Education: Salinity Data Exploration

Exploring salinity patterns is a great way to better understand the relationships between the water cycle, ocean circulation, and climate. Our data tools can be used to explore Aquarius Ocean Surface Salinity and Historical Ocean Data (salinity, temperature and density at the ocean surface and below).

Aquarius Ocean Surface Salinity Data
Google earth
Ocean surface salinity data from Aquarius are available by year, season, and month in two formats: 3D (GoogleEarth globe) and 2D (flat maps). Monthly data are presented as both means (i.e., data averaged over the month) and anomalies (i.e., differences from the annual average of Aquarius data). Animations of monthly means and anomalies are also available (2D format only).
Questions That Can Be Explored Using This Tool
  • Is salinity uniform throughout the ocean?

    • Global sea surface salinity
      No. Although the amount of salt in the ocean is relatively constant on time scales of years to decades, sea surface salinity (SSS) varies because freshwater input & output - part of the global hydrologic or water cycle - varies from place-to-place. The large scale pattern of evaporation and precipitation is established by earth's atmospheric convection cells. In the simplest case - for example, if earth were covered by oceans and not spinning - the atmosphere would move heat between the hot tropics and cold poles in a very simple way. Hot air would rise along the equator, creating a band of low atmospheric pressure. Cold air would sink at the poles, creating regions of high atmospheric pressure.

      The presence of continents and earth's rotation complicates the idealized "two cell" system. A fairly realistic model of earth's atmospheric circulation consists of six cells: three on each side of the equator. This pattern has atmospheric pressure highs at the poles, 30°N and 30°S. It also has atmospheric lows along the equator, 60°N and 60°S. Away from the poles, cloudiness and precipitation (P) dominate bands of low pressure: these latitudes host the world's rainforests. Dryness and evaporation (E) dominate bands of high pressure: these latitudes are home to deserts.

      In terms of SSS, lower salinity generally occurs where precipitation is greater than evaporation (P>E). Higher SSS generally occurs where evaporation is greater than precipitation (E>P). Compare the "six cell" model with the map of global average SSS (image upper right). How well do SSS patterns mirror bands of high and low atmospheric pressure? In places where they do not match, what other types of freshwater inputs & outputs might be occurring?
  • What is the range of ocean surface salinity in Earth's ocean?

    • Ocean surface salinity generally ranges from 32 practical salinity units (psu, which are roughly equivalent to parts per thousand) to 37 psu, but may be as high as 42 psu in the Red Sea or as low as 8 psu in the Baltic Sea.
  • Which is the saltiest ocean basin?

    • The Atlantic Ocean. Why? For the most part, the salinity of the surface waters in each of the ocean basins is established by the amount of local precipitation - which would tend to make the surface waters fresher - relative to the amount of local evaporation - which would tend to make the surface waters saltier. Areas where the local evaporation exceeds local precipitation tend to be saltier than those where precipitation dominates. The Atlantic Ocean is a basin with strong evaporation relative to precipitation, with this situation reversed in the Pacific.
  • Where are the saltiest waters in the Atlantic, Indian, and Pacific?

    • In all of these ocean basins, the saltiest regions are located at mid-latitudes, around 20°N and around 20°S.
  • Where, in general, are the freshest surface waters?

    • The freshest waters are found in the Antarctic Ocean and in the upper latitudes of the Northern Hemisphere, above 40°N.
  • How does large-scale atmospheric circulation influence global patterns on land and sea?

    • Six cell model of atmospheric circulation
      The large-scale pattern of evaporation and precipitation is established by Earth's atmospheric dynamics. Hot air rises above the equator and cools while it moves polewards, creating a band of low atmospheric pressure in the equatorial region. At the poles, cold air sinks and warms while moving equator-ward, creating regions of high atmospheric pressure near the poles.

      This heating pattern - coupled with the rotation of Earth - produces a general pattern with atmospheric pressure highs (red arrows) at the poles at at 30°N and 30°S. In these "high pressure" regions, evaporation (E) is greater than precipitation (P) and deserts are likely to occur. Along Earth's equator and at 30°N and 30°S, atmospheric lows (green arrows) are zones in which precipitation exceeds evaporation (P > E).
  • How does large-scale atmospheric circulation influence patterns on land?

