Water Cycle Synergy

Rain reigns... but soil moisture and ocean saltiness play key roles, too

Soil & Salt Synergy

You are familiar with the role rain plays in our daily lives... should you pack a raincoat or not?

Have you ever wondered... where the water in rain comes from?

Or where will it may go?

Or how we know?

This is our water cycle story. You may be surprised that two of our main characters are soil moisture and sea surface salinity.

Soil Moisture - Water stored in the upper layer of earth in which plants grow. It is affected by precipitation, temperature, soil characteristics, and more.

Sea Surface Salinity - Measure of dissolved salt ions in seawater at the sea surface. The total amount of dissolved salt in the global ocean is generally stable. So, salinity is affected by fresh water entering and leaving the ocean.

These two measurements are linked thanks to instruments on three satellites: Soil Moisture Ocean Salinity (SMOS), NASA Aquarius, and Soil Moisture Active Passive (SMAP). Each was designed to sense the  brightness temperature  at a frequency of 1.4 GHz. These measurements tell us about the saltiness of the ocean and whether earth's land surface is wet or dry.

LEFT TO RIGHT: Operated by the European Space Agency,  SMOS  was launched in November 2009.  Aquarius  onboard the Aquarius/SAC-D satellite began collecting data in August 2011. Its sea surface salinity measurements overlapped with those of  SMAP , which has been measuring salinity since April 2015. All of these instruments were designed to measure Earth's brightness temperature at L-band (~1.4 GHz), a frequency protected for radio astronomy purposes. 

Let's get to know soil moisture by watching it change over several years. You'll notice that some parts of the world have persistent dry surface conditions (yellow) including northern Africa, the Arabian Peninsula, and parts of Australia. Other regions have persistently wet soil (blue) such as the Amazon basin in South America and west central Africa.

Over rest of the globe conditions vary a lot as seasons change including snow and ice (white).

How does soil moisture look where you live?

Soil moisture observations from NASA's SMAP mission from early May 2015 through through mid January 2022. Color coding is shown below the map. On land, white indicates snow or ice and gray indicates missing data. Note there are some data collection gaps (e.g., around July 2019). (Source: Akiko Hayashi, NASA JPL)

Now let's check out how sea surface salinity changes over time. Some of the features are fairly stable. In the Atlantic Ocean, for example, there are two high salinity regions (red) on either side of the equator. In the Pacific Ocean, low salinities (blue) are found in the north and along the equator.

Some salinity features change a lot over time, can you guess why?

Salinity observations from NASA's SMAP mission from early May 2015 through through mid January 2022. Color coding is shown below the map. Note there are some data collection gaps (e.g., around July 2019). (Source: Akiko Hayashi, NASA JPL)

Now that you've gotten to know these players in the water cycle story, let's look at soil moisture and sea surface salinity at the same time time.

If you look carefully, you may notice places where high soil moisture and low salinities are connected.

You may see that some land regions with low soil moisture are found near ocean areas with high salinity.

As this story evolves, you'll see why some areas follow these patterns... and why others don't.

Salinity and soil moisture observations from NASA's SMAP mission from early May 2015 through through mid January 2022. Color coding for each measurement is shown below the map. On land, white indicates snow or ice and gray indicates missing data. Note there are some data collection gaps (e.g., around July 2019). (Source: Akiko Hayashi, NASA JPL)


Role of Rain

The most obvious contributor to soil moisture? Rain, snow or lack thereof. This data visualization shows Earth's average precipitation for each month based on almost 20 years of data from NASA satellite missions. High rainfall dominates around the equator, seen in red and yellow. Dark blues indicate where there is little to no precipitation... including over the ocean.

NASA has a comprehensive estimate of rain and snow covering nearly 20 years, IMERG – the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (GPM). IMERG combines information from a constellation of satellites are operating in Earth orbit to estimate precipitation over the majority of the Earth's surface. Shown here is average monthly precipitation rates in millimeters per hour. Cool colors (i.e., blue and green) are areas that receive low amounts of precipitation for a given month. Warmer colors (yellow, orange, and red) receive higher amounts of precipitation for the given month. (Source:  NASA Scientific Visualization Studio , SVS)


Flamingos & Fueling Hurricanes

We just looked at sea surface salinity and soil moisture during April 2021. Now, let's skip ahead four months to see how patterns have changed.

In the Atlantic, hurricane activity tends to increase as tropical ocean water heats up. This video shows the paths – and in some cases, category of – named hurricanes during August into early September 2021. Sea surface temperature is shown beneath the clouds, rain, and storm tracks; for example, deep reds correspond to water that is 29°C (~84°F) or warmer.

