Lower Evaporation in Polar Regions May Impact Ocean Circulation and Global Climate
It may seem counterintuitive that Earth’s coldest, iciest places are actually some of the driest places on our planet. But Earth’s polar regions see significantly less evaporation – the conversion of liquid water on Earth’s surface into gaseous water vapor in our atmosphere -- than other parts of our world, largely due to the presence of sea ice. New NASA research using a sophisticated satellite instrument shows the Southern Ocean surrounding Antarctica has been evaporating less water to the atmosphere, with potential impacts on global ocean circulation and Earth’s climate.
Evaporation is a key part of the global water cycle, the process by which water circulates continuously between Earth’s surface (land and ocean) and the atmosphere. As the Sun heats up water from lakes, rivers and the ocean, the resulting water vapor condenses to form clouds and then returns to the surface as precipitation as rain and snow. About 85 percent of atmospheric water vapor evaporates from the surface of Earth’s ocean; with tropical regions having the highest levels of evaporation, due to their warm temperatures and close proximity to the ocean. Evaporation plays a key role in weather and climate. Less evaporation means less water vapor in the air, which can change precipitation patterns around the globe.
An evaporation blocker
The presence of sea ice completely changes the dynamics of evaporation, however. This layer of frozen ocean water covers much of the Arctic Ocean and Southern Ocean, with the extent and thickness changing seasonally due to temperatures. “You can think of sea ice as a blanket that covers the ocean, similar to when people put covers over their swimming pools to keep evaporation down,” said Eric Fetzer, project scientist for NASA’s Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite. “So, when there’s a lot of sea ice, you just don’t get much evaporation.”
“The reason why we care about changing sea ice conditions in this region is because sea ice prohibits the interaction between the ocean and the atmosphere,” said Linette Boisvert, a research scientist at NASA’s Goddard Space Flight Center. “That can affect the water cycle, clouds, and precipitation patterns across the globe.”
Boisvert first became interested in the connection between sea ice and evaporation while working on her doctorate, when she used AIRS data to measure evaporation in the Arctic Ocean and the Greenland Ice Sheet. Inspired by this previous work, she decided to do a similar study in Antarctica. “We wanted to see if we could use AIRS to estimate evaporation from the Southern Ocean and Antarctic sea ice surfaces, using the same method and same data we’d used in the other studies,” she said.
Data from satellite instruments are extremely valuable in Antarctica, due to the region’s lack of ground observation data. “There just aren’t a lot of observations in the Southern Ocean, because it’s hard to get ships down there to take measurements,” said Boisvert.
The reason why we care about changing sea ice conditions in this region is because sea ice prohibits the interaction between the ocean and the atmosphere...That can affect the water cycle, clouds, and precipitation patterns across the globe.
AIRS was chosen for the study because of its ability to accurately chart temperature and humidity changes over both the Arctic Ocean and the Southern Ocean. The instrument is also a workhorse when it comes to “seeing through” cloud cover, which poses an obstacle to most instruments. To produce daily estimates of evaporation from the Southern Ocean, the study also used wind speeds from NASA’s Modern-Era Retrospective Analysis for Research and Applications (MERRA-2), a global modeled atmospheric data set.
The Southern Ocean's decreasing trend
The team’s findings revealed a decreasing trend in annual evaporation over most of the Southern Ocean for the study period 2003-2016. According to Boisvert, one possible reason for the decrease is that until a few years ago, Antarctic sea ice extent was increasing slightly (this contrasts with the Arctic, which has seen a consistent decline in sea ice extent since the late 1970s.) More sea ice would have insulated the ocean surface from the atmosphere and reduced evaporation. Since 2015, however, the Southern Ocean has actually seen a large decrease in sea ice cover. “If the trend of decreased sea ice continues, it will be interesting to see if that changes the evaporation,” said Boisvert.
Boisvert says the findings are important because if the rate of evaporation in polar regions decreases, it can potentially impact other elements of the climate process, such as ocean circulation.
The ocean circulation connection
Ocean circulation refers to the large-scale movement of water that transports heat around the planet via surface and deep ocean currents. While surface currents are easy to visualize because they’re triggered by winds, deep ocean currents are like invisible forces that work behind the scenes, driven by water density.
When ocean water evaporates, most of the salt is left behind; hence, more evaporation means saltier water. Because salt water is more dense than fresh water, it tends to sink. “Evaporation really matters in certain polar regions, because by evaporating water from the surface, you actually make the ocean saltier,” said Fetzer. “You’re basically concentrating salt in the ocean, and that heavy salt water then sinks.”
Fresh water freezes at 32 degrees Fahrenheit, but seawater freezes at approximately 28.4 degrees Fahrenheit. So when the more dense, salty water sinks, the fresher water left at the surface can freeze more easily. “Evaporation affects whether and how sea ice forms, and conversely, evaporation is modulated by sea ice,” said Fetzer. “So, you have this interplay between evaporation and sea ice formation, and it all ties into the ocean circulation around Antarctica.”
The Ross Sea's increasing trend
While the results showed that evaporation decreased in most of the Southern Ocean, the study also revealed an increase in evaporation in the Ross Sea, which is adjacent to an ice shelf that spawns strong katabatic winds (cold and dry winds that blow downslope from higher elevations). “There is less sea ice coverage in that area, due to these katabatic winds which force the sea ice way from the ice shelf, and when the wind blows over this area of open water in the Ross Sea, you get a lot of evaporation occurring there,” said Boisvert. Her next step, she said, is to look specifically at this increase in the Ross Sea area, and AIRS data will again be an integral part of the study.
“People don’t really use AIRS too much in the polar regions—it’s used more widely in the mid-latitudes,” she said. “But being able to use these satellite-derived datasets to estimate evaporation at the poles can give us lots of insight into what’s going on in those regions.”
“In the global evaporation picture, the polar regions are not all that important because there is so much evaporation happening in the tropics. But locally, it matters a lot,” said Fetzer. “By changing sea ice and evaporation in the polar regions, you can actually change the entire deep ocean circulation, sea ice formation and local cloud cover, and that could have a big effect on climate.”
Read the paper
Boisvert, L., Vihma, T., & Shie, C.‐L. (2020). Evaporation from the Southern Ocean estimated on the basis of AIRS satellite data. Journal of Geophysical Research: Atmospheres, 125, e2019JD030845. https://doi.org/ 10.1029/2019JD030845