溫鹽環流在 2010 年後上升趨勢的物理機制

Abstract

大西洋經向翻轉環流（AMOC）在地球氣候系統內的經向熱傳輸中發揮重要作用。許多 OMIP2 的模式無法很好地模擬 2010 年後 AMOC 再次上升的趨勢(Tsujino et al., 2020)，此研究探討了驅動 AMOC 回復及 AMOC 年際間變化的主要物理機制。研究證實，北大西洋濤動（NAO）透過表面熱通量影響混合層深度（MLD）和密度異常的變化，導致 AMOC 的年際變異。本文也闡明了拉布拉多海和溢流 (overflow) 的作用。透過使用 CMIP6 OMIP2 大氣強迫的敏感性實驗，我們發現海洋 K-Profile 參數化（KPP）在 AMOC 強度變化下中扮演關鍵角色。這些結果顯示在海冰區域下合適的垂直混合率可以更好地描繪拉布拉多海中的深水形成以及通過丹麥海峽的溢流狀態，且隨著變得更淺的溫躍層，等密面進而向上移動，使得溫鹽性質更接近觀測資料，從而更好地模擬 2010 年後的 AMOC 回復。 本研究使用基於 OMIP2 實驗的兩個不同的海洋-海冰耦合模型進行敏感性實驗，並採用 JRA55-do 資料集作為強迫場，該資料集涵蓋 1958 年至 2018 年。通過TIMCOM 和 POP2 海洋模型的敏感性實驗發現，降低海冰面積下的垂直背景擴散係數會使 45°N 以北水域的密度增大，尤其在北極海。隨後，較高密度的水被輸送到北歐海，推高格陵蘭-冰島-蘇格蘭海嶺上的等密面，導致穿過丹麥海峽的高密度水溢出，進而影響拉布拉多海域，增強AMOC。我們進一步比較不同年份下的機制，根據 AMOC 的上升趨勢，將其區分為丹麥海峽溢出水和拉布拉多海域鹽度剖面指標，並發現 AMOC 傳輸通量的上升幅度與丹麥海峽溢出水影響拉布拉多海域呈正相關。這一平衡態機制在不同時間段內保持穩定且一致，可以良好闡釋模式內 AMOC 的上升。

Recent AMOC recovery after 2010 cannot be well simulated by many OMIP2 mod els(Tsujino et al., 2020). The primary physical mechanism driving this recovery and interan nual variability of AMOC is investigated in this study. North Atlantic Oscillation (NAO) is confirmed to affect the changes of mixed layer depth (MLD) and density anomalies lead ing to the interannual variability of AMOC through surface heat flux. The roles of Labrador Sea and overflow are clarified. CMIP6 OMIP2 forcing sensitivity experiments are used to show the key role of K-profile Parameterization (KPP) on the AMOC strength. These results suggest the required modification of KPP under sea ice area to improve the AMOC simulation. An appropriate boundary layer can better represent deep water formation in the Labrador Sea and the overflow through Denmark Strait, leading to a better simulation of AMOC recovery after 2010. This study uses sensitivity experiments based on OMIP2 experiments with two independent ocean-sea ice coupled models, utilizing the JRA55-do dataset as the forcing field, covering the period from 1958 to 2018. Our study indicates that sensitivity experiments with the TIMCOM and POP2 ocean models reveal that reducing the background diffu sivity under sea-ice region increases water density north of 45°N, especially in the Arctic Ocean. Consequently, denser water is transported to the Nordic Seas, raising the isopyc nal surfaces over the Greenland-Iceland-Scotland Ridge, leading to an overflow of denser water through the Denmark Strait. This overflow subsequently impacts the Labrador Sea, strengthening the AMOC. We further compared the mechanisms across different years, clearly distinguishing between the Denmark Strait overflow and the salinity profiles in the Labrador Sea as indicators of AMOC enhancement. The increase in AMOC transport positively correlates with the influence of Denmark Strait overflow on the Labrador Sea, which has been particularly significant over the past decade.