探討風應力變異與海洋熱傳輸變化在北極海冷卻中之作用

Abstract

海冰快速融化以及北極暖化放大效應是近年來人為暖化效應下顯著的特徵。儘管輻射反饋效應(radiative feedbacks)已被發現對北極區域性的暖化有所貢獻，大氣熱傳輸以及海洋熱傳輸也可能佔有非常重要的角色。在本篇研究中，我們分析了利用地球系統模式Community Earth System Model version 2「Mechanically Decoupled」實驗，模擬西元1850至1980年，將海洋所受應力改為氣候平均值下的氣候。因此，全耦合實驗 (Fully Coupled)與「Mechanically Decoupled」實驗ensemble mean的差異即為應力變動驅動海洋產生變動的部分對於西元1850至1980年間氣候變化的貢獻。我們發現應力驅動海洋變動對氣候所造成的影響使傳送到北極的海洋熱傳輸(ocean heat transports)減弱、北極地區海洋到大氣與海冰的熱通量減少，北極周圍近地表大氣溫度及北極海表溫度皆下降。若將海洋熱傳輸拆解為由Barents Sea Opening (BSO)、Fram Strait (FS)、Bering Strait (BS)以及Davis Strait (DS)進入北極海的熱傳輸則可發現Barents Sea Opening主導了整體海洋熱傳輸的變化。藉由進一步的分析，我們發現海洋熱傳輸的改變受到了表面風場驅動的海面高度(Sea Surface Height)改變所造成的海洋地轉流場變化影響。最後，我們還分析了海洋熱傳輸減少對於北極海垂直剖面上的溫度、鹽度以及北極表層水(Arctic Surface Water)厚度變化。本次研究發現了應力驅動海洋動力的變化對於北極氣候所扮演的角色以及其對北極海生地化變化、生態系統與社會經濟可能的潛在影響。

The accelerated decline of Arctic sea ice and intensified Arctic warming stand out as prominent features of anthropogenic climate change. While radiative feedbacks are recognized as significant regional contributors, the remote impacts of atmospheric and oceanic heat transports are also crucial. In this study, we analyzed large-ensemble historical simulations using the mechanically-decoupled technique. This method involves relaxing the wind stress forcing on the ocean to seasonal climatological values during the period from 1850 to 1980, using the Community Earth System Model version 2. By comparing the ensemble means of fully-coupled and mechanically-decoupled simulations, we isolate the wind-driven component and its role in historical simulations. Our analysis reveals that wind-driven influences on the Arctic Ocean lead to weakened ocean heat transport (OHT) into the region, reduced ocean-to-atmosphere heat fluxes, and cooler ocean and near-surface air temperatures in the Arctic. Breaking down the total OHT to the Arctic according to entry points such as the Barents Sea Opening (BSO), the Fram Strait (FS), the Bering Strait (BS), and the Davis Strait (DS), we observe that the OHT through the Barents Sea Opening dominates in total OHT changes. Further examination suggests that these changes in OHT are linked to geostrophic currents induced by wind-driven sea surface height anomalies. Additionally, we explore the impacts of reduced OHT on the vertical distributions of oceanic temperature, salinity, and the thickness of Arctic Surface Water. Our findings underscore the significance of wind-driven ocean dynamical responses in shaping Arctic climate and emphasize potential implications for Arctic biogeochemistry, ecosystems, and socioeconomics.