Irminger Sea Carbon Cycle

By Amber Stronk | October 31, 2024 | Comments Off on Irminger Sea Carbon Cycle
Coastal and Global Scales Nodes Science Highlight Q4

The high-latitude North Atlantic, a region with high phytoplankton production in the spring and deep convection in the winter, is of particular importance for the global carbon cycle. The vertical transport of carbon from near the surface into the deep ocean, by combination of biological and physical processes, is known as the biological carbon pump. The carbon pump is particularly active in the Irminger Sea, yet the carbon budget, and its seasonal and interannual variability, are poorly known.  A study by Yoder et al. (2024) used carbon system data from multiple observational assets (moorings and CTD casts) of the OOI Irminger Sea Array to assess net community production in the mixed layer and the implications for the biological pump in this region.

Data analysis was challenging, because it involved working with multiple instrument types, gappy records, calibration offsets, and other idiosyncrasies. In addition, data from multiple instruments and observing platforms needed to be combined to produce continuous records. The primary sensors utilized were pH and pCO2. These are difficult sensors to work with, to the extent that a community workshop was convened to develop a “users guide” to best practices for analysis (Palevsky et al., 2023). Yoder et al. were able to quality control, cross-calibrate, and merge data from the OOI surface mooring, flanking moorings, gliders and shipboard CTD casts (Fogaren and Palevsky 2023; Palevsky et al. 2023) to create the first multi‐year time series of the inorganic carbon system for the Irminger Sea mixed layer. This remarkable data set, based on instruments with sample rates of 1-2 hours, provides a seven-year record with near-daily resolution (Figure 28).

The time series results (Figure 3) showed that carbon system variables (dissolved inorganic carbon (DIC), total alkalinity (TA), and partial pressure of CO2 (pCO2)) co-vary through the annual cycle, with minimums in late summer at the end of the productive season and maximums in winter. The summer draw-down of pCO2 indicates that biophysical effects, rather than temperature, are the primary drivers of pCO2 variability. The influence of vertical mixing and primary productivity can be clearly seen in DIC and TA. In the subpolar North Atlantic, shoaling of the mixed layer in spring is generally associated with spring phytoplankton blooms, as indicated by increasing chlorophyll (Chl) concentration. Interestingly, it is found that highest integrated rates of DIC removal from the mixed layer via photosynthesis take place prior to mixed layer shoaling.

After a thorough analysis that included mixed layer budgets of DIC and TA, followed by assessment of gas exchange, physical transport, and the hydrologic cycle, the authors conclude that strong biological drawdown is the primary removal mechanism of inorganic carbon from the mixed layer. Furthermore, they point out the importance of interannual variability in both the drivers of and resulting magnitude of surface carbon cycling. This is primarily due to variability in net community production. Acknowledging the challenges taken on by OOI to maintain an array in the Irminger Sea, the authors note that “collecting observational data is both costly and challenging, however if only 1 year of data is collected or multiple years are averaged together, [carbon system dynamics] … will be misrepresented.”

This project shows the potential for OOI data, with appropriate processing and analysis, to provide unique insights into the ocean carbon system. It is notable that the authors made a substantial effort to calibrate and combine data from multiple instruments and moorings, and to take advantage of ancillary data (e.g. gliders, OOI CTD casts, and non-OOI CTD casts) in their processing. Enabling this type of analysis was a goal in the design of the multi-platform OOI Arrays and shipboard validation protocols.

[caption id="attachment_34983" align="alignnone" width="623"]Coastal and Global Scales Nodes Science Highlight Q4 Time series of dissolved inorganic carbon (DIC), total alkalinity (TA), partial pressure of CO2 (pCO2) temperature, chlorophyll-a (Chl), and mixed layer depth (MLD) in the Irminger Sea mixed layer from 2015-2022. Colors identify annual cycles. From Yoder et al., 2024.[/caption]

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References:

  1. Fogaren, K. E., Palevsky, H. I. (2023) Bottle-calibrated dissolved oxygen profiles from yearly turn-around cruises for the Ocean Observations Initiative (OOI) Irminger Sea Array 2014 – 2022. Biological and Chemical Oceanography Data Management Office (BCO-DMO). Version Date 2023-07-19 doi:10.26008/1912/bco-dmo.904721.1
  2. Palevsky, H.I., S. Clayton and 23 co-authors, (2023).OOI Biogeochemical Sensor Data: Best Practices & User Guide Global Ocean Observing System, 1(1.1), 1–135. https://doi.org/10.25607/OBP-1865.2
  3. Palevsky, H. I., Fogaren, K. E., Nicholson, D. P., Yoder, M. (2023) Supplementary discrete sample measurements of dissolved oxygen, dissolved inorganic carbon, and total alkalinity from Ocean Observatories Initiative (OOI) cruises to the Irminger Sea Array 2018-2019. Biological and Chemical Oceanography Data Management Office (BCO-DMO). Version Date 2023-07-19 doi:10.26008/1912/bco-dmo.904722.1
  4. Yoder, M. F., Palevsky, H. I., & Fogaren, K. E. (2024). Net community production and inorganic carbon cycling in the central Irminger Sea. J. Geophys. Res., 129, e2024JC021027. https://doi.org/10.1029/2024JC021027
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Summer Science Tours: CGSN Engages Young Environmentalists

