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Climate Variability, Ocean Circulation, and Ecosystems

The ocean stores heat and carbon dioxide, and redistributes them via its large-scale circulation. Both the storage capacity and redistribution patterns are affected by climate change, and the ocean in turn may modify climate through various feedback mechanisms. Variability in ocean-atmosphere feedback on climate time scales may have profound effects on ecosystems through habitat changes (e.g. increasing mean temperature) or through modulation of shorter time scale phenomena (e.g. the El-Nino/Southern Oscillation cycle).

Learn more about the various themes within Climate Variability, Ocean Circulation, and Ecosystems:

Climate Variability and Ecosystems

Atmospheric forcing at seasonal to interdecadal time scales strongly influences the structure of marine food webs.  Understanding when and how marine ecosystems shift between equilibrium states is a widely debated and central issue for both the research and marine resource management communities because many ocean food webs appear to be undergoing major shifts.

For example, limited time series in the subtropical North Pacific Ocean show substantial changes in the phytoplankton, zooplankton, and pelagic fish biomass during the mid-1970s and 1980s.  Early evidence indicates another shift may have occurred in the late 1990s.  The North Atlantic has also exhibited recent shifts in circulation, with corresponding declines in copepod and cod stocks.

Untangling and understanding the causes, processes, and consequences of the different ocean climate cycles is an important problem confronting oceanographers.  These cycles span years to decades upon which episodic events (e.g., storms) are superimposed. Thus, scientists require high-frequency (minutes), sustained (decades) time-series data across a range of ecologically relevant spatial scales.  High-frequency data are needed to resolve the physical structuring of marine food webs, which can be heavily influenced by short-lived episodic events.  While sustained data collection over decades will capture these interdecadal cycles in atmospheric forcing.

The OOI network of distributed assets measures atmospheric and in situ physical, chemical, and biological properties.  Vertical resolution of the system ranges from less than one meter to tens of meters; horizontal resolution ranges from meters to hundreds of kilometers.  For many ecosystem questions, OOI data resolves the chemistry and the particulate matter in the upper 200 m of the water column.

Given that many of the signatures of these large-scale processes are resolved at local and regional scales, the network includes coastal and global sites.  Network nodes in the North Pacific and subpolar Southern Ocean obtain data on the El Niño Southern Oscillation and Pacific Decadal Oscillation cycles, while nodes in the subpolar and subtropical Atlantic obtain data on the North Atlantic Oscillation.

Related Science Questions

  • How do climate signals due to forcing at ENSO, NAO, and interdecadal time scales (e.g., PDO) lead to changes in water column structure and chemical and biological properties?
  • What are the effects of climate signals on variability in water column structure, nutrient injection in the photic zone, primary productivity, and vertical distribution and size structure of particulate material?

Ocean Circulation, Mixing, and Ecosystems

Water column mixing is central to driving ecosystem productivity by replenishing nutrients to the euphotic zone; however if mixing is too vigorous, overall productivity is suppressed by light limitation. The nonlinear interaction between mixing and light availability, and the corresponding ecosystem response remains a central question to biological and chemical oceanography.  These nonlinear processes impact overall phytoplankton community composition, which in turn affects entire food webs.

In the past it has been difficult to measure the impact of mixing on ecosystem dynamics. Traditional approaches have not allowed scientists to maintain a persistent presence in the ocean to quantify the role of high- and low-frequency mixing events. The relative role of episodic and seasonal mixing events on the overall productivity of marine ecosystems remains an open question; their importance relative to large cyclical phenomena (El Niño Southern Oscillation, Pacific Decadal Oscillation, North Atlantic Oscillation) remains difficult to evaluate.

The OOI provides the infrastructure to persistently observe mixing processes in the ocean and assess the corresponding impact on the marine ecosystems.  The distributed OOI assets measure parameters necessary for studying air-sea exchange processes, mixed-layer depth dynamics, material exchange across the base of the mixed layer, internal wave dynamics, the evolution of benthic boundary layers, and changes in the composition and size distribution of the phytoplankton.

