English
黒潮, 100 km, 100-1000倍, 乱流, トカラ
日本語
ボタン
Kuroshio, 100 km, 100-1000-fold, Turbulence, Tokara Strait
かごしま丸(鹿児島大学)
鹿児島湾の夕焼け
長井らが開発した乱流を自由落下曳航式に観測する装置を用いた観測に参加した学生たち。
本研究における乱流渦の縦方向の大きさは、大きく見積もって海山の高さと同等の100m程度で、これが砕けて乱れが鎮まるまでには30分程度しかかからない。黒潮の約1m/s程度の流れで30分この乱流渦が流されても2 km弱しか下流には強い乱流域を広げることができない計算になる。しかし、実際には著しく強い乱流が100 km程度続いている。一体どの様にしてこの100 kmの強乱流混合海域が維持されているのだろうか。長井らは、かごしま丸(鹿児島大学)での航海で複数回の乱流の自由落下曳航式観測を高解像度で実施し、黒潮が海山の片側で時計回りの渦を形成し、それが慣性不安定と言う不安定現象を引き起こすことで、黒潮が海山背後で形成する流れの水平的な勾配から乱流混合を支えるエネルギーが抽出され、観測された100 kmにおよぶ強乱流層を形成することを突き止めた。 この発見は、海洋の風成循環がどの様にしてその平衡状態を保つのか、そして海洋内部の海水がどの様に混ざっているのかを理解する上で非常に重要な意味を持つ。これら二つは、長年にわたり、海洋学の問題であった。 海洋風成循環は、約1TWの風によって駆動される。風は吹き続けて平均的に海流を加速する方向に働くので、これと反対に働くプロセスが1TWで逆に循環を止める様に作用しなければ、循環は加速され続けてしまう。実際にはこれらがバランスすることによって海洋の循環はある程度の平衡状態を保っていると言うことができる。流れを止める作用を持つプロセスは摩擦である。見かけ上、乱流は海流を妨げる“摩擦”として働く。例えば飛行機で乱気流の中を飛行する際には飛行機は安定して飛行できず燃料を多く使わなければならない。あたかも乱気流が“摩擦力”として飛行機の飛行を妨げている様だ。これと同様に乱流は海流や風成循環を妨げる。しかし、どこでどの様にこの乱流が発生して、平衡状態を保っているのかは詳細にわかっていない。これまでの研究から、海流が海底の起伏上を流れる際に山岳波が生成されて、これによって風成循環を形作る海流のエネルギーが0.2TWの率で失われているらしいことが指摘されている。しかしながら、山岳波の多くは生成海域で乱流に砕けることなく、遠方へ伝播する。遠方へ伝播する際には再び海流が山岳波のエネルギーを吸収し得るため1TWの“摩擦”がどの様に形成されているかは現在も謎のままである。
トカラ海峡の海山上を流れる黒潮で100kmにわたる通常の100-1000倍の強さの乱流を発見!
かごしま丸観測に参加した研究者の集合写真
と言うタイトルで東京海洋大学長井らがCommunications earth & environment に論文を出版しました。
Nagai, T., Hasegawa, D., Tsutsumi, E. et al. The Kuroshio flowing over seamounts and associated submesoscale flows drive 100-km-wide 100-1000-fold enhancement of turbulence. Commun Earth Environ 2, 170 (2021). https://doi.org/10.1038/s43247-021-00230-7
EndFragment
この発見は、黒潮のパラドクスを解く重要な鍵である可能性がある。黒潮は表層が貧栄養塩であるにもかかわらず、上流域では多くの魚類が産卵を行い、魚卵や稚魚は栄養の少ない黒潮によって下流へ運ばれる。貧栄養は、つまり餌が少ないことを意味する。また、黒潮海域は魚類生産が高いため、なぜ稚魚の生命をリスクの高い黒潮に委ねるのか、なぜそこで高い魚類生産が維持されているのかは、黒潮パラドクスとして知られている。したがって、黒潮上流のトカラ海峡で観測された著しく強い100 kmスケールの乱流ホットスポットは、黒潮への栄養塩供給を促し、黒潮を流れる生物を支える役割を担っている可能性があるため、黒潮パラドクスを解く鍵となり得る。 さらに、黒潮と黒潮続流海域は、地球を取り巻く大気にとって主要なCO2吸収域であることが知られており、本研究によって明らかとなった黒潮内での大規模な乱流ホットスポットは、それが栄養塩を植物プランクトンに供給し得ることから、下流域でのCO2吸収を促進していることを示唆する。しかしながら、温暖化予測モデルは、今後数10年の間に黒潮や黒潮続流域の表層が温められ、表層の成層が強化されることで、乱流によってそれを破壊することがより困難となることを予測している。この成層強化は、植物プランクトンへの栄養塩供給を減少させるため、CO2の吸収だけでなく、それに依存する動物プランクトン、魚類生産量を減少させることが推察される。実際にどの様な影響がどの程度発生するかは、さらなる現場観測と数値実験などを用いた詳細な研究が不可欠である。