By Amber Stronk | August 28, 2024 | Comments Off on Summer Science Tours: CGSN Engages Young Environmentalists

The U.S National Science Foundation (NSF) OOI Coastal and Global Scale Nodes (CGSN) Team at WHOI has had a busy summer of talks and tours. With the help of Mashpee Wampanoag WHOI Tribal Liaison and Native Land Conservancy (NLC) founding board officer, Leslie Jonas, CGSN hosted two notable sets of visitors in July and August 2024. The NLC is an Indigenous-led land conservation nonprofit based on Cape Cod that seeks to preserve land for future generations.

As a part of their Preserving Our Homelands (POH) summer science program, a group of students from the Mashpee Wampanoag tribe visited WHOI on 18 July. The POH program provides 6th, 7th, and 8th grade native students with hands-on science experiences in order to deepen their understanding of the environment from a western science perspective and its relationship to tribal culture, and traditional ecological knowledge. Their visit included a stop at the LOSOS facility, where CGSN team members talked about the scientific and technical aspects of the OOI program and provided an opportunity to see ocean observing technology up close. CGSN is grateful to WHOI engineer Ben Weiss and Sea Grant Marine Educator, Grace Simpkins, for organizing the visit and looks forward to ongoing interactions with the POH program.

Before the excitement from the POH tour had died down, a second group of visitors was hosted in early August. The group was made up of about 20 members of the Black, Indigenous, and People of Color (BIPOC) environmental science community. This included the NLC Executive Director, Diana Ruiz, and thirteen members of the Massachusetts Audubon Society and four NLC First Light Fellows. First Light is a paid summer fellowship program for rising Native American conservationists ages 18-25. With mentors from Mass Audubon, Fellows develop individual projects with topics in areas of ecological research, wetland restoration, water quality or land protection that build career skills and advance the NLC’s work. The fellowships combine indigenous culture, environmental sciences, and career development in order to open up career pathways. The four Indigenous Fellows who visited WHOI are studying at Brown, Yale, and Salish Kootenai College and got exposure to real-world instrumentation and engineering tools used to address pressing questions in ocean science research.

Read more about the NLC Fellows.

[caption id="attachment_34683" align="alignnone" width="640"] WHOI Senior Engineering Assistant Diana Wickman discusses the operation of an OOI ocean glider with Mashpee Wampanoag POH visitors. Photo credit J. Lund.[/caption] [caption id="attachment_34684" align="alignnone" width="640"] The August group included Native Land Conservancy First Light Fellows and members of the Massachusetts Audubon Society. Photo credit: L. Jonas.[/caption] Read More

Deep-Ocean Vertical Structure

By Amber Stronk | July 31, 2024 | Comments Off on Deep-Ocean Vertical Structure

It is often assumed that, at frequencies below inertial, the vertical structure of horizontal velocity and vertical displacement can be reasonably described by a single dynamical mode, e.g. the lowest order flat-bottom baroclinic mode. This is appealing because it would mean that first-order predictions of deep-ocean velocity structure could be determined from knowledge of density and surface currents. However, there is a relative paucity of full ocean depth data to test this idea. A study by Toole et al. (2023) used full ocean depth data from five sites – four of which are Ocean Observatories Initiative (OOI) arrays (Station Papa, Irminger Sea, Argentine Basin and Southern Ocean) – to address the question “does subinertial ocean variability have a dominant vertical structure?”

Data analysis was challenging, because it involved working with gappy records as well as combining information from multiple instruments on different moorings. As noted by the authors, “no single OOI mooring sampled velocity, temperature and salinity over full depth.” Wire-following profiler data from Hybrid Profiler Moorings were combined with ADCP and fixed-depth CTD data from adjacent moorings. While the authors note that “depth-time contour plots of the velocity data from each OOI site clearly reveal the shortcomings of the datasets” they also recognized that despite the shortcomings, “these observations constitute some of the only full-depth observations of horizontal velocity and vertical displacement from the open ocean.”

It was possible to obtain 2-3 years (non-contiguous in some cases) of near-full ocean depth data from each site. Inertial and tidal variability was removed, and the data were filtered over 100 hr (~4 days). Empirical Orthogonal Function (EOF) decomposition was used to identify an orthogonal basis set that described horizontal velocity and vertical displacement. In addition, dynamical modes were determined for three cases: flat bottom, sloping bottom and rough bottom. Note that computing the dynamical modes requires the vertical density profile, which was taken as the mean over each deployment. Analysis was focused on the lowest modes, which accounted for the majority of the variance.

