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Peter G. Strutton and Francisco P. Chavez

Abstract

Changes in phytoplankton concentration, mixed layer depth, and incident radiation strongly modify the upper- ocean heat budget. An extreme example occurred during the 1997/98 El Niño–La Niña. In the central equatorial Pacific, biological heating of the mixed layer increased from ∼0.1°C month−1 in December 1997 (El Niño) to ∼1.0°C month−1 in July 1998 (La Niña). This change was due to 1) shoaling of the mixed layer from ∼100 to ∼20 m (∼56% of the 0.9°C month−1 increase); 2) a twentyfold increase in surface chlorophyll concentrations (∼29% of the increase), coincident with a shoaling of the subsurface chlorophyll maximum from ∼100 to ∼50 m; and 3) an increase in incident shortwave radiation from ∼175 to 275 W m−2 (∼15% of the increase). The observed range of heating rates (0.1°–1.0°C month−1) corresponds closely to the mean condition of the western (oligotrophic) and eastern (mesotrophic) equatorial Pacific, respectively. Increased phytoplankton concentrations act to retain heat near the surface and should result in shallower mixed layer depths. The influence of decadal changes in chlorophyll concentrations on heat storage was also quantified. The observed chlorophyll variability leads to interannual changes in penetrative heat flux (E d,SW,PEN, the irradiance flux out of the bottom of the mixed layer) of the order of 5 W m−2, or from 65% to 170% of the mean. This variability is significant when compared with recent work that describes couplings between tropical and global ocean temperature dynamics. The analyses presented here show that satellite and buoy data can be used to accurately and simply estimate the biological contribution to heating for basin-scale studies, and possibly for future improvement of ocean circulation models.

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Francisco P. Chavez, Dave Wright, Robert Herlien, Michael Kelley, Farley Shane, and Peter G. Strutton

Abstract

A shutter mechanism for reducing the effects of biofouling on bio-optical instruments deployed on oceanographic moorings has been designed, built, and tested. The initial development was carried out on a spectroradiometer. The optics of the spectroradiometer are protected by copper shutters that rotate out of the field of view prior to a measurement and rotate back after the measurement is completed. The shutter system can sense an obstruction and, if one is detected, attempt to rotate in the opposite direction. The controlling software stores the home position in the memory so the shutter can return to cover the optics, irrespective of direction of rotation. The system has been tested in the equatorial Pacific, where it has provided five months of data that are unaffected by biofouling.

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Francisco P. Chavez, J. Timothy Pennington, Robert Herlien, Hans Jannasch, Gary Thurmond, and Gernot E. Friederich

Abstract

A telemetering electronics/control unit (OASIS) has been developed for use on moorings and free-floating drifters. The OASIS controllers are part of a long-term “coastal ocean observatory,” consisting of an infrastructure of ships, submersibles, an array of remote sensing platforms (moorings, drifters), communication links between sensors and laboratory, and data management facilities. The OASIS controller coordinates retrieval of data from a varying array of up to 28 oceanographic sensors, which may output digital, analog, or frequency data. The controller provides scheduling, sensor control software, data logging, preliminary data processing, and two-way telemetry between remote platform and ship or shore station. Telemetry allows real-time access to data and permits users to alter control parameters as necessary.

Two OASIS moorings have been successfully deployed off the central California coast since 1992. Real-time access and two-way telemetry has allowed the moorings to become testbeds for the deployment of new sensors and widely used observational and planning tools. Over longer timescales the moorings will be an important tool for tracking environmental variability. OASIS drifters have been tested in 11 deployments off California and 3 deployments in the equatorial Pacific. This paper describes the OASIS controller, its deployment on moorings and drifters, and presents oceanographic data that demonstrate the types of information obtained both from central California and the equatorial Pacific.

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Yuichiro Takeshita, Brent D. Jones, Kenneth S. Johnson, Francisco P. Chavez, Daniel L. Rudnick, Marguerite Blum, Kyle Conner, Scott Jensen, Jacqueline S. Long, Thom Maughan, Keaton L. Mertz, Jeffrey T. Sherman, and Joseph K. Warren

Abstract

The California Current System is thought to be particularly vulnerable to ocean acidification, yet pH remains chronically undersampled along this coast, limiting our ability to assess the impacts of ocean acidification. To address this observational gap, we integrated the Deep-Sea-DuraFET, a solid-state pH sensor, onto a Spray underwater glider. Over the course of a year starting in April 2019, we conducted seven missions in central California that spanned 161 glider days and >1600 dives to a maximum depth of 1000 m. The sensor accuracy was estimated to be ± 0.01 based on comparisons to discrete samples taken alongside the glider (n = 105), and the precision was ±0.0016. CO2 partial pressure, dissolved inorganic carbon, and aragonite saturation state could be estimated from the pH data with uncertainty better than ± 2.5%, ± 8 μmol kg−1, and ± 2%, respectively. The sensor was stable to ±0.01 for the first 9 months but exhibited a drift of 0.015 during the last mission. The drift was correctable using a piecewise linear regression based on a reference pH field at 450 m estimated from published global empirical algorithms. These algorithms require accurate O2 as inputs; thus, protocols for a simple predeployment air calibration that achieved accuracy of better than 1% were implemented. The glider observations revealed upwelling of undersaturated waters with respect to aragonite to within 5 m below the surface near Monterey Bay. These observations highlight the importance of persistent observations through autonomous platforms in highly dynamic coastal environments.

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