干、湿大气环流模式中地表增温的经向分布及其机制

使用耦合了平板海洋的三维大气环流模式, 探究理想条件下极地增温放大现象的产生机制。实验中关闭海冰和云的辐射效应, 固定地表反照率, 并将海洋经向热量输送设置为零。通过控制地表蒸发的有无, 模拟湿大气和干大气两种情形。模拟结果显示, CO₂浓度加倍后, 湿大气环流模式中存在极地增温放大的现象, 而干大气环流模式中不存在这种现象。在干大气环流模式中, 地表增温幅度基本上不随纬度变化, 即均匀增温。湿大气环流模式中, CO₂浓度加倍导致的直接辐射强迫和水汽反馈导致的辐射效应都是热带比极地更强, 唯一能够解释湿大气中极地增温放大原因的是从赤道向极地的大气能量传输增强。在干大气环流模式中, 从赤道向极地的热量输送及其变化比湿大气弱很多, 因此无法支持极地增温放大现象。干大气中的均匀增温是CO₂的直接辐射强迫和普朗克效应相互竞争的结果。研究结果表明, 与水汽相关的经向热量输送是地球极地增温放大的关键因素, 而在基本上没有水汽的火星上, 可能不会出现极地增温放大现象。

Meridional Patterns of Surface Warming and the Underlying Mechanisms in Dry and Moist AGCMs

Using an atmospheric general circulation model (AGCM) coupled to a slab ocean, the mechanisms for producing the PWA (polar warming amplification) in idealized conditions are investigated. In the simulations, both ice albedo feedback and cloud radiative effects are turned off, but realistic radiative transfer of greenhouse gases (such as CO₂) and atmospheric heat transport are included. Surface albedo is fixed and meridional oceanic heat transport is set to zero everywhere. Through turning on or off surface evaporation, the model is a moist AGCM or becomes a dry AGCM. Results show that under doubling atmospheric CO₂ concentration, PWA occurs in the moist AGCM but not in the dry AGCM. In the dry AGCM, the increases of surface temperatures are nearly uniform from the equator to the poles. The radiative forcing of increased CO₂ and water vapor feedback are stronger in the tropics than those in the polar region, so that the only mechanism for driving the PWA in the moist AGCM is an enhanced meridional heat transport. In the dry AGCM, the poleward heat transport also increases with a much smaller magnitude, so that it is not able to support a PWA. This study emphasizes that water vapor and its associated meridional heat transport are necessary for the PWA on Earth, and PWA may not occur in a dry atmosphere such as Martian atmosphere.