convective envelopes over the warm pool. The MJO is sometimes argued as the destabilization of the moisture mode and/or Kelvin–Rossby wave couplet dynamics, and these theories try to explain the MJO horizontal scale and propagation speed selection using a linear stability analysis. However, we have yet to reach consensus on it because of a lack of observational justification of parameters and assumptions in the proposed theoretical models. To address this issue simply but without losing complexity in moist processes and wave dynamics, we statistically scrutinize MJO-like disturbances in a zonally asymmetric aquaplanet simulated with the Nonhydrostatic ICosahedral Atmospheric Model (NICAM). About 5-year simulations at four horizontal resolutions (dx=110, 56, 28, and 14 km) with explicit convection are conducted, and all of them reproduce MJO-like disturbances.
Results suggest that the horizontal scale and propagation speed of MJO-like disturbances are distinctively increased in dx=28-km and 14-km simulations compared to the others. This resolution dependency, which could be related to how predominant the moisture-convection feedback is, raises a hypothesis that the MJO horizontal scale is selected via the competition between two zonal scales: (1) advection length scale (Ladv) corresponding to a timescale on which upper-tropospheric divergent flows subside to convergence altitudes; and (2) wave-propagation length scale (Lwave) corresponding to a timescale on which gravity waves excited by convection are thermally damped. Lwave represents the zonal scale of upper (lower)-tropospheric warm (cold) anomalies associated with high-order baroclinic waves, which can support the upper-cloud formation by local static instability and/or excite deep convection. In fact, the scale expansion of MJO-like disturbances found in dx=28-km and 14-km simulations is well explained by increased Lwave (> Ladv) due to reduced radiative cooling, and this scale selection is realized by the cloud-radiation interaction. We also find that the propagation speed of MJO-like disturbances is positively correlated with the ratio of spectral power for eastward- to westward-propagating synoptic-scale disturbances, suggesting the importance of wave activities embedded in the MJO to the MJO propagation.
Results suggest that the horizontal scale and propagation speed of MJO-like disturbances are distinctively increased in dx=28-km and 14-km simulations compared to the others. This resolution dependency, which could be related to how predominant the moisture-convection feedback is, raises a hypothesis that the MJO horizontal scale is selected via the competition between two zonal scales: (1) advection length scale (Ladv) corresponding to a timescale on which upper-tropospheric divergent flows subside to convergence altitudes; and (2) wave-propagation length scale (Lwave) corresponding to a timescale on which gravity waves excited by convection are thermally damped. Lwave represents the zonal scale of upper (lower)-tropospheric warm (cold) anomalies associated with high-order baroclinic waves, which can support the upper-cloud formation by local static instability and/or excite deep convection. In fact, the scale expansion of MJO-like disturbances found in dx=28-km and 14-km simulations is well explained by increased Lwave (> Ladv) due to reduced radiative cooling, and this scale selection is realized by the cloud-radiation interaction. We also find that the propagation speed of MJO-like disturbances is positively correlated with the ratio of spectral power for eastward- to westward-propagating synoptic-scale disturbances, suggesting the importance of wave activities embedded in the MJO to the MJO propagation.