Emergence of an Advective Boundary Layer in Monsoon Cross-Equatorial Flow: Scaling, Dynamics, and Idealized Models

The conventional Ekman model of the tropical boundary layer neglects nonlinear momentum advection and breaks down near the equator, where Coriolis effects are weak. During South Asian monsoon onset, we identify a dynamical regime transition to an advective boundary layer (ABL). Reanalysis links this transition to a shift in the zonal momentum balance from frictional to meridional-advection control as cross-equatorial flow intensifies, accompanied by increasing local Rossby number and vanishing absolute vorticity, signaling the breakdown of Ekman balance. A scaling analysis shows that this transition occurs when the meridional length scales of geopotential and zonal wind contract such that their product approaches $φ/f^2$. In the resulting ABL regime, kinetic energy is governed by a balance between its generation and advection, yielding a linear diagnostic relation between meridional geopotential gradient and meridional wind. A simple theoretical model predicts that the sensitivity of this relation is controlled by an advective timescale that equals the inertial timescale ($1/f$) at the transition latitude, where zonal and meridional wind speeds become comparable. Testing this framework in idealized aquaplanet experiments confirms that stronger cross-equatorial pressure gradients and slower planetary rotation rates amplify advective effects and shift the transition latitude poleward. Across experiments, the sensitivity of meridional winds to the geopotential gradient remains tightly linked to $1/f$ at the transition latitude. Together, these results establish the ABL as a distinct dynamical regime, with important implications for monsoon onset, intraseasonal variability, and the representation of tropical boundary layer processes in climate models.