Weird weather can span the world

极端天气横扫全球

　　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，科罗拉多州丹佛市的气温达到了34℃，比往年同期正常水平高出6℃。这座城市被一个热空气构成的穹顶笼罩，这股热浪已经推动美国西部各地发生规模创纪录的火灾。第二天丹佛又开始下雪。到午夜时分，气温已跌至冰点以下。这是怎么回事?

　　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.

　　原因是极地喷射气流，这是一种环绕地球流动的高空风，由北方的北极气团和南方的较温暖气团之间的温差驱动。其蜿蜒曲折的路线是由被称作罗斯贝波的高低压模式决定的。而因为流体动力学本身很复杂，喷射气流反过来会对这些导向性的罗斯贝波施加影响。

　　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.

　　当喷射气流经过美国西海岸上空的高压带北侧时，热空气与北侧更远处的空气之间的巨大温差增加了喷射气流的能量。由于这种额外的推动力，气流从高压带东侧下行时又向西弯曲转回原方向。用气象学术语来说，罗斯贝波中断了。在本例中，波浪中断时生成的白色水花以大量冷空气的形式出现，这些原本来自加拿大北部的冷空气被突然拽向南方，又哗啦一下砸在科罗拉多州。

　　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.

　　这本身并不足以证明气候变化。但这种变化可能藏在暗处。回到9月的头几天，台风“美莎克”(Maysak)正北上经过朝鲜半岛上空，它是今夏造访该地的前所未见的"台风三连击"中的第二个，这要归功于南部的高温海域和气旋友好型条件。“美莎克"在中国东部上空释放的热量推动该区域的喷射气流达到了特别惊人的高速。美国国家气象局的菲利普·帕潘说，强化的气流继续在太平洋上空移动，可能诱发了天气系统五天后在丹佛发生的极端情况。

　　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.

　　并非所有经过朝鲜半岛上空的台风都会产生这样的效果。(“美莎克"之后的"海神"就没有。)它们需要恰到好处地撞击喷射气流。但温室变暖正在扩大热带地区的范围。模型显示，这将意味着更多的热带风暴会到达高纬度地区，在那里它们可以刺激喷射气流，致其路线更加蜿蜒曲折，就像一个孩子轻抖手腕就能让波浪沿着长绳传出那样。

　　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.

　　一些科学家担心，随着气候变化的加剧，罗斯贝波推动天气突变的能力将会增强。即使在今天，当环境条件合适时，它们也能带来“天气之鞭"。