    • Away from the poles, cloudiness and precipitation dominate bands of low pressure: these latitudes host the world's rain forests. Evaporation dominates bands of high pressure: these latitudes are home to deserts. These general patterns can be seen in an animation of Earth's Normalized Difference Vegetation Index (NDVI), which shows the density of vegetation.
  • How does large-scale atmospheric circulation influence ocean surface salinity patterns?

    • Average evaporation minus precipitation
      Lower ocean surface salinity generally occurs where precipitation is greater than evaporation. Conversely, higher salinity generally occurs where evaporation is greater than precipitation. Use the Aquarius Salinity Data tool to view an image of average evaporation minus precipitation.
  • What is the contribution of fresh water from rivers to the oceans?

    • It depends on the time and which river you're examining. Watch the animation Changes in Global Sea Surface Salinity to look for areas of low salinity (dark blue) that fluctuate over time and which are adjacent to continents. Likely regions are the mouths of the largest rivers of the world: Rio de Plata, Amazon/Orinoco, Mississippi, Niger and Congo, Ganges, and Huang rivers. Interestingly, the flux of fresh water coming from the rivers into the ocean is ten times less than the freshwater input from precipitation.
  • How far does sea ice extend in the polar oceans?

    • It depends on the season, year, and which pole you're examining. For example, the area of sea ice in the Antarctic that melts in summer is about 14 million square kilometers (~ 5.4 million square miles). This is equivalent to the area covered by the entire Antarctic landmass. Explore the weekly Brightness Temperature data maps from Aquarius, which are used to monitor the extent of ice coverage at the north and south poles.
Historical Ocean Data
We have developed a set of interactive tools to explore historical salinity, temperature, and density data sets (2005 NOAA World Ocean Atlas & Database for the "Flat Map Interface" and 2009 NOAA World Ocean Atlas & Database for the "GoogleEarth Interface"). These data are available as the following three distinct yet complementary tools:

Profile of salinity data
Annual Mean Data
Maps of salinity, temperature, or density data averaged over a year can be clicked to create in-water profiles at up to six locations. Plotted data will also be displayed in a table (and downloadable as Excel files). Sources include interpolated atlas data (i.e., data gaps are filled in) or actual measurements from the database (i.e., data gaps are not filled in).
Questions That Can Be Explored Using This Tool
  • Does salinity vary with depth?

    • Yes. Discover this for yourself by exploring the Historical Annual Mean Data tool. Be sure to click the "Plot" button, located below the "Selected Location List" box. In-water profiles from these locations will appear at right. The blue profile - from the equatorial Atlantic Ocean - shows relatively low salinity at the surface, an increase in the top 100 meters (328 feet), and then a decrease below 100 meters. The green profile is located in in the central South Atlantic Ocean, a region with very high surface salinity. It shows an unusual gradient with depth: salinity decreases from 37 Practical Salinity Units (psu) to 35 psu in the upper 300 meters (984 feet). Of these selections, the red profile shows the least variability in salinity in the upper 300 meters. The red profile is from the Southern Ocean, a region with low surface salinity. It's the only profile that shows a steady increase of salinity in the top 300 meters and constant salinity values below that. For the other locations, much of the salinity change occurs within the top 200 meters (656 feet), a salinity minimum is reached at about 1000 meters depth (3281 feet), while salinity is fairly constant below 1500 meters (4921 feet) depth. You can select other locations (i.e., by clicking on the map and hitting "Plot") to see how salinity varies with depth at various locations.
  • For the three locations explored in the previous question, does temperature vary with depth?

    • Yes. In the previous question, you saw salinity profiles for three pre-selected locations. Return to the Historical Annual Mean Data tool and click the "Temperature" button above the world map. The three corresponding temperature profiles will appear. The in-water temperature profiles are very different from the corresponding salinity profiles. The blue (equatorial Atlantic) and green (South Atlantic) profiles show higher temperatures at the ocean surface. The red profile (Southern Ocean), on the other hand, shows an increase in temperature at about 150 meters (492 feet) depth. Together, temperature and salinity determine the density of seawater and are thus key to how ocean water moves. To discover how these salinity and temperature profiles are related to density, investigate the next question.
  • For the three locations explored in the previous question, does density vary with depth?

