Oceanography - currents and circulation (2)

By Dr J Floor Anthoni (2000)
The world consists almost for 80% of water (oceans). Not surprisingly, these play a major role in the world's natural affairs. For instance, all rain originates from the oceans. Ocean temperatures are stable from day to day and change slowly with the seasons, as opposed to land temperatures. This chapter explores the circulations of water, in the air, in surface currents and in deep ocean currents. Because the planet rotates, all movement on its surface experiences a mysterious force, the Coriolis deflection. How does it work? How is water transported onto continents? How do surface currents influence climate? What is El Niño and how does it work?

please note that this document covers: part1 air circulation and part2 ocean circulation (this page)

Introduction to this chapter. 
Solar energy reaches the planet on its outer atmosphere. Some is reflected and some reaches the surface through a number of natural filters that protect life on earth. What reaches the surface, warms it up and makes plants grow. Atmosphere, ionosphere, exosphere, Van Allen radiation belts, magnetosphere, satellites, orbits.
solar radiation
The solar radiation warms the surface of the planet and its atmosphere. Various gases filter incoming and outgoing radiation, resulting in a planet which is liveable. Solar radiation, blackbody, terrestrial radiation, atmospheric gas filters, absorption and shade, photosynthetic absorption, seasons.
air circulation
The way air circulates in the atmosphere is surprisingly complicated. It governs the climate all over the world. During summer and winter, predictable areas of high and low pressure develop and the winds associated with them. General global circulation, Hadley Cell, jet stream, deserts,
water circulation The water retained as moisture in the air, brings life to the continents where it circulates almost independently from that above the oceans. Humans have disrupted this water cycle by changing forests into agriculture and urban development, which in turn changed the climate.
Currents and winds are deflected by Coriolis forces. How do they work? How do winds and currents react? What is an Ekman Spiral? How do layers of air or water move?
surface currents
The surface currents are driven by wind blowing over the ocean's surface. Where winds blow predictably, these currents are predictable also their associate climates. Map and names of currents.
deep currents
Most of a water particle's life is spent in the deep sea and nutrients befall the same fate. Yet at places on Earth, the cool bottom water is able to resurface and sea life here is bountiful. 
El Niño
Wind-driven currents depend on the vagaries of climate and climate depends on ocean currents. An introduction to the El Niño climate variation that affects nearly the entire world's climates.
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The water circulation
The water cycle What makes planet Earth so exceptional, is a very common substance called water. It is available in massive quantities in our oceans, ice caps, lakes and rivers. It cycles through the air and through the bodies of every living creature. 
In the diagram this water cycle is shown, with percentages attached for each component (Total amount of water on Earth= 1.4E9km3. Evaporation 100%= 4E5km3). As one can see, the circulation above land is essentially separate from that above the seas, but all water comes from the seas, since rivers eventually drain all terrestrial water back into it. About 6% is exchanged between the two. Note that the fertility of the land thus depends very strongly on how well the water cycles above the land. Most of the terrestrial cycled water is held in the living tissues of plants. Evaporation, being part of the cycle inside these plants, is part of the chemistry that makes plants grow, and that determines the biological productivity of the land. It is not surprising therefore, that natural ecosystems have developed ways to soak up rain, dew and snow and to store the water for gradual use.

Water, 'dissolved' in air as vapour, has a major influence on how the planet's temperature is stabilised. Everywhere on Earth, winds flow from warm to cold and back again, to transport the equator's heat to the poles. Without water vapour, air alone would not be able to do so, unless it flowed much faster than experienced today. The water's secret is held in its heat capacity (one calory heats one gram of water by one degree), and more so in the amount of heat required to evaporate it (about 400 calories per gram). Every gram of water evaporated, cools its environment by 400 calories. The water vapour rises high into the atmosphere, exchanging heat with the cool air, only to condensate back into rain again. In this manner, massive amounts of heat are exchanged with and between all layers of the troposphere (where surface winds blow).
Disrupting the water cycleDisrupting the water cycle
Through deforestation, agriculture and urbanisation, the terrestrial water cycle has been disrupted substantially, resulting in world-wide climate change. The diagram shows how. In the top part, it shows the natural situation, before Man changed the land. The land was forested, and the forests acted like a large sponge. Moisture for the land arrives mainly from the sea, precipitating in the coastal zone. In the forested situation, only a small part of the rain drains back through rivers to the sea. Most evaporates, rising to form clouds, only to rain down further inland. Successively the rains reach the centres of the continents, although in lesser volumes than at the coast. As a result, the centres of all continents have always consisted of arid land.

