18 October 2019

Equatorial Windows and Barriers for Stationary Rossby Wave Propagation

Science

The eastern Pacific and Atlantic regions are identified as common windows in both boreal winter and summer, while the Africa–Indian Ocean section exists as a window only in boreal summer. The western–central Pacific is found to be a barrier section. These results are consistent with correlation analysis of reanalysis data. Moreover, the dependence on the wavenumber of the cross-equatorial waves (CEWs) is investigated, revealing that they are restricted to long waves with zonal wavenumbers less than 6 and that their wavenumber vectors exhibit a northwest-southeast (southwest-northeast) tilt when they cross the equator from the NH to SH (from the SH to NH). This long-wave dominance of CEWs results from the spectral-selective filtering mechanism, which suggests that long waves have narrower equatorial barriers than short waves. Finally, the main wave duct associated with each window is obtained by the global passing CEW density distribution. The results indicate that the main CEW ducts roughly follow a great circle–like pathway, except for the Africa–Indian Ocean window in boreal summer, which may be modulated by the cross-equatorial monsoonal flow.

Impact

The eastern Pacific and Atlantic sections are revealed as common windows in both DJFM and JJAS, while the

Africa–Indian Ocean section appears as a window in JJAS. The western–central Pacific section is found as a barrier in both seasons by blocking most of the waves. These findings from the WRE results are supported by lead-lag correlation analyses, which present clear wave trains through the window sections and few signals near the barrier sections. In addition, the windows of boreal winter have basically equal CEW numbers for the two directions, while the boreal summer windows are dominated by NH-to-SH CEWs. The directionality in boreal summer is consistent with the clearer wavelike correlations in the SH for NH base points, while there are fewer correlations in the NH for SH base points.

 

Summary

To identify the equatorial windows and barriers for CEWs, we develop the WRE method based on the wave ray tracing method under horizontally nonuniform basic flow. Relative to the wave ray tracing method, the WRE method can quantify the propagation properties of Rossby waves based on the statistics of a large ensemble of rays. Hence, we can obtain much more detail on CEWs than in previous studies. Using the WRE, this study presents the location, seasonality, and directionality of the CEW windows and barriers. The windows can be categorized into ‘‘westerly windows’’ and ‘‘cross-equatorial-flow windows’’ based on the background flow according to previous studies. We propose the spectral-selective filtering mechanism as a uniform dynamical mechanism for the two types of windows. Here, the spectral selective filtering mechanism acts as a mutual selection between the background flow and the CEWs.

Contact
Aixue Hu
National Center for Atmospheric Research (NCAR)
Publications
Li, Y, J Feng, J Li, and A Hu.  2019.  "Equatorial Windows and Barriers for Stationary Rossby Wave Propagation."  Journal of Climate 32(18): 6117-6135.  https://doi.org/10.1175/jcli-d-18-0722.1.