    • Yes. Return to the Historical Annual Mean Data tool and click the "Density" button above the world map. Then click on "Plot" button at bottom. The three corresponding density profiles will appear. You will see that - for all locations - density increases with depth.
  • Compare profiles salinity, temperature, and density with depth: which is the most consistent?

    • Density is the most consistent because heavier water always sinks below less dense water.
  • What is the relationship between salinity, temperature, and density?

    • The density of sea water is directly dependent on salinity and temperature. Colder waters are denser than warmer waters: all other things being equal, colder water will sink below warmer water. In addition, the more you dissolve salt in water the denser it gets. In the Middle East lies a salty lake called the Dead Sea. The decrease of precipitation - which started more than 40,000 years ago - along with high rates of evaporation has resulted in its high salinity. The Dead Sea is now much saltier than any ocean area. In fact, its density is so high that a person can float on the lake with ease.
Time series graph of salinity
Changes in Monthly Mean Data
Create global maps of ocean conditions averaged over each month, from the surface down to 1500 meters (4921 feet). Monthly time-series graphs of salinity, temperature, or density can be plotted for up to six locations by clicking on the maps. Plotted data will also be displayed in a table (and downloadable as Excel files). Sources include interpolated atlas data (i.e., data gaps are filled in) or actual measurements from the database (i.e., data gaps are not filled in).
Questions That Can Be Explored Using This Tool
  • Is sea ice as salty as the sea water it forms from?

    • No. In winter, seawater with an average salinity of 34 (practical salinity units, roughly equivalent to parts per thousand) begins to freeze when its temperature drops to around -1.9°C (28.6°F). Since the salt ions in seawater cannot enter the ice crystal structure, sea ice is has a very low salinity (1-10 practical salinity units, psu). The remaining salt is ejected into the ocean, making it saltier (often called "brine"). In some cases, brine can be dense enough to sink to great depths, triggering deep ocean circulation. The Historical Changes in Monthly Mean Data tool has a pre-set location in the North Atlantic Ocean. Click on the "Plot" button beneath the "Selected Location List" to see how average surface salinity changes from January though December. Note that surface seawater salinity is always well above 10 psu (i.e., is saltier than typical sea ice).

      To see the salinity of deeper waters at this location, hit the "Refresh" button in your browser and use the "Depth" pull-down menu below the map to change the depth to "50" meters (164 feet). Click on the same location (i.e., blue dot between Canada and Greenland; it is fine if the new location is not exactly the same as the previous one). This should add a new entry to the "Selected Location List" that ends with "50m". Hit the "Plot" button to see the new profile from 50 meters depth. Note that the deeper seawater generally has higher density than the ocean surface.
  • Is the melting of sea ice a source of fresh water?

    • Yes. When air and water temperatures increase in spring/summer, sea ice melts and adds fresh water into the ocean, thereby decreasing the salinity of the ocean. Use the Historical Changes in Monthly Mean Data tool to look at the salinity time series for two pre-selected profiles in the polar regions. Click on the "Plot" button beneath the "Selected Location List" to see the time series and note the months when salinity decreases for each time-series. In the Arctic, ice melt begins in the northern spring (i.e., starting around April). In the Antarctic, ice melt begins about six months later.
  • Are there "salty" and "fresh" seasons in our ocean?

    • In some regions, yes. Large portions of the world ocean experience relatively small changes in ocean surface salinity (i.e., less than 0.3 practical salinity units, psu). In some regions, however, relatively strong variations in ocean surface salinity (i.e., greater than 0.3 psu) occur where evaporation and precipitation change drastically from season-to-season, such as near the mouths of major rivers, and in areas where sea ice forms and melts.