This data visualization shows hurricane tracks, clouds, IMERG precipitation, and sea surface temperatures from August 1st to September 5th, 2021. At right are the named hurricanes during the full 2021 season. Infrared cloud data are from the NOAA Climate Prediction Center (CPC) Cloud Composite dataset together with storm tracks from the NOAA National Hurricane Center (NHC) Automated Tropical Cyclone Forecasting (ATCF) model. Sea surface temperatures (SST) are also shown over the oceans, derived from the NASA Multi-sensor Ultra-high Resolution (MUR) dataset. (Source:  NASA SVS) 

Notice how the sea surface heats up off northeast coast of South America (bottom right of the video) until the last frame on 05-Sep-21. Can you guess what's happening with salinity at this time?

The close connection between salinity and soil moisture have sparked some novel science investigations. In fact, salinity has been used to improve forecasts of rainfall on land in the midwestern US and central Africa.


Averages & Anomalies

We've examined how soil moisture and salinity changed during 2021. But how did they compare to "the usual"? In other words, how much did they deviate from long-term average conditions?

Thanks to long data records from Earth-orbiting satellites, we can create anomaly maps. Scientists use these to see trends that are otherwise hidden in data.

AnomalySomething that deviates from what is standard, normal, or expected.

In this section, you'll scroll through sea surface salinity anomaly data maps for January - August 2021. These were created from the  Optimally Interpolated Sea Surface Salinity (OISSS) product . This unprecedented salinity product was recently made possible by over a decade of sea surface salinity observations!

Each map that scrolls by illustrates how that particular month's salinities vary from the long-term monthly average derived from the OISSS dataset. In the background, a movie loops through that month's daily maps of variations in precipitation (based on seasonal data); in other words, data show how that day's precipitation varies from the seasonal average.

In each salinity map, brown colors show saltier conditions while teal greens show fresher conditions (i.e., lower salinity).

In the background precipitation movie, brown colors show drier conditions while teal greens show rainier conditions.

These direct comparisons show where precipitation is a major driver in sea surface salinity such as the equatorial Pacific Ocean. They also show where salinity values are influenced by processes other than precipitation or evaporation over the ocean. For example, fresher-than-expected conditions are found in areas with substantial river outflow or melting ice.

Researchers use information about anomalies to study how the departure of salinity from normal conditions influences ocean density and currents. Why should we care? Ocean currents move heat and carbon and thus impact ecosystems, weather, and climate.

If anomalous conditions persist long enough, they can become "the new normal." That's why it's important to examine data from connected earth systems: ocean, land, and atmosphere. Such research will help us better anticipate – and prepare for – long-term trends.

There's a lot to learn and we're just getting started! Stay tuned by visiting the  NASA Salinity website .


Fun Facts & More

5 Things to Know about Salinity

  1. When it rains, carbon dioxide in the air gets dissolved, making the raindrops a little bit acidic. And, when it reaches rocks on land, it weathers and erodes them, releasing salt minerals that flow into the rivers and eventually into the sea. It’s estimated that globally rivers carry around 4 billion tons of dissolved salts into the ocean each year.
  2. Salt also comes from cracks in the seafloor, hydrothermal vents where the magma from the Earth’s core hyper-heats the seawater carrying minerals. There are also underwater volcanic eruptions that do the same and massive salt deposits known as salt domes — it’s the dissolved salts that create about 3.5% of the weight of seawater.
  3. The minerals deposited into the sea have built up over millions of years — becoming more and more concentrated. Scientists estimate that there’s so much salt in all our oceans that if we removed it and spread it over the Earth’s surface evenly, it would make a layer around 500 feet thick.
  4. The level of salinity of the oceans is hugely important because the saltier sea is actually heavy and can affect ocean current movement, which is how heat is transported around the globe, helping to regulate the Earth’s climate.
  5. Even more important is the salinity level’s impact on marine life. Too much salt can be disastrous. There’s a reason why one of the saltiest seas on the planet is called “The Dead Sea.”


LEFT TO RIGHT: Operated by the European Space Agency,  SMOS  was launched in November 2009.  Aquarius  onboard the Aquarius/SAC-D satellite began collecting data in August 2011. Its sea surface salinity measurements overlapped with those of  SMAP , which has been measuring salinity since April 2015. All of these instruments were designed to measure Earth's brightness temperature at L-band (~1.4 GHz), a frequency protected for radio astronomy purposes.