Guest Post by Wim Röst
The creation of ‘Hot Worlds’ and ‘Glacial Worlds’ during past geological periods was only made possible by warm and cold deep oceans, respectively, that were created by specific configurations of continents and oceans. Without warm deep oceans, no warm global climate state is possible. Without cold deep oceans, no glacial periods are possible.
The temperature of the oceans is mainly created by the kind of downwelling water. When warm very salty water is downwelling, warm oceans are created. When cold salty waters are downwelling, cold oceans are created.
What is oceanic downwelling?
Oceanic downwelling is the sinking of more dense seawater into a layer of less dense water. ‘More dense’ means: more weight (or mass) per unit volume.
Reasons for ‘density’
There are two reasons for seawater to be more dense than other seawater:
- the seawater is colder
- the seawater is saltier
We can expect downwelling to occur in areas with the highest density of surface waters. The following map shows where we find surface waters with the highest density.
Figure 1: Sea-surface density [kg m-3]. Annual mean sea surface seawater density calculated from World Ocean Atlas 2005 fields of temperature and salinity using the SEAWATER toolbox. Density here is in kg m-3. Blue and red squares are added.
The higher the density, the more chance there is for downwelling. The densities in the map above are annual densities. Because downwelling can occur every moment, this map only gives an indication of the annual potential for downwelling. In our current cold Quaternary Period, most downwelling potential is found in the North Atlantic and around Antarctica.
Two types of downwelling
There are two main types of downwelling:
- Warm and very salty
- Cold and salty
In the above map of figure 1 we find present warm and very salty downwelling in the red squares. The cold and salty downwelling areas are indicated by the blue squares.
(Nota bene, downwelling also occurs in the Pacific but the exact location(s) are unknown by the author)
As warm surface waters are transported to the poles they mostly have a higher salt content (a higher salinity) than other local waters because of the higher evaporation at the time the transported waters still were residing in the tropics and subtropics. But because this salty tropical or subtropical water also is very warm, it continues floating until it cools in polar areas. The now cold and salty water will sink into less dense water until it reaches equally dense water. We find this cold downwelling both in Arctic and Antarctic areas. Depending on the density of the downwelling waters, intermediate water (cold but shallower water) or deep ocean water is formed. The higher the density, the deeper the water will sink.
We don’t often find warm downwelling today, but warm downwelling does happen. Warm waters will also sink into colder waters when they are salty enough. Very salty warm water is that dense, and can sink into colder waters that have a lower salinity.
The role of salt
It is because the ocean water contains salt that this type of downwelling – warm downwelling – exists. As will be explained, this simple physical fact (the salinity of the oceans) is of decisive importance for the average background temperature of the Earth’s climate in different geological periods.
Simply said: if you only turn on the warm water tap of the big Jacuzzi in your bathroom, all bathwater will be warm. When you, after filling the bath close the door and return half a day later, the atmosphere in the bathroom will be warm.
But if you only turned on the (very) cold water tap and after filling the bath closed the door and returned half a day later, not only the bath will be very cold but you will also experience an unpleasant cold atmosphere in the bathroom.
If the Jacuzzi fills 71% of the bathroom, that situation would resemble the Earth whose surface is 71% ocean water. Cold water taps in our oceans are filling the deep ocean with ice cold water. We have two big chillers: one in the Arctic and one in the Antarctic. In recent geologic time, they have been perfectly positioned for cooling. Both are receiving more saline than average water from the Atlantic.
In our present Earth, unfortunately, our ‘warm water taps’ hardly function. We have ‘warm water taps’ in the Mediterranean, the Red Sea and the Persian Gulf. Also in the Pacific Ocean, there is some production of warm downwelling water. But as we will see, the total deep warm water production is minimal compared to the deep-and-cold water production which is, by far, dominating downwelling. Therefore, our Earth has a very, very cold Jacuzzi.
And because of the cold oceans, we experience very cold climates. Historically cold.
Every second around 40 million cubic meters (40 Sv, one Sv = one million cubic meters per second) are downwelling into the depth of the Earth’s oceans. Ninety percent of that 40 million cubic meters is very cold.
Cold downwelling: the downwelling in the Thermohaline Circulation
In the Thermohaline Circulation (THC) cold downwelling plays a significant role. Warm salty water from the Gulf Stream flows to the North Pole area, cools and sinks. This water flows as deep cold water to all of our oceans. All the deep oceans are connected to each other.
The deep ocean flow is not in the form of a simple ‘transport belt’ as is often shown in graphics like figure 2 below. Imagine that all deep and surface water is moving, sometimes more slowly, sometimes faster.
Figure 2: A popular image of the Thermohaline Circulation. Shown are the cold downwelling areas near the poles and the direction (not: quantity) of transport of deep waters (blue) and surface waters (red).
Less known: examples of warm downwelling. First: the Mediterranean
After the smaller Red Sea and the Arabian Gulf, the Mediterranean Sea has the highest salinity of all sea surface waters. In summertime in the Mediterranean evaporation is high and precipitation is nearly absent. Salt remains in the sea after evaporation and after some time all Mediterranean water will become very salty.
Figure 3: Surface salinity of the Mediterranean. Entering in the west at Gibraltar, surface waters become saltier after evaporation as we move to the east of the Mediterranean.
When this very salty and very warm Mediterranean water is cooling, it will sink below the less dense waters that enter at Gibraltar. The less dense water from the Atlantic overflows the dense, still warm and very salty waters that we find eastwards.
