Paul Myers, Professor at the University of Alberta, emphasizes the necessity for a new generation of very high-resolution computer models to effectively understand how ocean currents will change in a warming climate
The Summary for Policymakers of the Intergovernmental Panel on Climate Change (IPCC)’s recent ‘Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC)’ states, “Impacts of climate-related changes in the ocean and cryosphere increasingly challenge current governance efforts to develop and implement adaptation responses from local to global scales, and in some cases pushing them to their limits. People with the highest exposure and vulnerability are often those with lowest capacity to respond.” (1)
The same report emphasizes that cryosphere-mediated climate feedbacks on land and in the ocean have contributed to especially rapid warming in the Arctic, with surface temperature likely to have increased there at a rate at least twice the global average, over the past 20 years. In association with this warming, sea ice is thinning and disappearing in summer, and enhanced land and glacial runoff are dumping increasing amounts of freshwater into the ocean.
With this freshwater input comes inputs of organic carbon and nutrients from rivers that drain watersheds with thawing permafrost. It is widely recognized that altered light and nutrient availability, associated with climate forcing (e.g., diminished sea ice cover, increased freshwater flux, coastal erosion, and wind-driven mixing), are changing marine productivity within the Arctic Ocean.
Significant climate impacts within the Arctic
These changes have significant impacts within the Arctic. However, the heavily modified upper waters of the Arctic Basin do not remain in the Arctic. They are exported to the south, with significant fractions of the waters flowing through the Canadian North, the waters of Inuit Nunangat, and then into the Davis Strait.
Less recognized is the potential for major “downstream” teleconnections (via ocean circulation) and the impacts of Arctic change on both the global ocean and climate systems, as well as on coastal populations and productive ecosystems along the flow path of the exported water and ice.
On this route south, additional low-salinity meltwater comes from the rapidly melting Greenland ice sheet. Recent studies suggest that a significant fraction of this melt is driven by warm ocean waters being transported into fjords, providing heat to melt tidewater glaciers.
Globally, climate change is predicted to significantly impact ocean currents, slowing the large-scale circulation, raising sea levels, inducing biodiversity loss, and threatening food security. Within this high-latitude region, risks posed by ocean and climate change are particularly high, with many studies suggesting Greenland melt may act as a tipping point in the climate system, leading to, at the very least, a significant weakening of deep water formation and the Atlantic Meridional Overturning Circulation, which plays an important role in poleward heat transport, and the transport of oxygen and carbon dioxide to the deep ocean.
Modelling ocean currents in a changing climate
To further understand how ocean currents may transport warm waters into Greenland’s fjords, and where the resulting meltwaters may go, additional marine information is needed, at high spatial and temporal resolution to answer. Computational resources for simulating the ocean and climate system can be allocated in numerous ways: by incorporating multiple components of the climate system (e.g., atmosphere, ocean) coupled together, running multiple simulations to understand model uncertainties (ensembles), or by using very high spatial resolution to represent small-scale processes. All approaches are important, and different modelling teams need to explore aspects of this resource triad.
Focusing on the resolution limb, today’s computational resources now allow for models of ocean currents and properties to resolve ocean basins at scales of less than 1 km. This very high resolution is key, as many oceanic and cryospheric processes are small-scale, linked to the mesoscale and sub-mesoscale. These small-scale features are highly transient, generally with short lifetimes of days to months.
Spatially, oceanic mesoscale features, such as eddies, are typically assumed to have spatial scales less than 100 km. They are assumed to be oceanic equivalents to atmospheric weather systems. Sub-mesoscale processes occur at even smaller scales, such as 1-10 km and can potentially be compared to warm and cold fronts in the atmosphere that produce dramatic weather. Beyond physics, it is believed that the sub-mesoscale has significant connections to the ocean’s biogeochemistry and, consequently, to life in the ocean.
Additionally, such a resolution is necessary to accurately represent the coastlines and bathymetry of Greenland’s coastal fjords, where warm ocean waters can impact vulnerable ice. Despite these processes being individually of small size, the latest simulations show that the integrated impacts of such processes can combine to reach the largest scales, affecting deep water formation and potentially the currents associated with the Atlantic Meridional Overturning Circulation.
Priorities for ocean currents in a changing climate
Thus, to continue developing an understanding of these small-scale processes and to ensure they don’t lead to tipping points that would rapidly change the ocean’s currents and thus impact the climate, we need to continually work on developing and improving the latest generation of high-resolution ocean general circulation models. Resources for such simulations will also need to be provided by national supercomputer systems. The modelling community must also continue to collaborate with the observational community to produce the necessary observations for evaluating these models.
