Can the meridional flow variations explain the observed rising and declining phase asymmetry in the solar cycle?

¹ Département d’Astrophysique/AIM CEA/IRFU, CNRS/INSU, Univ. Paris-Saclay & Univ. de Paris Cité 91191 Gif-sur-Yvette, France
² Toulouse Univ., Institut de Recherche en Astrophysique et Planétologie, CNRS, CNES Toulouse, France

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. , This email address is being protected from spambots. You need JavaScript enabled to view it. , This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 September 2025
Accepted: 8 December 2025

Abstract

Context. Accurate forecasting of the 11-year solar cycle remains a central challenge in solar physics, with major implications for space weather prediction. A striking feature of the cycle is its asymmetry between the rising and declining phases, with the decay phase typically lasting much longer. This asymmetry could be due to variations in the Sun’s meridional circulation, though whether these variations are primarily deterministic – driven by Lorentz-force feedback – or stochastic remains debated.

Aims. We aim to establish whether deterministic variations, stochastic fluctuations, or a combination of both in the meridional circulation can reproduce the observed rise–decay asymmetry of the solar cycle.

Methods. We performed kinematic flux-transport dynamo simulations incorporating three classes of time-dependent meridional flow profiles: (i) deterministic variations, (ii) stochastic fluctuations, and (iii) hybrid combinations. To evaluate cycle asymmetry, we analysed four diagnostics: the rise-to-decay time ratio and correlations of cycle amplitude with rise time, rise rate, and decay rate near the preceding minimum.

Results. Solar cycle asymmetry is highly sensitive to the temporal evolution of the meridional flow. When both the meridional flow and the Babcock–Leighton mechanism are stochastic, the kinematic Babcock-Leighton solar dynamo model fails to produce cycles with decay times consistently longer than rise times. Physically motivated deterministic variations, inspired by Lorentz-force feedback and interpreted as a response to emergence and equatorward migration of active regions (i.e. the butterfly diagram) are able to reproduce the observed asymmetry. A representative case is obtained when the flow is modulated as δvsin²(2θ_max), where δv is the modulation amplitude and θ_max is the latitude of the toroidal field maximum. This formulation captures the essential feedback effect in the model: the meridional flow weakens near cycle maximum, remains suppressed afterward, and subsequently recovers. Hybrid scenarios combining deterministic and stochastic variability along with Babcock–Leighton fluctuations are also able to reproduce rise–decay asymmetry. Across all cases, a robust positive correlation emerges between cycle amplitude and rise rate, while correlations with rise time and decay rate remain weak but significant.

Conclusions. Meridional circulation variability plays a critical role in shaping solar cycle asymmetry in the flux transport dynamo model scenario. Improved observational constraints on its spatio-temporal behaviour are essential. Incorporating such variability into forecasting tools – such as Solar Predict – can enhance their physical realism and predictive skill.