An Idealized Numerical Modeling Study of Enhanced Coastal Upwelling By Topographic Variations Around Capes
The enhancement of coastal upwelling on the downwind (i.e., equatorial) side of large headlands is studied using a three-dimensional, high-resolution numerical model in an idealized domain. Early simulations show that positive (negative) wind stress curl in the northern (southern) hemisphere does not enhance upwelling compared to having steady winds with zero wind stress curl. Multiple idealized grids are examined to determine the main topographic variations driving enhanced upwelling. The grid with a cape and varying shelf width (widening downstream of the tip of the cape) exhibits the most upwelling enhancement. The widening shelf width contributes to upwelling through a fundamental process derived from the steady-state continuity equation in the shallow water equations. Depth-averaged flow divergence must be accompanied by depth-averaged onshore cross-isobath flow. We find the largest amplitude cross-shore transport occurs near the shelf break, where the shelf is widening downstream. The cape geometry still further enhances upwelling. Examination of the along-isobath momentum balance clearly shows a large pressure gradient just south of the tip of the cape, where the shelf is widening and just south of localized low pressure resulting from flow around the cape. Except for very near the coast, the flow remains largely geostrophic, with flow largely following lines of constant pressure. This results in enhanced onshore cross-isobath flow just south of the low pressure, in the lee of the cape. These combined effects are much larger in the model with both capes and changing shelf width than in models with either no capes or no changing shelf width, leading to the conclusion that the topographic variations work together to create environments particularly favorable to enhanced upwelling as well as those particularly unfavorable.