Profiling Floats Equipped with Rainfall, Wind Speed, and Biogeochemical Sensors for Use in the Tropical Pacific Observing System

PIs: Stephen C. Riser, School of Oceanography, University of Washington; Jie Yang, Applied Physics Laboratory, University of Washington

Description

This work is part of the Tropical Pacific Observing System (TPOS) New Technologies pilot initiative to build a suite of 15 Argo-type profiling floats over a period of 4 years with an advanced suite of sensors, and to deploy these floats in the tropical Pacific as part of a test of enhancing the Tropical Pacific Observing System. These floats all have Argotype capabilities in terms of providing basic CTD data in real-time through the Argo data stream and the GTS, and in addition carry sensors for measuring (1) dissolved oxygen; (2) pH; (3) chlorophyll fluorescence and particulate backscatter; (4) rainfall (using passive acoustic methods); and (5) wind speed (also passive acoustics). All of the float and sensor data are part of the real-time Argo data stream. While we have previously built floats with each of these capabilities, the work in TPOS2020 represents the first time that this particular set of sensors has been used together. Numerical models and analysis of the existing Argo dataset have shown that many of the capabilities of the TAO array in the Equatorial Pacific can be duplicated with an expanded array of Argo profiling floats at a reduced cost. Other parameters (e.g. the list given above) have been measured utilizing the float platforms at little cost in terms of the extra energy required, and not unreasonable increases in cost in terms of the extra hardware needed.

This project was funded by the NOAA’s Global Ocean Monitoring and Observing Program (GOMO) as one of its six technology development projects in support NOAA’s contribution to TPOS 2020.

Tracks of 15 floats deployed as part of this project since 2018. The float numbers are shown at the beginning of the tracks, and the dots along the tracks are shown at 60 day intervals.

Accomplishments

Since 2018 we have deployed 15 floats equipped with these sensors in the tropical Pacific, as can be seen in the accompanying plots. The floats, all drifting at 1000 m and profiling from 2000 m at 10-day intervals, show the known, strongly banded structure of the mid-depth flow near the Equator.

Each of the floats collects samples of temperature, salinity, dissolved oxygen, pH, chlorophyll fluorescence, and particulate backscatter between the sea surface and approximately 2000 m on each profile, as shown in the sample profiles below. The floats were deployed on several cruises between 2018 and 2020; some of the earliest floats have now collected 100 profiles.

When viewed together, the profile data show the evolution of these parameters over time, with the most variability in the upper few hundred meters.

Each float also carries a PAL (Passive Aquatic Listener) sensor that is used to estimate wind speed and rainfall using passive acoustic methods. The PAL samples the variability in acoustic pressure while the float is drifting at a depth of 1000 m between profiles; these data are returned and converted to hourly rain and wind estimates after the profiles are received, as can be seen in the accompanying diagram for float 12780.

Lessons Learned

The data indicate that it is clearly feasible to collect useful data in the Equatorial ocean using biogeochemical profiling floats over extended periods. As can be seen in the time/pressure plots for float 12792, the floats sample the strong seasonal variability in the upper ocean that is characteristic of the low-latitude Pacific; there are latesummer maxima in temperature, dissolved oxygen, and pH, accompanied by a late summer minimum in salinity. For floats located elsewhere in the region, the seasonal maxima and minima occur at different times and amplitudes. The wind and rainfall clearly also show seasonal signals (especially rain). These are new observational tools in the Equatorial ocean and hint at what might be possible with a much larger fleet of such floats in the region. It is also possible to use the data collected to infer other (unmeasured) biogeochemical parameters, such as pCO2, nutrients, silicate, and titration alkalinity. This has been previously done for other regions with BGC-floats, such as the Southern Ocean, and is now underway for the global ocean, including the Equatorial Pacific region. The float and sensor data collected in this work are now being used in several publications that are in preparation.

Publications

Carter, B. and coauthors (2021) Locally interpolated regressions version 3: updated algorithms for global seawater property estimation. In preparation.

Johnson, K, and M. Bif (2021) Primary productivity in the global ocean observed from Argo oxygen floats. Submitted to Nature.

Riser, S. and J. Yang (2021) Physical and biogeochemical variability in the Equatorial Pacific observed from an array of profiling floats. In preparation.

Data

All of the data are publicly available in near real-time. The profile and raw PAL data can be obtained via ftp through the Argo Global Data Assembly Center (GDAC) in netCDF format, at ftp://usgodae.org/pub/outgoing/argo or ftp://ftp.ifremer.fr/ifremer/argo. The hydrographic and profile data (minus the wind and rain) are also available in ASCII format from the MBARI FloatViz site (https://www.mbari.org/science/upper-ocean-systems/chemical-sensorgroup/floatviz/). The MBARI site includes biogeochemical quantities derived from the raw observations, such as pCO2, DIC, and total alkalinity. The adjusted wind and rainfall data are currently available by request from UW and will soon appear on a dedicated public website.

Profiles of temperature, salinity, dissolved oxygen, and chlorophyll fluorescence from floats 12792 (colored violet in the float trajectory plot) and 17534 (colored magenta in the float trajectory plot) in May of 2020.

Wind speed (left) and rainfall rate (right) along the path of float 12780 (the green track in the trajectory plot), during the period of July 2018 through March of 2021. These quantities are measured acoustically using a PAL (Passive Aquatic Listener) sensor on the float, while the float is drifting at a depth of 1000 m between profiles. The wind speed is variable, often peaking in the autumn of the year, while the rainfall tends to be largest in mid-winter and mid-summer, with minima in spring and autumn. During 2020 and 2021 a La Nina event likely influenced the rainfall, especially late in 2020 and early in 2021. All of the floats shown in the trajectory plots are equipped with PAL sensors.