The cold Humboldt Current of the Pacific Ocean flows toward the equator along the coasts of Ecuador and Peru in South America. When the current approaches the equator, the westward-flowing trade winds cause nutrient-rich cold water along the coast to rise from deeper depths to more shallow ones. This upwelling of water has economic repercussions. Fishing, especially for anchovies, is a major local industry.

Every year during the months of December and January, a weak, warm countercurrent replaces the normally cold coastal waters. Without the upwelling of nutrients from below to feed the fish, fishing comes to a standstill. Fishers in this region have known the phenomenon for hundreds of years. In fact, this is the time of year they traditionally set aside to tend to their equipment and await the return of cold water. The residents of the region have given this phenomenon the name of El Nifio, which is Spanish for "the child," because it occurs at about the time of the celebration of birth of the Christ child.

While the warm-water countercurrent usually lasts for two months or less, there are occasions when the disruption to the normal flow lasts for many months. In these situations, water temperatures are raised not just along the coast, but for thousands of kilometers offshore. Over the last few decades, the term El Nifio has come to be used to describe these exceptionally strong episodes and not the annual event. During the past 60 years, at least ten El Nifios have been observed Not only do El Niftos affect the temperature of the equatorial Pacific, but the strongest of them impact global weather.

The processes that interact to produce an El Nifio involve conditions all across the Pacific, not just in the waters off South America. Over 60 years ago, Sir Gilbert Walker, a British scientist, discovered a connection between surface pressure readings at weather stations on the eastern and western sides of the Pacific. He noted that a rise in atmospheric pressure in the eastern Pacific is usually accompanied by a fall in pressure in the western Pacific and vice versa. He called this seesaw pattern the Southern Oscillation. It was later realized that there is a close link between El Nino and the Southern Oscillation. In fact, the link between the two is so great that they are often referred to jointly as ENSO (El Nino-Southern Oscillation).

During a typical year, the eastern Pacific has a higher pressure than the western Pacific does. This east-to-west pressure gradient enhances the trade winds over the equatorial waters. This results in a warm surface current that moves east to west at the equator. The western Pacific develops a thick, warm layer of water while the eastern Pacific has the cold Humboldt Current enhanced by upwelling. However, in other years the Southern Oscillation, for unknown reasons, swings in the opposite direction, dramatically changing the usual conditions described above, with pressure increasing in the western. Pacific and decreasing in the eastern Pacific. This change in the pressure gradient causes the trade winds to weaken or, in some cases, to reverse. This then causes the warm water in the western Pacific to flow eastward, increasing sea-surface temperatures in the central and eastern Pacific. The eastward shift signals the beginning of an El Nifio.

Scientists try to document as many past El Nino events as possible by piecing together bits of historical evidence, such as sea-surface temperature records, daily observations of atmospheric pressure and rainfall, fisheries’ records from South America, and the writings of Spanish colonists dating back to the fifteenth century. From such historical evidence we know that El Nirtos have occurred as far back as records go. It would seem that they are becoming more frequent. Records indicate that during the sixteenth century, an El Nino occurred on average every six years. Evidence gathered over the past few decades indicates that El Ninos are now occurring on average a little over every two years. Even more alarming is the fact that they appear to be getting stronger. The 1997-1998 El Nifio brought copious and damaging rainfall to the southern United States, from California to Florida. Snowstorms in the northeast portion of the United States were more frequent and intense than in most years.

太平洋沿岸的洪堡德寒流[又称秘鲁寒流]沿着南美洲的厄瓜多尔和秘鲁流向赤道。当接近赤道时，西流的信风会导致沿海营养丰富的冷水从深处上升到较浅的地方。冷水上涌会带来经济上的影响。捕鱼业，尤其是鯷鱼的捕捞因此成为当地的主要产业。

每年十二月和次年一月之间，一股微弱的、温暖的逆流取代了通常寒冷的沿海水域。没有营养物质从海水深处涌上来喂鱼，渔业就停滞了。这片区域的渔民几百年前就已经知道这种现象了。事实上，这段时间向来也是渔民们修补设备、等待冷水回归的时间。该地区的居民已经替这种现象取了个名字叫“厄尔尼诺”，在西班牙语里意思是“圣婴”，因为它发生在庆祝基督诞生的这段时间。

温暖的逆流通常会持续2个月或更短，但正常的洋流中断几个月的情况也时有发生。在这种情况下，不只是海岸附近的海水温度升高，连数千公里之外的海面温度都会升高。在过去的几十年里，厄尔尼诺一词已经被用来描述这些异常强烈的事件，而不是一年一度的普遍现象。在过去的60年中，至少出现了10次厄尔尼诺现象。厄尔尼诺现象不仅影响赤道太平洋的温度，其中最强的还会影响全球的天气。

相互作用造成厄尔尼诺现象的过程包括整个太平洋的条件，而不仅仅是美国南部的水域。60年前，英国科学家吉尔伯特·沃克先生发现气象站播报的东西太平洋之间的气压有一定的关系。他注意到，当东太平洋气压上升时，通常会伴随着西太平洋气压的下降，反之亦然。他把这种交互式的现象叫做“南方涛动”。后来人们意识到，厄尔尼诺现象和南方涛动之间有着密切的联系。事实上，两者之间的联系是如此之大，他们往往将两者统称ENSO（厄尔尼诺-南方涛动）。

在一个典型的年份里，东太平洋的气压比西太平洋的气压高。这由东西向的气压梯度增强了赤道水域的信风，形成一股温暖的沿赤道由东向西移动的表层流。这样，西太平洋便形成了一个深厚的、温暖的水层，而东太平洋则有被上涌增强的洪堡德寒流。然而，在其他年份，南方涛动不知道是因为什么原因，朝相反的方向波动，使得西太平洋气压增加，东太平洋气压减小，极大地改变了前面所说的通常情况。这种压力梯度的变化使得信风减弱，在某些情况下还可能逆转。这就导致了西太平洋的暖水向东流动，提高了太平洋中部和东部的海平面温度。这种向东流动标志着厄尔尼诺现象的开始。

科学家们试图尽可能多地记录发生的厄尔尼诺现象。他们收集各种微小的历史证据，如海面温度记录、气压和降水的日常观察，南美洲渔业记录，以及追溯到15世纪的西班牙殖民者的作品。从这样的历史证据，我们知道，厄尔尼诺现象从有文字记载开始就发生过。而且看起来变得越来越频繁。记录表明，十六世纪平均每六年就发生一次厄尔尼诺现象。而最近几十年收集的证据表明，厄尔尼诺现象平均两年多就要发生一次。更令人吃惊的是，厄尔尼诺的威力越来越强。1997-1998年的厄尔尼诺给美国南部从加利福尼亚到佛罗里达带来了大量的、破坏性的降雨。美国东北部的暴风雪也比大多数年份更频繁、更猛烈。