El Niño and La Niña are opposite phases of what is known as the El Niño-Southern Oscillation (ENSO) cycle. The ENSO cycle is a scientific term that gives the detailed account of the variations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific (approximately between the International Date Line and 120 degrees West). La Niña is sometimes referred to as the cold phase of ENSO and El Niño as the warm phase of ENSO. These deviations from normal surface temperatures can have large-scale impacts not only on ocean processes, but also on global weather and climate (What are El Niño and La Niña, 2016).
The term El Niño means ...view middle of the document...
El Niño and La Niña events are a natural part of the global climate system. They occur when the Pacific Ocean and the atmosphere above it change from their normal state for several seasons. El Niño events are associated with a warming of the central and eastern tropical Pacific, while La Niña events are the reverse, with a sustained cooling of these same areas (What are El Niño and La Niña events?).
These changes in the Pacific Ocean and its overlying atmosphere occur in a cycle known as the El Niño–Southern Oscillation (ENSO). The atmosphere and ocean interact, strengthening each other and creating a 'feedback loop' which develops small changes in the condition of the ocean into an ENSO event. When it is clear that the ocean and atmosphere are fully connected an ENSO event is considered established. Even in a neutral state, temperatures in the Pacific Ocean vary from east to west – for example, the western Pacific 'warm pool' in the tropical Pacific has some of the warmest large-scale ocean temperatures in the world. During an ENSO event, ocean temperatures become warmer than usual or cooler than usual at different locations, which are reflected in ocean temperature gradients. The most important driver of ENSO is these temperature gradients across the Pacific, both at the surface and below the surface, particularly at the thermocline (Glantz, 2001:31)
Conditions for development of El Niño and La Nina
Under normal conditions, deep warm water in the western Pacific produces a low pressure region with heavy storm activity, while the eastern Pacific is a dry, high pressure area with shallow warm water. Surface winds blow from east to west, while upper winds blow from west to east. The thermocline is deeper, and sea level higher, in the western Pacific than in the east.
El Nino develops when trade winds become weaker over the central and western Pacific Ocean. It results in dramatic rise of surface temperature of the ocean and depression of the thermocline in the eastern Pacific Ocean. Such condition develops at the end of calendar year along the coasts of Ecuador and northern Peru. La Nina the direct effects consist of a strengthening easterly trade wind across the central Pacific and a cooling of the surface waters in the eastern and central Pacific basin (saha,2012:163).
For La Nina, the easterly trade winds strengthen. This blows more warm water west, and allows cold water below the ocean's surface to push towards the top near the South American coast to replace the warm water. In an El Nino, the opposite occurs. The easterly trade winds become weaker, and can even reverse direction. The warm Pacific Ocean becomes nearly stationary or pushes eastward and gains heat. The warm water builds up along the equator in the eastern Pacific. The warm ocean surface warms the atmosphere, which allows moisture-rich air to rise and develop into rainstorms(saha,2012:163).
Their frequency and times of occurrence
When do El Niño and La Niña...