The Southern Ocean exerts a major influence on the Earth’s climate. Powerful circumpolar winds, which are uninterrupted by land, and intense cooling that increases water density both act to mix intermediate and deep ocean waters to the sea surface. This intense mixing makes the Southern Ocean an interesting place to study ocean biogeochemistry—the study of the exchange and distribution of chemical elements and compounds within the ocean, as well as between the ocean and the atmosphere and land. This continuous upwelling of old, deep water also makes the Southern Ocean an important sink for the additional heat and carbon dioxide pumped into the atmosphere by human activities. Amazingly, as much as 90 percent of the excess heat produced in the atmosphere by greenhouse gases and nearly half of the excess carbon dioxide is absorbed by the Southern Ocean.
The Southern Ocean is also a major source of nutrients for the rest of the world ocean. Nutrients brought up to the surface by deep Southern Ocean mixing may be responsible for three quarters of the primary production in the ocean north of 30 degrees south. Primary production—a biological process in which plants use the sun, water, and air to produce their own food and grow—is important because this process forms the foundation of food webs in marine ecosystems.
The Southern Ocean is also particularly susceptible to ocean acidification, due to low carbonate ion concentrations created by deep mixing and low temperatures. This can have profound ecosystem impacts. Understanding these connections between the Southern Ocean, the atmosphere, and the rest of the world ocean is one of the primary research goals identified by the Scientific Committee for Antarctic Research.
Despite the importance of the Southern Ocean in global climate and biogeochemistry, it has been one of the least observed regions of the world ocean. The annual cycle of ice growth makes much of the region inaccessible in winter. The entire Southern Ocean may see only one or two research cruises each austral winter (June through September), other than those in the Drake Passage region where oceanographic ships make regular transits to Antarctica’s Palmer Peninsula. This sparse ship-based sampling is not sufficient to identify temporal changes that may occur rapidly in this system.
In a multi-institutional effort to learn more about the Southern Ocean, the SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling) science program was launched in September 2014 and funded for six years by the National Science Foundation with additional support by NOAA and NASA. The SOCCOM program operates profiling floats in the Southern Ocean as part of a global fleet of floats known as the Argo array. MBARI Scientist Ken Johnson serves as the associate director of the SOCCOM program.
The profiling floats drift freely, taking measurements from two kilometers depth to the surface every 10 days over a lifetime of five to seven years. To sample ocean biogeochemistry, profiling floats need simple, robust, and low-power chemical sensors that can operate independently for years. MBARI’s Chemical Sensor Group, led by Johnson, developed novel sensors that can measure nitrate and pH in the ocean. These sensors (the in situ ultraviolet spectrophotometer (ISUS) nitrate sensor and the Deep-Sea DuraFET pH sensor) have been successfully integrated on over one hundred SOCCOM profiling floats. Dissolved oxygen instruments are the only other chemical sensors that are routinely available on profiling floats.
The sensors are installed onto SOCCOM profiling floats at MBARI, and then sent to the University of Washington for final assembly, including the addition of commercial instruments that measure oxygen, chlorophyll in phytoplankton, and optical backscatter (used to measure water turbidity, the cloudiness of water caused by large numbers of individual particles, similar to smoke in the air). These floats are then deployed in the Southern Ocean (see animation below).
The Southern Ocean was previously the least studied ocean because its harsh conditions make it difficult for research ships to travel there. This vast ocean plays a huge role in global climate and carbon cycling between the ocean, atmosphere, and land. Profiling floats have allowed scientists to collect oceanographic measurements year-round.
Large phytoplankton blooms along the ice edge are a major feature of Southern Ocean waters. Because the profiling floats can operate under ice, the floats can observe these ice-edge blooms. The concentration of particulate carbon shown in the animation reflects the growth of phytoplankton, as confirmed by the chlorophyll sensor on the profiling floats.
As phytoplankton accumulate, they utilize and deplete nitrate in the surface waters, which reduces the concentration relative to the winter values (negative anomalies in nitrate). The animation shows the phytoplankton bloom starting in late austral winter around 35 degrees south and the bloom then proceeding towards the pole as the seasons progress into austral summer. Large accumulations of particulate carbon result when the bloom reaches the ice edge. Fresh water from the melting sea ice stabilizes the water column and allows a strong bloom to form. This ice-edge bloom follows the retreat of the ice to the south and the depletion of nitrate moves south with the bloom.
