Global impacts of shelf seas

Oceans 2025: Theme 3, work package 3.1

Steep slopes bordering shelf seas have impacts on ocean circulation and the transmission of signals around the ocean basins. Dense water formation and cascades at the shelf edge are thought to be important for water mass formation and off-shelf transports of carbon. We aim to quantify the water fluxes between the shelf and open ocean globally, including the development of methods to incorporate shelf effects into global models. This work will contribute to understanding the whole carbon cycle. The study is organized as three components with aims, rationale and outline plans as follows.

a) To quantify and predict dense-water formation, cascading, slope mixing and their effects on the ocean. These processes are believed to be important but are unquantified as controls on oceanic water masses and overturning circulation. The emphasis is on:

1. data for internal waves and mixing on the slope from experiments in 2005 (north of Scotland), 2007 (Gulf of Cadiz) and 2008 (planned; dense water from Svalbard)
2. modelling for cascading and mixing sensitivity to topography, ambient and initial conditions, forcing, internal waves.

b) To determine constraints on adjacent ocean circulation imposed by the shelf and slope. Some flow from ocean to shelf is possible, but how much and of what form? E.g. is shelf-ward baroclinic flow returned via mixing and in boundary layers; is depth-integrated flow diverted along the slope? Parallel aims for models are to find what is needed in global models to represent near-margin flow, how to represent the shelf and the possible benefit of unstructured grids. The study involves numerical model experiments, initially for a circular basin with uniform shelf. Semi-analytic, finite difference and unstructured-grid models will be used to investigate behaviour as function of flow scale, stratification, friction and mixing. Later, effects of parameterised mixing in an ocean model will be studied, and shelf contributions to global ocean energy dissipation and overturning.

c) To quantify and predict shelf-sea contributions to global biogeochemical budgets:

• ocean-margin fluxes and budgets (Phosphorus, Nitrogen, Carbon, production and CO2);
• shelf seas' role in large scale transport, especially of freshwater and pollutants.

These fluxes are important in global marine cycling, are not known and are poorly represented in ocean-wide models. In 2007-2009 the global representation of shelf seas is being developed in the NERC QUEST-related e-science project GCOMS in Theme 9, and ecosystem modelling in MarQUEST. This will be followed by:

• a 10-year hindcast
• model verification
• development of key shelf-sea process representation
• investigation of export pathways
• experiments with resolution of shelf-seas for their biochemical contribution in a coarse-resolution ocean model,
• parametrisations of net shelf effects where shelves are not resolved
• studies of sensitivity to atmospheric/oceanic forcing and model parametrisations

Team:
John Huthnance
Alan Davies
Phil Hall
Rob Hall
Jason Holt
Chris Hughes
Miguel Maqueda
Eleanor O'Rourke
Graham Tattersall
Jiuxing Xing