Seasonal and interannual variation of equatorial waves in ERA5 and GloSea5

This study presents an analysis of the equatorial waves in the ERA5 reanalysis and forecasts of the Met Office Global Seasonal Forecast System version 5 (GloSea5), by projecting dynamical fields onto theoretical equatorial wave modes. The seasonal and interannual variation of equatorial wave activity in GloSea5 is evaluated against ERA5 waves. We find that in ERA5 the seasonal and spatial variations of eastward-moving Kelvin wave activity are mainly determined by the ambient zonal flow, with wave activity being stronger in easterlies than westerlies. For the westward-moving mixed Rossby-gravity waves and equatorial Rossby waves, the seasonal and spatial variations of the upper-tropospheric wave activity are determined by both the ambient flow and extratropical forcing with stronger wave activity generally in the westerlies, whilst the lower-tropospheric wave activity is related to the ambient flow and tropical convection. In GloSea5, the Kelvin wave activity is not well simulated in the upper troposphere, and in general the wave activity and wave-related convective signal are too weak. In contrast, GloSea5 performs better for the westward-moving equatorial waves, including their Doppler-shifted eastward-moving components, despite a significant overestimation in the Atlantic and African region. The phase of ENSO has a substantial impact on equatorial waves in both ERA5 and GloSea5. The mechanism of the impact is through changes in the ENSO-related ambient flow, upper-tropospheric extratropical forcing, and tropical convection. GloSea5 can capture the low-level Kelvin wave-ENSO relationship reasonably well, with stronger wave activity over the eastern Pacific in El Nino, although the ENSO-driven variation is too weak. The wave-ENSO relationship for westward-moving waves is well represented in GloSea5, especially in the upper-level eastern-Pacific westerly duct region in the extended boreal winter and the low-level Indo-Pacific in all seasons. We conclude that in GloSea5 the representation of the wave-ENSO teleconnection is the key driver for the performance of interannual variability in the wave activity.