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

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