The nitrate anomalies observed by the floats lead to some unique constraints on the processes that occur in this critical ecosystem. Zooplankton continuously graze on the phytoplankton and export their biomass to depth in the form of fecal material, a process termed the “biological pump”. The strength of the biological pump is an important control on the ocean’s absorption of carbon dioxide. Understanding its magnitude and variability in the Southern Ocean has been a challenge. The amount of particulate carbon observed by float sensors does not reflect the total amount of phytoplankton production in the system, as it does not show the particulate carbon that has been consumed and exported to the deep.
How can we determine how much carbon from phytoplankton was produced and then removed by grazing if the floats can’t observe it? Nitrate provides the answer. In the animation, the accumulation of particulate carbon and the depletion of nitrate occur near a one-to-one ratio. That is, if about 10 micromoles per kilogram of particulate carbon accumulate, about 10 micromoles per kilogram of nitrate are depleted relative to winter concentrations. However, it is well established that when phytoplankton grow, the ratio of carbon to nitrogen in the biomass that results is about 6.6 to one. This proportion is termed the Redfield Ratio. While variations in the Redfield Ratio carbon-to-nitrate value can occur, they are generally in the direction of values higher than 6.6 to one.
If particulate carbon is accumulating at a ratio nearer to one to one, relative to nitrogen consumption (as is shown in the animation), then large amounts of particulate carbon have been lost from the upper ocean. It follows that some 5.6 units of carbon must have been produced and exported to depth for every unit of particulate carbon that remains in the surface waters. This conclusion is only possible with the supporting nitrate sensor data collected from the profiling floats. While satellite remote-sensing data provide the tools to observe particulate carbon accumulation (though with difficulty in the Southern Ocean due to extensive cloud cover), remote sensing observations of the particulate carbon loss have not been possible.
Analysis of data from the SOCCOM floats allow carbon export to be determined throughout the Southern Ocean without sending scientists to sea on ships for long periods of time. These detailed observations are not yet made in other ocean regions, except by ship-based experiments that occur only infrequently. The sensors on the floats enable a variety of other biogeochemical processes, such as air-sea gas exchange and ocean acidification, to be observed as well.
While hundreds of profiling floats represent a quantum leap forward in data collection, when compared to ship-based efforts, these floats still undersample a highly variable ocean. The next step is to merge the large volume of data returned by the SOCCOM fleet with high-resolution, data-assimilating models to enable new methods of data analysis and ocean visualization. The SOCCOM program is catalyzing this next step in ocean analysis with the Biogeochemical-Southern Ocean State Estimate (B-SOSE) model developed by Matt Mazloff and Ariane Verdy at Scripps Institution of Oceanography. An ocean-state estimate model is a real-time model that assimilates ocean observations and optimizes model inputs to minimize the difference between model estimates and observations, much like an atmospheric weather model. The B-SOSE accomplishes this by assimilating data from the floats and then adjusting biogeochemical boundary conditions so that the model more closely simulates observations.
The animation shows the high-resolution B-SOSE output for pH at 100 meters depth from the beginning of float deployments in 2014 through 2016 with the profiling float data superimposed on the model output. The model output clearly shows the variability in a property such as pH, which is far beyond our observational capabilities to fully sample. In the graphic, the float data are colored with the same scheme as the model output. In this format, if the observations are difficult to discern from the model results, then the model is performing well and scientists become more confident of the fine-scale structure revealed by the model. In regions where the float data stand out in the animation, the model must be improved. The SOCCOM plan is to implement as many floats as needed such that a model numerically approximating the laws of nature is able to fill in the gaps.
The profiling float data, processed and inspected for quality control at MBARI, are immediately placed in the public domain, where they have been utilized in dozens of research studies. The real-time data from SOCCOM floats have also been a valuable tool for outreach and education. MBARI’s Information and Technology Dissemination Division now operates an Adopt-A-Float program in partnership with Climate Central, one of the SOCCOM partner institutions. School classes from kindergarten through high school are paired with SOCCOM scientists who go to sea for float deployments. Students can name a float and follow its progress through blogs written at sea. The float data are made available through a special version of SOCCOMViz. A model float, constructed at MBARI, showcases the innards of a profiling float and is available for classroom instruction.
The SOCCOM program will continue to build and deploy profiling floats in the Southern Ocean for three more years. The experience and tools developed in this work, including improved sensors and novel analytical frameworks such as the B-SOSE model, are forming the basis for a global ocean observing system. This system will enable real-time monitoring of ocean health and the role of ocean biota in regulating global climate. The challenge for the ocean science community lies in finding the resources to implement such a system across the global ocean. The answers that lay in wait are not only of interest to the scientific community, but are also important to understanding how ocean and atmosphere interactions, far from where most people live, ultimately impact human health and the planet