Dynamical Axisymmetric Modes of the Hadley Circulation

The Hadley circulation is the primary large-scale meridional circulation in the tropics and is conventionally seen as axisymmetric. However, meridional dynamics in the tropics are far from zonally uniform and recent developments have highlighted the importance of contributions from regional and time confined meridional overturning circulations to the global Hadley regime.

In the present work, we decompose the Hadley circulation into axisymmetric modes (AMs) which retain the linearized dynamics of an axisymmetric atmospheric circulation. Such modes are the normal mode solutions of the linearized axisymmetric equations of horizontal atmospheric motion, which coincide with the zonal wavenumber zero (k = 0) normal mode solutions to the linearized equations of horizontal atmospheric motion (Laplace tidal equations). We propose a method for the decomposition into AMs which draws similarities to previously developed local identification methods for equatorial waves ([1] and [2]).

The diagnostic potential of the decomposition is shown by analysing the preferred AMs of the Hadley circulation and recalling their physical underpinnings. In the literature, axisymmetric theory and constraints are frequently employed in the study of zonally confined meridional circulations ([3]). Therefore, we also analyse the validity and applicability of the decomposition into AMs in the case of zonally confined regional overturning circulations. Our work aims to be a contribution to the study of different regional meridional overturning regimes and the analysis of the regional contributions to the global Hadley circulation.

 

References:

[1] Cruz, J.B., Castanheira, J.M. & DaCamara, C.C. (2024) Local identification of equatorial Kelvin waves in real-time operational forecasts. Quarterly Journal of the Royal Meteorological Society, 150(761), 2440–2457. https://doi.org/10.1002/qj.4717

[2] Cruz, J.B., DaCamara, C.C. & Castanheira, J.M. (2025) Local identification of equatorial mixed Rossby–gravity waves. Quarterly Journal of the Royal Meteorological Society, 151(770), e4978. https://doi.org/10.1002/qj.4978

[3] Geen, R., Bordoni, S., Battisti, D. S., & Hui, K. (2020). Monsoons, ITCZs, and the concept of the global monsoon. Reviews of Geophysics, 58, e2020RG000700. https://doi.org/10.1029/2020RG000700

 

This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020- https://doi.org/10.54499/LA/P/0068/2020 , UID/50019/2025, https://doi.org/10.54499/UID/PRR/50019/2025 , UID/PRR2/50019/2025, UID/50017/2025 (doi.org/10.54499/UID/50017/2025) and LA/P/0094/2020 (doi.org/10.54499/LA/P/0094/2020).