Is Pacific warming climate chaos?

Photo source: Wildfire in 2015, NASA 

Have you read the recent news about heatwave in US, bushfire conditions in East Australia or the flood in Mumbai? All these kinds of extreme events are often happening worldwide and many people think that the sole reason is due to climate change but actually, there are several climate drivers contributing to such devastating phenomenon in an always-changing atmosphere.

This blog will review what I believe is one of the major climate driver, El Nino. So what is El Nino? It is what causing sea surface temperature rise periodically and bringing huge effect on air and moisture movement the globe.

Image source: Bureau of Meteorology, Australia

Basically, El Nino is an extensive ocean warming occurs in the equatorial Pacific Ocean around Christmas. When the average Sea Surface Temperature (SST) anomaly greater than 0.4°C for at least six consecutive months in Nino 3.4 region, it is defined as El Nino. We call this standard of measure as the Southern Oscillation Index (SOI), a consistently negative SOI indicates the presence of El Nino.

The image above shows an El Nino event recorded in 2006. Warm sea surface temperature (Red colour) covered from the coast of Peru to the central tropical Pacific.

Image source: Bureau of Meteorology, Australia


What is Nino 3.4?

The tropical Pacific Ocean has been divided into a number of regions. The scientists observe SST anomaly in the area bounded by 5oN to 5oS and 170oW to 120oW, called Nino 3.4 index.

Image source: NOAA, United States of America.


Is every El Nino the same? YES, BUT NO

Although the formation process of every El Nino are same, each El Nino event has its own personality and different in strength and duration. It is a reoccurrence natural phenomenon occurs at irregular intervals of 2-7 years. The weather pattern usually returns to normal after a period of time. 2015-2016 is one of the three strongest El Nino year on record.


How does it happen? Why Northern Australia getting drier under El Nino?

Image source: NOAA, United States of America.

The normal weather pattern in Equatorial Pacific

Under normal situation, trade winds blow across the Equatorial Pacific from the east to the west, and pushing the warm sea surface water to move westwards. This causes western tropical Pacific to have warmer sea surface and higher sea level height than the eastern tropical Pacific.

This sea surface temperature difference across the Pacific causes air to rise to Australia’s North and descend near Peruvian coast. This regional scale air motion cycle called “Walker Circulation”. When warm moist air ascends over the west, it cools and condenses to form rainfall.  In contrast, the sinking motion in East Pacific creates a region of high surface air pressure, and brings a relative stable and cooling condition to the coast of Peru.

Image source: NOAA, United States of America.

El Nino Event in Equatorial Pacific

During El Nino period, air pressure over the west is higher than central tropical Pacific Ocean. This change in air pressure gradient weakens the easterly trade winds, or even reverses the wind direction. The normal westward flow of equatorial sea water is slower, or even pause. Warmer water drift back to the east.  The change in the ocean temperature pattern and sea surface pressure between the east and west alters the pattern of air circulation. With low surface pressure shifting east, the condensation and precipitation follows.  Precipitation in Australia is smaller than usual, hence a drier weather.


What is the impact of El Nino?


Data Source extracted from NOAA, U.S and Bureau of Meteorology, Australia.

There is more, but the message is clear:  The impact of El Nino is not simply a climatic issue, it also affects agriculture, industrial production, public health, tourism and livelihood at local and global scale. It is important for us mankind to keeping the eye on it. The impacts will emerge during the developing stage or mature stage of the El Nino. You may want to check out ‘ENSO Wrap-up’ for the current status of El Nino.

For more basic information on El Nino, please visit the following National Geographic’s video:

For more detailed on the specific analysis of El Nino events in the last century, please check: