北大西洋流或許會在下個世紀暫時停止
幸好在未來1000年內不會發生全面停擺
北大西洋流把墨西哥灣的溫暖海水帶到歐洲，使得歐洲西北部大部分的氣候相對來說較為溫和。但是科學家猜測格陵蘭的融冰以及過多的降雨，可能會中斷北大西洋流。格羅寧根大學和烏特勒支大學的科學家經由模擬顯示，由於北大西洋上空的降雨變化不會太大且較為短暫，因此洋流不可能會全面停擺。不過，未來一百年內仍有百分之15的可能性北大西洋流會暫時停止。這項結果於12月30日發表在期刊《科學報告》(Scientific Reports)。
上圖為北大西洋流。圖片來源：Sven
Baars, University of Groningen
「由於海洋儲存了相當多的能量，因此洋流對地球的氣候有重大影響。」格羅寧根大學的數值數學副教授Fred
Wubs表示。他和烏特勒支大學的同事Henk
Dijkstra合作研究海洋環流已有二十餘年。
盒狀模型
海洋科學家早已發現大西洋的洋流對海洋表面的淡水多寡相當敏感。由於氣候變遷會造成更多格陵蘭的融雪流入海裡並讓海洋上方的降雨增加，因此有科學家提出這或許會讓北大西洋流停止甚至逆轉，使得熱能無法被運輸至歐洲。
數十年來有許多模型都在模擬淡水對洋流的影響。「科學家用的模型分為兩種：根據描述流體如何流動的方程式所建構出來的高解析度模型，以及高度簡化過後的盒狀模型。」Wubs解釋。「我們在烏特勒支大學的同事創建了一個新的盒狀模型，比目前用來描述海洋大尺度作用的模型還要稍加精準。」
全面停擺
他們的想法是利用盒狀模型來估計淡水輸入量的微小改變，造成北大西洋流暫時減緩或全面停擺的可能性有多高。北大西洋流的行為是非線性的，意謂微小改變可能就會造成巨大影響。而想要得知盒狀模型描述的物理作用如何演變，就只能透過模擬來取得。Wubs表示：「我們預計我們要尋找的轉變現象是罕有事件，因此需要大量模擬才能預估它們發生的可能性。」不過這些荷蘭科學家發現有位法國科學家設計出一套方法可以選擇出最有希望得到目標現象的模擬，因此能減少通盤模擬的所需數量。
Wubs的博士生Sven
Baars很快地便把此方法接上烏特勒支大學的盒狀模型。接著Dijkstra的博士生Daniele
Castellana便開始進行模擬。Wubs表示：「模擬結果顯示，未來一千年內北大西洋流全面停擺的可能性小到可以忽略。」
暫時停止
比較有可能發生的是，相對溫暖的海水送到西北歐的過程會暫時停止。「在我們的模擬結果當中，未來一百年之內發生此事件的機率是百分之十五。」這種暫時發生的變化可能會為北大西洋帶來厄運，不過還需要進行後續研究才能證明。因此，目前這項研究只是評估風險有多高的第一步。此外，他們的模型並沒有考慮到北大西洋的淡水含量或許會出現極為可觀的變化，這是冰層融化可能造成的後果。Wubs說：「我們接下來的挑戰是透過高解析度的氣候模型來驗證我們的結果。」
North Atlantic
Current may cease temporarily in the next century
Total collapse in the next
1,000 years unlikely
The North Atlantic Current transports
warm water from the Gulf of Mexico towards Europe, providing much of
north-western Europe with a relatively mild climate. However, scientists
suspect that meltwater from Greenland and excessive rainfall could interfere
with this ocean current. Simulations by scientists from the University of
Groningen and Utrecht University showed that it is unlikely that the current
will come to a complete stop, due to small and rapid changes in precipitation
over the North Atlantic. However, there is a 15 percent likelihood that there
will be a temporary change in the current in the next 100 years. The results
were published on 30 December in the journal Scientific Reports .
'The oceans store an immense amount of energy and the
ocean currents have a strong effect on the Earth’s climate', says University of
Groningen Associate Professor in Numerical Mathematics, Fred Wubs. Together
with his colleague Henk Dijkstra from Utrecht University, he has studied ocean
currents for some 20 years.
Box model
Ocean scientists have found that the Atlantic Ocean
currents are sensitive to the amount of freshwater at the surface. Since the
run-off of meltwater from Greenland has increased due to climate change, as has
rainfall over the ocean, it has been suggested that this may slow down or even
reverse the North Atlantic Current, blocking the transport of heat to Europe.
Simulations of the effects of freshwater on the
currents have already been performed for some decades. ‘Both high-resolution
models, based on the equations describing fluid flows, and highly simplified
box models have been used,’ explains Wubs. ‘Our colleagues in Utrecht created a
box model that describes present-day large-scale processes in the ocean rather
well.’
Total collapse
The idea was to use this box model to estimate the
likelihood of small fluctuations in freshwater input causing a temporary
slowing down or a total collapse of the North Atlantic Current. The current
shows non-linear behaviour, which means that small changes can have large
effects. The evolution of the physics described by the box model can only be
obtained using simulations. ‘As the transitions we were looking for are
expected to be rare events, you need a huge number of simulations to estimate
the chance of them happening,’ says Wubs. However, the Dutch scientists found
that a French scientist had devised a method to select the most promising
simulations, reducing the number of full simulations required.
Sven Baars, a PhD student of Wubs, implemented this
method efficiently and linked it to the Utrecht box model. Daniele Castellana,
a PhD student of Dijkstra, performed the simulations. ‘These simulations showed
that the chances of a total collapse of the North Atlantic Current within the
next thousand years is negligible,’ says Wubs.
Interruption
A temporary interruption in the delivery of
relatively warm water to north-western Europe is more likely: ‘In our
simulations, the chances of this happening in the next 100 years are 15
percent.’ Such temporary transitions may cause cold spells in the North
Atlantic, although this needs to be verified in further studies. Therefore, the
current study is just a first step in determining the risk. The model does not
take into account considerable changes in freshwater in the North Atlantic,
which can be caused by melting of the ice sheets. Wubs: ‘Confirming our results
through simulation with a high-resolution climate model will be the next
challenge.’
原始論文：Daniele
Castellana, Sven Baars, Fred W. Wubs, Henk A. Dijkstra. Transition
Probabilities of Noise-induced Transitions of the Atlantic Ocean Circulation. Scientific
Reports, 2019; 9 (1) DOI: 10.1038/s41598-019-56435-6
引用自：University of Groningen. "North Atlantic
Current may cease temporarily in the next century”
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