Horizontal convection (HC) provides a minimal paradigm for studying buoyancy-driven transport that is central to ocean circulation and climate (Hughes & Griffiths 2008, Annu. Rev. Fluid Mech.). In the standard HC configuration, buoyancy is supplied and removed exclusively through the bottom boundary of a horizontal fluid layer, and the dynamics are commonly characterized by the Rayleigh and Prandtl numbers. However, an often underemphasized control parameter is the domain aspect ratio $\Gamma$, which can strongly influence the flow organization and, consequently, the convective heat transport. To quantify its impact systematically, we performed an extensive set of three-dimensional direct numerical simulations spanning a broad range of Rayleigh numbers and aspect ratios.

We find that domain geometry has a remarkably strong impact on horizontal convection. In particular, as $\Gamma$ increases, the onset of an organized circulation shifts to higher Rayleigh number. More importantly, we show that geometry controls the scaling laws of convective heat transport. Our simulations clarify how the system undergoes a transition from the Rossby-type laminar regime $I_l$ (Rossby 1965, Deep-Sea Res. Oceanogr. Abstr) to the $I_l^{*}$ regime predicted by the Shishkina–Grossmann–Lohse theory (Shishkina, Grossmann & Lohse 2016, Geophys. Res. Lett.). The transition to I$_l^{*}$ is controlled by vertical confinement, which forces the kinetic boundary-layer thickness to saturate in shallow configurations and leads to a distinct change in the observed Nu(Ra) scaling.

Overall, our results demonstrate that domain geometry is not a secondary detail but a key ingredient for interpreting laminar HC. Accounting for the effect of $\Gamma$ is therefore essential when formulating predictive scaling theories for geophysical applications.