These data were collected during cruise RR2004 on the R/V Roger Revelle using a Sea-Bird SBE911plus CTD. This cruise departed Honolulu, Hawaii on December 26, 2020 and the ship transited south along the great circle route from Honolulu to 30°S x 150°W, returning to Hawaii on February 23, 2021. CTD casts were performed at 103 stations, encompassing Subtropical, Subantarctic, and Polar waters in the Pacific Sector of the Southern Ocean.

Hydrographic profiles were performed along the transect with the CTD (the "full-water cast"), sampling for Freons, dissolved oxygen, DIC, alkalinity, extracted chlorophyll, nutrients, POC, PIC, biogenic silica, coccolithophore counts and FlowCAM analyses. Each CTD full-water cast was alternated with a "trip-on-fly" water cast. The latter casts involved tripping bottles at 24 depths "on the fly" as they passed the following 24 depth targets. These trip-on-fly casts served to provide greater resolution in hydrographic sections across the features.

Evaluation and calibration of CTD conductivity and oxygen sensors

Salinity:
For the first 62 stations of RR2004, there was a consistent salinity offset of 0.016 ± 0.033 between conductivity sensors. There was a 0.013 ± 0.006 difference in salinity estimated from the salinometer and conductivity sensor 1, and a -0.004 ± 0.034 difference for conductivity sensor 2. The large standard deviations in the offset for conductivity sensor 2 were due to casts 32 and 33, where the pump for CTD 2 became disconnected. Removing these stations reduces the salinometer/conductivity sensor 2 offset to -0.0005 ± 0.006. Given the high agreement of conductivity sensor 2 with the salinometer, conductivity sensor 1 was replaced for stations 63 through the end of the cruise. After switching out the sensor, the difference between salinities between conductivity sensor 1 and 2 was reduced to 0.004 ± 0.002 (see Figure S1 Supplemental File), with a difference between the salinometer and CTD 1 0.0001 ± 0.0233 and a difference between the salinometer and CTD 2 of -0.0039 ± 0.0233. The difference between CTD 1 and the salinometer and CTD 2 and the salinometer was statistically significant (Student's t-test, t = -49.9, 95% confidence interval: -0.0041 – -0.0038, p < 0.001, 452 degrees of freedom), and so the salinometer measurements are significantly better represented by salinities measured by CTD 1 than CTD 2 for stations 63 onwards.

To summarize, users of salinities from RR2004 conductivity sensors should consider:
1) applying a correction factor of ~0.01 to conductivity sensor 1 salinities, at least for stations 32 and 33 where conductivity sensor 2 was not operational;
2) using salinities from conductivity sensor 2 for stations 1-62, except stations 32 and 33; and
3) using salinities from conductivity sensor 1 for stations 63 onward.

Oxygen:
Systematically low measurements recorded by CTD oxygen sensor during casts 1 and 2 led to its replacement for the remainder of the cruise. For stations 3 and onward, there was good agreement between the O2 sensor and the oxygen measurements made in the Hydrolab (see Figure S2 Supplemental File), with

2 = 1.0079[CTD O2] + 0.0643

which has R² > 0.99. For stations 1-2, the calibration is

O2 = 1.5205[CTD O2] + 0.9017

which has R² = 0.91.