El Nino As Ocean-Atmosphere Interaction

Ocean-atmosphere interaction is as simple has it sounds it is usually referred to has the exchange of energy at the interface of the oceanxand atmosphere whether it be physical or chemical . However, besides energy there Is also an exchange of mass including that of fresh water from rain, sea ice melting etc and of gases that diffuse into seawater such as oxygen and methane. The oceans and atmosphere store and exchange energy in the form of heat, moisture, and momentum They also absorb heat more effectively than land and ice surfaces, and also store heat more.xOceanic heat is released more slowly than on land, keeping coastal areas more temperate. Changes in the energy balance between the oceans and atmosphere play an important role in the planet's climate change. A prime example of ocean-atmosphere interaction is the natural variability of the El Nino and southern Oscillation(ENSO), one of the largest and the most well known interannual variations of the earth’s climate. The ENSO can then be subdivided into two phases the El Nino and La Nina which will be investigated further.

So, what exactly is the El Nino and the Southern Oscillation, also known as ENSO, it is the periodic fluctuation in sea surface temperature and the air pressure of the overlying atmosphere across the equatorial Pacific Ocean. The Southern Oscillation describes a bimodal variation in sea level barometric pressure between observation stations at Darwin, Australia and Tahiti. The ESNO has Two components -sea temperature and atmospheric pressure which are strongly related. During an El Nino event, the easterly trade winds that are converging across the equatorial Pacific weaken. This in turns slows the ocean currents and draws surface water away from the western coast of South America and reduces the upwelling of cold, nutrient rich water from the deeper ocean allowing warm surface water to build in the eastern part of the basin. The alternating strength of the trade winds is because of changes in the pressure gradient of the atmosphere over the tropical Pacific the warming of the sea surface works to decrease the atmospheric pressure as winds move from a rea of high pressure to an area of low pressure. Therefore,xthe pressure gradient affects the sea surface temperatures, and the sea surface temperatures affect the pressure gradient. The El Nino and La Nina, together called the El Niño Southern Oscillation these warmer or cooler than normal ocean temperatures can affect weather patterns around the world by influencing high- and low-pressure systems, winds, and precipitation. ENSO may bring much needed moisture to a region while causing extremes of too much or too little water in others.

El Nino is the warm phase of ENSO which generally brings wetter conditions across the southern tier of the United States and parts of South America, and drought in the western Pacific. Under normal conditions the easterly trade winds, driven by the pressure difference between the eastern Pacific high and the western Pacific low push warm water towards the equator and across the Pacific Ocean towards Australia and Indonesia. This causes a pool of warm water to form near the equator in the western Pacific. It also causes the thermocline, A vertical column of water where temperature changes rapidly, to move closer to the surface off the coast of South America, bringing nutrient-rich waters to surface by upwelling This upwelling cold water cools the atmosphere above, and prevents rain clouds from forming off the coast of Peru the warm water pushed to the west by the trade winds, heats as it flows along the equator so when it reaches the western Pacific heat is added to the atmosphere causing it to rise, form clouds, and produce extensive rainfall. The then moisture depleted upper atmosphere then circulates back to the east where it descends off the coast of South America contributing to the dry conditions. But During El Nino conditions there is a weakening of the easterly trade winds and the warm waters of the western Pacific are pushed toward the east xThis causes the thermocline in the eastern Pacific to sink, preventing the upwelling of cold waters from below . As the warm water shifts eastward so does the development of atmospheric disturbances that lead to upwelling of the atmosphere to form thunderstorms. Rising bodies of moist air thusxoccur closer to the coast of the Americas, leading to increased storms not only in South America, but in North America as well. These low-pressure systems that develop in the eastern Pacific can move over thexcontinent and cause severe weather. In addition, they create upper level winds that tend to shear the tops off of developing tropical storms and hurricanes in the Atlantic Ocean and Gulf of Mexico, leading to a decrease in the number of intense tropical cyclones that develop in these regions. See figure 1 below

The La Nina is nearly opposite to its brother El Nino as it refers to cooler-than-normal temperatures in the eastern Pacific Ocean around the equator. The temperature distribution changes how air circulates high up in the atmosphere, altering the routes of the jet stream over the Americas and making wind and precipitation patterns fluctuate all over the globe.xLa Nina events sometimes follow El Nino events, which occur at irregular intervals of about two to seven years. Th e deviations from the normal sea temperatures are measured by scientist through the use of the Oceanic Nino Index La Nina events are indicated by sea-surface temperature decreases of0.5 degrees or more while El Nino is classified by a 0.5 degree increase in temperatures. La Nina events are caused by opposite process of El Nino the trade winds strengthen, and warm water is pushed to the far western equatorial Pacific over Indonesia. This results in cooler surface water in the equatorial Pacific Ocean and dry conditions in Pacific coastal South America and much wetter conditions in northern Australia and southeast Asia. In the winter of a La Niña year, these winds are much stronger than usual. This makes the water in the Pacific Ocean near the equator a few degrees colder than it usually is.The La Nina is also characterized by higher-than-normal pressure over the central and eastern Pacific. This results in decreased cloud production and rainfall in that region thus Drier-than-normal conditions arexobserved along the west coast of tropical South America, the Gulf Coast of the United States. This phenomenon also causes drier conditions across equatorial East Africa during the months of December to February. Extreme La Nina events are known to cause disastrous floods in northern parts of Australia. For example, following the 2010 La Nina event events in Queensland, Australia experienced the worst floods ever. As a result of this catastrophic event, over 10, 000 people were displaced and forced to evacuate. See Figure 2 below

Ocean atmosphere interactions comprises a critical part of the earth’s climate system. ENSO variations such as the El Nino and its little sister the La Nina play and integral and important role in the variation of climate as they both can significantly influence the weather patterns, ocean conditions and the marine fisheries around the globe specific for an extended period of time.