南極底層水淡化的速率高於預期

原文網址：www.sciencedaily.com/releases/2017/01/170125145741.htm

南極底層水淡化的速率高於預期

研究人員表示此改變可能會打亂海洋環流並加速海平面上升

南極底層水(Antarctic Bottom Water)是全球海洋環流系統的一部份，其沿著冰冷的深海海床，將氧氣、碳及其他營養成分供應至全球海洋。過去十年以來，科學家一直觀察著南極底層水的變化。由伍茲霍爾海洋研究所(WHOI)最新進行的研究指出，這些變化正往出人意料的方向進行，而可能對海洋和氣候造成重大影響。

刊登在1月25日《科學前緣》(Science Advances)期刊的論文中，由WHOI的海洋學家Viviane Menezes和Alison Macdonald領導的研究團隊發表南極底層水(AABW)在2017年至2016年期間以驚人的速率變淡許多――此改變可能會改變海洋環流而最終造成海平面上升。

本研究的主要作者，WHOI的博士後研究員Menezes表示：「如果環流發生了變化，海中的一切也會隨之改變。」海洋環流驅動了全球冷暖水體彼此之間的移動過程，因此對地球的溫度和氣候來說，它在熱量的儲存及調節環節上具有重要地位。「我們仍尚未知曉其中的所有情節。雖然我們取得了一些新線索，但還無法破解整個謎題。」

這道謎題本身存在已久：過往研究指出AABW在這數十年間經歷了重大變化。自1990年代起，一項國際計畫就已經反覆對全球特定幾個海盆進行週期性採樣，以追蹤不同地區的海流及環境因子如何隨時間變化。這些採樣位置，或稱「站位」中有一串是從南極延伸至印度洋東部，研究人員即沿著這串站位來追蹤AABW的特性。AABW為流經深海，低於0℃的極冷水層(由於含鹽量，即「鹽分」較高的緣故，因此仍處於液態)。當它環繞南大洋並往北流進三大洋盆時會跟暖水混和。

AABW形成於南極的冰棚，此處颳起的強風會冷卻稱作冰間湖(polynyas)的開放水域，造成部分海水開始凍結。由於水中的鹽分不會結凍，因此冰層附近未結冰的水體就會越來越鹹。而鹽分會增加海水的密度，使得這些海水沉往海底。

「這些海水被視為大尺度全球海洋環流的基石。」本研究的共同作者，WHOI的高級研究專員Macdonald表示。「南極底層水的性質有些來自於大氣，像是溶解其中的碳和氧氣。這些性質會被南極底層水送往海洋深處。之後隨著南極底層水流往世界各地，它會跟周圍的水團混和並且開始交換彼此之間的特性。這就像是深深吸入一口氣之後，花上數十年甚至數百年的時間極為緩慢地吐出一般。」

因此，這道凜冽的海流對全球海洋的環流、溫度、含氧量及養分的調控上都有重要地位。它也可以用來衡量氣候變遷的影響程度，同時也是氣候變遷的影響因子之一。

過往利用重覆測量得到的數據所進行的研究發現，1994年至2007年AABW變得較為溫暖且較淡(也就是鹽分降低)。當Macdonald和Menezes重新探訪這條測線，他們測量了自此之後AABW歷經了什麼樣的變化。

在2016年南半球的夏季，他們參與了研究船R/V Revelle從南極往北航行至澳洲的航次，期間他們每3海浬就得在頻繁發生的暴風雨當中採集數據。在船上實驗室，他們跟研究共同作者，斯克里普斯海洋研究所的Courtney Schatzman合力以鹽溫深感測器(CTD sensors)分析樣品的鹽度、溫度與其他性質。而原始數據目前由Schatzman持有。

研究團隊發現雖然步調減緩了一些，但先前觀測到的暖化趨勢仍持續著。然而，讓他們最感驚訝之處在於樣品缺乏鹽分的程度：本區的AABW在過去十年淡化的幅度是1994至2007年的四倍。

「當我看到鹽度變化時的念頭是：『哦，哇!』」Menezes說。「有時你收集到數據後得花上2到3年才能發現其中端倪，但這次我們在數個小時內就明瞭我們找到了什麼，而且還知道它有多麼出人意料。」

若全球皆發生如此巨幅的改變，可能會嚴重打亂海洋環流並且影響海平面甚鉅。

「當水變得較淡且較暖，它的密度也會跟著下降，造成水體膨脹且占據更多空間――這會促使海平面上升。」Macdonald表示。「如果這些海水不再下沉，會對全球海洋環流的模式造成十分深遠的影響。」

關於這項改變的成因仍然環繞著許多謎團。Menezes和Macdonald推測淡化可能起源於近年發生的一齣大幅改變地貌的事件。2010年，跟羅德島差不多大(比花蓮略小)的冰山撞上南極默茨(Mertz)冰川的前緣，而開鑿出一座超過1000平方英哩的冰河斷塊，並造成阿黛利地和喬治五世地海岸的冰原地貌發生劇變，此處正是本研究觀察的AABW的推測形成位置。之後該斷塊融化而大幅沖淡了此處的海水，可能進一步也讓AABW變得更淡。未來研究可以利用化學分析來追溯AABW的來源是否為那次撞擊及崩解事件發生的位置，以證實理論是否為真。

Antarctic bottom waters freshening at unexpected rate

Shift could disturb ocean circulation and hasten sea level rise, researchers say

In the cold depths along the sea floor, Antarctic Bottom Waters are part of a global circulatory system, supplying oxygen-, carbon- and nutrient-rich waters to the world's oceans. Over the last decade, scientists have been monitoring changes in these waters. But a new study from the Woods Hole Oceanographic Institution (WHOI) suggests these changes are themselves shifting in unexpected ways, with potentially significant consequences for the ocean and climate.

