OCE-RIG: Identifying the role of ocean circulation in polar climate change

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The high-latitudes have changed substantially over recent decades: the Arctic has warmed faster than anywhere else on Earth, concurrent with a rapid decline of its sea-ice cover, while the Southern Ocean has largely cooled and its sea-ice cover has modestly expanded. The dynamics giving rise to these observed changes are not yet understood - as evidenced in the large spread of past and future polar climate changes simulated by our state-of-the-art global climate models. Yet, improved understanding of the changing polar oceans is critical to both polar and global climate predictability. This study aims to establish the relative roles of, and interactions between, the oceans, atmosphere and cryosphere in polar climate change and identify the key processes at work. A further goal of the project is to introduce an audience of under-represented high-school students to the key role of the ocean in our changing climate.A major challenge to studying high-latitude climate change is the inherently coupled nature of the governing system dynamics. For instance, oceanic changes around Antarctica have been driven by a combination of factors, such as greenhouse gas forcing, increased freshwater input, and changes in surface winds due to stratospheric ozone depletion; the effects of each of these drivers have been further shaped by the background ocean circulation on which they act, making it difficult to attribute the primary causes of observed - or even simulated - polar climate changes, and precluding confident polar climate prediction over the coming century. This study seeks overcomes these challenges through numerical simulation with an oceanic general circulation model driven at the ocean surface by these distinct climate forcings - applied separately and together. This approach allows us to identify the key role of polar ocean circulation in setting the response to greenhouse gas forcing, and reveals that distinct circulations of the polar oceans have given rise to their contrasting changes over recent decades - with the Arctic warming at a far greater pace than the Southern Ocean due to differences in meridional ocean heat transport changes. The study further examines the response to the full range of climate forcings - including fresh water flux changes and variations in both large-scale and regional surface winds - allowing us to identify the role of the background ocean circulation in mediating the response to each. Furthermore, by driving the ocean model with climatological surface boundary conditions unique to different comprehensive coupled global climate models, we assess the influence of distinct background ocean conditions in the large spread of polar climate projections simulated across those coupled models.A major challenge to studying high-latitude climate change is the inherently coupled nature of the governing system dynamics. For instance, oceanic changes around Antarctica have been driven by a combination of factors, such as greenhouse gas forcing, increased freshwater input, and changes in surface winds due to stratospheric ozone depletion; the effects of each of these drivers have been further shaped by the background ocean circulation on which they act, making it difficult to attribute the primary causes of observed - or even simulated - polar climate changes, and precluding confident polar climate prediction over the coming century. This study seeks overcomes these challenges through numerical simulation with an oceanic general circulation model driven at the ocean surface by these distinct climate forcings - applied separately and together. This approach allows us to identify the key role of polar ocean circulation in setting the response to greenhouse gas forcing, and reveals that distinct circulations of the polar oceans have given rise to their contrasting changes over recent decades - with the Arctic warming at a far greater pace than the Southern Ocean due to differences in meridional ocean heat transport changes. The study further examines the response to the full range of climate forcings - including fresh water flux changes and variations in both large-scale and regional surface winds - allowing us to identify the role of the background ocean circulation in mediating the response to each. Furthermore, by driving the ocean model with climatological surface boundary conditions unique to different comprehensive coupled global climate models, we assess the influence of distinct background ocean conditions in the large spread of polar climate projections simulated across those coupled models.