Guest Writer Feature: The El Niño Southern Oscillation (ENSO) by Aldrin Gabuya


"You got the temperature rising like El Niño."

Remember this line from the old hit song by Tata Young? It actually describes the feeling in many regions of the world at this time, mostly in the mid-latitudes, because it is summer in the Northern Hemisphere. But have you ever wondered why we've been observing unusual weather phenomena across different parts of the world lately, such as hotter summers and fewer rainy days? 

[Photo credit: Karlos Manlupig, Greenpeace website]

What exactly happens during El Niño? 

Aside from the speculated climate change primarily caused by the immense amount of greenhouse gases present in the atmosphere, another phenomenon is greatly affecting global weather patterns: the El Niño Southern Oscillation, or ENSO. It is a quasi-periodic weather pattern associated with the anomalous fluctuations of sea and atmospheric temperatures across the equatorial Pacific Ocean, from the International Date Line to the South American Coast.

It has an irregular transitional period of 2-7 years on average per phase. The Spanish term El Niño (meaning "the little child") originated from the fisherman and coastal residents of Peru (a country with highly productive fisheries), who first observed the peculiar warming of the ocean which peaks around Christmas time on certain years. 

ENSO events often peak during the winter months in the Northern Hemisphere (December - February) and might extend up to the summer months depending on its strength.

Before we go deeper into it, we should know first how the typical weather pattern in the equatorial Pacific Ocean works. Air in the tropics, known as the trade winds or doldrums, regularly blow from east to west caused by the combination of the effect of the movement of air between pressure differences (from high to low) and the Earth's rotation (Coriolis Effect). The trade winds are responsible for piling up warm water from the Central Pacific to the Western Pacific, where most convective activity occurs. 

As the warm water piles up in the Western Pacific, it pushes the colder water underneath and is brought to the Central Pacific in a process called upwelling. Hence, the air over warm waters rises from the equator and sinks as it encounters cooler air in the mid-latitudes, bringing the air back to the equator, and creating an atmospheric circulation known as the Walker Circulation. This governs weather patterns all over the globe.


Diagram depicting the Walker circulation in the Pacific Ocean on an ENSO-neutral event.
[Photo credit: NOAA Pacific Marine Environmental Laboratory website]


ENSO has two main phases: the warm phase, El Niño, and the cold phase, known as La Niña. During an El Niño event, there is an increase of ocean-atmosphere coupled temperatures over the Central to Eastern Pacific, and a weakening of the prevailing trade winds which force the warm water to draw back in the Central Pacific. This causes a shift in the position of the Walker Circulation and the convergence zones. This causes the drier weather in areas in the Western Pacific, and wetter conditions in the Central and Eastern Pacific.

On the other hand, La Niña (Spanish for "young girl") is the exact opposite of El Niño, and is associated with the cooling of ocean-atmosphere coupled temperatures. In turn, prevailing trade winds strengthen and push more warm water into the Western Pacific. From there, the position of the convergence zones get extended westward to the Australia-Asia region, causing wetter conditions along regions in the Western Pacific and drier conditions in the Central and Eastern Pacific.



Diagrams depicting the atmospheric circulation during El Niño and La Niña events.
[Photo credit: NOAA Pacific Marine Environmental Laboratory website]


How do we know if there is an upcoming El Niño or La Niña event? The National Oceanic and Atmospheric Administration (NOAA), a prominent global weather agency, keeps track of the conditions of the oceans and atmosphere all over the world. The agency provides numerous weather models and data archives derived from observations depicting current and possible future conditions. They also make forecasts, which are then made available to the public.

NOAA scientists defined an upcoming El Niño as having ocean-atmosphere coupled temperatures to be warmer than normal (0.5˚C or above) for at least three consecutive months on the ENSO 3.4 region. 

On the other hand, for an upcoming La Niña, the ocean-atmosphere coupled temperatures must be cooler than normal (0.5˚C or above) for at least three consecutive months. The strength of an ENSO event depends on how much of an increase or decrease is observed from the normal temperature. 