Since Man has deforested the land for agriculture and urbanisation, the natural forest-sponge has disappeared, and the coastal rains flow back immediately to the sea. The amount evaporated from the coastal zone became much less, resulting in much less rain to fall further inland. This also warmed up the deforested lands, with the result that the clouds rose higher than normal, reluctant to condensate back into rain. The centres of the continents became expanding deserts. Note that it was sufficient to develop only the coastal zone to achieve this effect!

The reduction in moisture had another major effect on climate, since most of the heat transport from equator to poles happens by the repeated cycles of evaporation, transport, condensation and precipitation (rain). With less moisture available in the air above continents, the heat transport diminished, resulting in warmer equator, cooler poles and warmer continents. The temperture stabilising function of the winds also became less, and winds 'compensated' for this by flowing faster. It resulted in climate change notable for its heavy rains, flooding of rivers, inland droughts and more severe but stable winds, higher tropical and desert temperatures and cooler poles.

Reader please note, that this aspect of climate change has not received sufficient publicity and interest, all attention being focused on global warming instead. But here is something interesting news to support our findings.

Rising clouds leave forests high and dry
  26 March 2003 - Exclusive from New Scientist Print Edition 

The base of clouds that form over the north-eastern states of the US have been getting ever higher over the past 30 years. It is a change that could severely disrupt forests in the Appalachian Mountains. Rising cloud ceilings have been spotted before in other parts of the world. In 1999, scientists found that clouds in the Monteverde cloud forests of Costa Rica were not forming as far down the mountains as they once did. 

This effect was initially attributed to rising sea temperatures in the Caribbean, caused by global warming, but in 2001 Robert Lawton of the University of Alabama in Huntsville reported that the main driving force behind the change was warmer, drier air moving up from the lowlands, which had been cleared of trees. The rising cloud ceiling has seriously damaged the cloud forest ecology, causing an alarming decline in populations of toads and frogs.

Now Andrew Richardson of Yale University in New Haven, Connecticut, and his colleagues have uncovered a similar trend in the temperate Appalachian ecosystem. The study was prompted by an intriguing observation by team member Thomas Siccama in the 1960s, when he regularly travelled through the mountains. "He noticed that in the fall, a lot of the time the clouds were at a level that coincided with the change from deciduous to coniferous forests," says Richardson. Concerned about the effect of a rising cloud ceiling on this forest boundary, the researchers examined data from 24 airports located along the south-west to north-east axis of the Appalachians. Airports routinely measure the cloud ceiling because it is important to pilots. 

Richardson's team found that in the 18 most northerly airports, the cloud ceiling has climbed an average of six metres per year since 1973. "Over 30 years, that's 180 metres, which is about six tree heights," says Richardson. "It is pretty stunning."

Scavenged water 

With the cloud base moving up, many spruce and fir trees will not be able to scavenge water from the clouds. "That's got all sorts of implications for hydrology in the mountains," says Richardson. Normally, the scavenged water drips down to the forest floor, but if this supply began to run dry it would immediately affect moisture-loving animals such as toads and salamanders. If the trend continued, broad-leafed deciduous trees such as sugar maple and yellow birch could move up the slope, visibly shifting the deciduous-coniferous boundary. The study is due to appear in a future issue of the Journal of Climate.

Roger Pielke, a climatologist at Colorado State University in Fort Collins and a member of Lawton's team, says that deforestation and urbanisation in the north-eastern US may be causing the cloud ceiling to rise, just as in Costa Rica, because both changes lead to warmer air that has to rise higher before it condenses. Cleaner air, containing a lower level of particulates, may amplify this effect. Particulates normally act as condensation nuclei, encouraging clouds to form at lower altitudes. Pielke cautions that because the research data comes from airports, it may be unduly influenced by the local effects of urbanisation. "But these are real trends," he says. "If they actually occurred in the mountains, they would be of concern." 

Anil Ananthaswamy

img: Coriolis deflectionBecause the Coriolis deflection is a major influence on winds and currents, it pays to understand it a little better. In traditional old-fashioned playgrounds, among the swings, see-saws and slides, almost always a rotating platform was found. Pushing it by its side railing, the 4-5 metre flat disc could be spun around and one could then jump on to experience and conquer strange forces that would make a grown man stumble and fall as if he was drunk (figure A). Note that the platform rotates like the planet, in an easterly direction, or counter-clockwise  when seen from above. The immediate effect of the spinning platform is a steady force pushing one outward. It is easily conquered by leaning towards the centre of the disc (Fig B). The real fun starts when walking around. For every step forward one will be pushed off-balance, needing several quick correcting steps to recover. What could the mysterious force be that does this?

