On Christmas Eve, 1777, the HMS Resolution landed on a small, isolated island 2,160 km south of Hawaii. The captain of the HMS Resolution, James Cook, named the island for the day it was discovered. Over a century earlier, 9,000 km across the Pacific, fishermen off the coast of present day Peru noticed a periodic warming of Pacific waters that coincided with a drop in local fish population. This event, which occurs every few years around Christmas day, was named El Nino or Christ child. Today, the coral reefs off Christmas Island (aka Kiritimati Island) are a focal point for researchers who stand ready to measure potential coral bleaching as a consequence of this year’s El Nino. This event influences weather far away from Christmas Island, and as we’ll see, across the globe.

El Nino is a natural phenomena that occurs periodically and is not a recent climate development. In fact, climatologists have studied clam fossils to measure El Nino events going back 10,000 years. El Nino is caused by an oscillation of high and low pressure zones and ocean temperature in the eastern equatorial region of the Pacific. For this reason, El Nino is formally named by scientists as the El Nino – Southern Oscillation or ENSO. While El Nino is associated with warmer ocean waters, its counterpart, La Nina (Little Girl in Spanish) is marked by colder than normal waters in the same region. To understand one, you need to understand the other. The image below shows the Pacific temperature variation between the two events.

You’ll note that the temperature variation is not very great, just a few degrees Celsius. However, given the size of the area the temperature anomaly occurs, this can have a dramatic effect on atmospheric circulations in the region. Warm ocean waters transports heat into the atmosphere above it. Warm air rises, creating a low pressure area that tends to be unstable and results in precipitation. Cooler ocean waters stabilizes the air above it. Cold air tends to sink and this results in a region of high pressure which is marked by low participation. So, what causes this oscillation? Lets take a look at the global wind map below:

For those of us who live in the Northern Mid-latitudes such as the United States or Europe, we are used to winds prevailing from the west. However, in the equatorial tropics, where the ENSO takes place, the trades winds prevail from the East. Typically, these easterly trade winds push the Pacific waters towards the west. Normally the result is this:

The easterly trade winds cause warmer water to pool up in the Western Pacific by Australia. The heat from the ocean transfers to the atmosphere in the region, which in turn, causes instability in the air. As the air rises, it cools, releasing moisture in the form of rain. This pattern causes the wet season in Australia from November to March. Warm water also expands, which means sea level is higher in the Western Pacific than the Eastern Pacific. The warm water in the west sinks and becomes colder. The ocean circulation returns this cold water to the coast of South America. The upwelling of this cold water brings with it nutrients for fish to feed from. A La Nina event is an amplification of these conditions.

An El Nino event is a flip-flopping of this pattern. During El Nino, the easterly trade winds die down, causing warm water to migrate back towards the coast of Peru rather than around Australia resulting in the scenario below:

Throughout El Nino, the upwelling of colder, nutrient rich water off the Peruvian coast weakens. As their food supply drops, fish in the region migrate away. This means lower catch amounts for fishers, which was observed in the 1600’s. In the west, rain moves away from Australia bringing in drought conditions. In the east, the warmer than normal waters produce flooding on the west coast of South America. Globally, an El Nino can result in a spike in global temperatures. And here is why:

During El Nino years, warm Pacific waters are dispersed over a larger surface area. Have you ever seen rain water pool up on a race track? Typically, to dry the track faster, crews come out with blowers to disperse the water over a wider area of the track, making it easier for water to evaporate into the air. El Nino essentially does the same thing. By dispersing warm water over a wider area, it allows for greater rate of transport of ocean heat into the atmosphere. The 1997-98 El Nino was the strongest on record, and that was reflected as a surge in global temperatures in 1998:

The 1982-83 El Nino also brought about a rise in global temperatures. Thus, ENSO brings short term noise into global temperature data. Other climate factors do this as well. For example, powerful volcanic eruptions can eject sulfur dioxide into the stratosphere. This results in global cooling for a period of 2-3 years. You can see that above as the Mt. Pinatubo eruption in 1991 caused a brief drop in temperatures in the early 1990’s. To discern between short term and long term effects, trend lines are used which are in blue above.

Note that, contrary to what you may hear in some quarters, global temperatures have continued to trend upwards since 1998. Trend lines are typically regression calculations which minimize sums of the distances between the individual data points and the trend line. Some charts have trend lines starting at the 1998 point and ending at a later data point lower to “prove” temperatures have not risen since 1998.

