AbstractAn algorithm has been developed to track synoptic-scale, westward travelling mixed-Rossby gravity (MRG) wave-like disturbances with a cross equatorial component. Applied to space-time filtered, meridional wind data, this algorithm finds locations with Gaussian-shaped wind structure stated in the solutions of shallow water equations (SWE). Based on 850hPa meridional wind from the global National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) dataset, local and instantaneous wave properties including the occurrence time, wavenumber, intrinsic frequency and magnitude were examined. It was found that these low-level MRG-wave like disturbances can be classified into a longer wavelength group and a shorter wavelength group. While most waves identified in the eastern Pacific give longer wavelengths, disturbances in the western Pacific tend to have a wider range of wavenumbers. Composite analysis revealed that, east of ~140°E, low-level disturbances are characterized by cross-equatorial wind anomalies with alternating signs, thus consistent with the MRG wave solution. West of ~140°E, they appear as northeast-to-southwest tilted eddies that propagate northwestward. Examination of their energetics suggests that such a tilting structure is favorable to the maintenance of these transients, due to the meridional shear of background zonal wind west of ~140°E in off-equatorial Pacific. Farther east, the confluent nature of the low-level background flow plays a dominant role in maintaining the MRG-wave like disturbances due to barotropic conversion. Finally, there is evidence of downward energy dispersion in the mid to upper levels suggesting that that upper-level wave activity might be important in triggering these low-level waves in the Pacific basin.
Journal of the Atmospheric Sciences – American Meteorological Society
Published: Dec 4, 2017
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