Since the seminal work of Walter Munk in the 1960s ('Abyssal Recipes'), oceanographers have believed that the upwelling of cold, abyssal waters that regulates the deep ocean's ability to sequester heat and carbon for decades to millennia is mainly driven by centimetre-scale turbulent mixing associated with breaking internal waves in the ocean interior. Measurements of deep-ocean turbulence over the last >20 years, however, have contested this scenario, and instead suggest that mixing by breaking internal waves drives *downwelling* of abyssal waters. Inspired by this conundrum, recent theoretical investigations have developed an alternative view of the role of mixing in sustaining deep-ocean upwelling. In this new view, upwelling is driven by highly localised turbulence within thin (typically tens of metres thick) layers near the seafloor, known collectively as the bottom boundary layer. In the BLT Recipes experiment, we recently set out to test this new view, and figure out how it works, by obtaining the first set of concurrent, systematic measurements of (1) large-scale mixing and upwelling, (2) their interior and bottom boundary layer contributions, and (3) the processes underpinning these contributions, in a representative deep-ocean basin (the Rockall Trough, in the Northeast Atlantic). This talk will review the insights emerging from the BLT Recipes experiment, and offer an outlook on how they might re-shape our understanding of the way in which turbulence sustains deep-ocean upwelling.
