Warm River Water Can Break Through Arctic Ice Barriers and Raise Ocean Temperature

Warm water discharging from rivers into icy Arctic seas heats the water's surface layers and contributes to sea ice melting, according to a new study.

Writing in the journal Geophysical Research Letters a team led by NASA Jet Propulsion Laboratory (JPL) researchers reports that this process occurs uniquely in the north pole region. The researchers made their observations in 2012 by using satellite data to analyze a Canadian river emptying into the icy Beaufort Sea.

"River discharge is a key factor contributing to the high sensitivity of Arctic sea ice to climate change," said research leader Son Nghiem of the JPL. "We found that rivers are effective conveyers of heat across immense watersheds in the Northern Hemisphere. These watersheds undergo continental warming in summertime, unleashing an enormous amount of energy into the Arctic Ocean, and enhancing sea ice melt. You don't have this in Antarctica."

The research focused on the Mackenzie River in western Canada, and they limited their analysis of satellite data to the year 2012 because among the 30 years worth of Arctic satellite data, that year holds the record for the smallest total extent of sea ice measured across the Arctic.

Nghiem and his team also gathered data collected between 1979 and 2012 to examine the extent of the region's sea ice and compare it to the Mackenzie River's effect on sea ice.

"Within this period, we found the record largest extent of open water in the Beaufort Sea occurred in 1998, which corresponds to the year of record high discharge from the river," said study co-author Ignatius Rigor of the University of Washington in Seattle.

The team observed that in June of 2012 an area of landfast sea ice that contained river discharge close to the delta was broken through by river water. In the following month, the temperature of the open water the river discharged into increased by 11.7 degrees Fahrenheit (6.5 degrees C).

"When the Mackenzie River's water is held back behind the sea ice barrier, it accumulates and gets warmer later in the summer," said Nghiem. "So when it breaks through the barrier, it's like a strong surge, unleashing warmer waters into the Arctic Ocean that are very effective at melting sea ice. Without this ice barrier, the warm river waters would trickle out little by little, and there would be more time for the heat to dissipate to the atmosphere and to the cooler, deeper ocean."

"If you have an ice cube and drop a few water droplets on it, you're not going to see rapid melt," said co-author Dorothy Hall of NASA's Goddard Space Flight Center in Greenbelt, Md. "But if you pour a pitcher of warm water on the ice cube, it will appear to get smaller before your eyes. When warm river water surges onto sea ice, the ice melts rapidly."

There are 72 rivers around the world that carry water into the Arctic Ocean. The researchers calculated that based on an average summertime water temperature of 41 F (5 C), the rivers are carrying as much heat into the Arctic Ocean each year as all of the electric energy used by the state of California in 50 years at today's consumption rate.

Nghiem said more research is needed to establish better water temperature readings in Arctic-draining rivers to further understand their contribution to sea ice melt.