The Effects of El-Nino Southern Oscillation
on Mississippi Weather


A few centuries ago, South American fisherman in Peru and Ecuador observed a change in the way currents were running along their coast. Normally, a cool current would flow north but every now and then, a warm ocean current would begin flowing south, which would wreak havoc on their fishing with the sight of dead fish littering the water and beaches. They noticed the phenomenon always seemed to arrive around Christmas time so they used the Spanish word, El Nino, which means "The Christ Child." This abnormal warming of surface ocean waters is just one part of what's referred to as the Southern Oscillation. An oscillation is a cycle that has the shape of a wave, or see-saw pattern such as highs and lows, or hot and cold temperatures. The Southern Oscillation is the pattern of reversing surface air pressure between the eastern and western tropical Pacific. When the surface pressure is high in the eastern tropical Pacific it is low in the western tropical Pacific and vice-versa. Since the ocean warming of El Nino and the pressure reversals of the Southern Oscillation usually happen together, most people refer to this phenomenon as El Nino/Southern Oscillation or ENSO for short. El Nino episodes generally occur every three to five years but the interval has varied from two to seven years. They typically last around eighteen months. Sir Gilbert Walker, while attempting to explain and predict the cycle of the monsoon rains in India, found that a correlation existed between the difference in average monthly sea level pressure in Australia and Tahiti. This difference in average pressure became the first "index" of the SO, and is still used today. During an El Nino event, the SOI has a large negative value due to the lower than average air pressure at Tahiti and higher than average pressure at Darwin, Australia.

According to some scientists, El Nino is the most powerful climate force on Earth. During El Nino years, the jet stream is shifted southward and a deep trough builds over the central United States. One effect of this pattern for the southeast United States is that the fall season tends to be colder than normal and the spring and summer seasons tend to be warmer than normal. This pattern also causes precipitation totals to be higher than normal in the winter and spring but drier than normal in the summer. A recent study determined the effects of the El Nino Phase of the Southern Oscillation upon precipitation in the Jackson, Mississippi area are erratic and thus, hard to predict (Miller.) The data also verifies that El Nino years result in cooler than average temperatures for the winter and November through April periods in Jackson.

The change in winds with height is known as vertical wind shear. Hurricanes need fairly uniform winds and low vertical wind shear to form. El Nino events generally reduce the number of hurricanes in the Atlantic Ocean during the summer and fall due to the powerful high altitude winds which increase the vertical wind shear. Basically, these winds cut the tops off of tropical storms before a healthy circulation can form. This is particularly important for people living along the Mississippi Gulf coast.

The Mississippi basin is very vulnerable to floods and droughts. The corn, soybeans, and wheat grown in the basin feed the United States and much of the world. A significant El-Nino event will ruin these crops. The Mississippi River is the sixth largest river in the world in terms of discharge and is sometimes known as the heart of the United States. The volume and power of the river can make floods catastrophic. A prime example of the basin's El Nino vulnerability is the Great Flood of 1993. The period of April to June 1993 was the wettest observed in the basin in the last 99 years. In some places nearly thirty inches of rain fell which is about twice the normal amount for the area. Many theories have been presented as to why the jet stream had shifted much farther south than normal. The National Center for Atmospheric Research and others concluded that El Nino conditions in the tropical Pacific were probably responsible for the shift.

The Mississippi River is also a major player in the transport of coal , petroleum, chemicals, and concrete. Most of these things are too large or heavy to transport economically by road. El Nino's floods can cause the rivers to swell which could destroy bridges and make navigation impossible due to increased sediment. The El Nino droughts could do exactly the opposite while the winter freezes can stop shipping on the river completely.

Mississippi has a vast ecological system including many bird species from different parts of the world that use the seasonal wetlands as temporary breeding grounds and temporary shelters. El Nino events could bring drier conditions for migrating birds which could cause them to suffer from a loss of habitat. Furthermore, climate induced increases in the Mississippi river would increase pesticides and pollutants to the Gulf of Mexico. This would worsen the oxygen starved "dead zone" that the river already causes in the gulf and affect the ecological system as well.

Drinking water supply and sewage treatment are two things you normally don't think of when dealing with weather extremes and changing climates. Estimates show that over 18 million people depend on the Mississippi system for their drinking water as the source from which treatment plants draw their raw water supply. As with the ecological system impacts, the extra pollution that results from El Nino inspired climate changes, will increase the workload that the water treatment plants have to do to make the water potable. Also, the water treatment plants are often built close to rivers and are usually deemed inoperable by floods.

To summarize, the El Nino Southern Oscillation phenomenon is a weather factor that has a great influence on so many aspects of our life, especially in Mississippi. Scientists seem to be unsure of why the intensity varies so much with El Nino but no one is skeptical of the damage a full-blown ENSO event can cause. Hopefully, in time, more information will be available and we will be able to predict and prepare for the next "big one."


Cox, J. D. (2000). Weather for Dummies. Foster City, CA: IDG Books Worldwide, Inc

Miller, S (UNK). Southern Oscillation on Jackson, MS . Retrieved April 9, 2008,
from National Weather Service Forecast Office Web site:

Sample, S. (20030122). El Nino-and what is the Southern Oscillation anyway?!.
El Nino Making Sense of Weather, from