Dispersion Characteristics and Circulation Associated with Boreal Summer Westward-Traveling Mixed Rossby–Gravity Wave–Like Disturbances

Abstract

Abstract An algorithm has been developed to track synoptic-scale, westward-traveling 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 structures stated in the solutions of shallow-water equations (SWEs). Based on 850-hPa 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–southwest-tilted eddies that propagate northwestward. Examination of their energetics suggests that such a tilting structure is favorable to the maintenance of these transients because of the meridional shear of background zonal wind west of ~140°E in the 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 because of barotropic conversion. Finally, there is evidence of downward energy dispersion in the mid- to upper levels, suggesting that the upper-level wave activity might be important in triggering these low-level waves in the Pacific basin.