Introduction to El-Nino and La Nina
The terms El Nino and La Nina refer to Pacific Ocean weather phenomenon that oscillates temperature back and forth from warm temperature to cooler and vice versa. El-Nino refers to warming of the central and eastern tropical Pacific, whereas La Nina is the reverse phenomenon, where the trade winds pick up speed and warm water in the eastern Pacific moves towards western side of the Pacific. El Niño and La Nina events last nine to twelve months, but at times can extend for years. However, on average, El Nino and La Nina events occur every two to seven years. The combined effect of both El Nino and La Nina generate weather changes all across the globe also known as “El-Nino Southern Oscillation (ENSO)”. The interaction of ocean currents and atmosphere results in the formation of a “feedback loop” which intensifies little changes in the condition of the sea into an ENSO event. ENSO is established when there is complete bonding of ocean and atmosphere. In short, the most important driver of ENSO is temperature variation.
Phases of ENSO
The ENSO cycle is completed in three different phases: neutral phase, El Nino phase and La Nina phase.
Neutral Phase of ENSO
In the neutral phase, the trade winds blow from eastern Pacific Ocean to the western Pacific carrying warm surface water and moist air with it. The central Pacific remains relatively cool as warm water is pushed by the trade winds westwards. In the neutral phase, the process of atmospheric convection occurs where the warm air over the west Pacific rises into the atmosphere that causes the formation of cumulonimbus clouds, carrying rain. After it rains, the dry air moves east and descends over the cooler eastern Pacific. The process of air rising over the west Pacific and falling over the east Pacific and westward air movement over the ocean surface is known as Walker Circulation.
During the El Nino phase, trade winds are weak or may reverse. In this phase, trade winds are not powerful enough to push the ocean currents westwards. The warm surface water shifts to central and eastern Pacific Ocean, resultantly the western Pacific Ocean becomes cooler than normal. The process of atmospheric convection discussed in the neutral phase migrates from the west to central and eastern tropical Pacific Ocean resulting in increased rainfall in parts of the eastern Pacific and western American continent, whereas Australia and Asia receive relatively less rain fall.
La Nina Phase
La Nina phase, although it appears similar to the Neutral Phase, differs in intensity and effects. Trade winds are strong in this phase as compared to pre-El Nino phase. The strong trade winds blow from the east Pacific Ocean towards the west, intensifying the Walker Circulation process and greater atmospheric convection over this region. “Sea surface temperature (SST)” over central and eastern tropical Pacific Ocean becomes cooler than normal, causing the thermocline to raise and move closer to the surface i.e. cold water of deep ocean water rises to the surface. During La Nina event, atmospheric convection over western Pacific increases because of the stronger winds that provide moisture, thus, intensifying Walker Circulation.
Currently, the world is experiencing drastic climate change and when combined with the phenomenon of ENSO, rapid and fluctuating changes in the oceanic atmosphere becomes more damaging. Therefore, policy makers and disaster managers need to pay proper attention to the impact of both climate change and ENSO. It is vital to keep track of the changes in weather patterns, develop long-term forecasts that could make regions susceptible to the consequences resulting from the slightest deviations in rainfall and temperature as compared to what has normally been a regular pattern.
ENSO in South Asia
“El Niño Southern Oscillation” (ENSO) has probably added to the worst human calamities on the planet in the shape of dry spells and floods. In terms of economic impact, around 10 to 20 percent deviation in world GDP growth and consumer price indices occurs due to ENSO.
The South Asian Monsoon phenomenon identified that the Indian Monsoon Rainfall (IMR) has a significant effect on South Asian countries, including Pakistan, Bangladesh, Nepal, India and Sri Lanka and it has been influenced by ENSO. ENSO phasing strongly affects the inter-annual and inter-decadal variability of seasonal rainfall over the subcontinent. It has been observed that ENSO adversely affects the South Asian Indian summer monsoon and the subcontinent receives relatively less rainfall during ENSO monsoon years.
The raising unpredictability regarding beginning and time period of monsoon exercises an enormous impact on agriculture, water resources, economics, ecosystems, and human security across South Asia. South Asian countries are largely agrarian societies, therefore, the fate of these countries is dependent upon summer monsoon rainfall and any disruption in monsoon pattern can cause grave consequences. The “Intergovernmental Panel on Climate Change” has stated that “with the passage of time drought-hit areas might grow consistently, and heavy precipitation events would likely rise in intensity as a manifestation of impacts of climate change on fresh aquatic systems”. It is important to investigate how increasing temperature impacts monsoon patterns.
