WHAT IS SOUTHERN OSCILLATION ? HOW DOES IT AFFECT THE INDIAN MONSOON?

Southern Oscillation refers to the shifting of the surface air pressure between the tropical eastern Pacific and eastern Indian Ocean. The eastern Pacific Ocean experiences high pressure when the Indian Ocean experiences low pressure. During certain years, the pressure conditions are reversed and the Indian Ocean has a higher pressure in comparison to the Indian Ocean. This phenomenon is known as the Southern Oscillation. The Index of Southern Oscillation is computed from fluctuations in the surface air pressure between Tahiti and Darwin.

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Southern Oscillation refers to the shifting of the surface air pressure between the tropical eastern Pacific and eastern Indian Ocean. The eastern Pacific Ocean experiences high pressure when the Indian Ocean experiences low pressure. During certain years, the pressure conditions are reversed and the Indian Ocean has a higher pressure in comparison to the Indian Ocean. This phenomenon is known as the Southern Oscillation. The Index of Southern Oscillation is computed from fluctuations in the surface air pressure between Tahiti and Darwin.

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ENSO (El Niño Southern Oscillation) is a phenomenon comprised of the continued interaction between the oceans and the atmosphere. This interaction is so extreme that ENSO is the most dominant source of weather and climate variability worldwide. The impacts of ENSO on climate variability include, but are not limited to, increased rainfall, drought, monsoons, and other natural disasters.

During El Nino years the rain area that is usually centered over Indonesia moves eastward in to Central Pacific, resulting in a change in air currents. Although India has not typically fallen within the direct zone of impact of these phenomena, the increase in ENSO climate variability is predicted to have a more direct and intense effect on India’s monsoon season.

The graph below shows the increase in sea surface temperature (SST) anomalies after 1976. This demonstrates the increase in ENSO climate variability in the regions surrounding India. These changes in ENSO climate variability forecast more extreme variations in climate, including more extreme and variable wind patterns and thus more erratic and unpredictable monsoons. 

 

 

 

 

 

 

The amplitude of the cyclical changes of the Indian monsoon and ENSO appear to increase or decrease simultaneously and follow similar patterns of oscillation over periods of multiple decades. This implies a corellation, if not a causality, between the two.

 

 

Image: Year-On-Year Rain Change Index -- This NASA-created index map condenses 5 years of rainfall data from 1998 to 2003. The colored areas show annual changes in rainfall. Areas in blue receive more rain whenever areas in red experience a shortfall, and vice versa. The year-on-year change in rain is smallest in the green areas. The global pattern confirms that El Nino is the second biggest factor in changing how and where rain falls around the world. Credit: TRMM/NASA/JAX

In El Nino years, the monsoon is mostly cancelled out by drought-like conditions in India, caused by the weakening of normal Hadley Cell circulation. In La Nina years, on the other hand, the monsoon cycle is intensified by weather conditions resulting from the strengthening of the normal Hadley cell circulation. The Hadley Cell circulation pattern is associated with the buildup of warm water in the western pacific leaking past Indonesia to slightly warm the Indian Ocean, and because of the Clausius Clayperon condition, which states that warm air can hold warm water, this intensifies the monsoon rains. According to recent research, the 132 year historical record shows that whenever India experiences a severe drought, it has always been during an El Nino event (Bates and others 2006). While this correlation can lead us to some conclusions about t he coupling of these two processes, it must be noted that not all El Nino events are accompanied by Indian droughts. The correlation between La Nina events and the Indian drought is much stronger. 

For India’s population, climate is a deciding factor in the subsistence of everyday life. Because the economy is so reliant on agricultural success, even slight setbacks in agricultural production due to unforeseen climate anomalies have dire economic and social consequences. The monsoon rains, which usually occur from June to September, have a crucial impact on crop abundance. Because of this dependence, it is essential to be able to predict monsoons in a timely and accurate manner to give the people of India a chance to prepare for and possibly mitigate the detrimental effects of these climate anomalies. More accurate prediction give the Indian government the ability to structure monetary and welfare policy changes in accordance with the upcoming agricultural situation. In addition, a better understanding of these anomalous occurrences will allow for the development of infrastructure to protect the economic and social livelihoods the people of India and mitigate the harsh potential effects of these weather patterns. Therefore, a better understanding of sources of correlation and/or causation between ENSO and the monsoon cycle will surely prove invaluable to India’s population because it will allow them to predict and prepare.

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