Calculating the rate of Antarctic Bottom Water formation using new theory, fine-scale modelling and observations

Lead Research Organisation: University of Reading
Department Name: Meteorology

Abstract

The equatorial regions of the Earth receive more solar energy than the polar regions. The extra heat is transported polewards in approximately equal parts by the circulations of the atmosphere and ocean. Warm surface waters enter the polar regions, where they release their heat. As these surface waters cool, they can begin to freeze to form sea ice and this releases dense brine into the ocean. The combination of surface cooling and brine release causes the surface waters to become sufficiently dense to sink. The cold bottom waters so formed then flow equatorward, balancing the poleward surface flow. The location and mechanism of bottom water production affects the spatial distribution and intensity of the ocean circulation, which helps determine the weather.

Evidence from climate models and observations suggest that bottom water formation around Antarctica will be affected by global warming and will influence the climate and weather in both the Southern and Northern hemispheres. In order to accurately predict future climate change, climate models require an adequate representation of bottom water formation around Antarctica, which is presently lacking.

As a result of their limited spatial resolution and simplified physics, climate models are unable to represent adequately the small-scale processes responsible for the formation of bottom water. Climate models typically produce too little bottom water and this water is too warm and fresh, which has been ascribed to inadequate representation of winter freezing of seawater to form sea ice, which releases brine into the upper ocean. This proposal will use high resolution numerical modelling of the sea ice and ocean, combined with field observations to improve our understanding of the processes controlling bottom water formation around Antarctica.

We will focus on a particular region, over the continental shelf north of the Ronne Ice Shelf, since this region is believed to be responsible for about a third of the bottom water formed around Antarctica, is representative of other regions of bottom water formation around Antarctica, and because, relative to other areas in the Southern Ocean, we have a lot of data with which to test our model. We will explore the role of the Ronne polynya, an open water region in the sea-ice cover caused by winds blowing off Antarctica, and frazil ice formation, in controlling the bottom water formation in this region. Frazil ice consists of millimetre-sized ice crystals that form and grow when the seawater is below its freezing point. We will include the physics of frazil ice formation into our models. We will test our models with recently-acquired oceanographic and atmospheric data and satellite observations.

We will use our models, which will be the most physically-sophisticated, and highly-calibrated, models to date, to calculate the rate of bottom water formation over the Ronne continental shelf. The models, once calibrated for the Ronne continental shelf, will be used to calculate total bottom formation around the whole of Antarctica. In addition to new estimates for bottom water formation, we will develop new model physics and identify the optimal representation of ocean mixing.

Planned Impact

Meeting the specific objectives of our research will produce fundamental science outputs. These outputs are designed to address holes in our knowledge of important processes in polar oceanography. The natural end users of our work will be other scientists, in particular researchers and practitioners in climate modelling institutions. These end users are not in the same community of scientists as the research team.

The outputs of our work fall into three main categories: (i) new scientific knowledge, e.g. rates of High Salinity Shelf Water and Antarctic Bottom Water formation; (ii) new physics in climate models, i.e. frazil ice formation; and (iii) practical advice ("best practise") on the optimal set up of oceanographic mixing schemes in existing climate model components.

Although our research will not directly interface with the general public or industry, the credibility and reliability of climate model predictions depend on their ability to accurately simulate Antarctic bottom water formation. Our research will lead to improved climate model representation of bottom water formation, and thus to a more robust ability to simulate current and future climate. Impact on society at large, e.g. predictions used to help guide government and international policy on Greenhouse gas emissions, would be through the scientist end users of our work and would not be expected through our work directly.

More details are in the section on Academic beneficiaries.

Publications

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Frew R (2019) Sea Ice-Ocean Feedbacks in the Antarctic Shelf Seas in Journal of Physical Oceanography

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Heorton H (2017) A Model of Sea Ice Formation in Leads and Polynyas in Journal of Physical Oceanography

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Nicholls K (2013) Eddy-Driven Exchange between the Open Ocean and a Sub-Ice Shelf Cavity in Journal of Physical Oceanography

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Petty A (2013) Impact of Atmospheric Forcing on Antarctic Continental Shelf Water Masses in Journal of Physical Oceanography

Related Projects

Project Reference Relationship Related To Start End Award Value
NE/I023708/1 27/10/2011 26/10/2012 £168,817
NE/I023708/2 Transfer NE/I023708/1 01/11/2012 31/10/2014 £91,175
 
Description Seawater freezes to form sea ice in polar oceans. The ice can form as frazil ice, which are tiny ice crystals that form in supercooled water, or as columnar ice from the top of the sea surface down.

Frazil ice formation is particularly important in cracks, known as leads, in the sea ice cover in winter, which expose the ocean to the cold atmosphere, and in open water regions in the ice cover. As frazil ice collects in leads it forms grease ice and eventually consolidates to form a continuous ice cover. Frazil and grease ice formation accounts for approximately half of sea ice formation in the Southern Ocean.

A new model has been developed that accounts for the physics of frazil and grease ice formation and this model has been incorporated into a sea ice model component of a climate model. The importance of frazil ice has been studied and, particularly, its role in salt release into the ocean where it affects the formation of dense bottom waters.
Exploitation Route By developing the frazil ice module in a climate sea ice module that is used by many groups and climate modelling centres, including the UK Met Office, the fundamental outputs of our work - a more physically realistic and accurate frazil ice description - will be used by all of these groups.
Sectors Aerospace, Defence and Marine,Education,Environment,Government, Democracy and Justice

 
Description A paper has been submitted for peer-reviewed publication describing our work on frazil and grease ice: A new parameterisation of frazil and grease ice production in a climate sea ice model by AV Wilchinsky, HS Heorton, DL Feltham, PR Holland Another paper has been published in collaboration with colleagues at BAS: Marius Årthun, Paul R. Holland, Keith W. Nicholls, and Daniel L. Feltham, 2013: Eddy-Driven Exchange between the Open Ocean and a Sub-Ice Shelf Cavity. J. Phys. Oceanogr., 43, 2372-2387. doi: http://dx.doi.org/10.1175/JPO-D-13-0137.1 The frazil module is inside a local version of CICE and so is currently in use at CPOM. It will undergo additional tests, under separately funded work, prior to being uploaded to the official repository at Los Alamos (from where it is accessed by others, including UK Met Office, BAS, NOC).
First Year Of Impact 2014
Sector Aerospace, Defence and Marine,Education,Environment
 
Title Input to CICE sea ice climate model 
Description Research projects have developed new physics of sea ice processes. Under separate funding, but in collaboration with research projects, this has been turned into new physics modules in the sea ice climate model CICE. 
Type Of Technology Physical Model/Kit 
Year Produced 2017 
Impact The CICE sea ice model is used by climate modelling groups worldwide. In the UK this includes the UK Met Office, NOC and BAS.