Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers
Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers
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DOI：
10.5194/tc-6-821-2012
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发表时间：
2012-01-01
期刊：
影响因子：
5.2
通讯作者：
Steffen, K.
中科院分区：
文献类型：
--
作者：
Box, J. E.;Fettweis, X.;Steffen, K.
格陵兰冰盖的质量损失在过去十年中加速了，这是由于冰川排放和地表融水径流的增加。在夏季，吸收的太阳能主要在地表被反照率调制，是控制消融区地表融化变化的主要因素。利用卫星反演的地表反照率与校准的区域气候模拟的地表气温和地表向下的太阳辐照度，我们确定了自2000年以来12个夏季期间冰盖反照率反馈的空间依赖性和定量影响。我们发现，虽然净太阳短波通量和温度随时间的变化定义的反照率反馈在97%的冰盖上是正的，但当使用成对的年度异常定义时，在63%的积累面积上明显存在二阶负反馈。由于降雪与地表气温异常呈正相关，这种负反馈抑制了累积区对变暖的响应。当利用卫星获得的融化持续时间来确定净短波通量进入融化的时间时，集中在消融区的正异常测量反馈占融化总增量的一半以上。2007年以来，与持续的夏季北大西洋涛动极端有关的异常强的反气旋环流使反照率反馈最大化的三种放大机制成为可能：(1)西部冰盖暖空气平流增加，地表感热增加，进而增加雪粒变质率，进一步降低反照率；(2)地表向下短波通量增加，地表升温加剧，反照率进一步降低；(3)降雪率的降低维持了低反照率，使地表太阳加热最大化，多年来逐渐降低反照率。夏季高海拔堆积区的净红外辐射和太阳辐射在此期间趋于正值。因此，有理由预计在另一个类似的变暖十年里，冰盖上的融化面积将达到100%。
Greenland ice sheet mass loss has accelerated in the past decade responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area. Using satellite-derived surface albedo with calibrated regional climate modeled surface air temperature and surface downward solar irradiance, we determine the spatial dependence and quantitative impact of the ice sheet albedo feedback over 12 summer periods beginning in 2000. We find that, while albedo feedback defined by the change in net solar shortwave flux and temperature over time is positive over 97% of the ice sheet, when defined using paired annual anomalies, a second-order negative feedback is evident over 63% of the accumulation area. This negative feedback damps the accumulation area response to warming due to a positive correlation between snowfall and surface air temperature anomalies. Positive anomaly-gauged feedback concentrated in the ablation area accounts for more than half of the overall increase in melting when satellite-derived melt duration is used to define the timing when net shortwave flux is sunk into melting. Abnormally strong anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007, enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward shortwave flux, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net infrared and solar radiation for the high elevation accumulation area approached positive values during this period. Thus, it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.