Underway Sea Surface Temperature, Salinity, and Position

Underway sea surface temperature and conductivity were measured using a Falmouth Scientific thermosalinograph (OCM-TH-212) as part of the R/V Knorr improved meteorological (IMET) sensor system. Readings were averaged and recorded at one-minute time intervals together with the global positioning system (GPS) time and location. Underway salinity was calculated relative to the 1978 practical salinity scale from the calibrated temperature and the raw conductivity readings using the equations of Lewis (1980). These data were quality controlled by examining all of the points recorded in two-day intervals and outliers were discarded based on visual inspection. Values were generally discarded when they were more than two standard deviations away from a time local mean. The exact value for the cut, therefore, depended on the instrumental noise at the time. Questionable points were generally left in the data set. The temperature, salinity, latitude, and longitude were then matched to the times when xCO₂ data were recorded. Linear interpolation was used to fill in for values cut in the QC process.

Both the temperature and salinity values were calibrated against the WOCE preliminary surface bottle values at each station. Although the exact trip time is not generally recorded in the WOCE ".SEA" files, the ".SUM" files do record the beginning and ending times of each cast. Since the Niskin bottles were tripped on the upcast, the surface bottle was tripped immediately before the rosette was brought aboard and the cast was completed. The end time for the cast was, therefore, taken as the trip time for the surface bottle at each station. The surface station data were then tied to the underway data by calculating the mean and median values of the underway data for the 15 minutes prior to the recorded cast end time. Although the ship was not underway while the cast was in progress, there was the potential that differences between the underway temperature readings and the discrete samples could have been real in very-high-gradient regions. Stations where the mean and median values were greater than 0.01 units apart were, therefore, flagged as questionable and not considered in the calibration fits.

Since the salinity measurements are a function of temperature, the temperature calibration was performed first. As noted earlier, the temperature data from section I8S/I9S were considerably noisier and appeared to have a different correlation with the CTD data than had data from the other legs. There were no significant differences among the remaining eight cruises, so they were all fit with a single function. Of the 1096 stations occupied after leg I8S/I9S, 201 were flagged as questionable. The remaining data were calibrated with a linear fit to the CTD temperature (Fig.2). The fitted slope of 1.0013 ± 0.0003 indicates that the sensor had a nearly ideal response. The intercept of 0.095 ± 0.007 indicates that the ship's sensor was reading nearly 0.1°C high. The final calibrated underway temperature values were within ±0.026°C of the CTD values at the stations. The data from section I8S/I9S have a slightly different calibration function because the pump with the thermosalinograph was shut down early in the cruise. Without a constant flow of fresh water across the sensor, the response relied more on diffusion and turbulent mixing at the intake. For this cruise, the sensor slope was significantly different from 1 (1.068 ± 0.007), and the offset was 1.53 ± 0.07°C (Fig.3). The standard deviation of the difference between the I8S/I9S CTD surface temperatures and the calibrated underway temperatures estimated at 131 stations was 0.44°C.

Underway salinity was calibrated to the preliminary WOCE bottle salinity results. Examination of the salinity data suggested that the calibration for the salinograph varied on a timescale of approximately 1 month (Fig.4). No obvious correlation was observed between the variability and the in situ temperature or salinity. On average, the uncalibrated underway salinity values were approximately 1.3 lower than the bottle salinity values. The reason for the varying offset is not known, but given this variability the underway data were fit to the station data for each leg individually.

Table 3 lists the coefficients for each leg. The problems with the pump shutdown on line I8S/I9S had a much more drastic effect on salinity than on temperature. The underway salinity values on that cruise did not track the station salinity values and were, therefore, deemed unreliable. The salinity values given in the I8S/I9S data set are simply a linear interpolation of the station data. The thermosalinograph gave much better results on all of the legs after I8S/I9S. The standard deviation of the difference between the WOCE bottle salinities and the calibrated underway salinity values at the stations occupied on legs I9N through I2 was ±0.058.

Table 3. Coefficients for linear calibration of underway salinity