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

<b>1. Parameters introduction</b><b>Date (UTC/GMT+08:00)</b>：Sampling time<b>Longitude</b>: measurement longitude, from zero to 360, in decimal degrees East<b>Latitude</b>: measurement latitude in decimal degrees North<b>SST</b>: measured sea surface temperature in degrees Celcius<b>Salinity</b>: measured sea surface salinity on the Practical Salinity Scale<b><i>T</i></b><b>eq</b>: equilibrator chamber temperature in degrees Celcius<b><i>P</i></b><b>eq:</b> equilibrator chamber pressure in hectopascals<b><i>P</i></b><b>:</b> air pressure measured at 10 m height in hectopascals<b>Air </b><b><i>x</i></b><b>CO</b>_{<strong>2</strong>}：measured mole fraction in dry air (<i>x</i>CO₂) in parts per million by volume in dry air<b>Air </b><b><i>p</i></b><b>CO</b>_{<strong>2</strong>}：atmospheric <i>p</i>CO2 in micro-atm, computed from SST, salinity and air <i>x</i>CO2 as follow:Air <i>p</i>CO2 = <i>x</i>CO₂^Ai^r×[<i>P</i> - <i>p</i>H₂O] (1)Where <i>P</i> is atmospheric pressure and<i> p</i>H₂O is the water vapor pressure at 100% humidity, calculated using the surface temperature and in situ salinity (Weiss and Price, 1980) as follow:<i>p</i>H₂O = exp(24.4543-6745.09/(SST+273.15)-4.8489×ln((SST+273.15)/100)-0.000544×Salinity).<b>Water </b><b><i>p</i></b><b>CO</b>_{<strong>2</strong>}：in situ <i>p</i>CO₂ in micro-atm was computed from the raw data of <i>x</i>CO₂. 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 <i>x</i>CO₂. Then,<i> x</i>CO₂ was converted to <i>p</i>CO₂ in the equilibrator (<i>p</i>CO₂^eq) as follows:<i>p</i>CO₂^eq = <i>x</i>CO₂ × (<i>P</i> − <i>p</i>H₂O), (2)where <i>P</i> is the atmospheric pressure and <i>p</i>H₂O is the water vapor pressure at 100% humidity, calculated using the equilibration temperature and in situ salinity. Note that <i>P</i>^eq was unstable and thus the air pressure (<i>P</i>) was used to calculate <i>p</i>CO₂^eq.A temperature coefficient of 4.23% °C⁻¹ (Takahashi et al., 1993) was used to calculate sea surface <i>p</i>CO₂ (<i>p</i>CO₂^in_situ):<i>p</i>CO₂^in_situ = <i>p</i>CO₂^eq × e[0.0423×(SST−<i>T</i>^eq)], (3)where <i>T</i>eq is the equilibrator temperature.<b>Wind speed</b>：measured wind speed at 10 m height in meter per second<b>Chl a</b>：estimated from underway water fluorescence measurements and validated using field-measured Chl a in milligrams per cubic meter<b>In-situ Durafet-pH</b>_{<strong>NBS</strong>}: 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.<b>2. Cruise</b>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 <i>a</i>. 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 <i>Xiangyanghong</i> 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 <i>p</i>CO₂. 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 <i>p</i>CO2. 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 (<i>x</i>CO₂). 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 <i>p</i>CO₂ measurement was &lt;1% as constrained by our standard gases. Underway Chl <i>a</i> data were estimated from underway water fluorescence measurements and validated using field-measured Chl <i>a</i>.<b>Refer to:</b> Li, C.-L., Zhai, W.-D., Qi, D. (2022) Unveiling controls of the latitudinal gradient of surface <i>p</i>CO₂ in the Kuroshio Extension and its recirculation regions (northwestern North Pacific) in late spring. <i>Acta Oceanologica Sinica</i> (English Edition), 41(5): 108–121. http://www.aosocean.com/article/doi/10.1007/s13131-021-1949-1