Local Identification of Equatorial Kelvin Waves in Real-Time Operational Forecasts

Abstract

Equatorial Kelvin waves (KWs) are associated with a variety of equatorial atmospheric phenomena. They are related to tropical convection, cloud and precipitation variability, tropical cyclogenisis, the onset of monsoon season over Africa and India, and even equatorial oscillation patterns, namely the quasi-biennial oscillation (QBO) and the Madden-Julian oscillation (MJO). As such, if operational forecasts are to improve near the tropics, it is important that they correctly represent KWs. This paper aims at developing a novel methodology for identifying KWs in real-time operational forecasts at specific longitudes using only the meridional structures of the solutions to the free Laplace tidal equations, known as Hough vector functions. The main advantages of this newly proposed methodology are that 1) it can univocally identify Kelvin waves at given longitudes and 2) very low computational cost is needed to apply it to real-time operational forecasts. The application of the method to 2015-2017 ECMWF analysis and operational forecast data reveals a topography related bias in the representation of the amplitude and phase of the Kelvin waves by the ECMWF operational forecast model.