Abstract
Abstract We investigate the influence of the background wind regime on interannual variability in equatorial Kelvin waves (including both freely propagating ones and convectively coupled ones) in the upper troposphere and lower stratosphere using the ERA5 reanalysis data. We focus on variability in the number of Kelvin wave cases as a function of the background westerly wind that controls wave propagation, given by the zonal wind index (ZWI) in the equatorial eastern Pacific that is a measure of the strength of the upper branch of the Walker circulation in the Western Hemisphere. The ZWI correlates well with sea surface temperature in the Niño-3.4 region, though one-third of the peaks of negative ZWI (weak westerly) cases occur during seasons other than December–February which is the typical El Niño season. In the positive ZWI (stronger westerly) cases, both the convective activity over the western Pacific and the extratropical Rossby wave activity over the central and eastern Pacific are enhanced. Kelvin waves over the Western Hemisphere appear frequently at 200 hPa but barely reach 100 hPa due to the strong westerly wind below this level that prohibits upward wave propagation. In the negative ZWI period, the number of Kelvin wave cases at 200 hPa decreases due to weaker convection; Kelvin waves reach 100 hPa and propagate even further upward into the stratosphere. The increasing (decreasing) tendency of Kelvin waves already starts 6 months before the weakest (strongest) westerly wind month in the Western Hemisphere (i.e., the ENSO peak seasons). Significance Statement Equatorial Kelvin waves play a critical role in the upward transport of momentum driving the equatorial stratospheric circulation. Our results reveal that background winds in the upper troposphere and the lower stratosphere act as a filter for equatorial Kelvin waves, regulating the number of their packets propagating upward. An investigation of the role of Kelvin wave dissipation in the westerly duct region, i.e., in the upper troposphere over the equatorial eastern Pacific, is needed.
Published Version
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