El Niño Southern Oscillation: correnti oceniche, gyres e thermoclino (ITA).
The general circulation of the atmosphere shows several anomalies. El Niño Southern Oscillation (ENSO) is among the most important. It is a macro-scale atmospheric phenomenon triggered through oceanographic processes.
In fact, its genesis is due partly to the oceanic current, both shallow and deep, and partly to the low latitudes atmospheric circulation (from the Equator to ca 30° N and S).
At first, let’s take a look to the role which the oceanic currents play respect to such atmospheric anomaly.
Oceanic waters, likewise air masses, suffer the Coriolis Effect. Such force deflects them from their original path to the right in the Boreal hemisphere and to the left in the Austral hemisphere. According to this reason, shallow oceanic currents produce macro-scale permanent vortices, which rotate clockwise on northern hemisphere an anticlockwise on southern hemisphere. Such oceanic structures called gyres and we can divided them into polar gyres and subtropical gyres. Subtropical ones are wider and there are two of them into Pacific and Atlantic oceans. They are arranged roughly symmetrically from the Equator. There is only one into southern Indian Ocean due to the presence of the Asiatic continent into northern hemisphere. Gyres operate a crucial function in exchange the heat through the different latitudes and so they carry warm water masses toward the high latitudes and the cold ones toward the low latitudes.
Although the oceanic currents influence the atmospheric circulation everywhere on our planet, such interaction on the Pacific sector appears somewhat strong owing to the complete absence of landmasses, which would cut off the oceanic dynamics.
The southern Pacific gyre is constituted by an Antarctic origin cold current that rises up toward north flowing along the South American coasts up to 5° S, where merges along with the south Equatorial current. This warmer water flows, in its turn, towards W up to 140°-130° of longitude E. Once reached the eastern Australian coasts it flows south toward the high latitudes and merges with the circumpolar Antarctic current again.
How subtropical gyre develops on northern hemisphere
It is exactly at the southern Pacific Ocean tropical and subtropical latitudes that processes disturbing the typical oceanic circulation take place. That happens because of the shallow oceanic currents dynamics. The process can be briefly explained through the following way: the cold Humboldt Current, which corresponds to the eastern sector of the gyre, in association with the Coriolis Effect and the winds flowing through the low troposphere levels, take away water masses from the South American coasts. Such mechanism transport water westward, piling it up along the west side of the southern and equatorial Pacific Ocean, hereupon we measure a difference in water depth level between both sides of the Ocean in about ten inches. All this triggers the rise of the deeper cold water, which counteracts the mass disequilibrium along the South American coasts through the coastal upwelling phenomenon. Then, a thermal gradient between both sides of the ocean takes shape. Such a horizontal gradient is noticeable through the depth of the thermocline. Thermocline is a thermal transition layer interposed between the upper mixed layer and the deeper partially unperturbed one. Within thermocline, the thermal gradient is very high. This transitional layer is between 200 and 1000 meters of depth and its thickness is different according to the season. So due to the gyre effect, “normal” thermocline will be shallower along the South American coasts, along which colder waters will flow through, and it will be deeper off the Australian coasts.
Through the next article we will find out how “typical” thermocline can suffer periodic modification that induce a sharp or a weak thermal gradient of the ocean waters between both sides of the Pacific sector. Furthermore, we are going to deal with the correlation between the oceanic current dynamics and the atmospheric circulation.
Shallow oceanic circulation in case of "normal" thermocline