El Niño-Southern Oscillation and PNG

Nari, Normal

The National, Tuesday December 22nd, 2015

 SOME provinces received rains (despite amount not universal) in the past month, following the nation-wide El Nino-induced drought. Extremes of rainfall in PNG are closely associated with the El Niño-Southern Oscillation (ENSO) phenomenon. El Nino induced droughts are brought on by ENSO. This is a brief on ENSO, its effects on PNG and how people can manage.


What is ENSO?

ENSO stands for El Nino Southern Oscillation. El Nino is the ocean component of ENSO. It is observed when there are major sea surface temperatures (SSTs) changes in the tropical Pacific. The term El Nino originally applied to an annual weak warm current that ran southwards along the coast of Peru and Ecuador in South America about Christmas-time, hence Nino which means Christ-child or the boy. It subsequently became associated with the occasional unusually large warmings, “El Nino events”, that occur every few years with catastrophical local and regional effects. This coastal warming has often been associated with extensive unusual warming of the ocean all across the tropical Pacific basin to about the dateline. Hence, it has been increasingly regarded as similar to the Southern Oscillation (SO).

The SO is principally a see-saw in the atmospheric mass involving exchanges of air between eastern and western hemispheres centred in tropical and subtropical latitudes with centres of action located over Indonesia and the tropical South Pacific Ocean. The centres of action are linked by a zonal east-west circulation along the equator, with rising motion in the western Pacific and sinking motion in the east called the Walker cycle (refer to Figure 1). Normally, warm air rising in the western Pacific (PNG side) travels to the east where it sinks. In short, SO refers to the movement of sea surface temperature, air pressure over sea and air circulation across the sea from one extreme to another.

Thus ENSO events are those in which both an SO extreme (pressures high at Darwin and low at Tahiti) and an EN occur together. Warming of the eastern Pacific reverses the normal east-west air circulation to a west-east motion. Westward winds that bring rain to Papua New Guinea are reversed as air moves eastward to replace warm air rising in the eastern Pacific (refer to Figure 2).

ENSO events are often indicated by SST changes and the Southern Oscillation Index (SOI). The SOI is a standardised index (number) resulting from the difference in air pressure measured between Darwin (Australia) and Tahiti, used since the 1870s. Cold air is heavier (high pressure) than warm air (low pressure). When seas near PNG are warm, warm air rises and the pressure measured at Darwin is low while that at Tahiti is high, giving a positive SOI. The reverse causes high pressure at Darwin with the SOI being negative. During an ENSO event, the SOI is constantly a negative number for several months while the SSTs around PNG are lower than normal. Severity of impact depends on how strong or weak ENSO is with reference to long-term records.


How does ENSO affect Papua New Guinea?

ENSO can bring both drought and flooding rain to PNG. During a drought, cooler drier air that carries little water (rain) descends over PNG. Cloudless days and nights are experienced with the nights being very cold, often causing frosts in higher parts of the highlands. After a drought a lot of plant cover is dried up and when the rains return, floods are experienced and exposed soil is washed away.

The country usually receive high rainfall due to its high mountain ranges which force the prevailing northwest and southeast (trade) winds to go over them. Therefore, most PNG agriculture and food production systems are adapted to deal with too much soil-water. As soils dry up, crops fail to produce food. 

In dry parts of PNG, agriculture and food production is adapted to deal with the dry conditions and so are less likely to be severely affected than the wetter parts. 

The frost that accompanies drought in the highlands damages food crops and leads to food shortages. 

Little water means sago cannot be ‘washed’ for consumption. Thus, areas depending on sago can run short. Bush fires that are often lit under dry conditions destroy bush foods and animals that can be used as food. 

Flooding destroys food gardens and access to food supplies. Various pests and diseases of crops become problems during these periods. When food is scarce, people suffer. 

An ENSO can cause water sources to dry up or contaminated during drought, often resulting in associated disease outbreaks like typhoid and diarrhoea. Services like schools and health centres close during droughts, whereas flooding destroys roads and bridges, limiting access to services. 

Lack of preservation and storage knowledge and lack of financial savings means limited food supply and buying options for the hard times. 


How can people survive an ENSO Event?

Although the effects of ENSO sound grim, there are strategies that can and have helped people to survive. Some are traditional strategies; others have been developed by NARI following the 1997/1998 El Nino-induced drought. These include short term and long- term strategies, which are available to the PNG farming communities. – NARI