      Find areas with high variability (i.e., month-to-month changes) using the Historical Changes in Monthly Mean Data tool. Areas to explore include the tropical Pacific and Atlantic, the Indo-Pacific Lombok Straits, much of the North Indian Ocean, the area between China and Japan, the eastern Caribbean, parts of the South Atlantic near the African and South American coasts, and areas with seasonal ice cover.
  • Which regions have high variability in ocean surface salinity because of changes in evaporation and precipitation?

    • In the tropical Pacific Ocean, much of the seasonal variability results from changes in rainfall patterns with shifts in the position of the Inter-Tropical Convergence Zone (ITCZ). The ITCZ is a continuous belt of low pressure near Earth's equator. It is formed by the rise of warm, moist air that moves toward the equator from the north and south. Regions in the ITCZ can experience rain more than 200 days in a year.

      Use the Historical Changes in Monthly Mean Data tool to explore four pre-set locations that are influenced by the ITCZ. After clicking on the "Plot" button beneath the "Selected Location List," time series data from the four locations will appear: two in the Pacific and two in the Atlantic Ocean. Each ocean basin has one pre-set location north of the equator and another south of the equator. Note how northern hemisphere locations have higher salinity in the first halves of the year, while southern hemisphere locations have higher salinity in the second halves of the year. This demonstrates the seasonal variability of the ITCZ.
  • Which regions have high variability in ocean surface salinity because of changes in river outflow?

    • Use the Historical Changes in Monthly Mean Data tool to explore five pre-set locations that are influenced by outflow from the following major rivers: Mississippi, Amazon, Congo, Ganges and Yangtze. Use these salinity data time-series plots to determine which rivers have the greatest influence on salinity variations. Also look at the timing of the "freshening" of the ocean (i.e., lowering of salinity) for each river to determine when the outflow is generally at its peak.

      Since the range of values shown on the vertical axis value adjusts to the data being plotted, you might wish to remove the time-series data from the river(s) with the highest variability to see month-to-month differences more clearly. This is done by clicking on specific entries in the "Selected Location List," hitting the "Remove" button and clicking the "Plot" button again.
Time series graph of salinity
Changes in Annual Mean Data
Create global maps of ocean conditions averaged over any year(s) from 1800 to 2005, from the surface down to 1500 meters (4921 feet). Annual time-series graphs of salinity, temperature, or density can be plotted by selecting up to six locations. Plotted data will also be displayed in a table (and downloadable as Excel files). Sources include interpolated atlas data (i.e., data gaps are filled in) or actual measurements from the database (i.e., data gaps are not filled in).
Questions That Can Be Explored Using This Tool
  • Can we determine if the oceans were as salty 100 years ago?

    • Yes because we have archived salinity measurements starting in the 1800's. With these data, we can begin to estimate global salinity changes from the sea surface to the deep layers of the world's oceans on the time scale of decades. To learn more about the amount of ocean conditions data available since 1800, use the Historical Changes in Annual Mean Data tool. Change the range of dates (e.g., from "1800" to "2005") and hit the "Update" button to see global maps of data coverage for the various data types.
  • What can be observed by exploring historical changes in salinity data?

    • Part I. By clicking on the radio buttons at upper left of the Historical Changes in Annual Mean Data tool, you can see the variation in cumulative (1800-2005) data coverage among the three data types: salinity, temperature, and density. Likewise, you can narrow the time span of data coverage: for example, by entering 100-year ranges in the "From Year" and "To Year" fields and hitting the "Update" button, you can observe changes in coverage of oceanographic data from century to century.

      Part II. An overall lack of global salinity data coverage makes long-term change studies challenging. However, trends towards saltier or fresher waters can be seen in the data and a few examples are provided to illustrate those trends. To see trends in increase of salinity, click this link to the Historical Changes in Annual Mean Data tool where two locations have been pre-set. Change the time range to 1950-1991 and pick a "Search Radius" of "200" kilometers (124 miles). Click the "Update" button and once the map is refreshed, click the "Plot" button (below the "Selected Location List").

