In an Oceanography article published 13 years ago, three of us identified salinity measurement from satellites as the next ocean remote-sensing challenge. We argued that this represented the next "zeroth order" contribution to oceanography (Lagerloef et al., 1995) because salinity variations form part of the interaction between ocean circulation and the global water cycle, which in turn affects the ocean's capacity to store and transport heat and regulate Earth's climate. Now, we are pleased to report that a new satellite program scheduled for launch in the near future will provide data to reveal how the ocean responds to the combined effects of evaporation, precipitation, ice melt, and river runoff on seasonal and interannual time scales. These measurements can be used, for example, to close the marine hydrologic budget, constrain coupled climate models, monitor mode water formation, investigate the upper-ocean response to precipitation variability in the tropical convergence zones, and provide early detection of low-salinity intrusions in the subpolar Atlantic and Southern oceans. Sea-surface salinity (SSS) and sea-surface temperature (SST) determine sea-surface density, which controls the formation of water masses and regulates three-dimensional ocean circulation.

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Comparisons of OSCAR satellite-derived sea surface currents with in situ data from moored current meters, drifters, and shipboard current profilers indicate that OSCAR presently provides accurate time means of zonal and meridional currents, and in the near-equatorial region reasonably accurate time variability (correlation ϭ 0.5-0.8) of zonal currents at periods as short as 40 days and meridional wavelengths as short as 8°. At latitudes higher than 10°the zonal current correlation remains respectable, but OSCAR amplitudes diminish unrealistically. Variability of meridional currents is poorly reproduced, with severely diminished amplitudes and reduced correlations relative to those for zonal velocity on the equator. OSCAR's RMS differences from drifter velocities are very similar to those experienced by the ECCO (Estimating the Circulation and Climate of the Ocean) data-assimilating models, but OSCAR generally provides a larger ocean-correlated signal, which enhances its ratio of estimated signal over noise. Several opportunities exist for modest improvements in OSCAR fidelity even with presently available datasets.

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The evolution during the 1990's of the cold halocline layer (CHL) of the Arctic Ocean is investigated using data from icebreaker and SCICEX submarine cruises. The CHL disappearance and subsequent partial recovery is described along repeated transects through the central Arctic Ocean from the Alpha Ridge to the Nansen Basin. Salinity at the top of the halocline is used as a measure of halocline development, with high salinity corresponding to a poorly developed halocline. In the Nansen, Amundsen, and Makarov basins, upper ocean salinity increased from 1991 to 1998 as the CHL disappeared, then decreased from 1998 to 2000 as it recovered. Salinity was higher over the study region through the 1990's than at any time during the prior 40 year period, hence the 1990's CHL recovery was only partial. Disappearance of the CHL from the Eurasian Basin in the early 1990's was due to a shift from the Laptev to East Siberian seas of the region for seaward flow of low salinity Siberian shelf waters. Ice velocities and sea level pressure fields suggest that the reappearance of the CHL in 1999 corresponded to a shift of this flow back to the Laptev Sea region.