Contributions of equatorial wave modes and parameterized gravity waves to the tropical QBO in HadGEM2

Abstract

The contributions of the equatorial waves to the quasi-biennial oscillation (QBO) are investigated using Hadley Centre Global Environment Model version 2 (HadGEM2). A gravity wave parameterization that couples its source spectrum to the convection is used. The equatorial wave modes are identified in the spectral domain, based on their distinct characteristics associated with momentum and heat fluxes. The Kelvin waves and parameterized gravity waves (PGWs) transport westerly momentum into the equatorial stratosphere by ~0.35 and 0.8 mPa, respectively, while easterly momentum is carried primarily by the PGWs (~0.75 mPa). The resolved inertio-gravity (IG) waves transport both easterly and westerly momentum by ~0.2 mPa. In the lowermost stratosphere, gradual dissipation of the Kelvin waves occurs, and the remaining waves induce eastward forcing in the westerly shear layer by 3-5 m s-1 month-1. The PGWs primarily dissipate within the sheared layer and provide large easterly and westerly forcing in the middle stratosphere (~17 m s-1 month-1), while the forcing in the lower stratosphere is comparable to the Kelvin wave forcing. The IG wave forcing is small because of the small wave dissipation rate. The Rossby and mixed Rossby-gravity (MRG) waves contribute to the westerly-to-easterly QBO transition by ~2 m s-1 month-1. The magnitudes of the simulated Kelvin and Rossby wave forcings are comparable with the Modern-Era Retrospective Analysis for Research and Applications, whereas the MRG and IG wave forcings are underestimated in the lower stratosphere. Differences between the simulation and reanalysis and the uncertainties in both are discussed.