Covariance based estimates of near-boundary diapycnal upwelling in a submarine canyon

We present direct covariance estimates of temperature flux from a conventional taut wire mooring placed in a narrow submarine canyon on the continental slope west of Ireland. Estimates of stratification from both moored sensors and vertical profiling instrumentation are used to facilitate the interpretation of these temperature transport estimates in terms of diathermal upwelling. At depths of 50-125 meters above bottom, the temperature flux is up-gradient, rather than down the mean gradient, and the corresponding vertical divergence of the diathermal temperature flux implies a diapycnal velocity of approximately 1 mm/s, consistent with diathermal migration estimated from a dye release study. Cospectra document that the temperature flux is related to a wave breaking process under-pinned by semi-diurnal frequencies. This up-gradient flux also results in a highly non-local temperature variance budget. We demonstrate that nonlinear temperature variance production at the bottom boundary is the root cause, with redistribution aloft determining the temperature flux profile.