1173 / 2019-08-05 11:35:01

Mechanism of the Northward Propagating Intraseasonal Oscillation: Insights from the Analytical Dynamic Model

9、大气科学与全球变化 > 专题1：大气环流对气候的影响

The northward propagating intraseasonal oscillation (NPISO) accounts for the major intraseasonal variability of the tropical atmosphere during boreal summer, which behaves closely associated with the Indian summer monsoon. However, the previous mechanisms related to the NPISO are still in debate. In our research, the NPISO and its process are attributed to the intraseasonal atmospheric dynamics modified by the background field. Such analytical dynamic model can capture major features of the NPISO in both the physical domain and the frequency-wavenumber spectrum domain. The interaction between the meridional shear of the vertical velocity at the intraseasonal scale and the mean easterly shear is expected to contribute to the NPISO from our analytical model. The positive vertical velocity shear in the north of the convection is favor to generate a new positive vorticity under the mean easterly shear. In the kinetic energy budget of the NPISO, the major balance of the generated vorticity is between gain from the potential energy (PE) at the same timescale and the work done by the pressure gradient force, which serves to move PE away from the region of convection to the generated vorticity. Consequently, such mechanism conduces the new convection development in the north. In addition, the seasonal mean position of ITCZ can regulate the mean easterly shear. However, the detailed effects of ITCZ can’t be reflected limited by the analytical model. Nonetheless, mechanisms of NPISO are expected to advance the understanding of the large-scale and lower-frequency modes interactions.