Measurements are made on horizontal scales of meters to kilometers and vertical scales of millimeters to meters. Water column data are collected at high frequency by profiling moorings, including the critical upper 200 m.  Data collected by the profiling moorings are spatially extended by running coordinated transects of AUVs and gliders.  Observations of resuspension and benthic boundary layer dynamics are enabled with sensors mounted at several depths above the seafloor.

The broadly distributed OOI sensor network allows for the measurement of numerous parameters, from the deep sea to the near-shore coastal ocean, enabling comparisons of a range of ecosystems.  For example, the high-latitude subpolar sites are representative of regions with severe weather, high CO2 flux, and oligotrophic and High Nutrient Low Chlorophyll (HNLC) areas.  Additionally, this broad distribution means that data from contrasting systems, such as nutrient mixing in the Pacific Northwest and the Middle Atlantic Bight, can comprehensively be compared and analyzed.

Related Science Questions

  • How do severe storms and other episodic surface mixing events affect physical, chemical, and biological water column processes?
  • What are the effects of variable strength storms on surface boundary layer structure and nutrient injection in the photic zone, primary productivity, and vertical distribution and size structure of particulate material?

 

Global Biogeochemistry and Carbon Cycling

The ocean modifies, and is affected by, climate; it serves as both reservoir and distributor of heat and carbon dioxide.  Understanding the processes of air-sea exchange and sequestration of CO2 (including anthropogenic CO2) in the oceans is critical in predicting the effect of CO2 emissions on climate and ocean ecosystems.

The exchange of CO2 between atmosphere and ocean is mediated by two general mechanisms: the solubility pump and the biological pump.  The solubility pump is driven by fluxes and mixing at the air-sea interface, ocean ventilation, and carbonate solubility. The biological pump is the conversion of dissolved CO2 into particulate and dissolved organic carbon by marine phytoplankton using light and nutrients.  Most of this particulate organic carbon is recycled through complex respiratory paths, however a fraction sinks and is sequestered for long time periods in the deep ocean or buried in marine sediments. Geological features on and below the seafloor such as volcanoes and gas hydrate formations, are also sources of ocean carbon in the form of carbon dioxide, carbon monoxide, methane, and other carbon compounds.

With sensors from the atmosphere to the seafloor, the OOI measures the oceanic carbon cycle as carbon dioxide as it moves from the atmosphere to the sea surface and is then assimilated phytoplankton. Seafloor instrumentation measure carbon exchange from the seafloor into the overlying ocean through hydrates and vents.

The rates of biological carbon fixation and sequestration are highly variable in the world’s oceans.  Increasing CO2 and climate change are projected to have significant impacts on ocean circulation, primary production, biogeochemical cycling, and ecosystem dynamics. Changes to atmospheric forcing, the heat content in the upper ocean, changes in ocean circulation will have regional effects on the exchange of CO2 across the air-sea boundary.  Climate variability influences nutrient distributions, phytoplankton growth, and phytoplankton community composition.

The broadly distributed OOI network allows for the examination of the oceanic carbon cycle on multiple spatial (latitudinal, depth), and temporal (seconds to decades) scales resolving levels of its current and evolving variability. For example, high latitude food webs, especially in the North Pacific and Southern Ocean, have been identified as particularly sensitive to changes in ocean pH.

Related Science Questions

  • What is the ocean’s role in the global carbon cycle?
  • What are the dominant physical and biological processes that control the exchange of carbon and other dissolved and particulate material (e.g., nutrients, organic matter, dissolved gases, and other materials) across the air-sea interface, through the water column, and to the seafloor?
  • What is the spatial (coastal versus open ocean) and temporal variability of the ocean as a source or sink for atmospheric CO2?
  • What is the seasonal to inter-annual variability in particulate (organic) flux?
  • What is the impact of decreasing pH to the chemistry and biology of the ocean?