A model for the Arctic mixed layer circulation under a summertime lead: implications for the near-surface temperature maximum formation

Leads in sea ice cover have been studied extensively because of the climatic relevance of the intense oceanatmosphere heat exchange that occurs during winter. Leads are also preferential locations of heat exchange and melting in early summer, but their oceanography and climate relevance, if any, remains largely unexplored during summertime. In particular, the development of a near-surface temperature maximum (NSTM) layer typically 10-30m deep under different Arctic basins has been observationally related to the penetration of solar radiation through the leads. These observations reveal that the concatenation of calm and wind events in the leads could facilitate the development of the NSTM layer. Using numerical modeling and an idealized framework, this study investigates the formation of the NSTM layer under a summer lead exposed to a combination of calm and moderate wind periods. During the calm period, solar heat accumulates in the upper layers under the lead. Near-surface convection cells are generated daily, extending from the lead sides to its center. Convection cells affect the heat storage in the mixed layer under the lead and the adjacent ice cap. A subsequent wind event (and corresponding ice drift) mixes and spreads fresh and cold meltwater into the warm layers near the surface. Surface mixing results in temperatures in the near-surface layers that are lower than in the deeper layers, where the impact of the surface stresses is weaker. Additionally, the warm waters initially located under the lead surface stretch and spread horizontally. Thus, an NSTM layer is formed. The study analyzes the sensitivity of the depth and temperature of the NSTM layer to buoyancy forcing, wind intensity, ice drift, stratification, and lead geometry. Numerical results suggest that the NSTM layer ap- pears with moderate wind and ice drift and disappears when the wind intensity is higher than 9ms 1. Depending on the background stratification, the calm period reinforces or becomes critical in NSTM layer formation. According to the results, ice drift is key to the development of the NSTM layer.

Automated summary

Studies have shown that leads in sea ice cover play an important role in climate change, especially in the intense heat exchange that occurs during winter. However, there is limited research on the oceanography and climate relevance of leads during summer. This study uses numerical modeling to investigate the formation of near-surface temperature maximum (NSTM) layer in summer leads, and finds that moderate wind and ice drift are crucial factors in the development of the NSTM layer.