While it is well known that the ocean is one of the most important component of the climate system, with a heat capacity 1,100 times greater than the atmosphere, the ocean is also the primary reservoir for freshwater transport to the atmosphere and largest component of the global water cycle. Two new satellite sensors, the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA Aquarius SAC-D missions, are now providing the first spaceborne measurements of the sea surface salinity (SSS). In this paper, we present examples demonstrating how SMOS-derived SSS data are being used to better characterize key land-ocean and atmosphere-ocean interaction processes that occur within the marine hydrological cycle. In particular, SMOS with its ocean mapping capability provides observations across the world's largest tropical ocean fresh pool regions, and we discuss from intraseasonal to interannual precipitation impacts as well as large-scale river runoff from the Amazon-Orinoco and Congo rivers and its offshore advection. Synergistic multi-satellite analyses of these new surface salinity data sets combined with sea surface temperature, dynamical height and currents from altimetry, surface wind, ocean color, rainfall estimates, and in situ observations are shown to yield new freshwater budget insight. Finally, SSS observations from the SMOS and Aquarius/SAC-D sensors are combined to examine the response of the upper ocean to tropical cyclone passage including the potential role that a freshwater-induced upper ocean barrier layer may play in modulating surface cooling and enthalpy flux in tropical cyclone track regions.

Nine CARbon Interface OCean Atmosphere (CARIOCA) drifters were deployed in the Southern Ocean (south of the subtropical front, STF) between 2001 and 2006. They recorded 65 months of measurements in all seasons between 57uS and 40uS. Hydrological fronts detected by altimetry indicate that one buoy explored the polar zone (PZ) of the Atlantic Ocean and the western Indian Ocean; the remaining buoys explored the northern and southern parts of the subantarctic zone (SAZ) from the mid-Indian Ocean (73uE) to the eastern Pacific Ocean (112uW). The air-sea CO 2 fluxes along the buoy trajectories are primarily driven by the spatial variability of the fugacity of CO 2 in seawater, fCO 2 : in the SAZ, they vary between 21.1 and 24.2 mol m 22 yr 21 , and the largest sinks occur close to the STF; in the PZ they vary between 21.6 and 0.6 mol m 22 yr 21 . When spatially extrapolated over each region, the yearly fluxes amount to 20.8 Pg C yr 21 in the SAZ and to 20.1 Pg C yr 21 in the PZ, with very small seasonal variation. In winter-spring, the sea-surface salinity and sea-surface temperature indicate mixing with deep water close to the subantarctic front and an episodic signature of north Atlantic deep water close to the polar front (PF). These events are associated with fCO 2 close to equilibrium. On a small scale (of a few km), close to the STF, fCO 2 variations of 1-2 Pa (10-20 matm) are associated with the presence of compensated mixed layers.