The warm deep water that is formed in the Mediterranean finally has a temperature of 12 – 14 °C (figure 4), which is warm compared to much colder deep water in the North Atlantic where temperatures below 1000 meter nearly always are less than five degrees C.
Figure 4: Temperature and salinity of Mediterranean water. The Y axis (depth) is logarithmic. The distance east- west is 3,700 kilometres. To compare with other oceans: the average salinity of all oceans is 34.9 PSU
At Gibraltar, the deep very salty warm water flows out of the Mediterranean into the Atlantic Ocean. In doing so, the Mediterranean creates a huge underwater waterfall: the dense very salty warm water sinks into the colder but less salty layers of the Atlantic Ocean. Figure 5.
Figure 5: The Mediterranean underwater waterfall at Gibraltar. Very salty and warm water (dark blue) dives deep down into colder Atlantic waters. The speed of the outflow can reach 2 metres per second: 7.2 kilometres an hour.
(Original: Marine Geology, Kuenen, p. 43, with a slight enhancement)
The warm downwelling water flows to a depth around 1000-1100 meter where it meets and mixes with colder but less saline waters. The inflow of Mediterranean water results in an estimated final temperature effect for the North Atlantic of between 0.1 and 0.3 °C.
The effect of the spreading and mixing of the Mediterranean water in the North Atlantic is also visible in the salinity map of figure 6.
The tongue of saline water from the Mediterranean outflow in the observations of Levitus, Burgett, and Boyer  at 1100 m depth. Contours show the salinity anomaly (practical salinity units)
Even deeper downwelling of very salty waters: Red Sea water
The Red Sea is not only warmer but also has a higher salinity than the Mediterranean: 40 – 41 PSU. Because of that high salinity, the outflow reaches far deeper waters in the Indian Ocean than the Mediterranean waters do in the Atlantic: we can find the warm Red Sea Intermediate Water at a depth of around 3 kilometres, see figure 7.
Figure 7: Very salty and warm Red Sea Intermediate Water at 3 km depth in the Indian Ocean: red oval
Source: Slide 47
Present downwelling: the numbers
In the present configuration of the Earth we find both warm and cold downwelling, but cold downwelling dominates. See table 1, red = warm downwelling, blue = cold downwelling. Based on estimations by Ganachaud and Wunsch (2000)
Source: Data from Plate 4b
Not mentioned in table 1 is the Mediterranean outflow of 1-2 Sv. If added, the total present warm deep-water production will be at least 3.5 Sv, still far less than the 36 Sv produced as ice cold Arctic and Antarctic deep waters. Ten times as much ice cold deep water is produced compared to warm deep-water production. As a result, present deep oceans are ice cold.
The present situation of ice cold oceans results in the historical cold climate state in the period we are living in, the Quaternary.
Figure 8: Phanerozoic Global Temperatures according to Christopher Robert Scotese The added red arrow indicates our present relatively warm interglacial temperature: but it is still an ‘Icehouse’ temperature.
Because yearly more than a million cubic kilometres of very cold deep waters are upwelling into the warm surface layer, the new surface water has a low starting temperature and diminishes the average temperature of the sea surface waters. Warmer upwelling waters would have a warming effect. As described in Cooling Deep Oceans – and the Earth’s General Background Temperature the cooled deep ocean cools the atmosphere, enabling our present very cold Quaternary, characterized by glacial periods.
In the next post, more about the development of the present configuration of the Earth that has led to our present ice cold oceans. As an introduction to the next posts some maps are presented in figures 9 and 10.
Figure 9: Two different configurations of the Earth’s continents and oceans. The map on the left has led to our present icehouse state, the situation shown on the map to the right would have led to a hothouse state.
Figure 10: Configuration of the Earth’s continents and oceans 65 million years ago, creating the warm deep oceans and the warm climates that were present at the start of the Eocene. Notice the Eurasian situation around 30N – the arid subtropics – where we presently find the relatively small warm and salty deep water producing seas (Mediterranean, Red Sea) described above. Light blue is ‘shallow’.
Surface water will become deep water when it is either cold and salty or when it is warm and very salty.
It is the special quality of salty seas that warm water can also downwell to great depths, due to a very high salt content. Because of the high salinity the density of the warm very salty water equals the density of much colder but less salty water. The very saline water will sink to the same depth as colder but less salty water, raising the average temperature of the deep ocean.
In the present configuration of the Earth the downwelling deep cold water dominates. As a result, the oceans and therefore the Earth’s climate are historically cold. As deep water wells up into the surface layer, the surface layer becomes relatively cold and because of the cold surface layer our present global atmosphere and climate is historically cold.
The present absence of large warm deep water producing seas, in combination with the large and well-functioning cold deep water production areas at the poles, ensure that our Earth is experiencing one of the coldest periods of the last 500 million years: the Quaternary Period. This is the very cold era we now live in, the glacial era.
With regards to commenting: please adhere to the rules known for this site: quote and react, not personal.
In commenting: please remember you are on an international website: for foreigners, it is difficult to understand abbreviations. Foreigners only understand words and (within the context) easy to guess abbreviations like ’60N’ or ‘SH’.
About the author: Wim Röst studied human geography in Utrecht, the Netherlands. The above is his personal view. He is not connected to firms or foundations nor is he funded by government(s).
Andy May was so kind to read the original text and improve the English where necessary. Thanks Andy!