A model for the Artic mixed layer circulation under a summertime lead: Implications on the near-surface temperature maximum formation
Abstract. Leads in the sea ice sheet have been studied extensively because of their climate relevance. In summer, there is an intense heat exchange between the ocean and the atmosphere in the leads. Leads are also preferential melting sites in early summer, but their oceanography and climate relevance, if any, remains largely unexplored during this period of the year. In particular, the development of a near-surface temperature maximum (NSTM) layer at typically 10–30 m deep under different Arctic Basins, has been related observationally to the penetration of solar radiation through the leads. The observations reveal that the concatenation of calm and wind events in the leads could facilitate the development of the NSTM layer. This study investigates, using numerical modelling and an idealized framework, 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. Also, the warm waters initially located under the lead surface stretch and spread horizontally. Thus an NSTM layer is formed. The study analyses the sensitivity of depth and temperature of the NSTM layer to buoyancy forcing, wind intensity and ice drift. Numerical results suggest that the NSTM layer appears with moderate wind and ice drift, and disappears when wind intensity is higher than 9 ms-1. According to the results, ice drift is key in the development of the NSTM layer.
Status: final response (author comments only)
RC1: 'Comment on tc-2022-233', Anonymous Referee #1, 05 Mar 2023
- AC1: 'Reply on RC1', Alberto Alvarez, 11 Apr 2023
RC2: 'Comment on tc-2022-233', Anonymous Referee #2, 16 Mar 2023
- AC2: 'Reply on RC2', Alberto Alvarez, 11 Apr 2023
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The manuscript, “A model for the Artic mixed layer circulation under a summertime lead: Implications on the near-surface temperature maximum formation” by Alvarez develops a simplified 2D model of a summertime Arctic lead using SHEBA data to examine the sensitivity of the near surface temperature maximum to changes in solar radiation, winds, and ice motion.
Global climate models have biased salinity and temperature distributions in the upper Arctic Ocean. It has been suggested this could be associated with resolution or parameterizations of vertical mixing (Rosenblum et al. 2020). Future changes in the upper ocean are also very uncertain as the changing sea ice state may modulate ocean circulation. The manuscript touches on an important issue of simulating processes controlling the structure of the upper Arctic Ocean but requires a clearer discussion of the novel conclusions that separate this study from previous studies of the summertime NSTM and to be put in a broader context in terms of the Arctic as a whole and other more complex models. I have listed these concerns below. My recommendation is major revisions.
Dewey, S., Morison, J., Kwok, R., Dickinson, S., Morison, D., & Andersen, R. (2018). Arctic ice‐ocean coupling and gyre equilibration observed with remote sensing. Geophysical Research Letters, 45(3), 1499-1508.
Rosenblum, E., Fajber, R., Stroeve, J. C., Gille, S. T., Tremblay, L. B., & Carmack, E. C. (2021). Surface salinity under transitioning ice cover in the Canada Basin: Climate model biases linked to vertical distribution of fresh water. Geophysical Research Letters, 48(21), e2021GL094739.