Life & Water Level

NASA is diving into the details of Earth's water and the life it sustains. This is thanks to its latest Earth-observing satellite missions:  Plankton, Aerosol, Cloud and ocean Ecosystem (PACE)  and  Surface Water and Ocean Topography (SWOT)  .

The technology underpinning each mission is unprecedented: both PACE and SWOT provide significant improvements in the data they deliver. But neither reveals all the details of life and its watery environment alone. Data from both missions are helping to untangle some mysteries about our living ocean... and on land, too!

Launched in February 2024, PACE is extending and expanding NASA's long-term observations of our living ocean. PACE detects phytoplankton – tiny marine algae – at the ocean surface. Phytoplankton make up the base of the marine food web, supporting ocean fisheries. Thus their abundance is a prime indicator of our ocean's health.

But that's only part of their contribution to the health of our planet. More than half of the oxygen production on Earth comes from the ocean with phytoplankton as vital contributors. Just as important, phytoplankton help draw carbon dioxide (CO ₂ ) out of the atmosphere, helping to control atmospheric warming.

Phytoplankton are key players in drawing CO ₂  down from our atmosphere. Their ability to export carbon to the deep ocean, a process that helps control the climate, is being studied by researchers around the globe.

PACE includes NASA's  most advanced color sensor ever , the Ocean Color Instrument (OCI). What's so special about the OCI? It's designed to help identify phytoplankton community types from space! Previous ocean sensors detected only a handful of colors (i.e.,wavelengths), the OCI covers a broad spectrum of wavelengths with no gaps, making it hyperspectral. By seeing the full rainbow of colors and beyond, PACE monitors global phytoplankton distribution and abundance with unprecedented detail.

PACE's unprecedented technology is revealing the diversity of phytoplankton found in our ocean on global scales. This is helping us understand the role that phytoplankton diversity has on life in the ocean. Moreover, PACE data is aiding the development of state-of-the-art computer programs that identify and quantify specific phytoplankton groups... including those that are potentially toxic. For example, PACE data are helping to build tools that better predict the extent and duration of harmful algal blooms.

Launched in December 2022, SWOT is taking an inventory of Earth's water. Its overall mission is to measure the height of water – both over land and the ocean – to help tackle pressing issues such as our changing oceans and coasts along with the availability of Earth's freshwater resources. A few months after launch, these data started being collected over much of the globe every 21 days.

SWOT has two primary science instruments that collect data below the satellite. One of these technologies, a nadir-looking altimeter, is a device that measures altitude or height. The precision of nadir altimeter data relies on other instruments both onboard (e.g., GPS) and on earth's surface (e.g., Laser Reflector Assembly).

SWOT carries on decades of data collected by previous satellites built collaboratively by NASA and the French Space Agency (Centre national d'études spatiales, CNES). SWOT's revolutionary new technology is the Ka-band Radar Interferometer (KaRIn), designed to collect swaths of data on either side of the nadir altimeter.

Among the most energetic – and arguably most fascinating – features in our ocean are eddies. Improved understanding of ocean eddies is a very promising area for fruitful collaboration between PACE and SWOT.

Let's begin with larger ocean features that have been studied for decades using traditional altimeters, known as eddies. Prior to SWOT, eddies with diameters less than 75 kilometers (~47 miles) could not be resolved because their signals got lost in the noise. SWOT improves the two-dimensional resolution of ocean surface topography down to between 15 and 45 kilometers (9 to 28 miles). And, under the right conditions, SWOT may detect small eddies with diameters of 7 to 20 km (~4 to 12 miles).

geostrophic current – an oceanic current in which the pressure gradient force is balanced by the Coriolis effect. The direction of geostrophic ocean flow is parallel to the lines of equal pressure.

Here's a simple way to think about it: water doesn't like to be piled up, so it starts to flow downhill as a result of gravity. This downhill flow is counteracted by rotation of the earth, which causes that "downhill" flow to curve to the right in the northern hemisphere (and to the left in the southern hemisphere). So, SWOT's observations allow us to estimate the speed and direction of ocean currents , including eddies, that are related to sea surface height.

These are motions that don't have that balance between the Coriolis effect and gravity... in other words, they are not the result of sea level differences. Such unbalanced motions that often involve the interaction of physics and biology in the ocean.

