Caltech's Linde Center Helps Navigate the Southern Ocean
At Caltech's Ronald and Maxine Linde Center for Global Environmental Science, researchers from diverse disciplines work together to investigate Earth's climate and its atmosphere, oceans, and biosphere; their evolution; and how they may change in the future.
In early February, the center hosted a three-day workshop focused on the Southern Ocean around Antarctica. Scientists from around the world working at the intersection of fluid dynamics and biochemistry gathered to summarize our current knowledge of the physical, chemical, and biological processes that are critical to the Southern Ocean's circulation and marine ecosystems. The researchers set out to identify areas where collaboration across disciplines is needed to push that understanding forward. Here are a few of the topics they covered.
The Use of Autonomous Underwater Vehicles for Observation
The Southern Ocean is one of the most inhospitable places on Earth. Despite the area's importance to the global climate, measurements and data are hard to come by because it is difficult to deploy research vessels in the region, especially in winter. Little, if any, data have been collected in some areas, especially in the deep ocean and underneath ice shelves.
But many new tools now exist to improve data collection and measurement in these remote regions. Autonomous gliders (shown above) have gathered information on currents, water density, and temperature at many depths, helping researchers like workshop participants Nicole Couto (Rutgers University), Mike Meredith (British Antarctic Survey), as well as Caltech's Andrew Thompson, assistant professor of environmental science and engineering, understand how warm waters are causing ice sheets to melt. Meanwhile, an extensive system of autonomous floats monitors temperature, salinity, dissolved gases and currents in the earth's oceans; moored instruments track what is happening beneath ice shelves; and even Antarctic seals outfitted with sensors provide scientists access to, and information about, some of the ocean's coldest and most inaccessible waters.
Iron Limitation on Phytoplankton Growth
Phytoplankton, microscopic algae that perform photosynthesis, the base of the Southern Ocean food web. These organisms require both nutrients and sunlight to survive. The Southern Ocean is a region where nutrients and sunlight (at least in summer) are plentiful, yet many parts of the Southern Ocean have extremely low phytoplankton concentrations. This is because not all nutrients are treated equally. Take iron, for example. Although iron is needed only in small amounts by phytoplankton, it is scarce throughout most of the Southern Ocean. Iron enters ocean waters by way of dust falling out of the atmosphere, from melting icebergs or glaciers, and from the ocean floor. Meeting participants Phil Boyd (University of Tasmania) and Nicolas Cassar (Duke University) are working to understand how sources of iron will respond to changing atmospheric and oceanic conditions, as well as how Southern Ocean ecosystems will adapt, are important research questions.
Phytoplankton distributions are largely observed by measuring ocean color from space. This image shows data from NASA's MODIS (MODerate resolution Imaging Spectroradiometer) satellite, which measures light coming off the ocean, NASA scientists use this information to determine the concentration of phytoplankton in the water. Here, yellow and orange colors indicate the presence of more phytoplankton.
The Importance of High Spatial Resolution in Ocean Models
The ocean is similar to the atmosphere in that much of the variability is contained in "weather systems," or high- and low-pressure areas. These weather systems create swirling currents, called eddies, that are the ocean equivalent of atmospheric storms. While storms in the atmosphere span hundreds of kilometers, eddies in the ocean only cover a few tens of kilometers. When numerical models, such as those run by meeting participant Andy Hogg (Australia National University), capture these smaller scales, the simulations explode with previously unseen dynamics and produce an energetic circulation that is more vigorous than seen in models that only simulate larger scales.
This image of Chatham Island, off the coast of New Zealand, was taken by MODIS. The blue wispy pattern (upper right) is a phytoplankton bloom that is being stretched and stirred by ocean eddies. Images like this one verify that high-resolution numerical models accurately reproduce oceanic motions and provide insight into how these small-scale currents influence Southern Ocean ecosystems.
Heat Input
Increasing carbon dioxide concentrations in the atmosphere warm the planet, with roughly 90 percent of the extra energy going into the oceans. The ocean warming that results is not uniform around the globe. Numerical models from the group of meeting participant John Marshall (MIT) suggest that the warming of the Southern Ocean will occur later than that of other oceans. The reason? The Southern Ocean provides a gateway where cold, dense waters, stored in the deep ocean, are brought up to the surface by the ocean circulation and are exposed to the atmosphere. These cold waters have the potential to store a large amount of heat. Understanding when this reservoir will be exhausted is critical to predicting future Southern Ocean temperature changes.
In this sea-surface temperature map created by a NOAA Geophysical Fluid Dynamics Laboratory model, Southern Ocean waters (green and blue) represent regions where cold water rises up to the surface, warms, and moves northward.
The Distribution of Sea Ice
The distribution of sea ice in the Southern Ocean is important for many reasons. For instance, sea ice can act as a cap on the ocean, limiting atmospheric interactions with the ocean surface that may trap carbon in the deep ocean. Recently, Caltech researchers including Thompson and Jess Adkins, professor of geochemistry and global environmental science, discovered a link between the distribution of sea ice in the Southern Ocean and differences in the ocean circulation in our present climate and at the Last Glacial Maximum.
As sea ice retreats, additional melting can be a source of iron to the ocean, influencing phytoplankton growth. The capacity for plankton and other organisms to survive the Antarctic winter is only just beginning to be understood, as explained in a recent review article on sea ice ecosystems by meeting participant Kevin Arrigo (Stanford University). Future under-ice observations are needed to improve our ability to estimate ecosystem changes in polar regions.
Credit: Hannah Joy-Warren, Stanford graduate student, taken during the Phantastic II cruise to the west Antarctic Peninsula (October/November 2014).
