Weird weather can span the world

極端天氣橫掃全球

　　On September 7th in Denver, Colorado, the temperature reached 34°C (93°F), 6°C above what is normal for the time of year. The city was sitting under the dome of hot air encouraging record fires across the American West. The next day snow started to fall. By midnight the temperature was below freezing. What happened?

　　九月七日，科羅拉多州丹佛市的氣溫達(dá)到了34℃，比往年同期正常水平高出6℃。這座城市被一個(gè)熱空氣構(gòu)成的穹頂籠罩，這股熱浪已經(jīng)推動(dòng)美國西部各地發(fā)生規(guī)模創(chuàng)紀(jì)錄的火災(zāi)。第二天丹佛又開始下雪。到午夜時(shí)分，氣溫已跌至冰點(diǎn)以下。這是怎么回事?

　　The immediate cause was the polar jet stream, a world-girdling high-altitude wind driven by temperature differences between Arctic air to the north and warmer air to the south. Its meandering path is set by patterns of high and low pressure known as Rossby waves. And because fluid dynamics are never simple, the jet stream exerts its own influence in turn upon these guiding waves.

　　原因是極地噴射氣流，這是一種環(huán)繞地球流動(dòng)的高空風(fēng)，由北方的北極氣團(tuán)和南方的較溫暖氣團(tuán)之間的溫差驅(qū)動(dòng)。其蜿蜒曲折的路線是由被稱作羅斯貝波的高低壓模式?jīng)Q定的。而因?yàn)榱黧w動(dòng)力學(xué)本身很復(fù)雜，噴射氣流反過來會(huì)對(duì)這些導(dǎo)向性的羅斯貝波施加影響。

　　As the jet stream passed to the north of the high pressure over America’s west coast, the big temperature difference between that hot air and what lay farther to the north added to its energy. This extra impetus meant that as the jet came down the east side of the high-pressure zone it curved back on itself towards the west. In meteorological terms, the Rossby wave broke. In this case the breaking wave’s white water took the form of large quantities of cold air from the Canadian north that were suddenly pulled south, and which crashed down on Colorado.

　　當(dāng)噴射氣流經(jīng)過美國西海岸上空的高壓帶北側(cè)時(shí)，熱空氣與北側(cè)更遠(yuǎn)處的空氣之間的巨大溫差增加了噴射氣流的能量。由于這種額外的推動(dòng)力，氣流從高壓帶東側(cè)下行時(shí)又向西彎曲轉(zhuǎn)回原方向。用氣象學(xué)術(shù)語來說，羅斯貝波中斷了。在本例中，波浪中斷時(shí)生成的白色水花以大量冷空氣的形式出現(xiàn)，這些原本來自加拿大北部的冷空氣被突然拽向南方，又嘩啦一下砸在科羅拉多州。

　　This is not in itself evidence of climate change. But that may lurk in the background. Go back to the first days of September and Typhoon Maysak was passing north over the Koreas, the second of an unprecedented troika of typhoons to do so this summer, thanks to hot seas and cyclone-friendly conditions to the south. The heat Maysak gave up over eastern China drove the jet stream there to particularly impressive speeds. That intensification travelled on over the Pacific, possibly predisposing the system towards what happened five days later in Denver, says Philippe Papin of America’s National Weather Service.

　　這本身并不足以證明氣候變化。但這種變化可能藏在暗處�；氐�9月的頭幾天，臺(tái)風(fēng)“美莎克”(Maysak)正北上經(jīng)過朝鮮半島上空，它是今夏造訪該地的前所未見的"臺(tái)風(fēng)三連擊"中的第二個(gè)，這要?dú)w功于南部的高溫海域和氣旋友好型條件�！懊郎�克"在中國東部上空釋放的熱量推動(dòng)該區(qū)域的噴射氣流達(dá)到了特別驚人的高速。美國國家氣象局的菲利普·帕潘說，強(qiáng)化的氣流繼續(xù)在太平洋上空移動(dòng)，可能誘發(fā)了天氣系統(tǒng)五天后在丹佛發(fā)生的極端情況。

　　Not all typhoons that pass over Korea will have such effects. (Maysak’s successor, Haishen, did not.) They need to hit the jet stream in the right way. But greenhouse warming is making the tropics larger. Models suggest this will mean that more tropical storms reach high latitudes, where they can invigorate the meanders of the jet stream in rather the same way that a child sends a wave down a skipping rope with a flick of the wrist.

　　并非所有經(jīng)過朝鮮半島上空的臺(tái)風(fēng)都會(huì)產(chǎn)生這樣的效果。(“美莎克"之后的"海神"就沒有。)它們需要恰到好處地撞擊噴射氣流。但溫室變暖正在擴(kuò)大熱帶地區(qū)的范圍。模型顯示，這將意味著更多的熱帶風(fēng)暴會(huì)到達(dá)高緯度地區(qū)，在那里它們可以刺激噴射氣流，致其路線更加蜿蜒曲折，就像一個(gè)孩子輕抖手腕就能讓波浪沿著長繩傳出那樣。

　　Some scientists are concerned that, as climate change worsens, the ability of Rossby waves to drive sudden changes in the weather will increase. Even today, they can bring about whiplash weather when the circumstances are right.

　　一些科學(xué)家擔(dān)心，隨著氣候變化的加劇，羅斯貝波推動(dòng)天氣突變的能力將會(huì)增強(qiáng)。即使在今天，當(dāng)環(huán)境條件合適時(shí)，它們也能帶來“天氣之鞭"。