Positive feedbacks in climate processes can make it difficult to identify the primary drivers of climate phenomena. Some recent global climate model (GCM) studies address this issue by controlling the wind stress felt by the surface ocean such that the atmosphere and ocean become mechanically decoupled. Most mechanical decoupling studies have chosen to override wind stress with an annual climatology. In this study we introduce an alternative method of interannually varying overriding which maintains higher frequency momentum forcing of the surface ocean. Using a GCM (NCAR CESM1), we then assess the size of the biases associated with these two methods of overriding by comparing with a freely evolving control integration. We find that overriding with a climatology creates sea surface temperature (SST) biases throughout the global oceans on the order of ±1°C. This is substantially larger than the biases introduced by interannually varying overriding, especially in the tropical Pacific. We attribute the climatological overriding SST biases to a lack of synoptic and subseasonal variability, which causes the mixed layer to be too shallow throughout the global surface ocean. This shoaling of the mixed layer reduces the effective heat capacity of the surface ocean such that SST biases excite atmospheric feedbacks. These results have implications for the reinterpretation of past climatological wind stress overriding studies: past climate signals attributed to momentum coupling may in fact be spurious responses to SST biases.

中文翻译：

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保留短期变化可减少风应力压倒性模拟中的平均状态偏差

气候过程中的正反馈可能会使确定气候现象的主要驱动因素变得困难。最近的一些全球气候模型（GCM）研究通过控制海洋表面感受到的风应力来解决这个问题，从而使大气和海洋在机械上脱钩。大多数机械解耦研究都选择用年度气候学来克服风应力。在这项研究中，我们引入了一种年际变化的替代方法，该方法可以维持表面海洋的较高频率动量强迫。然后，我们使用 GCM (NCAR CESM1)，通过与自由演化的控制集成进行比较，评估与这两种覆盖方法相关的偏差大小。我们发现，忽略气候学会导致全球海洋的海面温度 (SST) 偏差达到 ±1°C 的量级。这比年际变化的覆盖带来的偏差要大得多，特别是在热带太平洋地区。我们将气候学上最重要的海温偏差归因于天气和次季节变化的缺乏，这导致整个全球表面海洋的混合层太浅。混合层的浅滩降低了海洋表面的有效热容量，从而导致海温偏差激发大气反馈。这些结果对于重新解释过去的气候风应力压倒性研究具有重要意义：过去归因于动量耦合的气候信号实际上可能是对海温偏差的虚假响应。