トカラ海峡を流れる黒潮に沿った5段面にわたる高解像度乱流直接観測。色は、乱流運動エネルギー散逸率を示す。
8月23日にSpringer NatureのCommunications earth & environmentから出版された東京海洋大学の長井らの論文は、トカラ海峡の海山上を流れる黒潮に沿って周辺の海洋内部に比べて100-1000倍程度の乱流が100 km程度に渡って海山下流方向に広がっていることを初めて示した。この生成域から100 kmにわたって広がる乱流混合は、従来の海洋内部の乱流の理論では説明できない。これまで、海洋内部の乱流混合は、海洋内部を伝わる波:内部波の鉛直的な流速勾配によって生成されているとされてきた。もしそうであれば、海山を流れる際にできる鉛直的な流速勾配が観測された乱流の源であろう。
このパズルを解く一つのピースが、サブメソスケールと呼ばれるスケールで発生する現象にあることをこの論文は示唆する。サブメソスケールは数100mから数kmで海洋のフロント近傍で発生する現象のスケールで、それよりも大きなメソスケール(数10 kmから数100km)と、小さなマイクロスケール(数mmから数10m)の間にある。メソスケールの諸現象(海流や中規模の渦)は、解像度の粗い数値モデルや、理論的な研究で理解が進み、マイクロスケール諸現象(乱流など)については、乱流の観測や、小領域に限った高解像度の数値モデルによって研究が行われてきた。しかし、計算機や観測機器の能力の制限のため、これら二つのスケールが影響を及ぼすサブメソスケールを含めたスケール間の相互作用の理解はこれまで全く進んでいなかった。近年の計算機と観測技術能力の向上によって、それらが解像できるスケールがメソスケールとマイクロスケールの両方からサブメソスケールに向かって広がりを見せ、サブメソスケールの諸現象が、フロント域で非常に重要な役割を担うことが明らかとなりつつある。 本研究の結果、黒潮が海山の片側斜面で時計回りの渦流を生成し、それがフィラメント状に下流へ伸び、回転速度が地球の自転に伴う系の回転を上回った時にサブメソスケールの不安定現象を発生させ、著しく強い乱流が100-kmにわたって続くことが明らかとなった。
The Kuroshio flowing over seamounts and associated submesoscale flows drive 100-km-wide 100-1000-fold enhancement of turbulence
鹿児島湾の夕焼け
Group photos of the Kagoshima-Maru cruises
A recent paper published August 23, 2021, in the journal, Communications, earth & environment by Takeyoshi Nagai, an oceanographer at Tokyo University of Marine Science and Technology, Japan and colleagues including Prof. Amit Tandon at UMassD, who was a Takeyoshi’s mentor 17 years ago, reveals an unprecedented long-lasting vigorous turbulent streak over 100 km along the Kuroshio Current in the North Pacific, flowing over the seamounts in the Tokara Strait south of Kyushu Japan. This long-lasting turbulent streak cannot be explained by previous turbulence theories for ocean interior that consider only velocity vertical gradient of internal waves as the major source of turbulence. By conducting a series of intensive turbulence observations and numerical simulations, the authors find submesoscale (a few hundred meters – a few kilometers) instability caused by the Kuroshio flowing over steep seamounts can extract energy of the Kuroshio through its velocity gradients along horizontal as well as vertical directions and inject it to microscale (a few millimeters – a few 10th of meters) turbulence.
