Abstract. We explore a two-dimensional kinematic solar dynamo model in a full sphere, based on the helioseismically determined solar rotation profile and with an α effect concentrated near the solar surface, which captures the Babcock-Leighton idea that the poloidal field is created from the decay of tilted bipolar active regions. The meridional circulation, assumed to penetrate slightly below the tachocline, plays an important role. Some doubts have recently been raised regarding the ability of such a model to reproduce solar-like dipolar parity. We specifically address the parity issue and show that the dipolar mode is preferred when certain reasonable conditions are satisfied, the most important condition being the requirement that the poloidal field should diffuse efficiently to get coupled across the equator. Our model is shown to reproduce various aspects of observational data, including the phase relation between sunspots and the weak, diffuse field.

Whether or not the upcoming cycle 24 of solar activity will be strong is being hotly debated. The solar cycle is produced by a complex dynamo mechanism. We model the last few solar cycles by "feeding" observational data of the Sun's polar magnetic field into our solar dynamo model. Our results fit the observed sunspot numbers of cycles 21-23 reasonably well and predict that cycle 24 will be about 35% weaker than cycle 23.

Although systematic measurements of the solar polar magnetic field exist only from mid 1970s, other proxies can be used to infer the polar field at earlier times. The observational data indicate a strong correlation between the polar field at a sunspot minimum and the strength of the next cycle, although the strength of the cycle is not correlated well with the polar field produced at its end. This suggests that the Babcock Leighton mechanism of poloidal field generation from decaying sunspots involves randomness, whereas the other aspects of the dynamo process must be reasonably ordered and deterministic. Only if the magnetic diffusivity within the convection zone is assumed to be high, we can explain the correlation between the polar field at a minimum and the next cycle. We give several independent arguments that the diffusivity must be of this order. In a dynamo model with diffusivity like this, the poloidal field generated at the mid latitudes is advected toward the poles by the meridional circulation and simultaneously diffuses towards the tachocline, where the toroidal field for the next cycle is produced. To model actual solar cycles with a dynamo model having such high diffusivity, we have to feed the observational data of the poloidal field at the minimum into the theoretical model. We develop a method of doing this in a systematic way. Our model predicts that cycle 24 will be a very weak cycle. Hemispheric asymmetry of solar activity is also calculated with our model and compared with observational data.Comment: 17 pages, 18 figures, submitted to MNRA

The quasi‐biweekly mode (QBM) and the 30–60 day mode are two major intraseasonal oscillations (ISOs) in the tropics. The QBM is known to have a major influence in determining the active and break conditions of the Indian monsoon during the northern summer. A westward‐propagating equatorial Rossby wave with quasi‐biweekly period influences the Australian monsoon during the northern winter. Universality between the summer and winter QBM is established through analysis of daily circulation and convection data for 10 years. It is shown that the mean spatial structure of the QBM in circulation and convection resembles that of a gravest meridional mode equatorial Rossby wave with wavelength of about 6000 km and westward phase speed of approximately 4.5 m s−1. However, the maximum zonal wind occurs at around 5°N (5°S) during the northern summer (winter). The wave structure appears to be translated northward (southward) by about 5° during the northern summer (winter). The relationship between outgoing long‐wave radiation and circulation data indicates that the mode is driven unstable by coupling with moist convection. Similarity in temporal and spatial characteristics of the mode during the two seasons leads us to propose that the same mechanism governs the genesis and scale selection of the mode in both the seasons. An acceptable mechanism for genesis and scale selection of the QBM has been lacking. In the present study, a mechanism for genesis and scale selection of the observed QBM is proposed. A simple 2½‐layer model that includes a steady Ekman boundary layer (BL) formulation incorporating effect of entrainment mixing is constructed for the convectively coupled equatorial waves. Without influence of the background mean flow, moist feedback in the presence of frictional BL convergence drives the gravest meridional mode equatorial Rossby wave unstable with observed wavelength and period but with zonal winds symmetric about the equator. Potential temperature perturbation associated with the Rossby wave is in phase with relative vorticity perturbation at low level. The BL drives moisture convergence in phase with the relative vorticity at the top of the BL. Release of latent heat associated with the BL convergence enhances the potential temperature leading to a positive feedback. The mean flow over the Indian Ocean and western Pacific at low levels is such that the zero ambient absolute vorticity or the ‘dynamic equator’ shifts to around 5°N (5°S) during summer (winter) and results in a shift of the unstable Rossby waves towards the north (south) by about 5°. The resulting structure of the unstable Rossby mode resembles the observed structure of the biweekly mode. It is shown that neither evaporation–wind feedback nor vertical shear of the mean flow is crucial for the existence of the mode. However these processes marginally modify the growth rate and make the structure of the unstable wave more realistic. Copyright © 2004 Royal Meteorological Society

Context. Turbulent fluxes of angular momentum and enthalpy or heat due to rotationally affected convection play a key role in determining differential rotation of stars. Their dependence on latitude and depth has been determined in the past from convection simulations in Cartesian or spherical simulations. Here we perform a systematic comparison between the two geometries as a function of the rotation rate. Aims. Here we want to extend the earlier studies by using spherical wedges to obtain turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. In particular, we want to clarify whether the sharp equatorial profile of the horizontal Reynolds stress found in earlier Cartesian models is also reproduced in spherical geometry. Methods. We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs, and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results. For slow rotation we find that the radial and latitudinal turbulent angular momentum fluxes are directed inward and equatorward, respectively. In the rapid rotation regime the radial flux changes sign in accordance with earlier numerical results, but in contradiction with theory. The latitudinal flux remains mostly equatorward and develops a maximum close to the equator. In Cartesian simulations this peak can be explained by the strong "banana cells". Their effect in the spherical case does not appear to be as large. The latitudinal heat flux is mostly equatorward for slow rotation but changes sign for rapid rotation. Longitudinal heat flux is always in the retrograde direction. The rotation profiles vary from anti-solar (slow equator) for slow and intermediate rotation to solar-like (fast equator) for rapid rotation. The solar-like profiles are dominated by the Taylor-Proudman balance.