Time Scales and Mechanisms for the Tropical Pacific Response to Global Warming: A Tug of War between the Ocean Thermostat and Weaker Walker
Time Scales and Mechanisms for the Tropical Pacific Response to Global Warming: A Tug of War between the Ocean Thermostat and Weaker Walker
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DOI：
10.1175/jcli-d-19-0690.1
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发表时间：
2020-07-15
影响因子：
4.9
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.
不同的海洋和大气机制已被提出来描述热带太平洋对全球变暖的响应，但很大的不确定性仍然存在的相对重要性和潜在的相互作用。在这里，我们使用理想化的实验强迫与广泛的突然和渐进的CO2增加的耦合气候模式（CESM）与一个简化的箱模型，探索之间的相互作用，和时间尺度，不同的机制驱动步行者环流的变化。我们发现一个强大的瞬态响应CO2强迫在所有模拟，持续20至100年之间，这取决于如何突然的系统扰动。这种初始响应的特征是印度洋-太平洋纬向海温梯度的加强和步行者环流的向西移。相反，50-100年后出现的平衡响应的特征是冷舌变暖，纬向风减弱，步行者环流减弱。在完全耦合模式的平衡响应的大小主要是由增强的热带外变暖和较弱的海洋副热带细胞，减少冷水供应赤道上升流。相比之下，在板片海洋模拟中，步行者胞的减弱更为温和，并由沿着赤道的差异蒸发冷却驱动。对流层中层垂直速度也观察到大气能量学所暗示的“较弱的Walker”机制，但其地面表现并不稳健。正确诊断这些瞬态和平衡响应之间的平衡，将提高对热带太平洋正在进行的和未来的气候变化的理解。
Different oceanic and atmospheric mechanisms have been proposed to describe the response of the tropical Pacific to global warming, yet large uncertainties persist on their relative importance and potential interaction. Here, we use idealized experiments forced with a wide range of both abrupt and gradual CO2 increases in a coupled climate model (CESM) together with a simplified box model to explore the interaction between, and time scales of, different mechanisms driving Walker circulation changes. We find a robust transient response to CO2 forcing across all simulations, lasting between 20 and 100 years, depending on how abruptly the system is perturbed. This initial response is characterized by the strengthening of the Indo-Pacific zonal SST gradient and a westward shift of the Walker cell. In contrast, the equilibrium response, emerging after 50-100 years, is characterized by a warmer cold tongue, reduced zonal winds, and a weaker Walker cell. The magnitude of the equilibrium response in the fully coupled model is set primarily by enhanced extratropical warming and weaker oceanic subtropical cells, reducing the supply of cold water to equatorial upwelling. In contrast, in the slab ocean simulations, the weakening of the Walker cell is more modest and driven by differential evaporative cooling along the equator. The "weaker Walker'' mechanism implied by atmospheric energetics is also observed for the midtroposphere vertical velocity, but its surface manifestation is not robust. Correctly diagnosing the balance between these transient and equilibrium responses will improve understanding of ongoing and future climate change in the tropical Pacific.