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Ray Schmitt’s introduction to this issue underlines the importance of including the oceanic component of the hydrological cycle in considerations of Earth’s climate. Spatial and temporal salinity variations play an important role in diapycnal (acrossdensity-surface) mixing, affecting property distributions and ocean circulation, which may in turn affect the atmosphere through sea-surface temperature.
Unfortunately, the historical distribution of subsurface salinity observations is generally very sparse, especially lacking adequate temporal resolution to confidently study climate cycles and trends. Energetic, high-frequency salinity variations resulting from vertical motions of internal waves must also be resolved or appropriately filtered, or these signals will be aliased into lower frequencies. There are only a handful of deep ocean sites with requisite sustained high-vertical-resolution time series. Among them are the Hawaii Ocean Time-series (HOT) Station ALOHA, and the Bermuda-Atlantic Time series Study (BATS) and Panulirus stations off Bermuda (Karl et al., 2001). Station ALOHA, located 100 km north of Oahu, occupies a 10-km-radius circle centered on 22°45´N, 158°W. It is slightly south of the climatological center of the zonally elongated surface salinity maximum of the North Pacific subtropical gyre (see Figure 1b in Schmitt, page 18 this issue). This station has been occupied approximately monthly since October 1988, with support from the National Science Foundation and the State of Hawaii. Each ALOHA cruise plan includes numerous physical and biogeochemical observations (Karl and Lukas, 1996; Karl et al., 2001; annual data reports at www.soest.hawaii.edu/HOT_WOCE); the plans also include a 36-hour period of three-hourly repeat conductivity-temperature-depth (CTD) profiling to 1000 m to help resolve high-frequency phenomena such as baroclinic tides (cf. Chiswell, 1994) and avoid aliasing. Most cruises also call for at least one full-depth (4.8-km) profile. The salinity profiles are calibrated against water samples that are accurate to better than 0.003 and internally consistent at a level of 0.001 (Lukas et al., 2001).
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