That is a statistical no-no!

Doing so would flunk you out of introductory statistics as that would not meet the regression fit requirements. This is not the only area where the popular media confuses short term and long term trends. You see it when economic data such as the monthly job report comes out. Not only do monthly figures contain a lot of short term noise, but they are typically revised later. Stock and commodity prices are also pretty noisy and often media reports hyperventilate over insignificant daily trends. Students have complained to me that statistics is boring, but it can be one of the most useful, practical courses one can take.

Moving off my soap box and back to El Nino, what else can we expect to encounter during an El Nino year? The oscillation of high and low pressure zones in the Pacific can have a dramatic effect on the jet stream and weather tracks across the Americas.

During the La Nina phase, high pressure in the North Pacific deflects the jet stream into Alaska where it sweeps down across Canada into the Northern United States bringing polar air along with it. During El Nino, low pressure in the Pacific allows the jet stream to drop southward. The heat from El Nino strengthens the jet stream. This creates a strong storm track that can bring significant rainfall and flooding across California and the South. Polar air can be trapped north of the U.S. resulting in warm winters in the Midwest and Northeast. The 1982 El Nino brought in the warmest Christmas in Buffalo history, clocking in at 64 degrees. The 1997 El Nino brought in a year’s worth of rain, some 13 inches, to Los Angeles in February of 1998. Early detection of El Nino can help in preparations for flooding in areas such as Southern California.

In South America, closer to El Nino itself, the effects are amplified. During the 1997-98 El Nino, some areas in Peru received ten times the normal rainfall amounts. As a consequence, landslides claimed the lives of over 200 persons in both Peru and Ecuador. On the other side of the Pacific, El Nino brings abnormally dry conditions. During the same 1997-98 El Nino, drought conditions combined with slash and burn agriculture sparked wildfires in Indonesia that consumed 7 million hectares (17.3 million acres). In 2015, wildfires in Indonesia have again heralded the onset of El Nino. These fires are rich in carbon dioxide emissions and it has been estimated that so far, as much carbon dioxide has been released in Indonesia as an entire year in Japan.

How does the 2015 El Nino compare with the great El Nino of 1997? The early returns are that this is a comparable event. In fact, the 2015 El Nino has already eclipsed the one week record set in 1997 for Pacific warming. Climatologists use a three month baseline to determine El Nino strength and if 2015 does not match 1997 on that baseline, it will not lag very far behind. That being the case, we can expect a general rerun of the events of 1997-98. As the video below explains, there are always variations to each El Nino event, but we can make probabilistic predictions to what this winter holds.

Lying in the cross-hairs of El Nino are the coral reefs off of Christmas Island. The surge in water temperature can generate a bleaching of the coral reefs. During the 1997-98 El Nino, some 20% of the world’s coral population was lost to bleaching. Warm water causes corals to eject algae called zooxanthellae. This algae lives with the coral and produces nutrients for the coral to consume. The loss of these nutrients triggers the bleaching of the vibrant colors the corals are famous for.

As the Pacific waters have reached 31 C (88 F), scientists stand ready not only to record the effects of this year’s El Nino, but to utilize coral fossils to reconstruct El Nino’s history and project the future. While the bleaching of corals has historically occurred on a periodic basis, the corals typically have been able to recover. However, with the oceans temperatures trending upward as a result of global warming, this may inhibit future recoveries of coral bleaching events. El Nino is part of a naturally occurring cycle, nonetheless, it will provide us with important information on what to expect as we experience a long term, non-cyclical warming globe.

As we proceed into 2016, the El Nino will diminish and the ENSO cycle will eventually trend back towards La Nina. Global temperatures, just as happened after the 1997-98 El Nino concluded, may subside a bit. It will be important not to be fooled by the short term noise. That drop in temperature will not represent a long term shift, but only a return to the trend line. Afterwards, we should expect global temperatures to commence its rise again. Heat is energy, and as the global base temperature continues its climb, El Nino events in the future can be anticipated to be more powerful. And we will need to incorporate that, along with a lot of other implications of climate change, into our long term policy planning.

*Image on top of post is comparison of sea height anomalies between the 1997 and 2015 El Nino events. As water warms, it expands, causing sea levels to rise. Credit: NASA.