Dependence of Internal Wave Bolus Transport on Pycnocline Thickness.

A bolus is a vortex formed by an internal wave propagating upslope with the shoaling wave. The amount of sediment and biota transported by boluses travelling up the world's continental slopes and onto the continental shelves has not been established but could be vital for maintaining nutrient‐rich coastal ecosystems. Previous laboratory experiments and simulations of shoaling boluses have considered a two‐layer density stratification or a linear density profile. We present experiments on bolus formation and propagation in a stratification with a density profile ρ(z) represented by a tanh‐profile, which describes many ocean pycnoclines. Our laboratory experiments examine pycnoclines with thicknesses varying from nearly zero (a two‐layer system) to large thicknesses where the density varies almost linearly with depth. The bolus volume, displacement upslope, and available potential energy are measured as a function of pycnocline thickness. Maximum upslope displacement is found to be twice that observed for a two‐layer stratification. Plain Language Summary: Interfacial waves can be observed along the surface of the ocean, and as they approach the coastline the waves steepen, break, and ultimately surge up the beach. These waves are analogous to internal waves, which occur within the ocean at the horizontal interface of more and less dense water. These internal waves can be an order of magnitude taller than the waves seen at the beach, and the surge (called a bolus) can move kilometers along the ocean bottom. Previous efforts to model these boluses have approximated the ocean's density as two layers of different density water. However, the ocean has a more gradual transition between the layers, which can be over 100 meters thick. The impact of this transition layer, called a pycnocline, on bolus transport is poorly understood. Here, we perform laboratory experiments to demonstrate that the presence of a thick pycnocline can double the distance the bolus travels upslope and increase the amount of energy for mixing. This demonstrates the need for a better model of bolus propagation to understand how these breaking internal waves move material along the bottom of the ocean. Key Points: Boluses propagate further for pycnoclines of finite thickness than for a discontinuous stratification with two layers of uniform densityThe experiments reveal the existence of a pycnocline thickness that results in a maximum bolus propagation displacement upslopeThin pycnoclines yield large bolus volumes with high available potential energy despite short bolus propagation displacement [ABSTRACT FROM AUTHOR]