RAPID: Wintertime Hydrography of the Chukchi Sea

Water from the Pacific Ocean enters the Arctic domain through the Bering Strait, between the US and Russia, and subsequently flows northward across the wide and shallow Chukchi Sea. During the winter months, the water is near the freezing point and contains high concentrations of nutrients. As the sun returns in the spring, these nutrients spur large phytoplankton blooms that in turn support much of the marine life.

It has recently been hypothesized that, in addition to the remote source via the Bering Strait, nutrients can be mixed into the water locally from the sediments of the Chukchi shelf. This happens when there are small openings in the ice during winter that quickly re-freeze. This process makes the surface water denser, which then sinks to the bottom and stirs up the nutrients.

From December 2020 to February 2021, the US Coast Guard Cutter Polar Star will be conducting a mission in the Chukchi Sea, which offers a rare opportunity to collect measurements of the water during this critical time of the year. We propose to outfit the ship with an easy-to-use portable system whereby expendable probes are launched twice daily to obtain profiles of temperature and salt content.

This study will reveal how and where the cold water is being transformed. By comparing these winter data with measurements collected the previous fall and following spring by other oceanographic expeditions, we can better understand the role of the near-freezing winter water in the Arctic ecosystem. The results from the Polar Star cruise will be incorporated into an existing community outreach program that engages local communities.

Pacific-origin winter water that flows northward through the Bering Strait is the predominant source of nutrients that spur the large productivity occurring each spring on the Chukchi Sea shelf. During its transit across the shelf during the cold months of the year, the winter water can undergo modification when subject to coastal polynyas. Recently it has been argued that this water mass transformation also takes place in small leads throughout the entire Chukchi shelf.

If true, this has significant consequences for the regional ecosystem, since the convective overturning can readily reach the bottom and mix nitrate from the sediments into the water column. The recent decision for the USCG Polar Star to conduct a mission in the Chukchi Sea from December 2020 to February 2021 offers an extraordinary opportunity to collect profiles during the very time that the water mass transformation takes place.

This proposal seeks funds to outfit the Polar Star with two easy-to-use portable systems for launching temperature/salinity probes twice daily. This will provide a rare view of winter water formation and transformation, including the degree to which it occurs, the atmospheric forcing, and the pathways of the newly ventilated water. Comparing the winter data to measurements collected in the previous fall and following spring/summer will enable us to determine the evolution and fate of the cold, high-nutrient water and better understand its role in the regional ecosystem.

The investigator will be chief scientist on one of the summer cruises – an NSF funded field program in the Chukchi Sea in August 2021 that entails a broadscale hydrographic survey of the shelf. As part of that effort, an outreach program will be carried out including a PolarTREC teacher and visits to local communities. The results from the Polar Star winter cruise will be incorporated into this outreach program to provide a broader context for the summertime measurements.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.