Bottom Boundary Layer Turbulence and Abyssal Recipes (BLT Recipes)

Since the seminal work of Walter Munk in the 1960s, 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 (thereby shaping Earth's climate) is driven by centimetre-scale turbulent mixing associated with breaking waves in the ocean interior. Measurements of deep-ocean turbulence over the last two decades, however, starkly contest this scenario, and instead suggest that mixing by breaking waves drives *downwelling* of abyssal waters. Inspired by this conundrum, recent theoretical investigations have developed a tantalising 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 this proposal, we set out to assess the validity of this new paradigm, 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). To this end, we will conduct a year-long field programme involving three research cruises, in close collaboration with a group of U.S. project partners.

To measure (1), in the first cruise we will release a long-lived chemical tracer detectable at very low concentrations near the seafloor on the basin's eastern flank, where deep waters flow into the basin from the south. We will monitor how the tracer mixes and upwells as it fills the basin by sampling it at 6 and 12 months after its release, during the two subsequent cruises. To measure (2) and (3), we will adopt a two-pronged approach. First, we will assess mixing and upwelling in the ocean interior and the transition into the bottom boundary layer by deploying turbulence-measuring profilers at hundreds of sites throughout the basin, in the first and third cruises. Second, as these ship-deployed profilers are unsuitable to approach the seafloor closely, we will obtain targeted measurements of mixing and upwelling across the bottom boundary layer in two ways: (i) by conducting two 6-month-long deployments of a cluster of novel, turbulence-measuring instruments moored at the seafloor, at two sites on the basin's sloping boundary; and (ii) by performing a near-bottom release of a short-lived dye at each of the mooring deployment sites, and monitoring how the dye mixes and upwells over several days after its release.

We will integrate our measurements of (1)-(3) into the design of a realistic numerical model of the circulation in the Rockall Trough, and use the model to conduct a rigorous, quantitative test of the new paradigm of ocean mixing. We expect this test to provide the first evidence of the dominance of bottom boundary layer turbulence in driving deep-ocean upwelling and, in so doing, trigger a profound transformation in our understanding and modelling of deep-ocean circulation.

The aim of our proposal is to understand how centimetre-scale turbulence near the seabed shapes the rate at, and way in which, deep water masses upwell throughout the world ocean. While we will engage with the research community through our publications and presentations in at least 5 national and international conferences, we will seek to maximise the wider impact of our research as follows.

Who - the following two communities will particularly benefit from our proposed research:

1. Climate modellers, including those at the U.K. Met Office and the U.S. NOAA Geophysical Fluid Dynamics Laboratory;

2. School pupils, their teachers and the general public.

How and what - relevance of our research to these communities and what will be done to ensure that they benefit from our research:

1. The impact of BLT Recipes on climate modellers will be through providing guidance to their efforts to improve the representation of the effects of turbulence in climate models. To maximise this impact, we have developed a close partnership with two leading modellers in the U.K. and the U.S. who respectively specialise in the assessment of the climate impacts of deep-ocean upwelling in models, and of the upwelling's sensitivity to the models' representation of turbulence. We have entrained these modellers into the planning of our experiment and exploitation of our results (see Hewitt and Griffies Letters of Support).

2. The impact of BLT Recipes on school pupils, their teachers and the general public will be enabled by a range of outreach activities targeted at cultivating enthusiasm for and understanding of marine and climate science by those communities. These activities include the development of an interactive website, and of a video blog and trailer of the experiment's fieldwork respectively aimed at secondary schoolchildren and the general public; and the development of a teaching segment and a rotating-table demonstration of near-seabed mixing for showcasing in science festivals. Outreach activities for school pupils are designed to encourage them to take science subjects at school and university, thus contributing to the U.K.'s skillbase.