On the parameterisation of oceanic sensible heat loss to the atmosphere and to ice in an ice-covered mixed layer in winter

Abstract

In high-latitude oceans with seasonal ice cover, the ice and the low-salinity mixed layer form an interacting barrier for the heat #ux from the ocean to the atmosphere. The presence of a less dense surface layer allows ice to form, and the ice cover reduces the heat loss to the atmosphere. The ice formation weakens the stability at the base of the mixed layer, leading to stronger entrainment and larger heat #ux from below. This heat transport retards, and perhaps stops, the growth of the ice cover. As much heat is then entrained from below as is lost to the atmosphere. This heat loss further reduces the stability, and unless a net ice melt occurs, the mixed layer convects. Two possibilities exist: (1) A net ice melt, su$cient to retain the stability, will always occur and convection will not take place until all ice is removed. The deep convection will then be thermal, deepening the mixed layer. (2) The ice remains until the stability at the base of the mixed layer disappears. The mixed layer then convects, through haline convection, into the deep ocean. Warm water rises towards the surface and the ice starts to melt, and a new mixed layer is reformed. The present work discusses the interactions between ice cover and entrainment during winter, when heat loss to the atmosphere is present. One crucial hypothesis is introduced:`When ice is present and the ocean loses sensible heat to the atmosphere and to ice melt, the buoyancy input at the sea surface due to ice melt is at a minimuma. Using a onedimensional energy-balance model, applied to the arti"cial situation, where ice melts directly on warmer water, it is found that this corresponds to a constant fraction of the heat loss going to ice melt. It is postulated that this partitioning holds for the ice cover and the mixed layer in the high-latitude ocean. When a constant fraction of heat goes to ice melt, at least one deep 0967-0645/99/$ -see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 -0 6 4 5 ( 9 9 ) 0 0 0 2 8 -4 convection event occurs, before the ice cover can be removed by heat entrained from below. After one or several convection events the ice normally disappears and a deep-reaching thermal convection is established. Conditions appropriate for the Weddell Sea and the Greenland Sea are examined and compared with "eld observations. With realistic initial conditions no convection occurs in the warm regime of the Weddell Sea. A balance between entrained heat and atmospheric heat loss is established and the ice cover remains throughout the winter. At Maud Rise convection may occur, but late in winter and normally no polynya can form before the summer ice melt. In the central Greenland Sea the mixed layer generally convects early in winter and the ice is removed by melting from below as early as February or March. This is in agreement with existing observations.

Recent advances in understanding the Arctic climate system state and change from a sea ice perspective: a review

“…Barents Sea sea ice bottom heat uptake from the ocean is proportional to the water temperature (Rudels et al, 1999), and thus should have increased during the Barents Sea warming. On the annual average however, the ice bottom experiences freezing, while net melting occurs at the top.…”

Arctic Ocean circulation and variability – advection and external forcing encounter constraints and local processes

“…The larger part is lost to the atmosphere. Rudels et al (1999a) and Rudels (2010) explored the situation, when sea ice melts on warmer water under freezing conditions using a KatoPhillips energy balance model (Kato and Phillips, 1969). The entrainment velocity w e then becomes;…”

“…When the upper layer has reached freezing temperature the second terms become zero. Both buoyancy inputs influence the entrainment velocity, which will be given by: To proceed we make the crucial assumption that the heat flux to ice melt is a minimum (Rudels et al, 1999a;Rudels, 2010). This keeps the density step between the created low salinity surface layer and the underlying Atlantic water small and allows for a more rapid transfer of heat from the Atlantic water to the upper layer and to ice melt and to the atmosphere.…”

“…Once freezing temperature is reached in the upper layer the entrained heat is not sufficient to fulfil the heat flux to the atmosphere and to ice melt and new ice starts to form. This is considered to occur simultaneously to the ice melt (Rudels et al, 1999a) (Fig. 5).…”

Arctic Ocean Heat Impact on Regional Ice Decay: A Suggested Positive Feedback

“…Temperature at the freezing point is the necessary condition for initial ice formation. However, this condition may not be fulfilled permanently after the onset of freezing because of turbulent entrainment at the base of the mixed layer (Turner 1973), vertical haline convection (Rudels 1990), upwelling (Falk-Petersen et al 2015, or external advection. In all these cases, warm (above the freezing temperature) water intrudes into the mixed layer, increasing its temperature and elevating F w above zero.…”