High-bandwidth Moorings

Ocean conditions surrounding the OOI RSN are bracketed on the pelagic or open-ocean end by the OOI global site at Ocean Station Papa, and on the coastal end by the Endurance Array. The OOI RSN resides in a complex system of currents, where wind- and tide-forced motions lead to turbulent mixing that aids transport of chemical and biological species. The Pacific Northwest is one of the most biologically productive regions of the world, hypoxia, ocean acidification, and harmful algal blooms are observed with increasing frequency. These complex physical, biological and chemical processes are all intertwined, and respond to forcing on a wide range of spatial and temporal scales. The water column moorings at Hydrate Ridge and Axial Seamount are well suited to resolve these processes, and the system’s response to changing forcing conditions resulting from climate change. The unprecedented power (375V) and bandwidth (Gb/s) capabilities of these moorings allow for a broad suite of sensors that include real-time digital imaging and acquisition of high bandwidth sonar and hydrophone data for biological applications.

Though both moorings are in about 3000 m of water, they have very different oceanographic foci. The mooring at Hydrate Ridge is situated adjacent to the coastal continental slope at the end of the Endurance Oregon Line, and in concert with the northern Endurance Washington Line, provides a unique opportunity for investigating a variety of interdisciplinary coastal studies (Figure 4.4-8). The coastal region of the Pacific Northwest is a classic wind-driven upwelling system. However, the presence of the Columbia River plume and the range of trajectories with which it can impinge on the ocean, and the strong variability of the width of the continental shelf, all play strong roles in setting the system’s response and behavior. In addition, the aforementioned large-scale systems affect the coastal region by modulating the pycnocline, nutricline and oxycline depths and offshore pressure gradients, which in turn affect the onshore transport of physical, biological and chemical quantities. The presence of internal waves driven by waves and tides, their interaction with the larger-scale currents, and their eventual breakdown into turbulence, are also vital to setting properties in the coastal region. All of these are expected to change strongly over time, but will be well resolved by the measurements at Hydrate Ridge, the Endurance Array, and supporting shipboard work.

In contrast to the margin setting of Hydrate Ridge, Axial Seamount is far from the continental shelf and hence represents an open-ocean or pelagic site in the continuum of observing scales represented in the OOI’s cabled system. Here, large-scale currents including the North Pacific Current, the subpolar gyre and the northern end of the California Current interact. These currents transport heat, salt, oxygen, and biota, all of which are crucial to the region’s ecosystem. However, their variability arises from forcing as varied as tides and wind (0.5--5 day timescales) to interannual (El Niño) to decadal (Pacific Decadal Oscillation) timescales. Examples of relevant science questions represented in the OOI Science Requirements include 1) Internal tides are ubiquitous vertical motions formed by tidal currents flowing past bottom features such as Axial Seamount. How, and how strongly, do they break down into turbulence, and what are the feedbacks on the large scale current system? 2) What is the impact of long- and short-term forcing changes on the structure and transports of the large-scale current system – and what are their effects on the ecosystem? Together with the mooring at Ocean Station Papa, these processes can be studied with observing platforms in the water column at these two sites.