Climate is changing. And nowhere is it more evident than near Earth's poles. Life that has adapted to these extreme regions – including sea ice-dependent species from tiny algae to huge polar bears – is being impacted. Salinity is a "key ingredient" for high-latitude ocean ecological communities. Why? It affects seawater density which, in turn, influences the movement of water, heat, and carbon.
A basin surrounded by land, the Arctic Ocean has a cap of frozen seawater (a.k.a. "sea ice") that waxes and wanes. For years, satellites have tracked sea ice growth each winter and the dramatic extent of sea ice melt each summer. In the Arctic Ocean, thermohaline (i.e., temperature- and salt-controlled) layering is vital to maintaining the cold, relatively fresh surface waters that support diverse ecosystems. However, measuring cold-water salinity from space is challenging.
Antarctica is a large continent surrounded by the Southern Ocean. Because of this geography, sea ice has more room to expand in the winter. So, sea ice – along with other ice that originated on land such as icebergs – can move into warmer latitudes and melt. Being the only place where our seas circle Earth without being slowed down by land, the Southern Ocean is prone to very high winds. Along with cold temperatures, a wind-roughened sea surface hampers the accurate measurement of salinity from satellite.
Use the tool below – based on studies by Lind et al. (2018) and Haumann et al. (2016) – to investigate the characteristics of salinity in the Arctic and Antarctic. Use the buttons to toggle between locations.
- In the past, the Arctic Ocean was largely covered by perennial ice that lasted from year to year. Towards its outer edge – away from the North Pole – ice cover varied with the seasons.
- Melt water from ice kept the upper Arctic Ocean relatively fresh. The atmosphere kept the shallow upper ocean layers very cold.
- Cold, fresh upper layers had relatively low density, keeping them afloat above deeper waters. Where did the deep waters come from? The warmer, saltier and denser Atlantic Ocean.
- The density boundary between shallow waters and deeper waters allowed some upward flux of heat and salt.
- There has been a drastic reduction in Arctic Ocean ice cover, both year-to-year and seasonally. The warm, salty waters of the Atlantic have shifted northward.
- This term is used to describe the increased flow of Atlantic water into the Arctic. This flow decreases the depth of the shallow fresh, cold layer.
- Both melting sea ice and increased outflow from rivers deliver fresh water to the Arctic Ocean. These processes have helped to balance "Atlantification" to some degree.
- However, as Atlantic waters shift northward, there has been an increase in the upward flux of heat and salt. This weakens stratification in the Arctic Ocean.
- Moreover, loss of ice cover allows warm water to directly transfer heat into the atmosphere, warming our climate.
- The coast surrounding Antarctica has sea ice, floating ice shelves, and icebergs that have broken off the continent. Moving northward, it transitions to the open ocean.
- The ocean is layered by density. Relatively fresh waters are found near the surface with saltier layers below.
- The freezing of seawater to form sea ice moves freshwater out of the ocean, as shown by the green arrows. Salt is left behind, shown by red arrows.
- The melting of ice moves freshwater into the ocean, shown by green arrows pointing down.
- In addition, sea ice motion away from the continent of Antarctica moves freshwater towards the north.
- Studies reveal how Southern Ocean salinity changed from 1982 to 2008. Melting of land ice, ice shelves, and sea ice added freshwater to the coast. This is shown by the white arrow.
- Also, reduced formation of sea ice slowed down the movement of freshwater out of the ocean, as shown by green arrows.
- Farther away from the coast, the surface layer was freshened by increased ice melt, affecting Southern Ocean circulation.
- Scientists have noted that "recent salinity changes in the Southern Ocean are among the most prominent signals of climate change in the global ocean." The trends depicted here may be major contributors to these changes.
Changes in Arctic freshwater distribution impacts ocean circulation, climate, and life. This study explores the use of satellite-derived sea surface salinity (SSS) as a proxy for Arctic freshwater changes. It builds on previous work that used satellite‐derived sea surface height (SSH) and ocean bottom pressure (OBP) to infer depth‐integrated freshwater content changes. This proof‐of‐concept study analyzes the output of an ocean‐ice state estimation product, finding that SSS variations are coherent with SSH-minus-OBP across much of the Arctic basin.
Fournier, S., Lee, T., Wang, X., Armitage, T., Wang, O., Fukumori, I., and Kwok, R. (2020). Read the full paper.
Hudson Bay, the largest semi-inland sea in the Northern Hemisphere, is completely covered by ice and snow in winter. About six months each year, however, satellite remote sensing of sea surface salinity (SSS) can be retrieved over open water. This provides some insight into freshwater cycles in the Arctic Ocean where SSS data are scarce. The study found that the main source of the year-to-year SSS variability in Hudson Bay is sea ice melting. The freshwater contribution from surface forcing precipitation minus evaporation (P-E) is smaller in magnitude but lasts through the entire open water season. River discharge is comparable with P-E in magnitude but peaks before ice melt. View the one-pager.
Tang, W., Yueh, S., Yang, D., Mcleod, E., Fore, A., Hayashi, A., Olmedo, E., Martínez, J., and Gabarró, C. (2020). Read the full paper.
This study presents the first systematic analysis of six commonly used sea surface salinity (SSS) products from NASA and the European Space Agency in terms of their consistency among one another and with in-situ data. When averaged over the Arctic Ocean, the products show excellent consistency in capturing seasonal and year-to-year variations. The products also consistently identify regions with strong SSS variability over time. However, many challenges still exist in retrieving Arctic SSS because brightness temperature (TB) has lower sensitivity in colder waters at the frequency employed by today's SSS satellites (i.e., L-band). View the One-pager. Read about an evaluation and intercomparison of SMOS, Aquarius, and SMAP SSS products in our Research Insights.
Fournier, S., Lee, T., Tang, W., Steele, M., and Olmedo, E. (2019). Read the full paper.
- All About Sea Ice
- World of Change: Arctic Sea Ice
- World of Change: Antarctic Sea Ice
- Sea Ice Communities
- Sea Ice
- Research Insights – Arctic Ocean
- Arctic Ocean Circulation: Going Around at the Top of the World
- Understanding Climate: Antarctic Sea Ice Extent
- Sea-ice Transport Driving Southern Ocean Salinity
- Arctic Warming Hotspot in the Northern Barents Sea
- Greater Role for Atlantic Inflows on Sea-ice Loss
- Arctic Regional Climatology
- Polar Regions
- Can Seawater Freeze?
- Density and Ocean Circulation
- Melting Ice, Rising Seas: Studying Change in Ice Sheets & Glaciers
- Melting Ice, Rising Seas: Ice Melt & Sea Level Rise
- SMAP Maps: Sea Surface Salinity, Northern Hemisphere
- SMAP Maps: Sea Surface Salinity, Southern Hemisphere
- Aquarius Maps: Sea Surface Salinity at High Latitudes