For a weak ENSO event, the temperature anomaly should be at 0.5˚C or greater but less than 1.0˚C. A moderate one involves a temperature anomaly of 1.0˚C or higher but less than 1.5˚C, and a strong one has an anomaly of 1.5˚C or larger. Other meteorological agencies have their own standards in monitoring and forecasting ENSO events, such as in Australia, where they use an anomaly of 0.8˚C.

However, our current knowledge of the root cause/s of the mechanism of ENSO is not yet fully understood. 



A map showing the different ENSO regions used in monitoring ENSO events.
[Photo credit: NOAA website]



Graph showing historical sea surface temperature (SST) anomaly fluctuations from 1950-present used in keeping track of ENSO events
[Photo credit: NOAA website]


So how does this impact the climate of the Philippines?  

ENSO has a significant effect on our climate. When El Niño occurs, the country experiences prolonged dry conditions, receiving less rainfall than normal. In general, the country experiences much higher temperatures than we normally do. The onset of the Southwest Monsoon is also delayed; we still expect the normal number of tropical cyclones but of stronger intensities and of irregular movement, having its track displaced to the north. 

Prolonged dry conditions also increase the probability of severe drought over plain fields, and on rare occasions, forest fires. These can greatly impact our agricultural sector. Water and power shortages are other consequences, due to less rainfall filling up water dams and other bodies of water supplying water and hydroelectric power. In contrast, we receive more rainfall during a La Niña event. More convective activity is present over the Western Pacific, triggering more thunderstorms and tropical cyclones. Some of its major consequences include frequent flash floods and landslides. 



A map featuring the impacts of El Niño in different parts of the world during winter (December - February) and summer solstice (June - August).
[Photo credit: NOAA website]


A map featuring the impacts of La Niña in different parts of the world during winter (December - February) and summer solstice (June - August).
[Photo credit: NOAA website]

The intensity of its respective impacts also depends on the strength of the occurring ENSO phase itself. The strongest ENSO events on record happened in 1982-1983 and in 1997-1998 with temperature anomalies of +2.2˚C and +2.4˚C, respectively. Meanwhile, the strongest La Niña events occurred in 1973-1974 and mid 1998-early 2001 with temperature anomalies of -2.0˚C and -1.7˚C respectively. 

How will these events affect us locally in the coming months? 

According to the latest outlook of NOAA, the emergence of another strong El Niño event is more likely (around 90% chance) to take place from the Northern Hemisphere's winter months of 2015 and might extend to the early spring months of 2016. The DOST-PAGASA has warned the public about the upcoming El Niño event regarding its effects in the following months. Hence, we'll likely be expecting a warm winter season later this year and a much hotter summer next year.

For more information about the imminent El Niño event and ENSO in general, and also for the latest weather information in the Philippines, check out the official websites of DOST-PAGASA and the NOAA-CPC. Stay safe, and be a weather watcher!



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REFERENCES:

1. University Corporation on Atmospheric Research. El Niño, La Niña & ENSO FAQ. Retrieved from: http://www2.ucar.edu/news/backgrounders/el-nino-la-nina-enso#4
2. Dela Cruz, G. (November 8, 2014). "How El Niño could affect the Philippines in 2015. Rappler News. Retrieved from: http://www.rappler.com/science-nature/environment/73542-prepare-drought-less-rainfall-2015.
3. PAGASA. El Niño Watch. Retrieved from: http://kidlat.pagasa.dost.gov.ph/index.php/floods/general-flood-advisories/85-climatology-and-agrometeorology/climate-others/714-el-nino-watch
4. Climate Prediction Center. (August 13, 2015). "El Niño/Southern Oscillation (ENSO) Diagnostic Discussion." Retrieved from: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html
5. De Guzman, K. (August 19, 2015). "PAGASA: Strong El Niño affecting the country." Retrieved from CNN Philippines website: http://cnnphilippines.com/news/2015/08/18/PAGASA-strong-el-ni%C3%B1o.html


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