ENSO in Pakistan
Direction of Winds
Monsoon enters Pakistan from two different sides. First, the south eastern wind that travels from the Bay of Bengal that enters Pakistan travelling through India along the foot hills of the Himalayas. Jhelum, Lahore, Rawalpindi and Sialkot districts receive the first rains of monsoon in Pakistan. These areas are included in the “northern monsoon belt” – the main monsoon occurring areas in the country. Second pathway is through Arabian Sea where the moist rain carrying south western winds enter Pakistan. The south eastern region of Pakistan receives rainfall from these wind currents. Long-term studies for analyzing trends related to monsoon in Pakistan are quite limited, requiring greater attention. Analysis of the geographic zones of Pakistan highlights the decreasing monsoon precipitation trends after the 1970s.
In1998, an El Nino event was recorded in Pakistan as a result of which the country witnessed heavy snowfall on its mountains followed by four years of drought in lower Punjab, Khyber Pakhtunkhwa and Balochistan. Another milder event of El Nino was traced in 2009 that resulted in droughts followed by the devastating floods of 2010.
Tracing the Roots of ENSO
Pakistan is among the top ten countries of the world most vulnerable to climate change. Pakistan is sensitive to both change in precipitation and temperature. As per the latest seasonal forecast, Pakistan is expected to experience an extended winter season in 2021. According to the forecast, most parts in Pakistan would be subject to low temperature till mid-March due to La Nina effects. These changes, in turn, could increase forest, water resources and agriculture sector vulnerabilities in the country. The adverse impacts of El Nino and La Nina will include:
Changes in Summer Monsoon Rainfall
The summer monsoon in Pakistan is adversely impacted by ENSO event. Due to the low intensity of cyclogenesis over the Bay of Bengal during ENSO, the monsoon system is weak and it dissipates before reaching Pakistan, thus, the country receives less than normal monsoon rainfall during ENSO monsoon years. Whereas, La Nina, causes higher than normal rainfall due to strong cyclogenesis activity in the Bay of Bengal as sea surface temperature rises. As discussed earlier, the drought of 2009 and the subsequent devastating floods of 2010 in Pakistan happened due to El Nino event in 2009.
Increasing Natural Disasters
As discussed above, during El Nino, Pakistan receives less than normal rainfall, which can lead to scarcity of water creating drought-like conditions in a country where there is already shortage of water. The negative impact of ENSO can be highlighted in terms of enhanced de-glaciations, changing rainfall trends leading to surface runoff, landslides, soil erosion, biodiversity loss and avalanches, etc.
Lower Crop Yield
According to the Food and Agriculture Organization, the La Nina may affect the quality and yield of wheat crops, especially in the rain-fed areas of Pakistan. La Nina could hamper the production of the country’s staple food (wheat) for the year 2021. Moreover, limited snowfall during winter in northern Pakistan could decrease the irrigation supplies for the season of spring during which water is naturally obtained from snow melt. Given the current scenario of rising wheat flour prices, further price hike is expected if production falls short due to lack of proper irrigation, thereby adding to the misery of people due to further inflation in the prices of basic commodities.
Decrease in Water Resources
During El Nino year, Pakistan receives less than normal rainfall which also depletes its water reserves. To meet the water demand of farmers to irrigate their fields, dam water is used during less than normal rainfall years. Moreover, the two large water reservoirs i.e. Tarbela and Mangla are used for electricity generation, therefore, when there is less rainfall, water reserves of Pakistan deplete as these reservoirs do not receive enough water. Whereas, on the other hand, during La Nina, Pakistan receives more the normal rainfall, but, the country’s water storage capacity is limited as no major water reservoir has been constructed since the completion of Tarbela Dam in 1976 and the excess water goes into the Arabian Sea without being stored for future use.
Pakistan is among the countries that is enormously affected by ENSO. Therefore, the Meteorological Department of Pakistan should have state-of-the-art facilities that can be used to better monitor and predict weather patterns. Further, ENSO also adversely affects our economy which is largely dependent upon agricultural produce. Therefore, to mitigate the adverse effects of ENSO and to adapt to changing weather phenomenon, forecasting and broadcasting ENSO-related information for the agriculture department/farmers could help in the better selection of crops and better yield. This will not only avert crops destruction, but may also lead to additional production. Secondly, construction of big water reservoirs is a matter of survival for Pakistan. Pakistan loses large quantities of water during the monsoon season, which can otherwise be used during low rainfall or drought times. Furthermore, infrastructure and agricultural damage can be prevented by constructing dams. In Pakistan, due to the declining surface and underground water resources, we cannot afford drought-like situation due to ENSO which can be averted through the construction of dams.
Lastly, most countries include ENSO as a variable while formulating their short/long- term economic policies, hence, Pakistan’s policy makers should also consider including ENSO as an effecting factor while formulating economic, agriculture and developmental policies.
Zuhaib Anwar works as a researcher at the Centre for Aerospace & Security Studies (CASS), Islamabad, Pakistan. He can be reached at email@example.com
Image Source: Saeed, F. 2016, “El Niño: A looming disaster.” The Third Pole, 2 March. https://www.thethirdpole.net/en/climate/el-nino-a-looming-disaster/