El Niño Lecture Notes There is a huge link between the atmosphere & ocean. The oceans influence the atmosphere to affect climate, but the atmosphere also influences the ocean, which can also affect climate. Together, atmosphere & ocean make up what we call the climate system Peruvian Fisheries Case Study Historically, the waters off Peru are particularly good fishing grounds. As of 2010, approximately 20% of the world s total fishing catch comes out of the ecosystem supported by the Peru (Humbolt) Current, which flows from Antarctica, up the west coast of South America. Specifically, the majority of the fish caught off the coast of Peru are anchovies. In the graph, you can see the trend in the size of the anchovy catch from 1950, when the commercial fishing industry started in force. You can see that from 1950 the number of fish caught each year steadily increased until there was a precipitous decline in 1972. This was followed by another crash in 1982 and another in 1998. So, the question is: Why did this fishery show this pattern of repeated collapse with recoveries in between? Why was the fishing so good? To start answering this question, you need to first understand why the fishing was so good in Peru to begin with. The answer is an phenomenon called upwelling. This phenomenon occurs when winds (and surface currents) are blowing from the poles to the equator, so this coincides with cold water surface
currents. (There is a similar phenomenon called downwelling that occurs in areas with warm water currents.) Looking at the map above, you can identify the upwelling areas based on the location of these cold surface currents. In additon to the Peru Current, other major upwelling areas include the California Current, the Benguela Current and the Canary Current. These currents cause some interesting phenomena as they interact with continents. As the wind is blowing along the surface of the ocean, it will drag the top layer of the water along with it. But, because of the Coriolis effect, this top layer will curve (or deflect) at about a 45 angle. It will be deflected to the left in the southern hemisphere and to the right in the northern hemisphere. This top layer of water will drag the layer below it in that direction, but that layer will also be deflected (due to the Coriolis effect) at about a 45 angle. This continues to occur deeper and deeper in the water column down to about 300-500 feet. You can see this in the diagram. However, each layer of water gets slightly less energy from the wind than the layer above it; this is shown in the diagram with the decreasing size of the vectors as depth increases. This phenomenon is called the Ekman Spiral and results in a net movement of water at 90 to the direction of the wind. This net motion of water is consequently called Ekman Transport. In areas with a cold water current, this Ekman transport moves water offshore. (In areas with a warm water current, the flow is opposite and moves water onshore.) Upwelling As the Ekman transport moves water offshore, there isn t any water at the surface to replace the water it. Therefore, the only water available is deep water, which moves in to replace it. Deep water is colder and saltier than the surrounding water. This can influence the weather in nearby areas, making it cool and causing moisture in the air to condense into fog. The deep water is also more nutrient-rich. The nutrients in these waters come from the decomposition of sinking dead material. In particular, these nutrients are nitrates & phosphates. Once these nutrients reach the surface the result is high NPP-
phytoplankton flourish in these areas and support robust food chains. (In fact, 25% of the world s marine fish catch comes from the 5 main upwelling areas, which make up 5% of the Earth s ocean area.) Normal SST in the South Pacific This upwelling pattern off the coast of Chile and Peru is intensified by the normal movement of the currents in the South Pacific. At the equator, water normally is warm due to the high level of sunlight received. The Trade Winds at the equator blow from East to West and pull the surface water along with it. This causes the warm water to accumulate in the western Pacific (near Australia), leaving more room for deep, cold water to upwell near Peru. You can see this pattern of SST (sea surface temperature) in the diagram below. Walker Circulation The pattern of the currents is in turn amplified by the normal circulation in the atmosphere. In addition to the Hadley, Ferrel and Polar cells that create circulation going North and South, there is an East-West Pattern of circulation at the equator called the Walker circulation. This circulation pattern wraps around the entire globe at the equator causing air to rise and sink and helps to explain why rainforests don t exist everywhere at the equator. Here you can see how the Walker Circulation pattern fits in with the Hadley, Ferrel & Polar Cells. The portion of the Walker Circulation in the South Pacific is shown in the diagram on the next page. As the warm water is pushed across the Pacific, it accumulates near Indonesia/ Australia. This causes the air over Australia to also
warm up and then the warm air will rise. The result is more rain over Australia/ Indonesia than over Peru. So the climate of Peru is even cooler and drier than one would expect just based on the cold Peru current or just the Upwelling. What happened in 1972-73 (and 82-83 And 97-98)? This is the normal situation in the South Pacific, but during El Nino, this entire pattern is disrupted. This is what happened in each of the years where the Peruvian fisheries saw major collapses. El Nino is a disruption in the normal pattern of surface currents & upwelling. It is also called El Niño- Southern Oscillation (ENSO). ENSO is a phenomenon that happens primarily in the southern pacific, though it has global effects (such as wetter winters in California and warmer winters in the midwest/ northeast US). In El Nino years, something causes the trade winds to slacken so they don t blow as strongly from east to west and the equatorial current is not as strong. No one fully understands what causes this, but as a result less water moves from east to west and because of the reduced surface currents, warm water spreads out across the Southern Pacific ocean. This results in decreased upwelling in Peru and increased rain due to warmer, wetter air off the Peruvian coast. In addition to warm water spreading across the Pacific during an El Niño event, the change in SST also triggers changes in the way the atmosphere circulates. Since the oceans and atmosphere are intimately linked, when one is disrupted on such a large scale, it will ultimately impact the other. You can see the impacts
in the diagram of the disrupted Walker Circulation below. Notice that the rain is concentrated over Peru and the upwelling off the Peruvian coast is supressed. This area is not capable of absorbing so much water at once, so flooding can result. On the other side of the Pacific, there is decreased rain due to less warm, moist air off the Australian coast, which can lead to droughts and wildfires in Australia. La Niña There is also a third state to this Southern Oscillation, called La Niña. The state is an intensification of the normal pattern as shown below. Both the Trade Winds and surface currents are more intense, so water is pushed more strongly toward Australia & Indonesia. This increases the normal amount of rain in Australia & Indonesia, while amplifying the upwelling near Peru. Teleconnections Although the ENSO phenomenon is located in the South Pacific, it can have impacts around the globe. These impacts that occur a great distance apart are called teleconnections. In North America, we experience these teleconnections because the disruptions to the Walker Circulation push the jet stream out of its normal path. The jet stream is a high-altitude river of air that is
responsible for much of the weather over all of North America. As you can see in the diagram below, during La Niña years, as well as normal years, the jet stream predominately moves north of California and brings rain to the Pacific Northwest. However, during El Niño, the jet stream dips south. It brings rain to California as well as to the southest United States. This disruption in the jet stream also results in warmer weather through the midwest and northeast regions of the US. These teleconnections are summarized in the map here. El Niño teleconnections extend beyond the Pacific and North America, however. Because the Walker Circulation wraps all the way around the equator (see diagram at the bottom of page 3), any change in the Walker Circulation disrupts the whole pattern. So, when El Niño causes the Walker Circulation to change in the Pacific, there is a domino effect around the globe, redistributing precipitation patterns. This can cause drought as far away as East Africa.