NASA Salinity: Air-Sea Interactions: Hurricanes

Hurricanes intensify as they travel over warm ocean water.
Hurricanes intensify as they travel over warm ocean water, threatening lives with powerful winds, heavy rainfall, storm surges, and coastal and inland flooding.
Around 70% of hurricanes that reached Category 5 crossed the Amazon/Orinoco river plume.
Around 70% of hurricanes that finally reached Category 5 crossed the Amazon/Orinoco river plume, according to a 2007 study.
River plumes can extend thousands of miles offshore.
These plumes can extend thousands of miles offshore. Their waters are usually fresher, warmer, and carry more dissolved organic matter than the surrounding ocean.
Monitoring the Amazon/Orinoco River is an important application of salinity data.
The Amazon/Orinoco River plume extent changes not only over weeks and months ... but also from year-to-year. Monitoring the plume is an important application of salinity data.

"Even in the middle of a hurricane, the bottom of the sea is calm." - Wayne Muller

Watching hurricanes slowly creeping toward land can be alarming. However, it is also important to track these storms' path over the ocean, which greatly influences their strength.

Satellite data such as sea surface temperature and height help scientists understand the ocean heat content available for storm intensification. Current forecasting tools do not include the effects of salinity, but new research shows that low sea surface salinity caused by the outflow of major rivers can have a considerable impact on how storms intensify. Why?

Outflow from rivers can form and maintain thin layers at the sea surface. Underneath, there can be a "barrier layer" where salinity changes dramatically, isolating the upper ocean from water below. This type of stratified water column can intensify passing hurricanes. How?

The answer is heat. Thinner surface layers warm up more quickly than surrounding waters. Not only that, the underlying barrier layer makes "stirring up" the water column by hurricanes - which brings up cold water in most places – more difficult. So, monitoring how certain river plumes change over time is key to understanding interactions between the ocean and hurricanes.

Use the tool below – based on a 5-year study by Fournier et al. (2017) – to investigate the characteristics of the Amazon/Orinoco plume during 2010 and 2014. Located in the eastern Caribbean Sea, the study area has been shown to lie directly in the path of many Atlantic hurricanes. Use the buttons to toggle between years or among data types.

Sea Surface Salinity

Defined by low salinity, the Amazon/Orinoco plume covers most of the study area.

Sea Surface Temperature

Defined by low salinity, the plume's water is warmer than average. Together, these properties help the plume "stay afloat".

Mixed Layer Depth

The depth of surface mixing – shown approximately in this diagram – differs between the plume and the surrounding ocean.

Dissolved Organic Matter

Dissolved organic matter from the Orinoco River is a major contributor in 2010. More dissolved particles, especially with darker color, help trap more heat.

Winds

The large extent of the 2010 plume was helped by low winds in the region. These winds allowed the plume to move well offshore.

Sea Surface Salinity

Defined by low salinity, the Amazon/Orinoco plume covers only 60% of the study area.

Sea Surface Temperature

Defined by low salinity, the plume’s water is warmer than average. Together, these properties help the plume "stay afloat".

Mixed Layer Depth

The depth of surface mixing – shown approximately in this diagram – differs between the plume and the surrounding ocean.

Dissolved Organic Matter

During 2014, organic matter is low and concentrated in the southern portion of the study area.

Winds

The 2014 extent is relatively small because of winds blowing toward South America. In addition, ocean eddies transported plume waters along the coast (rather than offshore).

First light image: Amazon and Orinoco Rivers Outflow

Amazon and Orinoco Outflow

Amazon River Delta

Barrier Layer Diagram

NASA’s Aquarius satellite data shows how ocean salinity (saltiness) changes during the year

NASA Tracks Amazon Plume

Hurricane Katia Track

Mixed Layer Impacts

Land/Ocean Response

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To view all salinity publications, visit the publications page.

Difference in salinity after the hurricane passed over the plume

Difference in temperature after the hurricane passed over the plume

Use the slider to compare changes in salinity and temperature of the Amazon River plume, outlined in black. For both images, crosses (+) show daily locations of Hurricane Katia in late August and early September 2011. The size of each cross indicates strength of sustained winds on that day. Colors indicate differences in conditions after the hurricane passed over the plume. For example, red areas of salinity got saltier and blue areas of temperature cooled down, indicating erosion of the surface barrier layer by the storm. Later hurricanes (Maria and Ophelia) did not intensify as the eroded barrier layer (due to Katia passage) allowed an increase in upper ocean cooling (Androulidakis, et al., 2016).

Featured Publications

Five years of summertime storm tracks for North Atlantic tropical cyclones within a 2 month period between 15 July and 15 September in the years 2010–2014

This study investigates sea surface salinity and sea surface temperature variations in the tropical Atlantic east of the Lesser Antilles, a region where freshwater advection from the Amazon and Orinoco Rivers - the Amazon-Orinoco Plume - may potentially impact air-sea interaction. Persistent sea surface temperature gradients are often found near the plume edge, which may have implications for ocean-atmosphere coupling associated with atmospheric convection.

Reference

Fournier, S., Vandemark, D., Gautier, L., Lee, T., Jonsson, B., and Gierach, M.M. (2017). Read the full paper.

Salinity distribution over the river plume area formed by the Amazon and Orinoco River outflows

The evolution of three successive hurricanes (Katia, Maria, and Ophelia) is investigated over the river plume area formed by the Amazon and Orinoco river outflows during September of 2011. The study focuses on hurricane impacts on the ocean structure and the ocean feedback influencing hurricane intensification.

Reference

Androulidakis, Y., Kourafalou, V., Halliwell, G., Le Hénaff, M., Kang, H., Mehari, M., Atlas, R. (2016). Read the full paper.

Aquarius SSS difference between after (5–10 SEP/2011) minus before (25AUG–1SEP/2011) the passage of Katia

At its seasonal peak the Amazon/Orinoco plume covers a region of 106 km² in the western tropical Atlantic with more than 1m of extra freshwater, creating a near‐surface barrier layer that inhibits mixing and warms the sea surface temperature to >29°C. Here new sea surface salinity observations from the Aquarius/SAC-D and SMOS satellites help elucidate the ocean response to hurricane Katia, which crossed the plume in early fall, 2011.

Reference

Grodsky, S., Reul, N., Lagerloef, G., Reverdin, G., Carton, J., Chapron, B., Quilfen, Y., Kudryavtsev, V., and Kao, H. (2012). Read the full paper.

Resources

Get the data:

View maps of sea surface salinity from SMAP and Aquarius
Learn how to access the data from PO.DAAC

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