Group seminar on 6. June, 14:15 CET

Seasonal cycle and ENSO-related variation of equatorial waves in ERA5 and GloSea5

Dr. Gui-Ying Yang

Equatorially trapped waves are fundamental components of the tropical atmosphere. They are closely related to fundamental phenomena of the tropical climate and weather. Understanding equatorial wave activity is key to understanding tropical atmospheric variability. Evaluating model ability to represent equatorial waves is hence crucial for improving weather forecast in the Tropics beyond a few days ahead. For sub-seasonal to seasonal forecast, it is important to evaluate model ability to represent the seasonal cycle of equatorial wave activity and its ENSO-related variation. 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. To understand/interpret the behaviour of the equatorial wave activity some theoretical mechanisms of ambient zonal flows impacting the occurrence and behaviour of equatorial waves are discussed. It shows that the observed seasonal cycle and ENSO-related variation of equatorial wave activity are mainly determined by the tropical ambient zonal flow, the associated upper-tropospheric extratropical forcing and convective activity. The extent to which GloSea5 is able to represent the observed equatorial wave behaviour is examined.