托福阅读材料：厄尔尼诺
先来恶补一下地理学相关知识，学习文章中一些疑难词：
厄尔尼诺(El Niño Phenomenon)又称厄尔尼诺海流，是太平洋赤道带大范围内海洋和大气相互作用后失去平衡而产生的一种气候现象，正常情况下，热带太平洋区域的季风洋流是从美洲走向亚洲，使太平洋表面保持温暖，给印尼周围带来热带降雨。但这种模式每2—7年被打乱一次，使风向和洋流发生逆转，太平洋表层的热流就转而向东走向美洲，随之便带走了热带降雨，出现所谓的“厄尔尼诺现象”。
拉尼娜(La Niña)是厄尔尼诺现象的反相，指赤道附近东太平洋水温反常下降的一种现象，表现为东太平洋明显变冷，同时也伴随着全球性气候混乱，总是出现在厄尔尼诺现象之后。
疑难词：
characterized adj. 以…为特点的
atmospheric adj. 大气的，大气层的
convection n. [流][气象] 对流；传送
modulate vt. 调节；调整vi. 调制；转调
precipitation n. 沉淀，沉淀物；降水
hypothesize vt. 假设，假定vi. 假设，假定
correlate adj. 关联的vi. 关联
tornado n. [气象] 龙卷风；旋风；暴风
persist into持续至
阅读文章：
Influence of the El Niño/Southern Oscillation on tornado and hail frequency in the
United States
美国：厄尔尼诺/南方涛动影响暴雨和冰雹频率
The El Niño(西班牙文)/Southern Oscillation (ENSO) is characterized by changes in sea surface temperature (SST) and atmospheric convection in the tropical Pacific, and modulates global weather and climate.
厄尔尼诺现象发生在热带太平洋，特点是海面温度及大气对流变化，会造成全球气候变化。
The phase of ENSO influences United States (US) temperature and precipitation and has long been hypothesized to influence severe thunderstorm occurrence over the US. However, limitations of the severe thunderstorm observational record,combined with large
year-to-year variability, have made it difficult to demonstrate an ENSO influence during the peak spring season. Here we use environmental indices that are correlated with tornado and hail activity, and show that ENSO modulates tornado and hail occurrence during the winter and spring by altering the large-scale environment. We show that fewer tornadoes and hail events occur over the central US during El Niño and conversely more occur during La Niña conditions. Moreover, winter ENSO conditions often persist into early spring, and consequently the winter ENSO state can be used to predict changes in tornado and hail frequency during the following spring. Combined with our current ability to predict ENSO several months in advance, our findings provide a basis for long-range seasonal prediction of severe thunderstorm‘activity.
厄尔尼诺影响美国的气温和降水，长期以来被认为会影响美国雷暴发生。但是，因为缺少强雷暴的观测记录，而且每年情况都有变化，很难去描述春季高峰时期的厄尔尼诺影响。所以我们利用与暴雨及冰雹活动相关的环境指数来描述它的影响。指数显示在冬春交接时期，厄尔尼诺通过大范围改变环境状况来影响暴雨和冰雹的发生。可以看到，在厄尔尼诺发生时期，暴雨和冰雹发生次数减少，而在拉尼娜时期，又有增多。而且，冬季厄尔尼诺经常会持续到春季，所以它在冬季的状态可以用来预测春季暴风雨冰雹的频率。加上我们目前预测能力可以提前预测到几个月后的状况，我们的发现会为强雷暴活动的大范围季节性预测提供信息。
这些来自文章的长难句可能会影响到理解：
1.However, limitations of the severe thunderstorm observational record, combined with large year-to-year variability, have made it difficult to demonstrate an ENSO influence during the peak spring season.
句子类型：插入语
句子拆分：
主干：limitations of the severe thunderstorm observational record have made it difficult to…
插入语：combined with large year-to-year variability，为附加成分，所以后面动词跟limitations一致。
翻译：但是，因为缺少强雷暴的观测记录，而且每年情况都有变化，很难去描述春季高峰时期的厄尔尼诺影响。
2. Here we use environmental indices that are correlated with tornado and hail activity, and show that ENSO modulates tornado and hail occurrence during the winter and spring by altering the large-scale environment.
句子类型：定语从句+宾语从句
句子拆分：
主干：Here we use environmental indices.
定语从句：that are correlated with tornado and hail activity and show that…that 引导的定语从句，先行词indices
宾语从句：show that ENSO modulates tornado and hail occurrence during the winter and spring by altering the large-scale environment.
翻译：所以我们利用与暴雨及冰雹活动相关的环境指数来描述它的影响。指数显示在冬春交接时期，厄尔尼诺通过大范围改变环境状况来影响暴雨和冰雹的发生。