Some examples are included in this conceptual diagram of coastal ocean features driven by wind. Small, non-geostrophic eddies can spin off coastal currents. Phytoplankton-rich waters – depicted in green – can be diverted into small filaments. These types of coastal systems can include areas of upwelling or downwelling, which can also transport phytoplankton.

When these types of features affect ocean color, they can be imaged by PACE. Likewise, its spatial resolution of 1 kilometer (0.6 mile) will capture the details of ocean eddies, both large and small!

Where in the world do we find balanced versus unbalanced motions? Before the launch of SWOT, a team of researchers ( Qiu et al., 2018 ) used a state-of-the-art global ocean simulation that is initialized by output of the Estimating the Climate and Circulation of the Ocean (ECCO) project.

Their results are shown in the slider below. You can swipe to see where balanced motion versus unbalanced motion dominates our ocean. The color scales' units are Eddy Kinetic Energy (EKE), a proxy for variability in the ocean due to eddies. The balanced motion map (left) shows high EKE where larger eddies spin off of major ocean currents such as the Gulf Stream, Kuroshio Current (off Japan's east coast), and in the Southern Ocean. The unbalanced motion map (right) has higher EKE values in regions with prominent bottom topographic features, for example, the Philippine Sea in the northwestern Pacific; off the Aleutian Islands; the Solomon, Coral, and Tasman Seas in the southwestern Pacific; and the continental slopes off the northeast US.

I'm excited to combine the powers of SWOT and PACE! SWOT currents will provide finer-scale resolution estimates of transport and stirring. It will be used to identify fronts, eddies and meanders that separate, trap, transport, or nourish phytoplankton. PACE products will provide new methods to identify the signatures of small currents that will not be resolved by SWOT. This can help to separate balanced vs. unbalanced motion.

Ocean motion information from ECCO is not only useful for SWOT, it is highly relevant for PACE. For example, the goal of a collaboration between ECCO and the   Darwin Project   is to model realistic connections between phytoplankton and their physical environment. Data from PACE can be used to evaluate and adjust the ECCO-Darwin model over time. The overall goal is understanding the ocean's uptake, storage, and transport of carbon with an eye towards improved climate prediction.

So far we've focused on the ocean synergies between PACE and SWOT. However, a groundbreaking capability for SWOT is measuring water height in lakes. And PACE carries on the "ocean color tradition" of providing information on phytoplankton populations in some lakes.

SWOT gives a significantly clearer picture of Earth’s freshwater bodies. It provides data on more than 95% of the world’s lakes larger than 62,500 square meters (15 acres). Currently, freshwater scientists have reliable measurements for only a few thousand lakes around the world. SWOT pushes that number into the millions.

SWOT provides data on surface area and water height. Combined with information about lake's bathymetry, changes in water volume can be calculated. As a result, SWOT data help us track changes in amount of freshwater available for drinking and agriculture.

SWOT can get lake level information from remote lakes such as those in the mountains of Tibet. Ocean color sensors can also provide data about the health of lakes in some of these "hard to reach" places.

Closer to home, PACE contributes to the important task of identifying the potential blooms of cyanobacteria in large lakes. A type of blue-green algae, blooms of cyanobacteria can pollute lakes and be toxic to people and animals.

The  Cyanobacteria Assessment Network (CyAN)  is a multi-agency project among the Environmental Protection Agency, NASA, National Oceanic and Atmospheric Administration (NOAA), and US Geological Survey (USGS) to develop an early warning indicator system to detect algal blooms in US freshwater systems. Likewise, PACE data can be applied to other investigations into lakes around the world. An integral group, known as PACE Early Adopters, are using PACE data to study the health of lakes from the US to New Zealand.

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• NASA (2022)  NASA Launches International Mission to Survey Earth’s Water . Press Release 22-133
• Mahadevan, A. (2014)  Eddy Effects on Biogeochemistry . Nature, 506, 168–169
• Morrow et al. (2019)  Scientists Invited to Collaborate in Satellite Mission's Debut .  Eos, 100
• Qiu et al. (2018)  Seasonality in Transition Scale from Balanced to Unbalanced Motions in the World Ocean . Journal of Physical Oceanography, 48, 591-605
• Tuerena et al. (2019)  Internal Tides Drive Nutrient Fluxes Into the Deep Chlorophyll Maximum Over Mid-ocean Ridges . Global Biogeochemical Cycles, 33(8)