Ocean temperature and heat content
Over the past 50 years, the oceans have absorbed more than 80% of the total heat added to the air/sea/land/cyrosphere climate system (Levitus et al, 2005). As the dominant reservoir for heat, the oceans are critical for measuring the radiation imbalance of the planet and the surface layer of the oceans plays the role of thermostat and heat source/sink for the lower atmosphere.
Domingues et al (2008) and Levitus et al (2009) have recently estimated the multi-decadal upper ocean heat content using best-known corrections to systematic errors in the fall rate of expendable bathythermographs (Wijffels et al, 2008). For the upper 700m, the increase in heat content was 16 x 1022 J since 1961. This is consistent with the comparison by Roemmich and Gilson (2009) of Argo data with the global temperature time-series of Levitus et al (2005), finding a warming of the 0 - 2000 m ocean by 0.06°C since the (pre-XBT) early 1960's.
Ocean salinity and freshwater content
Among the major societal impacts of climate change is an increase in the global cycle of evaporation and rainfall caused by a warmer ocean surface layer. Changes in the patterns and magnitude of rainfall and storms affect nearly every facet of society, from agriculture and urban water supplies to disease and health, to housing, transportation and insurance impacts of severe weather. While the impacts are local and regional, the causes and patterns are global.
Regionally, the ocean becomes fresher or saltier where the balance between evaporation minus rainfall tips in one direction or the other over time. As an integrating measurement made with high accuracy, freshwater content (salinity anomaly over a layer) is the most sensitive yardstick available for observing the global fingerprint of a changing hydrological cycle.
A second application of salinity is to diagnose the global volume of ice. Melting of either floating ice or glaciers and ice sheets lowers ocean salinity.
Recent analysis of Argo data in relation to the historical record show an increase in salinity in evaporative mid-latitude regions and a freshening at high latitudes and tropical convergence zones. This pattern may imply an increase in the global hydrological cycle by several percent (Hosoda et al, 2009, Johnson and Lyman, 2008).
Steric sea level
Steric sea level provides a great example of Argo's complementary relationship with other observing system elements, particularly the altimeter Jason. Argo provides the capability to understand sea level change by measuring its component due to subsurface temperature and salinity. The steric component is dominant over the mass component in regional sea level variability and on a global basis it accounts for about 1/3 of total sea level increase in the past half century (Domingues et al 2008). Accurate projections of future sea level require an understanding of the causes of sea level change in the modern record.
On seasonal and longer time-scales, sea surface height is dominated by changes in subsurface density. Thus, by measuring temperature and salinity as a function of depth, Argo reveals not only how much of sea surface height variability is steric in origin, but also how the steric signal is distributed over depth and between temperature and salinity. Combining sea surface height measurements from the Jason altimeter and Argo's ability to see below the ocean surface, climate related basin-scale signals on interannual and decadal timescales, such as a 15-year spin-up of the South Pacific gyre described by Roemmich et al, 2007 are becoming apparent. On global scales, Argo and Jason, together with satellite gravity measurements, partition global sea level rise into its steric and mass-related components (Willis et al, 2008, Cazenave et al, 2009, Leuliette and Miller, 2009, Wunsch et al, 2007).
The oceans are not only reservoirs for heat and water in the climate system. They are dynamically active, redistributing heat and water by means of an ocean circulation that responds to changes in wind and thermohaline forcing. Argo presently observes only the interior upper-ocean circulation, so a complete observing system that includes boundary currents and deep measurements is essential for understanding the entire ocean circulation. Some recent papers describing upper-ocean circulation include Roemmich et al's (2007) paper on Argo contributing to estimating changes in gyre-scale circulation, Gille's (2008) paper on the Antarctic Circumpolar Current and Hernández-Guerra et al's (2008) paper on the Atlantic meridional overturning circulation.