Title: Emergence of an Advective Boundary Layer in Monsoon Cross-Equatorial Flow: Scaling, Dynamics, and Idealized Models
Abstract: The low-level Somali jet is the primary conduit of moisture transport for the South Asian monsoon and shares key features of monsoon evolution, including rapid onset and slower seasonal retreat. Conventional Ekman balance, which neglects nonlinear momentum advection, becomes inadequate near the equator where Coriolis effects weaken. Using ERA5 reanalysis, scaling theory, and idealized aquaplanet experiments, we show that monsoon onset involves a transition to an advective boundary layer (ABL). In the Southern Hemisphere, the easterly flow feeding the jet is broadly Ekman-like, with kinetic-energy generation balanced by frictional dissipation. Near the equatorial transition region, however, an advective balance emerges, in which kinetic-energy generation by northward flow is balanced primarily by kinetic-energy advection. As the jet matures, this evolves into a three-way balance among kinetic-energy generation, advection, and dissipation.
The seasonal regime transition to an ABL is marked by increasing boundary-layer local Rossby number, near-vanishing absolute vorticity, and a shift in the zonal momentum budget from frictional to meridional-advection control. A scaling analysis shows that the transition occurs when the meridional length scales of geopotential and zonal wind contract such that their product approaches a threshold. In the ABL regime, meridional winds become linearly related to meridional geopotential gradients, yielding a precise quadratic dependence of kinetic-energy on pressure gradients that helps explain rapid jet onset and slower retreat. Idealized experiments confirm that stronger cross-equatorial pressure gradients and slower rotation amplify advective effects and shift the transition latitude poleward. The latter part of the talk will propose a low-dimensional dynamical-systems interpretation using kinetic energy and flow angle that links the seasonal transition to changing stability of distinct dominant balances.
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