Friday, July 11, 2014

What is El Niño, and How Does it Affect Hawai'i?

If you ever pay attention to the weather forecast, you’ve probably heard about a phenomenon called El Niño.  However, if you’re like most people, you probably don’t have a clue as to what El Niño refers to or what it means.  You may have the vague notion that it causes changes in the weather, but you might be unsure at to the nature of those changes.  For example, does it make it rain more?  Less?  Does it make it hotter or cooler?  This confusion is understandable, because the effects of El Niño vary depending on where you are!  In this post we’re going to dispel some of the mystery and confusion surrounding El Niño in general and explain how this phenomenon affects Hawai’i.  Looking at El Niño gives us the chance to examine a number of other aspects of the oceanic-atmospheric relationship, and helps us to think about oceans and the atmosphere as a big system.  You may find this information useful, since many climatologists are predicting unusually strong El Niño conditions for 2014.

Explaining ENSO

Map from Wikipedia.
El Niño, which means “the (male) child” in Spanish, is actually part of a larger periodic cycle that affects ocean currents around the equator in the Pacific Ocean.  This cycle is called the El Niño Southern Oscillation, or ENSO for short.  Before explaining ENSO, though, let’s look at how the equatorial Pacific Ocean usually works.  A good place to start is the trade winds.  In Hawai’i we know that the trade winds generally blow from the northeast, and are generally very reliable in the summer.  These winds provide a nice breeze and keep the weather pleasant most of the time.  The trade winds are part of a pattern of atmospheric circulation called the Hadley Cell circulation, which we’ve explained elsewhere.  These winds flow across the Pacific towards the equator, and because of the Coriolis effect (from the rotation of the earth), they also blow towards the west.  Related to this is another pattern called the Walker circulation, which causes the prevailing winds to blow towards the west along the equator.  This is because the air pressure is normally high around Tahiti due to sinking air.  The wind always blows out of high pressure areas.  In contrast, by Australia and Indonesia the air pressure is low, which means that air is rising up through the troposphere.  Wind always blows into low pressure areas.  The wind pushes the water towards the west, and so the warm equatorial water tends to pool in the western Pacific around Southeast Asia and Australia.  These are the normal conditions, as you can see in the diagram.  There are some interesting aspects to this circulation pattern.  For example, it causes the thermocline to be deeper in the western Pacific.  It also makes sea level slightly higher in the western Pacific.  When this pattern is particularly strong we call it “La Niña”, which in Spanish means “the (girl) child”.  La Niña conditions tend to decrease rainfall in the eastern Pacific in places like southern California.

As you can see from the maps in the above paragraph, this means that the water temperature is higher in the western Pacific.  This means that there is a lot of evaporation, which provides moisture to the equatorial regions of South and Southeast Asia.  At the same time, the water is much cooler off the coast of South America.  Since the wind normally blows to the west, this draws water away from South America, which in turn pulls up cold water from the depths below.  This nutrient-rich water creates the richest fishing area on the planet.

Graphic from Windows to the Universe.

Reversing the Walker Circulation

Every now again, however, the westward flow of the Walker Circulation weakens or even shuts down all together.  On average, this happens about every 2-8 years.  When the winds weaken, the warm water that has pooled in the western Pacific starts to move back towards the east (1), since the wind is no longer pushing it.  The difference in air pressure between Tahiti and Indonesia is much less in this case, and in some instances the trade winds will actually blow in the opposite direction.  The term “El Niño” refers to these conditions.  El Niño has significant effects not only on ocean temperatures, but wind and weather throughout the Pacific and beyond.  Areas in the eastern Pacific get abnormally wet, whereas the western Pacific is drier.  For example, the strong El Niño in 1998 caused widespread storms across the western part of the United States.

This has indirect effects on humans, as it can lead to droughts in India, China, and other places in Asia.  Historically El Niño conditions have contributed to some of the most devastating famines in history (2).  There are economic effects as well.  For example, when El Niño occurs it shuts down the cold water upwelling off the coast of Peru that plays such a large role in the fisheries there.  The fisheries industry is extremely hard hit when this happens, which in turn affects the entire economy of Peru.  At the same time, the Polynesian wayfinders that initially discovered and settled in Hawai’i and other islands in the Pacific had a keen understanding of El Niño.  The periodic reversal of the trade winds allowed them to expand to islands and archipelagoes that would have been much more difficult to reach under normal conditions.

El Niño’s Impacts on Hawai’i

Now that we understand the basics of El Niño, we can begin to look at how this curious phenomenon affects the weather here in Hawai’i.  Both La Niña and El Niño seem to affect our rainfall patterns.  As you know, Hawai’i has two seasons: the dry Kau season, which runs from about May to September or October, and the wetter Ho’oilo season, which lasts from October to May.  When there is a particularly strong La Niña, the Kau season tends to be significantly wetter, whereas the El Niño phase generally correlates with much lower rainfall totals (and even drought) during the Ho’oilo season.  In addition, during La Niña the normal wet season is often abnormally wet.  For example, in early 2006 Hawai’i experienced 40 straight days of rain during the La Niña wet season.  You can see the pattern illustrated in the rainfall maps of Maui and Kahoolawe.  Since this year is an El Niño year, many experts are expecting drier than average conditions starting in around October.  However, as we were writing this blog it was still uncertain as to how strong the 2014 El Niño would turn out to be.

Climatologists and other researchers at the University of Hawai’i are currently carrying out work to deepen our understanding of ENSO’s effects on the archipelago.  For example, one research project is focusing on how microclimatic variables such as solar radiation, relative humidity, temperature and potential evapotrasporation respond to ENSO phase changes in different seasons here.  Other work focuses on quantifying the effects of ENSO on the dry season, since the dry season is often downplayed as ENSO is generally discussed as a wet season phenomenon in the context of Hawai’i.

Although we understand the basics of ENSO, looking into the future there is still a lot of uncertainty surrounding the phenomenon.  One big unknown is how the ENSO cycle will be affected by climate change.  It is likely that one or more of the physical processes that are responsible for determining the characteristics of ENSO will be modified by climate change, but it isn’t yet possible to reliable speculate as to whether ENSO activity will be enhanced or dampened, or if the frequency of events will change.  Thus there are exciting frontiers of climate research that you might someday contribute to as you continue your studies of geography and atmospheric processes.


(1)  This reverse flow is called a “Kelvin wave”.

(2)  For an outstanding account of how El Niño, coupled with colonial administrative policies, contributed to famines in 19th century, see Mike Davis’s Late Victorian