全球暖化下東南太平洋低雲對熱帶海溫分布之影響

Abstract

熱帶太平洋海表溫度分布在調節全球能量平衡中起著至關重要的作用。這種現象被稱為海溫分布效應（pattern effect），主要是透過短波雲反饋達成。我們藉由分析耦合模式比較計畫第六階段（Coupled Model Intercomparison Project Phase 6, CMIP6）的四倍二氧化碳模擬中，海表溫度和雲在模擬的頭25年反應來研究雲—海表溫度耦合。此外，我們使用社群地球系統模式（Community Earth System Model, CESM）第1版及第2版進行「鎖雲」實驗，以檢視在暖化情境下東南太平洋的低雲對赤道太平洋海表溫度之緯向梯度的影響。 首先，根據文獻，在赤道太平洋海表溫度梯度的前25年反應中，我們區分出了兩類氣候模式。赤道緯向海表溫度梯度在7個模型中增加，在10個模型中減少。如同海溫分布效應提出的機制所預期，兩組模式在副熱帶東南太平洋地區（雲反饋最大模式間差異的區域）表現出相反的雲反饋。具有增強的緯向海表溫度梯度和暖池區域變暖較多的模式，表現出負的雲反饋和增加的東南太平洋低雲。這很可能是由於暖池海表溫度變暖所引起的自由大氣增暖，在東南太平洋上空增強了邊界層穩定性。與之相對的，具有減弱的緯向海表溫度梯度和暖池區域變暖較少的模式則呈現相反情況：由海表溫度變暖所引起的東南太平洋邊界層穩定性下降，並未被與暖池相關的高空變暖所補償，導致這些模式中的東南太平洋低雲減少。 除了上述海表溫度分布對雲的影響外，我們假設了東南太平洋的雲反饋有助於兩組模式在赤道緯向海表溫度梯度反應中的差異。為了闡明東南太平洋雲對熱帶海表溫度分布的影響，我們使用兩個在四倍二氧化碳模擬初期階段表現出相反的赤道緯向海表溫度梯度和東南太平洋雲反饋之反應的模式進行鎖雲實驗。結果支持了我們的假設：在全球暖化的情況下，東南太平洋雲反饋將通過由一連串低雲—風—海表溫度反饋形成的遙相關途徑強化赤道海表溫度梯度之反應。定量來說，東南太平洋的雲反饋解釋了兩個模式中約30%的赤道東西向海表溫度梯度變化。

The tropical Pacific sea surface temperature (SST) pattern plays a crucial role in modulating the global energy budget. This phenomenon, primarily led by the shortwave cloud feedback, is known as the "pattern effect." With the goal of investigating cloud-SST coupling, we examine the first 25-year responses of SST and cloud in the abrupt-4xCO2 experiments of Coupled Model Intercomparison Project Phase 6 (CMIP6). Additionally, we conduct "cloud-locking" experiments with the Community Earth System Model version 1 (CESM1) and version 2 (CESM2) to examine the impact of low clouds in the southeastern Pacific on equatorial Pacific SST zonal gradient under a warming scenario. First, consistent with the literature, two categories of CMIP6 models based on the responses in the equatorial Pacific SST gradient during the first 25 years are identified. The equatorial zonal SST gradient increases in 7 models and decreases in 10 models. As expected from the mechanisms put forward by the "pattern effect," the two groups exhibit opposite cloud feedbacks in subtropical southeastern Pacific, the region with the largest inter-model spread of cloud feedback. Models with intensified zonal SST gradient and stronger warming over the warm pool exhibit negative cloud feedback and increasing low clouds over the southeastern Pacific. This is likely due to the strong warming in the free atmosphere induced by SST warming over the convective warm pool, which enhances the boundary layer stability over the southeastern Pacific. In contrast, models with reduced zonal SST gradient and less warming over the warm pool project the opposite: the decreased boundary layer stability induced by SST warming over the southeastern Pacific is not compensated by warming aloft linked to the warm pool, leading to a decrease in low clouds over the southeastern Pacific in these models. In addition to the aforementioned SST pattern’s influence on clouds, we hypothesize that the cloud feedbacks in the southeastern Pacific contribute to the diverse responses in the equatorial zonal SST gradient in the two groups. To clarify the impact of southeastern Pacific clouds on tropical SST pattern, we conducted cloud-locking experiments using the two models that exhibit opposite equatorial zonal SST gradient and southeastern Pacific cloud feedback responses in the initial stage of the abrupt 4xCO2 experiments: CESM1 and CESM2 models. The results support our hypothesis: under global warming, southeastern Pacific cloud feedback will reinforce the equatorial SST gradient responses via a teleconnection pathway shaped by a chain of low cloud-wind-SST feedback. Overall, cloud feedback in southeastern Pacific explains about 30% of the equatorial zonal SST gradient changes in both models.