The results (Figure 32) showed that there is an EOF consistent with a dynamical mode at most sites. However, the appropriate dynamical mode is different for each site – no single dynamical accounted for a dominant fraction of variability across all sites. The authors note that differences in bathymetry, stratification and local forcing complicate the picture, with different dynamical processes dominating at different sites. Prior studies (not full ocean depth) that appear to show a “universal” vertical structure may be misleading

This project shows the potential for OOI data, with appropriate processing and analysis, to provide unique insights into ocean structure and dynamics. The researchers have made the combined vertical profile data available to the community on the Woods Hole Open Access Server. The dataset DOI (https://doi.org/10.26025/1912/66426) is also linked here: https://oceanobservatories.org/community-data-tools/community-datasets/.

[caption id="attachment_34586" align="alignnone" width="624"] Mode 1 EOFs for velocity (u, red; v blue; cm/s) and vertical displacement (black, decameters) for OOI arrays at (from left) Argentine Basin, Southern Ocean, Station Papa and Irminger Sea. Adapted from Toole et al., 2023.[/caption]

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References:

Toole, J.M, R.C. Musgrave, E.C. Fine, J.M. Steinberg and R.A. Krishfield, 2023. On the Vertical Structure of Deep-Ocean Subinertial Variability, J. Phys. Oceanogr., 53(12), 2913-2932. DOI: 10.1175/JPO-D-23-0011.1.

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Wave Statistics from 3-Axis Motion Sensors on OOI Surface Buoys

By Darlene Trew crist | October 11, 2023 | Comments Off on Wave Statistics from 3-Axis Motion Sensors on OOI Surface Buoys

The Ocean Observatories Initiative (OOI) Pioneer Array at the New England Shelf (Pioneer-NES) collected data for nine years from November 2013 through November 2022 across the shelf break. Of the three Surface Moorings in the array (Inshore – ISSM (40.37°N, 70.88°W); Central – CNSM (40.14°N, 70.77°W); Offshore – OSSM (39.94°N, 70.89°W), only CNSM was equipped with a surface wave sensor: the Axys Technologies Tri-Axys Directional Wave Sensor (WAVSS). This meant that observations on wave data were limited to a single location within the array. Recognizing that data from a single location could be restrictive for some types of analysis, the Coastal and Global Scale Nodes Group (CGSN) identified an opportunity to increase the number of surface wave observations and extend their geographic extent to the full cross-shelf span of the Pioneer-NES Moored Array. This was accomplished by using the engineering data collected by the MicroStrain 3-axis motion sensors (MOPAK) deployed on all three surface moorings. The data collected by the MOPAKs can be used to compute the bulk and directional wave statistics at each Surface Mooring in the array.

The MOPAK sensors collected triaxial acceleration, angular rate, and magnetic orientation for 20-minutes at 10 Hz once-an-hour. These data are used to compute the buoy displacements and velocities. A zero-crossing algorithm, which identifies the number of times the buoy vertical displacement (heave) crosses zero (indicative of wave motion), is used to calculate six bulk wave statistics: significant wave height (Hsig) and period (Tsig); wave height (H10) and period (T10) of the highest 10% of waves; and the mean wave height (Havg) and period (Tavg).

The wave power and cross-spectrums are used to compute five directional wave statistics: peak wave height (Hs) and period (Tp); mean wave direction and spread; and an alternative method for significant wave height (Hm0). The MOPAK-derived wave statistics were validated against, and showed excellent agreement with, both the WAVSS dataset from the CNSM mooring and wave datasets collected by National Data Buoy Center wave buoys 44097 (Block Island, RI – 40.97°N, 71.12°W) and 44008 (Nantucket, MA – 40.50°N, 69.25°W).

[media-caption path="https://oceanobservatories.org/wp-content/uploads/2023/10/Screenshot-2023-10-31-at-3.12.55-PM.png" link="#"]Figure 1. The significant wave height (Top) and mean wave period (Bottom) at the Pioneer-NES Central Surface Mooring for Deployment 11 (Apr. 2019 – Sept. 2019) as measured by the WAVSS (blue), calculated from the MOPAK (red), and from the two nearest located NDBC buoys – Nantucket (green) and Block Island (grey).[/media-caption]

The Python code to process a MOPAK dataset into a wave dataset is available to users as the process_mopak.py module in the public OOI Data Exploration GitHub repository. The wrapper function calculate_wave_statistics in the module accepts a deployment’s worth of MOPAK data and returns a new dataset with the calculated wave statistics, including attributes, units, and associated metadata, which may be saved as a new netCDF file. This process triples the number of surface wave datasets at the Pioneer-NES Array, allows for validation of the existing WAVSS wave dataset, and opens new possibilities for studying the wind-wave field across the NES-break.

We encourage users to work with the MOPAK code to generate surface wave statistics, and to submit any questions to the OOI HelpDesk.

 

 

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