The effects of ocean warming are profound and well-documented. But sometimes changes in the patterns of winds and ocean currents cause seawater to suddenly cool, instead.

海洋暖化的影響，深遠且證據充分。不過，有時於風及洋流模式中的改變，反而導致海水突然變冷。

Surface temperatures can plummet rapidly — by 10℃or more over a day or two. When these conditions persist for several days or weeks, the area experiences a “coldwave”, which is the opposite of more familiar marine heatwaves.

表面溫度會在一兩天內，迅速下降達10°C或更多。當這些情況持續幾天或幾週時，此地區經歷與更常見之海洋熱浪相反的“寒潮”。

When a “killer coldwave” manifested along South Africa’s southeast coast in March 2021, it killed hundreds of animals across at least 81 species. More worrying still was the fact these deaths included vulnerable manta rays and even specimens of notoriously robust migratory bull sharks.

於2021年3月，沿著南非東南海岸出現“殺手寒潮”時，奪走包括至少81個物種，數百隻動物的生命。更令人擔憂的是，這些死亡包括，脆弱的蝠鱝及甚至臭名昭彰、強壯，行遷徙之牛鯊實例的事實。

In southern Africa, bull sharks, whale sharks and manta rays have previously washed up dead following such sudden cold events, especially over the past 15 years. As we report in Nature Climate Change, the conditions that can drive these killer coldwaves have grown increasingly common over the past four decades.

在南部非洲，牛鯊、鯨鯊及蝠魟，先前曾繼此類突發寒冷事件之後，被沖上岸死亡，特別是在過去15年間。如同，我們在《自然•氣候變遷》期刊中，報導的那樣。在過去四十年間，能驅動這些殺手寒潮的狀況，已經變得日益普遍。

Ironically, strengthening winds and currents as a result of climate change can also make these deadly localised coldwaves more likely in places such as the east coasts of South Africa and Australia, potentially putting even highly mobile species such as sharks in harm’s way.

諷刺的是，由於氣候變遷，增強的風及洋流，也會使這些致命的局部寒潮，更有可能出現於，諸如南非及澳洲東海岸等地。潛在上，甚至使諸如鯊魚等，高度流動性的物種，受到傷害。

Certain wind and current conditions can cause the sea surface to cool, rather than warm. This happens when winds and currents force coastal waters to move offshore, which are then replaced from below by cold water from the deep ocean. This process is known as upwelling.

某些風及洋流狀況會導致海面變冷，而非暖化。當風及洋流迫使近海的海水移向海岸，之後被來自深海的冷海水，從下方取代時，發生此情況。這過程，被通稱為上升流。

In some places, such as California on the US west coast, upwelling happens regularly along hundreds of kilometres of coastline. But localised upwelling can occur seasonally on a smaller scale, too, often at the edges of bays on the east coasts of continents due to interactions of wind, current and coastline.

在諸如位於美國西海岸的加利福尼亞州等，一些地方經常沿著數百公里的海岸線，發生上升流。不過，由於風、洋流及海岸線的相互作用，局部的上升流也會季節性，以較小的規模發生。經常在大陸東海岸的海灣邊緣。

Previous research had shown climate change induced changes in global wind and current patterns. So we investigated the potential consequences at particular locations, by analysing long-term wind and temperature data along the south-eastern coast of South Africa and the Australian east coast.

先前的研究已經證實，氣候變遷引起了，於全球風及洋流模式上的改變。因此，我們藉由分析，沿著南非東南海岸及澳洲東海岸，長期的風及溫度數據。調查研究了，於特定地點可能發生的影響。

This revealed an increasing trend in the number of annual upwelling events over the past 40 years. We also found an increase in the intensity of such upwelling events and the extent to which temperatures dropped on the first day of each event – in other words, how severe and sudden these cold snaps were.

這揭露了，過去40間，每年上升流事件數量，日益增加的趨勢。在此類上升流事件的強度，及每次事件於第一天，溫度下降的程度上，我們也發現增強。換句話說，這些寒潮是多麼嚴重及突然。

During the extreme upwelling event along the southeast coast of South Africa in March 2021, at least 260 animals from 81 species died. These included tropical fish, sharks and rays.

於2021年3月，沿著南非東南海岸的極端上升流事件，至少來自81個物種、260隻動物死亡。此些包括，熱帶魚、鯊魚及鰩魚。

To investigate the ramifications for marine fauna, we took a closer look at bull sharks. We tagged sharks with tracking devices that also record depth and temperature.

為了調查研究，對海洋動物的影響，我們更縝密觀察了牛鯊。我們使用，同時記錄深度及溫度的追蹤裝置，標記了鯊魚。

Bull sharks are a highly migratory, tropical species that only tend to travel to upwelling regions during the warmer months. With the onset of winter, they migrate back to warm, tropical waters.