“Our finding may have big implications for understanding how the wind-driven ocean general circulation is maintained and how the interior of the ocean is mixing. Both remain uncertain for decades, and the latter is very important for nutrient supply for phytoplankton” said by Amit.
Ocean general circulation, that consists of numerous mesoscale eddies and large scale major ocean currents, is driven by about 1 TW power input by wind. To achieve its equilibrium, energy has to be dissipated at the same rate. The energy dissipation can occur by fluid friction only at the smallest scale for ocean flows (a few millimeters), while large scale flows tend to transfer energy to larger scales. Therefore, it has been a puzzle on how the energy of the general circulation reaches to the scale to the dissipation. One of the candidates is the lee wave. Lee waves can be generated by ocean currents flowing over the bottom topography. Several studies pointed out that lee waves can extract energy from general circulation at a rate about 0.2 TW. However, a previous study suggested that a large fraction of the lee wave energy is not dissipating near the generation sites and radiates away. These radiated waves can be reabsorbed to the large scale ocean current and do not necessarily dissipate their energy.
Takeyoshi continues “Missing pieces could be hidden in the submesoscale”. The submesoscale processes have spatial scales that range from a few hundred meters to a few kilometers, which are in between microscale (a few millimeters – a few 10th of meters) and mesoscale (a few 10th of kilometers – a few hundred meters), both are relatively well-explored. ““We are now in a very exciting era, in which observationally and numerically resolvable scales are emerging both from microscale and mesoscale toward the submesoscale”” This was said by Amit sensei when I started a job at UMassD as a postdoc of him, and it got me very interested in my work with him. I have been working on the submesoscale observationally and numerically since then” said by Takeyoshi. The word “sensei” means a teacher in Japanese.
Tow-yo turbulence profiler, Underway-VMP, which consists of VMP250 (Rockland Scientific International) and UCTD Winch (Teledyne Ocean Science), and students who worked together with the UVMP.
Turbulence dissipation rates (strengths of turbulence) in color for 5 vertical sections along the Kuroshio flowing over seamounts of the Tokara Strait
Takeyoshi continued “Our finding may have very important implications in nutrient supply to the Kuroshio, and this could be a key to solve the Kuroshio Paradox”. The Kuroshio surface water is known to be nutrient poor and yet many fish species spawn in the regions southwest of Kyushu where their eggs and larvae are advected from by the nutrient poor Kuroshio Current and utilize the Kuroshio to migrate north. Nutrient depletion usually means less food availability for higher trophic levels. Why many fish species spawn there, risking their larva’s lives under food depleting environment? Why are there so many fish in the Kuroshio regions, one of the major fishing sites of the North Pacific, even with less food? This has been called the “Kuroshio Paradox”. The observed strong turbulence can inject nutrients, which are abundant in the deeper layers of the Kuroshio, toward the sunlit surface, where phytoplankton can photosynthesize. Then, zooplankton eat phytoplankton and are eaten by fish in the downstream. But the question remains why the Kuroshio surface water remains nutrient poor even with the observed large nutrient injection. "This is possibly because these biological responses are happening in the subsurface layers". Also, another recent research pointed out that these trophic transfers may occur very quickly in nutrient depleted environment. "In other words, as everybody is so hungry, they eat as fast as possible so that nobody can see it (like leaving souvenirs on the table at your lab). We need to have more observations to figure out who eats souvenirs".
This is not the end of the story, because the Kuroshio and the Kuroshio Extension regions are found to be one of the major net CO2 sinks for the Earth’s atmosphere. The diffused nutrients toward the shallower layers and associated phytoplankton growth are very important for the CO2 uptake in these regions. “However, climate model predicts the upper layers of these regions will be warmed leading to more strongly stratified layers, which will become more difficult to mix by turbulence within several decades. If this is the case, then phytoplankton will decrease and CO2 uptake in these regions will also decline. Furthermore, dependent higher trophic levels including commercially valuable fish are expected to decrease too” said by Takeyoshi.
High-resolution turbulence measurements reveal a large-scale mixing hotspot in the Kuroshio flowing seamounts south of Kyushu Japan!
Takeyoshi Nagai has published a manuscript in Communications earth & environment.
