36
G. Lagerloef and J. Font
the year 2010 marks the turning point that will begin the era of space borne salinity
measurements.
3.2 Scientific Background – Links Between the Ocean
Circulation, Water Cycle and Climate
The 1990s saw a confluence of two important developments for sea surface salinity
(SSS) remote sensing. Scientifically, there was increasing awareness of the connection between surface salinity, ocean circulation and climate variability (e.g.
Broecker, 1991). Meanwhile, advances in microwave radiometer technology were
making it feasible to measure SSS at the levels of accuracy, spatial and temporal resolution needed to address important scientific questions (Lagerloef et al.,
1995). By the time of Oceans from Space 2000, the key scientific themes had been
identified for applying satellite SSS data study the links to ocean circulation and
climate. These included tropical air sea interactions and El Niño, high-latitude convection and salinity anomalies, mid-latitude subduction processes, and the relation
of salinity to changes in the global water cycle. The mission concepts were evolving to address these topics with measurement capabilities of ∼0.2 pss accuracy
on ∼100–200 km and 10–30 day resolutions.
Scientific interest continued to grow during the past decade, reflected, for example, in the Journal of Geophysical Research special section on ocean salinity
(Lagerloef, 2002). This included about two dozen peer-reviewed papers addressing the role of SSS on upper ocean dynamics, air-sea interaction and climate
based on observational and modeling studies. Ocean salinity’s critical importance
to understanding and predicting climate variability was further documented in the
report of the US CLIVAR Salinity Working Group (US CLIVAR, 2007) and the
Intergovernmental Panel on Climate Change (IPCC, 2007).
These assessments identified salinity variability as a key index of the marine
hydrologic cycle. SSS is a tracer for varying evaporation and precipitation, runoff
and ice processes. These have important consequences for oceanic currents and mixing dynamics that influence the ocean’s capacity to absorb, transport and store heat,
freshwater and carbon dioxide. The assessments also reviewed clear observational
evidence of decades-long changes, for example, of decreasing salinity in the subpolar North Atlantic and Southern Ocean, while the near surface salinity in the
subtropics was increasing.
More recent studies reveal new features in these trends and links to water cycle,
circulation and anthropogenic climate change. Stott et al. (2008) attributed to human
influence the recent increases in the observed salinity in the Atlantic (20–50 ◦ N).
Gordon and Giulivi (2008) found opposing trends, increasing since the late-1980s,
in the sub-tropical gyres of North Atlantic and North Pacific, with the latter experiencing a relative freshening (Fig. 3.1). The authors attributed this to increasing
atmospheric transport of fresh water from Atlantic to Pacific via the trade winds
across Central America. The North Atlantic and Nordic Seas upper ocean freshening trend of the 1960s–1990s has reversed over the last decade (Holliday et al.,
2008). This may be attributed to changes in the ocean circulation (Hakkinnen and
G. Lagerloef and J. Font
the year 2010 marks the turning point that will begin the era of space borne salinity
measurements.
3.2 Scientific Background – Links Between the Ocean
Circulation, Water Cycle and Climate
The 1990s saw a confluence of two important developments for sea surface salinity
(SSS) remote sensing. Scientifically, there was increasing awareness of the connection between surface salinity, ocean circulation and climate variability (e.g.
Broecker, 1991). Meanwhile, advances in microwave radiometer technology were
making it feasible to measure SSS at the levels of accuracy, spatial and temporal resolution needed to address important scientific questions (Lagerloef et al.,
1995). By the time of Oceans from Space 2000, the key scientific themes had been
identified for applying satellite SSS data study the links to ocean circulation and
climate. These included tropical air sea interactions and El Niño, high-latitude convection and salinity anomalies, mid-latitude subduction processes, and the relation
of salinity to changes in the global water cycle. The mission concepts were evolving to address these topics with measurement capabilities of ∼0.2 pss accuracy
on ∼100–200 km and 10–30 day resolutions.
Scientific interest continued to grow during the past decade, reflected, for example, in the Journal of Geophysical Research special section on ocean salinity
(Lagerloef, 2002). This included about two dozen peer-reviewed papers addressing the role of SSS on upper ocean dynamics, air-sea interaction and climate
based on observational and modeling studies. Ocean salinity’s critical importance
to understanding and predicting climate variability was further documented in the
report of the US CLIVAR Salinity Working Group (US CLIVAR, 2007) and the
Intergovernmental Panel on Climate Change (IPCC, 2007).
These assessments identified salinity variability as a key index of the marine
hydrologic cycle. SSS is a tracer for varying evaporation and precipitation, runoff
and ice processes. These have important consequences for oceanic currents and mixing dynamics that influence the ocean’s capacity to absorb, transport and store heat,
freshwater and carbon dioxide. The assessments also reviewed clear observational
evidence of decades-long changes, for example, of decreasing salinity in the subpolar North Atlantic and Southern Ocean, while the near surface salinity in the
subtropics was increasing.
More recent studies reveal new features in these trends and links to water cycle,
circulation and anthropogenic climate change. Stott et al. (2008) attributed to human
influence the recent increases in the observed salinity in the Atlantic (20–50 ◦ N).
Gordon and Giulivi (2008) found opposing trends, increasing since the late-1980s,
in the sub-tropical gyres of North Atlantic and North Pacific, with the latter experiencing a relative freshening (Fig. 3.1). The authors attributed this to increasing
atmospheric transport of fresh water from Atlantic to Pacific via the trade winds
across Central America. The North Atlantic and Nordic Seas upper ocean freshening trend of the 1960s–1990s has reversed over the last decade (Holliday et al.,
2008). This may be attributed to changes in the ocean circulation (Hakkinnen and