      Part III. To see a freshening trend (i.e., decrease in salinity over time), click this link to the Historical Changes in Annual Mean Data tool where a location has been pre-set. Change the time range to 1975-2002 and pick a "Search Radius" of "200" kilometers (124 miles). Click the "Update" button and once the map image is generated, click the "Plot" button below the "Selected Location List." Information on long-term salinity trends have been published by Roemmich & Gilson (2009) in "Progress in Oceanography," which is available online and by Durack & Wijffels (2010) in the "Journal of Climate," which is summarized online.
  • How else can we learn about historical salinity?

    • Even though direct salinity measurement is sparse before the 1950s, there is a broad variety of other techniques to indirectly reconstruct the state of oceans salinity tens hundreds, thousands or tens of thousands years ago: e.g., dissolved oxygen, chemical traces in fossil shells, ice cores. We know for instance that the volume of the six mightiest rivers draining into the Arctic Ocean is now greater than it was when routine measurements began 70 years ago, having increased about 7%. Moreover, rising temperatures are making sea ice melt more rapidly and the huge Greenland ice sheet is currently thought to be shrinking rapidly. One study reconstructed a 45,000 to 60,000 year old record of ocean salinity. As Earth experienced cold and warm periods during this interval of time, the Atlantic Ocean respectively got saltier and fresher. For example, during the colder periods of Ice Age, the Intertropical Convergence Zone was pushed further south than today, keeping rainfall and - as a result - freshwater out of the northern Atlantic Ocean.
  • Does surface salinity affect the deep ocean?

    • Yes because the ocean surface is where most seawater properties are initially set. Temperature differences arise from heating and cooling at the sea surface. Salinity differences result from freshwater entering (e.g., rain, snow, rivers, melting ice) and leaving (e.g., evaporation, ice formation) the ocean surface. Together, temperature and salinity determine the density of seawater, making surface waters sink or float. In some regions, where seawater is very cold and salty, it can sink to great ocean depths.

      Click this link to the Historical Annual Mean Data tool to see a map of ocean surface density with four pre-set locations in the North Atlantic Ocean. Click the "Plot" button (below the "Selected Location List") to create four density profiles. Density always increases as you go deeper, since denser waters sink below less dense waters. In these examples, notice how northern waters are denser than tropical ones. Switch the radio button (at upper left) to "Temperature" to see that colder waters are located in the north.
  • Is density-driven circulation part of the so-called "Global Conveyor Belt"?

    • Yes, density-driven deep ocean circulation is often associated with a "Global Conveyor Belt," a model which does not capture the complexity of the real ocean. In this very simple model, surface circulation carries warm upper waters poleward from the tropics. Along the way, it distributes heat from the ocean surface to the atmosphere. Near the poles, deep-water formation comes from sinking of cooled water masses. In those regions, very high surface water densities are reached. While the "Global Conveyor Belt" model itself is not particularly accurate, deep-water formation does indeed takes place in regions such as the North Atlantic Ocean.

      Click this link to the Historical Changes in Monthly Mean Data tool with two pre-set locations in the Labrador and Greenland-Norwegian Seas. Click the "Plot" button (below the "Selected Location List") to see how salinity in these locations changes from month-to-month. Then click the "Density" radio button above the map (the time-series plot should change automatically; if not, hit the "Plot" button again). Apart from a dip in summer, density is relatively high all year long: thus waters in these regions are dense enough to sink during much of the year. Now add a location in the tropical Atlantic and hit the "Plot" button again: you'll find that surface density is much lower there.
  • Does density-driven circulation play a role in climate?

    • Yes. Density-driven circulation plays a key role because deep ocean currents are connected to major surface currents such as the Gulf Stream. Surface currents can absorb, store and redistribute vast amount of the sun's heat around the globe. Without this circulation, places at the same latitude across the globe would generally have the same average temperatures.

      Click this link to the Historical Changes in Monthly Mean Data tool with two pre-set locations at the same latitude on the western and eastern edges of the North Atlantic Ocean. Hit the "Temperature" radio button above the map and then the "Plot" button (below the "Selected Location List"). The month-to-month time series data will show how ocean temperatures influence the climates in these regions (i.e., eastern Canada is generally colder than Great Britain).