Nagai, T., Hasegawa, D., Tsutsumi, E. et al. The Kuroshio flowing over seamounts and associated submesoscale flows drive 100-km-wide 100-1000-fold enhancement of turbulence. Commun Earth Environ 2, 170 (2021). https://doi.org/10.1038/s43247-021-00230-7
EndFragment
Training vessel Kagoshima-Maru (Kagoshima University)
Twilight in Kagoshima Bay after the cruise
Takeyoshi continued “Our finding may have very important implications in nutrient supply to the Kuroshio, and this could be a key to solve the Kuroshio Paradox”. The Kuroshio surface water is known to be nutrient poor and yet many fish species spawn in the regions southwest of Kyushu where their eggs and larvae are advected from by the nutrient poor Kuroshio Current and utilize the Kuroshio to migrate north. Nutrient depletion usually means less food availability for higher trophic levels. Why many fish species spawn there, risking their larva’s lives under food depleting environment? Why are there so many fish in the Kuroshio regions, one of the major fishing sites of the North Pacific, even with less food? This has been called the “Kuroshio Paradox”. The observed strong turbulence can inject nutrients, which are abundant in the deeper layers of the Kuroshio, toward the sunlit surface, where phytoplankton can photosynthesize. Then, zooplankton eat phytoplankton and are eaten by fish in the downstream. But the question remains why the Kuroshio surface water remains nutrient poor even with the observed large nutrient injection. "This is possibly because these biological responses are happening in the subsurface layers". Also, another recent research pointed out that these trophic transfers may occur very quickly in nutrient depleted environment. "In other words, as everybody is so hungry, they eat as fast as possible so that nobody can see it (like leaving souvenirs on the table at your lab). We need to have more observations to figure out who eats souvenirs".
This is not the end of the story, because the Kuroshio and the Kuroshio Extension regions are found to be one of the major net CO2 sinks for the Earth’s atmosphere. The diffused nutrients toward the shallower layers and associated phytoplankton growth are very important for the CO2 uptake in these regions. “However, climate model predicts the upper layers of these regions will be warmed leading to more strongly stratified layers, which will become more difficult to mix by turbulence within several decades. If this is the case, then phytoplankton will decrease and CO2 uptake in these regions will also decline. Furthermore, dependent higher trophic levels including commercially valuable fish are expected to decrease too” said by Takeyoshi.
Their paper finds that when the Kuroshio flows over the steep seamounts in the Tokara Strait, clockwise spinning flow, as opposed to the Earth’s spinning, is generated from one side of the slopes, producing a streak of water spinning clockwise that triggers submesoscale instability, called inertial and symmetric instabilities and associated strong microscale turbulence. Although these instabilities have already been reported along the continental slopes in the abyssal Southern Ocean, there has been no observational evidence that this mechanism provides a large scale mixing hotspot in the major ocean currents flowing over seamounts in relatively shallow layers until this study.
Turbulence is intermittent. That’s why scientists need to measure it at high spatial resolutions to know where mixing occurs. But measuring turbulence is not easy, because turbulence probes are very sensitive and easily detect unwanted signals caused by instrument motions by rope’s tension and electronic noises. “In fact, it can detect noises of your steps when it placed at the lab” added by Takeyoshi. His team developed the method to measure turbulence at 1-2 km horizontal resolutions using a tow-yo microstructure profiler without stopping a ship. They use 1.5 mm diameter Dyneema rope to let it sink quasi-freely without severe unwanted contamination from the rope’s tension and recover it to the surface repeatedly while a ship is steaming at a slow speed, 1-2 m/s. While yo-yo profiling is to keep deploying and recovering the instrument vertically at one location like a yo-yo, this method is called tow-yo profiling. Using this new technique and Training vessel Kagoshima-Maru (Kagoshima University), the authors and participants of cruises found the enhanced turbulence by 100-1000-fold compared to typical interior ones, that spreads over 100 km scale along the Kuroshio behind the seamounts.
Twilight in Kagoshima Bay after the cruise
Tow-yo turbulence profiler, Underway-VMP, which consists of VMP250 (Rockland Scientific International) and UCTD Winch (Teledyne Ocean Science), and students who worked together with the UVMP.