Changes in the polar water cycle will have global impacts due to the sensitivity of the overturning circulation and heat transport to changes in freshwater input (Broecker, 1997; Clark et al., 2002). Observations suggest that changes in the global water cycle may already be apparent in changes in ocean stratification (e.g. Durack and Wijffels, 2010). The stratification of the Southern Ocean is delicately poised and particularly sensitive to changes in the freshwater balance (Gordon, 1991). Substantial uncertainty remains with regard to the high-latitude contributions to the global water cycle, the sensitivity of the water cycle to climate change and variability, and the impact of changes in the high-latitude water cycle on the remainder of the globe.
Freshwater fluxes from melting sea ice, sub-ice shelf melting and precipitation are of the same order of magnitude in the Southern Ocean, and all three components need to be measured. Variables that need to be measured include atmospheric circulation (winds, storms, evaporation, precipitation, moisture flux); the horizontal and vertical circulation of the ocean, including exchange between high and low latitudes and the circulation beneath the sea ice, through the annual cycle; sea ice extent, thickness and distribution; and the contribution of glacial ice (ice shelf melt and iceberg production). New satellites promise synoptic observations of aspects of the freshwater balance, including snow and ice thickness, that cannot be measured at high spatial or temporal resolution using conventional means, but these new sensors are in critical need of data sets for validation.
At the 2013 Scientific Steering Committee meeting in Shanghai, China, the SOOS Steering Committee identified the top gaps in observations for each of the 6 SOOS Science Themes that should be identified as "priority observations" for the coming years. SOOS encourages the community to develop field initiatives to address these key gaps and to highlight their contribution to the international SOOS effort through SOOS endorsement or other connections.
Repeat Hydrography, Argo floats, gliders, underway observations from ships, animal-borne sensors, surface drifters, bottom landers, moorings and sea-ice drifters.