By nature and even daily experience, we are led to believe that the cold moves downwards, that the cold air falls towards the earth and that only from space can there be a significant cooling of the air that surrounds us and above us. On the other hand, heat can only come from below, from the earth and above all from under the ground.
If this reasoning works quite well for the emerged lands, it is not true for the large mass of water in the oceans, which is associated with an average temperature much lower than the atmospheric temperature and of the emerged lands in general, especially in the deep sea, and already below 500-800m.
Below this altitude there is only absolute darkness, the sunlight cannot penetrate and the cold reigns supreme with shivering temperatures between 4 and 5-7°C, regardless of latitude, but with local variations depending on the slow vertical movements of the water masses, depth currents, topography, the presence of submerged volcanic structures, etc.
But if the atmosphere undergoes very rapid and frequent horizontal and vertical movements, of the order of tens of km per hour, the ocean water masses certainly do not stay still, although the movements are much, much slower: in some cases, as in abyssal waters, lasting months or years, even decades. This is the case of deep ocean currents, which give rise to real rivers of cold and salty water which crawl very slowly on the seabed of a large part of the planet, and which draw what is known as the great global thermohaline circulation.
What sets these circulatory phenomena in motion? THEessentially the same forces that move all the fluids on the earth, as happens with the atmosphere, or the magma of the mantle, but also the waters of every water basin, or rather all those whose volume and consistency depend on the earth’s rotation and by the different incidence of solar and cosmic radiation in general.
In the case of water masses, the joint action of the Earth’s rotation (apparent Coriolis force) and the internal phenomena connected to the overlying air masses (Ekman transport), and in particular to the constant winds that are generated across the the equator and medium-high latitudes, causes water transport generally in an east-west direction in the tropical and sub-polar belt, and west-east at the equator and mid-latitudes.
First of all, a superficial shift of the water masses occurs, so that some areas of the ocean appear to be “higher”, and others more “depressed”, with differences of the order of tens of cm (for example between the two coasts of Panama, the Atlantic and the Pacific, there is a difference in sea level of more than 20 cm). But that’s not all, in fact the movements of water masses in depth, although slower, are also more varied and with a periodicity that is not yet clear in most cases.
In fact, along the western coasts of the continents, the deep waters are dragged by the earth’s rotation to slam against the continental slopes, then to rise upwards (a phenomenon known as upwelling). In their movements they are influenced both by the forces and phenomena mentioned previously and by the regime of the winds on the surface.
The result is a net movement towards the surface and in the direction of the equator, or towards the south for the currents of the northern hemisphere (Canary current, California current, Labrador current); northwards for those of the southern hemisphere (Humboldt or Peru current, Benguela or south-west Africa, Western Australia).
The seasonal, annual and multi-annual alternation of the atmospheric circulatory regime also determines different effects on the upwelling of deep waters, resulting in a greater or lesser contribution to the movement of the thermocline (transition layer between the cold deep water and the warm surface water). , but above all to the extent and speed of the surface flow of cold currents, once the deep water has reached the surface.
The periodic upwelling of deep waters therefore conditions various phenomena in and above ocean waters, from their productivity, in terms of plankton and fish, to their ability to cause evaporation and provide more or less energy to the atmospheric system above. This is the case of various areas of tropical cyclone formation, conditioned in their development and strength by the presence of underlying warm waters.
As is easy to imagine, the phenomenon of tropical storms is reduced in frequency and power due to the dilution of warm surface waters, with those colder in depth; a fact that occurs more or less constantly in the southern Atlantic and the south-eastern Pacific, precisely where the upwellings and the most extensive and persistent cold currents are rampant.
Thanks to satellite observations, for a few decades we have been able to appreciate that the mixing zones of water on the surface are characterized by considerable vorticity, with very bizarre local effects, both in the distribution of the water masses and their planktonic, nektonic, as well as salinity.
Among the most complex and extensive effects, among those that most influence the distribution of heat on the earth, we have the so-called ENSO cycle (El Nino Southern Oscillation), which, despite its name, is anything but cyclical. The phenomena known as “El Nino” correspond to an overheating of surface waters close to the Equator, but the most intense episodes, such as in 1998 and 2016, are rare and not equally distributed.
Neutral conditions, or those linked to the opposite effect (La Nina), are more frequent and long-lasting, constituting climatic “normality” in the imagination of the populations of the South Pacific. To tell the truth, over the last 50 years the phases linked to “El Nino” have become rarer and less long-lasting, although more intense. Could this be one of the effects of recent climate changes?
One fact is certain, predictability is still very poor, and relegated to a few months; while in these cases we are talking about annual, if not multi-annual, phenomena. What we observe, however, is that in the case of the heating of surface waters, the air temperature has an impact, but above all the dominant atmospheric currents which, by accumulating the surface waters in smaller sectors, force them to heat up. Nothing new under the sun, as they say; especially when it comes to heating.
In the case of cooling, however, this is caused almost exclusively by the phenomena of upwelling and rising of deep cold waters, as can be seen in the most recent NOAA returns (sequences of the last 3-4 months), regarding the distribution of water masses sub-superficial. We note how the colder water tends to overthrow and replace the warmer water, starting from the east, i.e. from the coasts of South America.
Below 300m, and the image should not be deceiving, it is not that the water is less cold, but the white simply means that the water does not present appreciable anomalies. In reality below 300-400m there is only cold water, and indeed increasingly freezing towards the abyssal depths, where the temperature just exceeds 4°C (attached graph).
To summarize, the most superficial layer, approximately 100-150m m, presents an almost constant temperature, with variations of the order of 2-3°C; followed by an area where the temperature drops sharply (Thermocline), up to 800-1000m, with a drop of up to 20-25°C, as at the equator. The decrease then continues, proceeding more gradually in depth, up to a deep homo-thermal layer, below 3500-4000m, where the temperature fluctuates around 4°C and depends mostly on salinity.
It is therefore easy to understand how much cold is present in the oceans, and that if the cold can affect the climate, this can only come from below, that is, from the ocean depths. But have the oceans always been this cold? Actually no, in fact they were much warmer than today, and even deeper; but that’s another story.
Prof. Giuseppe Tito
