El Niño: its causes and effects on the planet climate

The causes of El Niño are to be found in the presence of two types of waves in the atmospheric circulation

by Lorenzo Ciotti
El Niño: its causes and effects on the planet climate

El Niño is a periodic climatic phenomenon that causes a strong warming of the waters of the Central-Southern and Eastern Pacific Ocean in the months of December and January on average every five years. Developing countries that are heavily dependent on agriculture and fisheries, particularly those bordering the Pacific Ocean, are most affected, although it is thought to have effects on a global scale as well through changes in atmospheric circulation throughout the planet.

The causes of El Niño are to be found in the presence of two types of waves in the atmospheric circulation (Kelvin waves and Rossby waves), which are directed in opposite directions to each other. Kelvin waves, faster, are directed towards the east; Rossby waves are, on the other hand, slower and westward.

This generates an accumulation of water on the South American coasts in the Pacific, which results in a rise in the water level of almost a meter. When, at the end of December, the winds directed from east to west weaken or even reverse, if we also witness the Southern Oscillation, a variation in atmospheric pressure between Australia and the Pacific, the energy accumulated on the eastern coasts of the Pacific is released to the west.

This therefore causes heavy rains and lower temperatures on the South American Pacific coasts and drought and higher temperatures on the western Pacific coasts. From the analysis of the speed of the Kelvin and Rossby waves, the time taken to restore the normal situation is calculated, which is approximately equal to 12-18 months.

El Niño: its causes and effects on the planet climate

The El Niño phase is established due to the overheating of the oceanic surface waters of the eastern Pacific which, through increased convection, in turn modify the equatorial circulation of the winds and with it the distribution of rainfall, regulating the alternation of periods of drought and increased rainfall throughout the Equatorial Pacific.

From the point of view of the atmospheric circulation, as a consequence of the thermal variations of the ocean, with the El Niño phase a circulation is established with ascending air in the eastern Pacific and a descending one in the western one, i.e.

there is a shift of the circulation of Longitudinal walker to the east. In addition to the direct effects on site or on the Pacific, ENSO is considered the best known cause of interannual variability of meteorological and climatic conditions on a global scale in the world, with a frequency ranging from three to eight years.

The main consequences of this phenomenon can be found in the Pacific, Atlantic and Indian Oceans. In the Pacific Ocean, in periods of thermal oscillation in which there is a strong increase in water temperature, the phenomenon of El Niño occurs, while with a strong decrease, La Niña occurs.

The ENSO events are substantially in phase between the Pacific and Indian Oceans, while a twelve to eighteen month lag is noted between the Pacific and Atlantic ENSOs. Another fundamental aspect that characterizes the effects of El Niño on the environment, or rather on the oceanic ecosystem, is the variation in the nutritional supply of food that the phenomenon causes in the Pacific Ocean.

The warm current that the Niño carries eastward is in fact extremely poor in nutritional elements, ending up replacing entirely the cold Humboldt current (present instead in the Niña phase) which, through the ascent of the deep waters, favors the transfer from the oceanic depths of the plankton, the which ensures food for large quantities of fish.

If this situation continues for long periods, the marine faunal balance is distorted ending up having a heavy repercussion on the economy of the South American populations of Ecuador, Peru and Chile who live mainly from fishing.