In a paper published January 25 in Science Advances, a team led by WHOI oceanographers Viviane Menezes and Alison Macdonald report that Antarctic Bottom Water (AABW) has freshened at a surprising rate between 2007 and 2016 -- a shift that could alter ocean circulation and ultimately contribute to rising sea levels.

"If you change the circulation, you change everything in the ocean," said Menezes, a WHOI postdoctoral investigator and the study's lead author. Ocean circulation drives the movement of warm and cold waters around the world, so it is essential to storing and regulating heat and plays a key role in Earth's temperature and climate. "But we don't have the whole story yet. We have some new pieces, but we don't have the entire puzzle."

The puzzle itself isn't new: past studies suggest that AABW has been undergoing significant changes for decades. Since the 1990s, an international program of repeat surveys has periodically sampled certain ocean basins around the world to track the circulation and conditions at these spots over time. Along one string of sites, or "stations," that stretches from Antarctica to the southern Indian Ocean, researchers have tracked the conditions of AABW -- a layer of profoundly cold water less than 0°C (it stays liquid because of its salt content, or salinity) that moves through the abyssal ocean, mixing with warmer waters as it circulates around the globe in the Southern Ocean and northward into all three of the major ocean basins.

The AABW forms along the Antarctic ice shelves, where strong winds cool open areas of water, called polynyas, until some of the water freezes. The salt in the water doesn't freeze, however, so the unfrozen seawater around the ice becomes saltier. The salt makes the water denser, causing it to sink to the ocean bottom.

"These waters are thought to be the underpinning of the large-scale global ocean circulation," said Macdonald, a WHOI senior research specialist and the study's co-author. "Antarctic Bottom Water gets its characteristics from the atmosphere -- for example, dissolved carbon and oxygen -- and sends them deep into the ocean. Then, as the water moves around the globe, it mixes with the water around it and they start to share each other's properties. It's like taking a deep breath and letting it go really slowly, over decades or even centuries."

As a result, the frigid flow plays a critical role in regulating circulation, temperature, and availability of oxygen and nutrients throughout the world's oceans, and serves as both a barometer for climate change and a factor that can contribute to that change.

A past study using the repeat survey data found that AABW had warmed and freshened (grown less saline) between 1994 and 2007. When Macdonald and Menezes revisited the line of stations, they measured how AABW has changed in the years since.

During the austral summer of 2016, they joined the crew of the research ship R/V Revelle and cruised north from Antarctica to Australia, braving frequent storms to collect samples every 30 nautical miles. In a shipboard lab, they analyzed the samples using data from conductivity-temperature-depth (CTD) sensors, which measure the water's salinity, temperature and other properties, with support from study co-author Courtney Schatzman of the Scripps Institution of Oceanography, who processed the raw data.

The team found that the previously detected warming trend has continued, though at a somewhat slower pace. The biggest surprise, however, was its lack of saltiness: AABW in this region has grown fresher four times faster in the past decade than it did between 1994 and 2007.

"I thought, 'Oh wow!' when I saw the change in salinity," said Menezes. "You collect the data and sometimes you spend 2 to 3 years to find something, but this time we knew what we had within hours, and we knew it was very unexpected."

Such a shift, were it global, could significantly disrupt ocean circulation and sea levels.

"The fresher and warmer the water is, the less dense it will be, and the more it will expand and take up more space -- and that leads to rising sea levels," Macdonald said. "If these waters no longer sink, it could have far reaching affects for global ocean circulation patterns."

Questions remain around the cause of the shift. Menezes and Macdonald hypothesize that the freshening could be due to a recent landscape-changing event. In 2010, an iceberg about the size of Rhode Island collided with Antarctica's Mertz Glacier Tongue, carving out a more-than-1,000-square-mile piece and reshaping the icescape of the George V/Adelie Land Coast, where the AABW observed in this study is thought to form. The subsequent melting dramatically freshened the waters there, which may have in turn freshened the AABW as well. Future studies could use chemical analysis to trace the waters back to the site of the collision and calving and confirm the hypothesis.

原始論文：Viviane V. Menezes, Alison M. Macdonald and Courtney Schatzman. Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean. Science Advances, January 2017 DOI: 10.1126/sciadv.1601426

引用自：Woods Hole Oceanographic Institution. "Antarctic bottom waters freshening at unexpected rate: Shift could disturb ocean circulation and hasten sea level rise, researchers say." ScienceDaily. ScienceDaily, 25 January 2017.