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

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.

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.