Concluding Remarks

It has been found that the set of inviscid normal modes of a two-localized-jet basic flow that broadly resembles the Pacific and Atlantic jets consists of three groups: single-jet modes, low-frequency modes and synoptic-frequency modes. The synoptic-frequency modes are most relevant to the disturbances in the model storm tracks jointly influenced by the two jets. The differential friction associated with the land and ocean sectors in the model is shown to have significant impact on the eigenvalues as well as the structure of the normal modes. It is noteworthy that some synoptic-frequency modes do not have a stronger localized magnitude downstream of the Atlantic jet. The relative intensity of the Pacific and Atlantic storm tracks therefore cannot be interpreted from the perspective of modal linear dynamics.

Nonlinear simulation of this forced flow produces a slightly more intense model Atlantic storm track with uniform friction. Seeding disturbances arise from wave-wave interactions among the major constituent wave components in a general flow field. Differential friction enhances the difference in the seeding disturbances, leading to an even greater difference in the relative intensity of the model storm tracks. We may conclude from this analysis that, given two pertinent localized jets as a proxy forcing, their locations and the related differential friction coefficients are the most important factors.

There are obvious limitations to the model results. For example, the model height variability has two regions of maximum values straddling each jet, with pronounced symmetric tilts against the shear of the background flow. Such features are intrinsic characteristics of barotropic unstable disturbances. This symmetry also reflects the constraint due to the use of ,3-plane approximation. In a spherical domain, wave disturbances would be dispersed preferentially toward the low latitudes. This study illustrates that the validity of the hypothesis for the relative intensity between the Pacific and Atlantic storm tracks can be demonstrated even in a barotropic model setting. While this is a significant contributing factor, it is unlikely to be the only important factor.

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