Synchronization of Surface Measurements
Profiles of in situ temperature and salinity in surface seawater, as measured by the CTD probe at the seawater intake, and the different seawater fCO2 profiles had to be matched and synchronized. Given the strong gradients in surface seawater temperature encountered during some periods of the cruise, this was very important for a reliable estimation of the differences between equilibrator and in situ temperatures. At the beginning of the exercise, all systems were switched to Universal Time Coordinated (UTC) time. The UTC time readings of all measurements are therefore the primary criterion for matching the data sets. However, UTC time alone would not produce a proper synchronization of the profiles for two reasons. The first is the different time the water travels from the seawater intake (where its temperature and salinity are being measured) to a given equilibrator (where the equilibration temperature is determined). This depends mainly on the individual flow rate of water and to some extent also on the location of the equilibrator in the supply line. By running two separate supply lines (port and starboard side line), which were kept at roughly the same total flow rate, we tried to make the supply flow characteristics comparable for all systems. With a single supply line, the ratio of water consumed for analyses to the water bypassing through this supply line would have changed more strongly en route with unknown implications for the water characteristics (such as the temperature deviation). With the chosen setup (see also Fig. 3), we tried to make the supply similar for all systems. The second, rather trivial, reason for an insufficient synchronization of the profiles based on UTC readings alone is that there are errors in the UTC readings themselves, which in some cases appear to account for 1 to 2 min during the course of the exercise.
The final matching of the profiles is based on the assumption that the profiles of in situ and equilibrator temperature should be connected by a fixed daily temperature offset. This is a first-order approximation, because the offset certainly depends on the stability of the water flow rate and the difference between seawater and ambient temperature. Flow rates were usually kept constant during the course of the exercise. The change in seawater temperature was significant, but its effect was minimized by matching the profiles on a daily basis. The matching procedure involved correcting every fCO2 profile with daily time lags in 1-min steps until the standard deviation of the difference between in situ and equilibrator temperature reached a minimum. This could always be achieved by time lags of < or = 3 min. In other words the fCO2 profiles were shifted minute-wise backwards in time against the CTD readings until the two temperature profiles showed the best match with the smallest standard deviation of the resulting offset.
This procedure proved very necessary. Even a mismatch of 2 min could cause a bias in the calculated temperature difference of up to 1°C and more (i.e., > or = 10 µatm) in the strong gradient regime. In the more stable regime, toward the end of the exercise the effect of this synchronization procedure is less pronounced or even negligible. On the other hand, the profiles could not be synchronized to better than 1 min, which still allows errors of the order of several µatm in some cases only because of temporal mismatch. This is an important aspect which restricts the interpretability of the results during passage of the very strong gradients.
Even after correction of all equilibrator temperature readings and after this synchronization procedure, the remaining uncertainty is on the order of 2 µatm for the largest portion of the cruise. To put it the other way round, any differences of < or = 2 µatm between the final fCO2 profiles are not significant under the circumstances of this exercise. During passage of the strongest gradients, the overall uncertainty is definitely higher than 2 µatm, at least for short periods, and may account for a mismatch of up to 5 µatm.
