SeaBird Scientific has provided a commercial O2 sensor, the SBE63, for a number of years. This sensor has been evaluated by the oceanographic community and found to be as good as any sensor available for long-term use on profiling floats. To date, the sensor has been used only on SeaBird Navis floats. The SBE63 has the drawback that it must be mounted inside the pumped fluid circuit of the CTD unit; this allows the sensor to sample the same water as seen by the temperature and conductivity sensors, but because of the mounting it is not possible to collect O2 samples in air while the float is transmitting data on the sea surface. It has been shown that collecting in-air measurements in this manner and using them to correct instrument drift in the O2 sensor is vital to producing data
of the quality required in biogeochemical studies of the ocean. A number of papers have been published showing how to do this, using known values for atmospheric O2 concentration estimated from NCEP observations wherever the float surfaces.
In order to make such in-air measurements, the SBE83 has to be moved outside of the CTD fluid circuit and mounted on the float endcap. SeaBird has now made this change in a prototype sensor, known as the SBE83, and the new sensor has been mounted on a specially-designed prototype float for testing in the ocean. The idea was to put 3 separate O2 sensors on a single prototype float: the first, the new SBE83; the second, a SBE63 mounted inside the fluid circuit; and third, an Aanderaa 4330 Optode sensor mounted externally near the SBE83. The 4330 and SBE63 are known to work well, so the idea was to test the new SBE83 against them in order to examine the stability of the SBE83 over time. This test float was deployed near Hawaii in November of 2020 and has been collecting profiles at intervals from 2 to 5 days since then, as shown in the accompanying figure.
Laboratory tests of an improved pH sensor, compensated for pressure, are now underway, with the first deployment on prototype floats planned for later this year. Analogous deployments will be done with the SeaBird OCR504 radiometer sensor. The design work for an improved hull is now underway. In general, this project is on track to meet its goals over its 3-year timeline.
An additional part of this program is to derive a biogeochemical climatology for the Equatorial Pacific that can be used to help derive parameters such as pCO2, total alkalinity, and DIC from the standard biogeochemical parameters measured by profiling floats. Dr. Brendan Carter and his colleagues at NOAA have made considerable progress on this problem and are preparing a paper based on their results.
SeaBird has successfully engineered an improved O2 sensor that will likely be used extensively on commercially available BGC profiling floats in the future. The availability of these floats is crucial to large programs such as GOBGC. It is our hope that the other goals of this NOPP project will be met in a timely manner; the design and testing of a new float hull is the most difficult job remaining and is underway. The partnership of UW, MBARI, and SeaBird has generally worked well. We have had weekly teleconferences and have communicated well, even in the COVID era.
Carter, B. and coauthors (2021) Locally interpolated regressions version 3: updated algorithms for global seawater property estimation. In preparation.
At this time, only one float, an engineering prototype, has been deployed, as shown in the figure above. These data are not yet being made available on public websites, as they consist of very preliminary observations and the sensor design is still undergoing refinement. When floats with the final design are deployed, sometime in the fall of 2021, the data will be made available through the Argo data portal.