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.

Driving of the SAO by gravity waves as observed from satellite

“…In addition, modeling efforts are currently underway to improve the representation of the tropics and, in particular, the QBO in weather and climate models (e.g., Yang et al, 2009;Kim et al, 2013;Kim and Chun, 2015;Lott et al, 2012Lott et al, , 2014Krismer and Giorgetta, 2014;Maury and Lott, 2014;Richter et al, 2014). With a more realistic model representation of the QBO, potentially the coupling toward higher latitudes and even seasonal weather prediction might be improved (e.g., Gerber et al, 2012;Scaife et al, 2014).…”

Tuning of a convective gravity wave source scheme based on HIRDLS observations

“…For example, GWs are important in driving the quasi-biennial oscillation (QBO) (e.g., Dunkerton, 1997;Ern and Preusse, 2009;Alexander and Ortland, 2010;Evan et al, 2012;Ern et al, 2014;Kim and Chun, 2015) and the semiannual oscillation (SAO) . Moreover, they are assumed to be the main driver of the summer-time branch of the stratospheric Brewer-Dobson circulation (Alexander and Rosenlof, 2003;Fritts and Alexander, 2003) and play a significant role in wind reversals in the mesosphere and lower thermosphere (Lindzen, 1981;Matsuno, 1982;Ern et al, 2013).…”

Response to Anonymous Referee #2

“…The simulated QBOs can also be quite "fragile", which means sensitive to many different aspects of model formulation depending on the model. For example, the QBO in the Canadian Middle Atmosphere Model (AGCM3-CMAM) is sensitive to the balance of resolved and parameterized wave forcing , while in different versions of the Met Office Unified Model (MetUM) the QBO is sensitive to the specification of stratospheric ozone Bushell et al, 2010) and/or the parameterized gravity waves (Bushell et al, 2010;Kim et al, 2013). Sensitivity to vertical resolution has been reported by numerous studies, for example by Giorgetta et al (2006) for the middle atmosphere version of the ECHAM5 (MAECHAM5) model and by Geller et al (2016) for the NASA Goddard Institute for Space Studies (GISS) climate model.…”