牛鯊是一種，傾向僅在較暖和月份期間，前往上升流地區之高度遷移性的熱帶物種。隨著冬季到來，它們遷移回溫暖和、熱帶的水域。

Being mobile, they should have been able to avoid the local, cold temperatures. So why were bull sharks among the dead in this extreme upwelling event?

是遷移性的，它們應該已經能夠避開地區性、冷的氣溫。那麼，為何在這次極端的上升流事件中，死亡的包括牛鯊？

圖2. 於南非，一次極端上湧海流事件後，被沖上岸的死牛鯊之一。

One of the dead bull sharks that washed up after an extreme upwelling event in South Africa. 

Bull sharks survive environmental conditions that would kill most other marine life. For example, they’re often found several hundred kilometres up rivers, where other marine life would not venture.

牛鯊能存活於，會殺死大多數其他海洋生物的環境條件中。譬如，它們經常出現於，其他海洋生物不會冒險之河流上游的數百公里處。

Our shark tracking data from both South Africa and Australia showed bull sharks actively avoid areas of upwelling during their seasonal migrations up and down the coast, even when upwelling isn’t too intense.

我們來自南非及澳洲的鯊魚追蹤數據顯示了，牛鯊在沿著上、下海岸進行季節性的遷徙期間，積極避開上升流區域，即使上升流不太強烈時。

Some sharks take shelter in warm, shallow bays until the water warms again. Others stick close to the surface where the water is warmest, and swim as fast as they can to get out of the upwelling.

有些鯊魚躲在暖和、淺的海灣中，直到海水再次暖化。另一些緊貼最暖和的海水表面，且盡可能快速遊離上升流。

But if marine coldwaves continue to become more sudden and intense, fleeing or hiding may no longer be enough even for these tough beasts. For example, in the event in South Africa that caused the death of manta rays and bull sharks water temperatures dropped from 21°C to 11.8°C in under 24 hours while the overall event lasted seven days.

不過，倘若海洋寒潮繼續變得更突然及強烈，即使對這些頑強的動物而言，逃跑或躲藏可能不再是足夠。譬如，在南非導致了，蝠鱝及牛鯊死亡的事件中，水溫在24小時內，從21°C降至 11.8°C，而整個事件持續了7天。

This sudden, severe drop paired with the long duration made this event particularly deadly. If future events will continue to become more severe, mass deaths of marine life could become a more common sight – especially along the world’s mid-latitude east coasts.

該結合了，持續時間長之突然、嚴重的下降，使得此事件特別致命。倘若，未來的事件繼續變得更嚴重，海洋生物大規模死亡，可能會成為一種更常見的景象。特別是在，世界中緯度地區的東海岸。

圖3. 在極端上湧海流事件後，死亡生物中有蝠鱝。

Manta rays were among the dead after the extreme upwelling event.

Overall, our oceans are warming. The ranges of tropical and subtropical species are extending towards the poles. But along some major current systems, sudden short-term cooling can make life difficult for these climate migrants, or even kill them.

總的說來，我們的海洋正暖化中。熱帶及亞熱帶物種的分佈範圍，正向兩極延伸中。不過，對這些隨氣候遷移的動物而言，沿著一些主要洋流，突然的短期降溫，會使生存變得困難，甚至奪走它們的生命。

Especially if events like the one in South Africa become more common. Tropical migrants would increasingly be living on the edge of what they are comfortable with in these areas.

特別的是，倘若像於南非那樣的事件，變得更普遍。熱帶的遷移動物，將日益生存於，它們感到舒適之此些地區的邊緣。

Our work emphasises that climate impacts can be unexpected or even counterintuitive. Even the most resilient life forms can be vulnerable to its effects. While we do see an overall warming, changes in weather and current patterns can cause extreme cold events as well.

我們的研究強調，氣候影響會是意想不到的。亦即，甚至是違反直覺的。即使最具恢復力的生命形式，也會容易受到其影響。儘管，我們確實看到整體的暖化。不過，於天氣及洋流模式上的改變，也會導致極端寒冷事件。

This really shows the complexity of climate change, as tropical species would expand into higher-latitude areas as overall warming continues, which then places them at risk of exposure to sudden extreme cold events. In this way, species such as bull sharks and whale sharks may very well be running the gauntlet on their seasonal migrations.

這確實顯現了，氣候變遷的複雜性。因為，隨著整體暖化持續，熱帶的物種會擴展到較高緯度的地區。在那情況下，使它們冒著曝露於，突然極端寒冷事件中的風險。

The need to limit our impacts on the planet by reducing greenhouse-gas emissions has never been more urgent, nor has been the need for research into what our future might hold.

藉由減少溫室氣體排放，來限制我們對地球之影響的需求，未曾如此迫切。也不曾需要，有關我們的未來，可能擁有什麼的研究。

網址：https://theconversation.com/climate-change-is-causing-marine-coldwaves-too-killing-wildlife-227781

翻譯：許東榮