気候変動における海の役割(The role of the ocean in climate change)

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2024-02-29 デラウェア大学 (UD)

◆デラウェア大学のシャオハイ・ヤン教授に指導された学生のケルシー・エドウィングが科学雑誌Geophysical Research Lettersに論文が受理された。また、同じジャーナルで別の学生であるレイ・ファンの論文も受理された。
◆エドウィングの論文は、極めて暖かい海水のイベントである海洋熱波(MHWs)が米国東海岸で果たす役割と、それが海洋炭酸塩系に与える影響を調査した。
◆一方、ファンの研究は、インド洋の南西部における温暖化と海面上昇の物理的原因を調査した。

<関連情報>

海洋熱波がアメリカ東海岸の大気-海洋CO2フラックスに与える影響 Impact of Marine Heatwaves on Air-Sea CO2 Flux Along the US East Coast

Kelsea Edwing, Zelun Wu, Wenfang Lu, Xinyu Li, Wei-Jun Cai, Xiao-Hai Yan
Geophysical Research Letters  ppublished: 02 January 2024
DOI:https://doi.org/10.1029/2023GL105363

Details are in the caption following the image

Abstract

Marine heatwaves (MHWs) are extremely warm ocean temperature events that significantly affect marine environments, but their effects on the coastal carbonate system are still uncertain. In this study, we systematically quantify MHWs’ impacts on air-sea carbon dioxide (CO2) flux anomalies (FCO2′) in the Mid-Atlantic Bight (MAB) and South Atlantic Bight (SAB) from 1992 to 2020. During the longest MHW in both regions, oceanic CO2 uptake capabilities substantially decreased, primarily due to significant increases in the seawater partial pressure of CO2 (pCO2sea). For all cases, MHWs played a more significant role in driving pCO2sea changes in the MAB than the SAB, where non-thermal drivers dominated pCO2sea variability. In the MAB, weakened wind speeds related to wintertime atmospheric perturbations increase ocean temperatures and pCO2sea, further reducing CO2 uptake during winter MHWs. This work is the first to connect extreme temperatures to coastal air-sea CO2 fluxes. The reduction in CO2 absorption noted during MHWs in this study has important implications for coastal regions to act as continued sinks for excess CO2 emissions in the atmosphere.

Key Points

  • Marine heatwaves (MHWs) primarily generated positive sea surface pCO2 (pCO2sea) anomalies in the Mid-Atlantic Bight (MAB) and South Atlantic Bight (SAB) but had a larger impact on air-sea CO2 flux anomalies in the MAB
  • Reduced wind speeds amplified MHW contributions during CO2 sink months and counteracted them during CO2 source months
  • In the MAB, wintertime atmospheric perturbations related to zonal shifts in the jet stream produce slower wind speeds which aid in generating air-sea heat flux type MHW events that ultimately reduce oceanic CO2 uptake

Plain Language Summary

The transfer of carbon dioxide (CO2) between the atmosphere and ocean is sensitive to sea surface temperature (SST) changes because warmer SSTs increase the sea surface partial pressure of CO2 and reduce the ocean’s ability to absorb CO2 from the atmosphere. It is, therefore, conceivable that marine heatwaves (MHWs), which are extremely warm ocean temperature events, could modify how carbon moves between the ocean and the atmosphere. This study provides the first attempt to evaluate the impacts of MHWs on the air-sea CO2 flux (FCO2) anomalies along the US East Coast, encompassing the Mid-Atlantic Bight (MAB) and South Atlantic Bight (SAB) during 1992–2020. Both regions experienced reduced CO2 absorption in response to the longest MHWs in each region. These extreme temperatures had a larger impact on CO2 absorption in the MAB compared to the SAB, where non-temperature factors were more influential. The coastal ocean plays an important role in helping to mitigate human-induced climate change by absorbing excess CO2 from the atmosphere. As such, the demonstrated reduced absorption of the ocean associated with MHWs in this study, which might also apply to other coastal locations, has vital implications for the efficiency of the ocean in offsetting global warming impacts.

熱帯南西インド洋における最近20年間の急激な海面上昇 Rapid Sea Level Rise in the Tropical Southwest Indian Ocean in the Recent Two Decades

Lei HuangWei ZhuangWenfang LuYang ZhangDeanna EdwingXiao-Hai Yan
First published: 27 December 2023  
https://doi.org/10.1029/2023GL106011
Details are in the caption following the image

Abstract

It has been reported that the sea level falls in the tropical Southwest Indian Ocean (SWIO) from the 1960s to the early 2000s. However, a rising trend of 4.05 ± 0.56 cm/decade has occurred during the recent two decades with our analysis showing that manometric sea level contributes 41% to this sea level rise. 30% of this rise is due to steric sea level (SSL) change in the upper 2,000 m with SSL rise in the upper 300 m of secondary importance. Conversely, thermal expansion below the thermocline (300–2,000 m), likely caused by water mass spread from the Southern Ocean, induces major contribution to SSL changes. Compared to existing studies demonstrating the contribution of thermal variations above the thermocline to sea level variability in the tropical SWIO, this study emphasizes the importance of ocean mass and deeper ocean changes in a warming climate.

Key Points

  • Rapid sea level rise occurs in the tropical Southwest Indian Ocean (SWIO) since the early 2000s
  • The ocean mass addition and the upper 2,000 m ocean warming contribute significantly to the total sea level rise
  • The upper 2,000 m ocean warming is primarily attributed to thermal expansion below the thermocline associated with the spread of water masses

Plain Language Summary

Global ocean sea level change is spatially and temporally nonuniform due to oceanic and atmospheric dynamics. The tropical Southwest Indian Ocean (SWIO) experienced a sea level fall from the 1960s to the early 2000s. However, a rapid sea level rise has occurred over the last two decades in the tropical SWIO that is faster than the global average. The ocean mass increase due to extra water input leads to an essential impact on sea level rise in the tropical SWIO. Compared to previous studies demonstrating the effect of thermal expansion in the upper 300 m, this study shows larger contributions from deeper ocean (300–2,000 m) warming over the past two decades. Overall, this study highlights the importance of ocean mass and deeper water thermal structure in regulating tropical SWIO sea level rise in a changing climate, as well as the need for observations and direct assessment of the abyssal ocean beneath 2,000 m.

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1702地球物理及び地球化学
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