LAUSR

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 re-analysis data. We focus on variability in the number of Kelvin wave cases as a function of the background westerly wind that controls the 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 the 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 to February which is the typical El Niño season. In the positive ZWI (stronger westerly) cases, both convective activity over the western Pacific and 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 the 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).