Large Scale Dynamics Associated with Super Cloud Cluster Organization over the Tropical Western Pacific

[摘要] Using a nonlinear primitive equation model of the tropical atmosphere, we have studied the large scale dynamics associated with the organization of super-cloud clusters (SCC) and the effect of nonlinear mean flow interaction and lateral forcing on the evolution of SCC and the Madden and Julian Oscillation (MJO) over the tropical western Pacific. It is found that the nonlinear interaction between convection and the large scale circulation associated with the organization of SCC can excite a variety of symmetric divergent motions along the equator and rotational motions away from the equator. The former can be identified as eastward propagating moist Kelvin waves and westward propagating inertia-gravity waves and the latter as Rossby waves. The interaction between the SCC and a basic flow induced by a northern wintertime heat source distribution gives rise to quasi-stationary modes over the western Pacific which may be identified with mixed Rossby-gravity wave. Westerly vertical wind shear over the western Pacific modifies the vertical tilt of the MJO and favors its growth.During the northern winter, cold surges from the East Asian continent exert strong control on the development of SCC in the equatorial regions by inducing pressure differential across the maritime continent and the western Pacific, leading to enhancement of surface wind convergence. When the influence of the subsidence motion associated with a pre-existing SCC/MJO is weakened, the cold surge-induced pressure anomaly can lead to the development of new SCC with intermediate time scales. The time interval between intermediate SCCs is about 8-10 days which is approximately the transit time of the MJO across the span of the warm pool of the western Pacific.During the northern summer, the SCCs in the equatorial region are much less organized with mixed eastward and westward propagating signals. In the northern subtropics (15°N-20°N), westward propagating synoptic scale waves are found in regions with strong westerly shear. These waves grow by latent heating as well as by extracting wave energy from the mean westerly vertical shear in the lower troposphere. Overall, the results suggest that the organization and location of SCC and the associated multi-scale motions are strongly dependent on the evolving seasonal mean flow.

[学科分类] 大气科学