A stronger versus weaker Walker: understanding model differences in fast and slow tropical Pacific responses to global warming
A stronger versus weaker Walker: understanding model differences in fast and slow tropical Pacific responses to global warming
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DOI：
10.1007/s00382-021-05818-5
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发表时间：
2021-05-29
期刊：
影响因子：
4.6
通讯作者：
Burls, Natalie J.
中科院分区：
文献类型：
--
作者：
Heede, Ulla K.;Fedorov, Alexey, V;Burls, Natalie J.
热带太平洋对辐射强迫的反应仍然不确定，因为预测的沃克环流和海温模式的未来变化在气候模型之间存在很大差异。在这里，我们研究什么决定了响应的幅度和时间尺度，以及为什么它们在不同模型中存在差异。具体来说，我们在水平分辨率和平均偏差不同的同一模型系列 (CESM) 的两种配置中比较了热带太平洋对二氧化碳突然增加（2、4、8、16 x CO2）的快速和缓慢响应。我们发现，分辨率更高的模型显示出对二氧化碳强迫的瞬态海洋恒温器响应，沃克单元更强，并且东太平洋信风带没有变暖。这种快速反应持续约 50 年，随后沃克细胞轻微减弱和赤道变暖。第二个模型具有较粗的分辨率，显示出微弱且短暂的海洋恒温器响应，随后是明显的沃克细胞减弱和东赤道太平洋变暖，类似于之前研究中指出的长期模式。这些快速和缓慢的反应也体现在二氧化碳逐渐增加的实验中。我们将快速海洋恒温器响应的幅度与赤道温跃层的结构联系起来，设定比耶克内斯反馈的强度。赤道东太平洋延迟变暖的幅度和时间与放大海洋恒温器的正反馈与海洋地下变暖侵蚀恒温器的影响的竞争有关。海洋副热带气流的减缓和热带外变暖的加剧进一步放大了后一种效应。这些影响之间的不同平衡可以解释模型对热带太平洋未来预测的巨大差异。
The tropical Pacific response to radiative forcing remains uncertain as projected future changes to the Walker circulation and SST patterns vary substantially among climate models. Here, we study what sets the magnitude and timescales of the response and why they differ across models. Specifically, we compare the fast and slow responses of the tropical Pacific to abrupt CO2 increases (2, 4, 8, 16 x CO2) in two configurations of the same model family (CESM) that differ in horizontal resolution and mean biases. We find that the model with a higher resolution shows a transient ocean thermostat-like response to CO2-forcing, with a stronger Walker cell and lack of warming in the eastern Pacific trade wind belts. This fast response lasts for about 50 years and is followed by a slight Walker cell weakening and equatorial warming. The second model, with a coarser resolution, shows a weak and short-lasting ocean thermostat response, followed by pronounced Walker cell weakening and eastern equatorial Pacific warming, similar to the long-term pattern noted in previous studies. These fast and slow responses also manifest in experiments where CO2 is gradually increased. We relate the magnitude of the fast ocean-thermostat response to the structure of the equatorial thermocline, setting the strength of the Bjerknes feedback. The magnitude and timing of the delayed eastern equatorial Pacific warming are related to the competition of positive feedbacks amplifying the ocean thermostat against the effect of ocean subsurface warming eroding the thermostat. The latter effect is further amplified by the slowdown of oceanic subtropical cells and enhanced extra-tropical warming. Different balances between these effects could explain the large spread in the model future projections for the tropical Pacific.