Enhanced turbulence due to the superposition of internal gravity waves and a coastal upwelling jet

Abstract

The upwelling‐driven coastal jet off Oregon is in geostrophic balance to first order. The accompanying thermal wind shear is stable to shear instability. Yet enhanced turbulence is observed in the upwelling jet, typically as long, thin patches with horizontal to vertical aspect ratios of 102 to 103 (median value ∼300). These patches are clearly defined by regions of low Richardson number and occur where and when the linear superposition of the three dominant shear constituents (near‐inertial, M2, and thermal wind) interferes constructively. This is most pronounced at the base of the coastal jet, where the thermal wind shear is largest. While the effect of the turbulence stress divergence on the jet is small compared to geostrophy (∼1%), it is significant in the second‐order force balance governing secondary circulation. The timescale associated with the decay of the thermal wind shear via turbulence stress is O(10) days. We confirm that the vertical salt flux due to mixing is comparable to the net Ekman transport of salt onto the shelf within the bottom boundary layer. Because numerical models of coastal circulation lack turbulence in midwater column, any vertical transport of scalars, including salt and heat, must be achieved inshore of the 40‐m isobath. This is inconsistent with the observations presented in this study, in which significant vertical turbulent salt transport is found to exist across the entire shelf.