El Nino and La Nina

Differences between El Nino and La Nina: Description

ENSO cycle, which stands for El Nino-Southern Oscillation cycle is a three phase natural cycle consisting of El Nino which is the warm phase, a neutral phase and La Nina which is the cold phase. They may last as long as a couple of years and usually happen every 3-5 years. They happens in central-eastern Pacific Ocean. They usually form in the summer, get strongest around December and then wither away between May and July of the next year. Both El-Nino and La-Nina are phenomena are deviations from normal weather patterns. Meteorologists and climatologists are still studying El Nino and La Nina. Though both begin with either weakening or strengthening surface winds in the Pacific, what causes the change in wind is still unknown.

 

Differences between El Nino and La Nina: Winds

During an El Nino event, the winds blowing east to west are not as strong as normal, so a lot of that warmer water gets left in the eastern part of the ocean. As the water gets warmer, it weakens the winds even more, which then leaves more warm water in the eastern part of the Pacific, which then weakens the winds even more and we get a self-perpetuating cycle. The opposite happens in La Nina.

Differences between El Nino and La Nina: Ocean Currents

Under El Nino the atmospheric and oceanic conditions are such that the ocean currents cause a movement of warmer water to the eastern Pacific Ocean.El Nino causes rising motion in the Pacific, and sinking motion in the Atlantic. La Nina causes sinking motion in the Pacific, and rising motion in the Atlantic.

Differences between El Nino and La Nina: Surface Heating

El Nino phenomenon usually occurs when the surface of the ocean heats up by more than half a degree Celsius above normal. La Nina on the other hand occurs when the reverse happens; that is the surface temperature of the ocean falls half a degree Celsius below normal.

Differences between El Nino and La Nina: Upwelling and Down-welling

El-Nino prevents water from deep within the ocean which is rich in nutrients to come up. This causes the fish in the ocean to migrate further south. La Nina on the other hand is the outcome of diametrically opposite conditions. It occurs when warm water builds up in the west part of the Pacific Ocean

Differences between El Nino and La Nina: Frequency

El Nino is the more prevalent phenomenon of the two whereas La Nina makes an appearance every few years.

Differences between El Nino and La Nina: Kelvin and Rossby waves

As El Nino grows large, it creates Rossby waves. These are similar to tidal waves, where large amounts of water move in the same direction toward land. In a Rossby wave, the top part of this massive ocean wave moves really slowly in one direction and the bottom part of the wave moves in the other direction. It takes months for Rossby waves to reach shore, and when they do, they get bounced back towards the ocean again. This slow-moving return wave from shore is called a Kelvin wave, and it brings enough cool water with it to cancel out the warming from the El Nino event. If enough cool water tags along with the Kelvin wave, we may get a La Nina event. For this reason, a La Nina event often follows an El Nino, but this isn’t always true. Though we tend to think of El Nino and La Nina as brother and sister, La Nina can occur independently. Because both events tend to build to their peaks during the winter months, this is usually when we see the greatest effects.

El Nino and La Nina: Impact on North America

The effects of El Nino are strongest in the winter, and cause different weather conditions across the United States.  All of these effects can be felt year round because El Nino can last anywhere from 9 to 12 months, but the strongest consequences are seen in the winter months, and can linger into early spring.

El Nino and La Nina: Impact on India

The Indian monsoon which affects India as well as other regions of south and south-east Asia and Australia is directly linked to the El Nino Southern Oscillation (ENSO) phenomenon. In summer months, temperatures over much of India rise to as high as 45 degrees Fahrenheit while the Indian Ocean is much cooler. Consequently, the warm air over the land rises and cooler moisture-bearing air blows in from the sea, bringing heavy rains to the region.  ENSO-induced warm zones in the Pacific cause the warm air over them to rise and initiate circulation cells. Such cells could have their downdraft sides over a nascent monsoon circulation cell in the Indian Ocean, which would disrupt its formation, causing poor monsoon rains over the subcontinent. This Indian monsoon model thus implies that El Nino years will see a weak monsoon and draught like conditions. La Nina has the opposite impact on Indian monsoon and usually results in better than normal monsoon in India.

 

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