Abstracts
SEARCH Open Science Meeting
October 27, 2003
Seattle, Washington, USA
The Ice/Ocean Interface During Summer: Implications for Ice-Albedo Feedback
Miles G. McPhee1
1McPhee Research Company, 450 Clover Springs Road, Naches, WA, 98937, USA, Phone 5096582575, Fax 5096582575, mmcphee@starband.net
An important, perhaps dominant, component of the ice-albedo feedback is absorption of solar energy by the upper ocean when sun angles are relatively high. The basic concept is simple: as solar radiation penetrates open water (or thin ice) with relatively low albedo, temperature of the mixed layer rises. Ocean-to-ice heat flux increases, enhancing ice melt at the base and exposed edges, creating more low albedo area, increasing energy absorption, and so on. Under certain conditions the effect can be quite dramatic; however, two factors tend to ameliorate the strength of the feedback.
(A) Storage and sequestration of heat in the upper ocean. Melting at the ice/ocean interface occurs by the transfer of heat and salt through thin sublayers adjacent to the interface where molecular effects dominate. Since the molecular diffusivity of salt is smaller than thermal diffusivity (by a factor of about 200), salt controls the rate of melting. In early summer, this allows a more or less steady increase in mixed layer temperature, meaning that a significant fraction of the solar energy entering the upper ocean heats seawater instead of melting ice. Later in the season, much of this stored heat is absorbed by melting ice, but at a time when sun angles are considerably lower, decreasing the albedo feedback effect. From several observational studies, heat and salt transfer processes are relatively well understood and generally incorporated in most numerical models of ice-ocean interaction. Less well understood is the sequestration of heat in the lower part of the early summer mixed layer. This occurs in mid-summer when relatively rapid "flushing" of fresh water from the surface creates a seasonal pycnocline that protects the water below from surface mixing. During the AIDJEX (1975-1976) year in the Beaufort Gyre, this trapped summer heat never was recovered, meaning that over the annual cycle, the net ocean heat flux was downward.
(B) Underice melt ponds and false bottoms. In summer, fresh melt water running off at floe edges and percolating through porous, relatively warm ice, collects in concavities under thin ice. Thin layers of fresh ice, called false bottoms, form at the interface between these "underice melt ponds" (at 0 deg C) and the underlying seawater (typically -1.6 deg C). This reverses the usual temperature gradient at the interface, so that even though seawater is above freezing, the heat flux under false bottoms is often downward. Simple modeling shows that a relatively small areal coverage of false bottoms can significantly decrease the aggregate bulk ocean-to-ice heat transfer. The obvious importance for albedo feedback is that not only do the false bottoms decrease overall transfer of heat from the ocean, but also preferentially protect thin ice from basal melting.
Abstract Categories: Physical Feedbacks
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