Surface Energy Fluxes During Arctic Freeze-Up

This presentation uses atmospheric and ocean mixed-layer observations from three cruises during the past two years to examine the magnitude and variability of the autumnal air-ocean energy fluxes, the sources of the variability, the impact of the fluxes on the ocean mixed-layer thermal structure, and how these surface energy fluxes are affected by the forming and remaining sea ice. These are the first direct observations of autumnal air-ocean heat fluxes since the significant loss of sea ice during the past 10-20 years. The measurements were made during the ACSE, Mirai, and Sea State field programs. The first two obtained measurements near the ice edge in the Laptev and Chukchi Seas in September 2014 and the last along the advancing ice edge in the Beaufort/Chukchi Sea in October 2015. These time periods include the onset of continuous ocean heat loss, the initial episodic ice formation, and the core period for southward advance of the ice. Frequent atmospheric soundings and continuous remote-sensor measurements provide the vertical kinematic and thermodynamic structure of the lower troposphere. Broadband radiometers, turbulent flux sensors, surface temperature sensors, surface characterization instruments, and basic meteorological instrumentation provide continuous measurements of all surface energy flux terms, allowing the quantification of the total energy exchange between the ocean and the atmosphere. Furthermore, each cruise provided continuous measurements of the upper-ocean temperature and salinity and frequent CTD measurements of the ocean temperature and salinity profiles, providing estimates of upper-ocean energy evolution. Various methods for characterizing the ocean surface allow linking energy changes with changes in ocean surface conditions.

Initial analyses of the September and October conditions show persistent ocean heat loss after Sep. 15 because of the reduction of downwelling shortwave radiation and strong impacts of off-ice airflow on turbulent heat fluxes and downwelling longwave radiation. During the month of October, the average net heat loss was ~90-110 W m-2, though multi-day events produced local losses up to 500 W m-2. Atmospheric cooling in the lowest 300-500 m over adjacent ice and subsequent cold-air advection was crucial to rapid heat loss over the nearby ocean, emphasizing the importance of ice surviving the summer melt for facilitating autumn freeze-up. Freeze-up and formation of pancake ice was associated with thermal changes primarily in the upper few meters of the ocean, though upward mixing by waves of sub-surface ocean heat at times retarded ice growth. Sub-surface lateral heat advection may also have played a role for heat loss and ice formation in some regions. Observations from all three cruises demonstrate the importance of both the atmospheric and oceanic boundary layers, and the air-ocean-ice interactions at their interfaces, for the autumnal evolution of the sea ice. This autumnal heat loss provides the primary Arctic forcing for processes recently hypothesized to impact mid-latitude circulation; these measurements should be used to validate models used in such large-scale studies.