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

Abstract

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.

Figures (17)

• Figure 1: Composite evolution of 900 hPa (a) zonal momentum and (b) meridional momentum terms during the seasonal transition. Day 0 corresponds to the onset of cross-equatorial flow (see section \ref{['5.2.2']} for the definition). Contribution of each term to the (c) zonal and (d) meridional momentum budget at 900 hPa 25 days before (blue) and after (red) the onset of cross-equatorial flow. Note that the scale of terms before (on the left) and after the onset (on the right) is different for both panels. The error-bars indicate the standard error for each term. All the terms are averaged over the "N.Eq." region (50$^\circ$E-60$^\circ$E, 2$^\circ$N-10$^\circ$N).
• Figure 2: Evolution of daily climatology (1979-2020) of $B_{\phi}B_{u}$ and $\phi/f^2$ over the "N.Eq." region. The length scales are estimated using an exponential fit. The vertical dashed line indicates the day when $\eta$ changes sign from positive to negative. The yellow line represents the daily climatology (1979-2020) of rainfall (in $mm/day$) averaged over the core monsoon zone (CMZ) based on India Meteorological Department (IMD) data.
• Figure 3: Meridional profile of zonal momentum terms for different SST maximum experiments.
• Figure 4: (a) Scatter plot between daily averaged meridional geopotential gradient (in $m/s^2$) and cross-isobaric meridional wind (in $m/s$) for composite periods based on ERA5 reanalysis data for 1979-2020. The least-square linear fit line after the development of ABL (day 26 to 80) is also plotted. (b) Steady-state zonally averaged meridional geopotential gradient (in $m/s^2$) and cross-isobaric meridional wind (in $m/s$) averaged for the $0-10^\circ$N latitude zone, with different SST maximum experiments. The least-square linear fit line starting from experiment $\mathrm{SST_{10N}}$ till $\mathrm{SST_{25N}}$ is also plotted in blue. The equation of the fit is also indicated for both panels.
• Figure 5: Steady-state precipitation (shading in mm/day) for (a) $0.25\Omega$, (b) $\Omega$, and (c) $2\Omega$ planetary rotation rate for maximum SST located at 25$^\circ$N. Winds at 900 hPa and zero absolute vorticity ($\eta=0$) contour are also shown for each case.