Underway-pumping pH, pCO2 and auxiliary data along the cruise track over the Kuroshio Extension and its recirculation regions (northwestern North Pacific) in late spring 2018

1. Parameters introduction

Date (UTC/GMT+08:00)：Sampling time

Longitude: measurement longitude, from zero to 360, in decimal degrees East

Latitude: measurement latitude in decimal degrees North

SST: measured sea surface temperature in degrees Celcius

Salinity: measured sea surface salinity on the Practical Salinity Scale

Teq: equilibrator chamber temperature in degrees Celcius

Peq: equilibrator chamber pressure in hectopascals

P: air pressure measured at 10 m height in hectopascals

Air xCO₂：measured mole fraction in dry air (xCO₂) in parts per million by volume in dry air

Air pCO₂：atmospheric pCO2 in micro-atm, computed from SST, salinity and air xCO2 as follow:

Air pCO2 = xCO₂^Air×[P - pH₂O] (1)

Where P is atmospheric pressure and pH₂O is the water vapor pressure at 100% humidity, calculated using the surface temperature and in situ salinity (Weiss and Price, 1980) as follow:

pH₂O = exp(24.4543-6745.09/(SST+273.15)-4.8489×ln((SST+273.15)/100)-0.000544×Salinity).

Water pCO₂：in situ pCO₂ in micro-atm was computed from the raw data of xCO₂. In the system, a nondispersive infrared spectrometer (LI-COR Model LI-7000, Lincoln, NE, USA) coupled to a showerhead-type equilibrator was used to measure xCO₂. Then, xCO₂ was converted to pCO₂ in the equilibrator (pCO₂^eq) as follows:

pCO₂^eq = xCO₂ × (P − pH₂O), (2)

where P is the atmospheric pressure and pH₂O is the water vapor pressure at 100% humidity, calculated using the equilibration temperature and in situ salinity. Note that P^eq was unstable and thus the air pressure (P) was used to calculate pCO₂^eq.

A temperature coefficient of 4.23% °C⁻¹ (Takahashi et al., 1993) was used to calculate sea surface pCO₂ (pCO₂^in_situ):

pCO₂^in_situ = pCO₂^eq × e[0.0423×(SST−T^eq)], (3)

where Teq is the equilibrator temperature.

Wind speed：measured wind speed at 10 m height in meter per second

Chl a：estimated from underway water fluorescence measurements and validated using field-measured Chl a in milligrams per cubic meter

In-situ Durafet-pH_NBS: in the system, a Honeywell Durafet^® pH sensor was used to measure the pH of the flowing water. The Durafet pH electrode features an integral automatic temperature compensator in a one-piece construction and is suitable for varying pH and temperature ranges. The Durafet pH electrode was calibrated with three standard buffers (pH_NBS = 4.01, 7.00 and 10.01 at 25 °C, Thermo Fisher Scientific Inc., USA). NBS stands for the National Bureau of Standards, which is now the National Institute of Standards and Technology of the U.S. Department of Commerce.

2. Cruise

The northwestern North Pacific has two strong Pacific western boundary currents: the Kuroshio and the Oyashio. The southward-flowing Oyashio Current carrying low-salinity/cold subarctic water meets the high-salinity/warm water of the Kuroshio Current, forming a frontal zone with relatively high concentration of Chl a. The Kuroshio Current flows northeastward, and it turns eastward off the coast of Japan at approximately 35°N, 140°E to form the Kuroshio Extension. To the south of the KE, a deeper mixed layer develops in the Kuroshio Recirculation, which is associated with oceanic heat loss. The Kuroshio Extension and Kuroshio Recirculation regions are important CO₂ sinks, whereas the southern subtropical gyre is annually in equilibrium with atmospheric CO₂.

The cruise of the R/V Xiangyanghong 3 began on May 10 and ended on June 7, 2018. During the cruise, we conducted underway analyses of sea surface temperature (SST), salinity, fluorescence, and pCO₂. The seawater samples were pumped from ~5 m below the sea surface. Underway SST was recorded continuously every 5 s along the cruise track using an onboard Sea-Bird flow-through thermosalinograph (SBE 38, Sea-Bird Scientific, USA). An automated flowing measuring system (AS-P2, Apollo SciTech, Inc., USA) was used for analyses of sea surface salinity, fluorescence, and pCO2. In the system, a nondispersive infrared spectrometer (LI-COR Model LI-7000, Lincoln, NE, USA) coupled to a showerhead-type equilibrator was used to measure the mole fraction in dry air (xCO₂). The LI-COR 7000 was calibrated every 4 h with four CO₂ gas standards (100, 247, 405, and 545 parts per million by volume in dry air). The overall uncertainty of the pCO₂ measurement was <1% as constrained by our standard gases. Underway Chl a data were estimated from underway water fluorescence measurements and validated using field-measured Chl a.

Refer to: Li, C.-L., Zhai, W.-D., Qi, D. (2022) Unveiling controls of the latitudinal gradient of surface pCO₂ in the Kuroshio Extension and its recirculation regions (northwestern North Pacific) in late spring. Acta Oceanologica Sinica (English Edition), 41(5): 108–121. http://www.aosocean.com/article/doi/10.1007/s13131-021-1949-1