Compensation of Hemispheric Albedo Asymmetries by Shifts of the ITCZ and Tropical Clouds
Compensation of Hemispheric Albedo Asymmetries by Shifts of the ITCZ and Tropical Clouds
复制标题
ITCZ 和热带云移动对半球反照率不对称的补偿
DOI：
10.1175/jcli-d-13-00205.1
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发表时间：
2014
影响因子：
4.9
通讯作者：
T. Mauritsen
中科院分区：
文献类型：
--
作者：
A. Voigt;B. Stevens;J. Bader;T. Mauritsen
尽管半球在晴空反照率上存在明显的不对称性，但对地球辐射收支的观测表明，两个半球的全天反照率是相同的。在这里，用大气环流模式ECHAM6耦合一个平板海洋进行水行星模拟，以研究云在晴空反照率中补偿半球不对称的程度和机制。由于热带辐合带（ITCZ）转移到黑暗的地表半球，云适应以补偿强加的不对称。这种热带补偿机制的强度与ITCZ移动的大小有关。在某些情况下，ITCZ的移动非常强烈，以至于过度补偿了晴空反照率的半球不对称性，从而产生了晴空反照率较低的半球全天反照率较高的气候范围。ITCZ位移对对流方案和板状海洋深度敏感。云辐射反馈解释了对流方案的部分敏感性，因为它们在Tiedtke （TTT）方案中放大了ITCZ偏移，而在Nordeng （TNT）方案中则没有作用。较浅的板块海洋深度，减少了下垫面的热惯性，增加了季节周期，稳定了ITCZ对年平均位移的影响。尽管没有迹象表明两个半球的反照率必须完全相同，但研究结果支持了气候系统调节以使半球反照率不对称最小化的观点。
Despite a substantial hemispheric asymmetry in clear-sky albedo, observations of Earth’s radiation budget reveal that the two hemispheres have the same all-sky albedo. Here, aquaplanet simulations with the atmosphere general circulation model ECHAM6 coupled to a slab ocean are performed to study to what extent and by which mechanisms clouds compensate hemispheric asymmetries in clear-sky albedo. Clouds adapt to compensate the imposed asymmetries because the intertropical convergence zone (ITCZ) shifts into the dark surface hemisphere. The strength of this tropical compensation mechanism is linked to the magnitude of the ITCZ shift. In some cases the ITCZ shift is so strong as to overcompensate the hemispheric asymmetry in clear-sky albedo, yielding a range of climates for which the hemisphere with lower clear-sky albedo has a higher all-sky albedo. The ITCZ shift is sensitive to the convection scheme and the depth of the slab ocean. Cloud–radiative feedbacks explain part of the sensitivity to the convection scheme as they amplify the ITCZ shift in the Tiedtke (TTT) scheme but have a neutral effect in the Nordeng (TNT) scheme. A shallower slab ocean depth, and thereby reduced thermal inertia of the underlying surface and increased seasonal cycle, stabilizes the ITCZ against annual-mean shifts. The results lend support to the idea that the climate system adjusts so as to minimize hemispheric albedo asymmetries, although there is no indication that the hemispheres must have exactly the same albedo.