Wind at Work

wind work – transfer of motion-related energy at the air-sea interface

This study uses a simulation to estimate the global wind work on ocean circulation. It resolves five terawatts, which is significantly more than could be resolved by earlier studies. However, this is still a small fraction of the total wind work on the global ocean, which itself is a small fraction of the total wind energy available in the full atmospheric column.

How much is five terawatts? It's about equivalent to 8.5 trillion lightbulbs lit up at the same time. To add more perspective, our global installed wind energy capacity reached 1 terawatt in June 2023.

Observing and interpreting winds over our ocean is one of NASA's oldest traditions. Records from wind-observing satellites are key to understanding interactions between the huge, restless systems that drive our climate: the ocean and atmosphere.

For decades, instruments called scatterometers have measured global ocean winds from space. A similar instrument – with the addition of “Doppler” capability – can measure ocean winds and ocean surface currents simultaneously. The resulting Doppler scatterometer enables new ways to understanding the ocean, the atmosphere, and how they interact.

ODYSEA's design is based on scientific recommendations by the  National Academies 2017-2027 Decadal Survey for Earth Science and Applications from Space , "Thriving on our Changing Planet." It also aligns with  NASA Earth Explorers  mission specifications related to cost and technical components (e.g., data sampling, launch vehicle requirements).

Our assessment starts with an ODYSEA measurement simulator to estimate global wind work. It is fed by outputs from a high-resolution simulation that combines a  Goddard Earth Observing System (GEOS)  atmosphere model coupled to an  Estimating the Circulation and Climate of the Ocean (ECCO)  ocean model. This coupled atmosphere-ocean simulation is the perfect tool to design and study the capabilities of a satellite mission concept like ODYSEA. The ODYSEA simulator generates global-scale science data sampled at high resolution.

A key objective is assessing the potential capabilities of ODYSEA to simultaneously measure winds and surface currents. Why? The pairing of wind and ocean currents is key to diagnosing wind work in our global ocean.

The figure below compares two examples of the sampling expected from ODYSEA. Moving the slider, you can see that faster ocean surface currents are often co-located with higher ocean surface winds.

What do we see in our simulated data? Surface ocean currents involve much smaller scales than winds; they also evolve more slowly. The wind field involves large scales (~1000 km or ~620 mi) resulting from atmospheric weather patterns that propagate rapidly. Embedded within these large-scale patterns are smaller-scale patterns (as small as 100 km or 62 mi); some of these smaller wind patterns propagate with the large-scale ones. Highly energetic surface ocean currents include NIWs, mesoscale eddies, and zonal jets.

Now let's look at an examples of ODYSEA's global coverage over 1.5 days. Within this short time period, ODYSEA is able to sample more than 90% of the global ocean with a spatial resolution of 5 km (3.1 mi)!

Ocean current speed is shown at left. Away from the equator, there is a smooth transition of surface currents between swaths. Also, slowly evolving surface currents such as large-scale currents and mesoscale eddies are well sampled by the temporal resolution of ODYSEA.

Wind stress at the ocean surface is shown at right. At mid- and high latitudes there are some discontinuities between swaths; for example, west of the tip of South America and south of the Alaska peninsula. These discontinuities are due to ODYSEA's 12-hour sampling, which cannot fully resolve the fast propagation of atmospheric storms.

Do these discontinuities show up in ODYSEA's wind work data? No! Check out the map below, which takes into account both ocean currents and wind. It does not have the discontinuities that appear in the wind field (slider above, right).

Note that red areas show where motion-related energy from the atmosphere is injected into the ocean. Conversely, blue areas show where the ocean is injecting motion-related energy into the atmosphere.

Large positive values of wind work are well captured at mid-latitudes (23° to 66° N and S). These areas are where large-scale atmospheric patterns are located. On the other hand, the simulator has under-sampled areas (i.e., diamond-shaped areas) around equatorial latitudes where the simulator has data gaps because of its 12-hour sampling.

Now let's look at seasonal wind work fields, which are averaged over three months during "opposite" seasons. In the slider below, January-February-March (winter in the northern hemisphere) is at left and July-August-September (winter in the southern hemisphere) is at right.

Comparing the two maps, there are significant differences observed at mid-latitudes. In each hemisphere, wind work is intense during the winter season. This intensification is because mid-latitude atmospheric storm tracks are more energetic in winter than in summer.

At tropical and equatorial latitudes – between 30°S and 30°N – wind work patterns are generally oriented east-west and have weaker seasonal variation. Such zonal patterns are known to be associated with west-blowing trade winds. In the Jul-Aug-Sep (map at right), there are some curious north-south oriented highs south of Japan. Associated with fast-moving atmospheric tropical cyclones, they are well captured by the ODYSEA simulator.

This study has explored the future capabilities of a new satellite mission, ODYSEA, to diagnose wind work over the globe. Wind work a major driver of ocean motion, which impacts our weather and climate change. Thus, ODYSEA is a new and exciting potential next step to extend and expand NASA's long history of observing ocean winds from space.

Our results indicate that the ODYSEA instrument performs well globally thanks to its wide swath, twice-a-day coverage at mid-latitudes, and daily coverage at low latitudes. We have also demonstrated that wind work – a key climate driver – can be derived from ODYSEA's co-located estimates of winds and ocean currents. – Hector Torres, Study Lead

Torres, H., Wineteer, A., Klein, P., et al. (2023)  Anticipated Capabilities of the ODYSEA Wind and Current Mission Concept to Estimate Wind Work at the Air–Sea Interface , Remote Sens., 15, 3337, doi: 10.3390/rs15133337