It is the Coriolis force, described and formulated by the French engineer and mathematician Gustave-Gaspard Coriolis in 1835. He realised that objects resting on a circular platform have different circular speeds, depending on their distance to the centre. In the centre the speed is zero, whereas it increases as the radius increases. An object moving inward (red arrows= path without Coriolis force) would have too much orbital speed for the new position and would land to the right, in front of it (dotted black arrows =  path with Coriolis force). It seems as if the object is deflected to the right (Fig C). When moving outward, the object's orbital velocity will be short of the new position, landing the object behind, again to the right, of its intended direction.
When moving with the rotation, one's speed would be too high for the radius, resulting in a push to the right. Moving against the rotation would reduce one's orbital speed, causing one to fall inward. In all cases a pull to the right is experienced, no matter where one goes or where one is on the platform! The mysterious pull appears to depend only on the platform's rotational speed and the speed of an object relative to this platform.

Coriolis formulated this force in a simple formula: pull = 2 x spin x speed   (see box below). Because Earth spins rather slowly, the Coriolis force is correspondingly small. For instance, a motorbike rider of 100 kg, on a bike of 200 kg, moving at 100 km/hr (30 m/s) would experience a side-ways pull at mid latitudes of only 0.9 kg. By contrast, gravity pulls him with 300 kg.
At this point it is important to observe that the Coriolis pull works only in a plane parallel to the rotating disc (blue arrows), and that up and down movements (jumping) do not invoke it, but movements parallel to the rotating disc do.

When looking from the north pole down on the planet, it looks like a rotating disc in fig C, turning counter-clockwise. Thus all movements on the northern hemisphere are deflected to the right. Likewise all movements on the southern hemisphere are deflected to the left. Figure D shows the path of winds travelling from an anticyclone (H) to a cyclone (L) with right-handed deflection.

Figure E shows a cross-section of the planet with E the equator and N the north pole. The equator rotates eastward towards the viewer. The dark green bars are conceptual spinning discs as in figure B. The blue arrows give the directions of the Coriolis forces (parallel to the spinning discs). Now it can be seen that moving north-south on the equator does not invoke Coriolis pull, whereas east-west movements do. Orbital velocity here is about 1600 km/hr.
At mid latitudes (45º), movements in all directions invoke about equal Coriolis pull. Near the poles, the Earth's surface resembles that of a spinning disc most, and here the Coriolis effect is largest.

A number of other interesting observations can be made.

The Coriolis deflection (force per unit mass) experienced on the flat spinning disc is equal everywhere, depending only on the object's velocity  v and the rotational spin  (radian/sec): f = 2 x spin x v, where spin is the Earth's rotational spin of 360 degrees, 2 x pi radians per 24 hours = 7.27E-5 rad/s . f = 1.45 x v  (m/s/kg )
The effect of latitude adds a sine factor as can be seen in fig. D: f = 1.45 x v x SIN( latitude
For example: f = 0, 0.25E-4, 0.50E-4, 1.0E-4, 1.45E-4 at latitudes 0, 10, 20, 43, 90 degrees.
The deflection caused by the turning earth is called geostrophic (Greek: Geos= Earth, trephos = to turn). Geostrophic wind and geostrophic currents are examples.

img: Ekman spiralEkman spiral
The Coriolis force acts on the transfer of movement by friction, from one layer of air or water to another. In theory, the water's surface is pushed at a 45 degree angle to the wind, as shown in the diagram for the southern hemisphere (left-hand deflection). As the movement is transferred from layer to layer, the speed decreases while the angle increases. Speed becomes zero in a direction opposing the wind. When all speeds and directions are taken into account, the net flow of mass is perpendicular to the wind. This theory explains the movements of water in the open ocean, but near coasts, water layers are not as free to move according to theory. Here one finds the water seldom moving further than 30 degrees from the direction of the wind.

Surface currents
The map and table below show the main surface currents of the world. Red arrows for warm currents, blue arrows for cold currents and purple arrows in between. When comparing currents with the average wind maps above, a good match is found. Monsoon winds that blow in opposite directions for part of the year, do not produce significant currents. Upwellings are shown in light blue. They are found mainly where cold currents hug a continent. In these places no thermocline exists that keeps the water layered (stratified). Coriolis forces and surface winds make the currents rotate, turning cold and nutrient-rich bottom water to the surface and carrying warmer water out to sea. In these places the richest fisheries of the world are found.
The orange lines on the map mark the places where warm water meets cold water. A front (subantarctic front) is formed because the two water masses won't mix easily. Cold water is pushed down to the deep ocean and in the front eddies are formed that bring nutrient-rich water to the surface. These places are also highly productive.

a Labrador Current
b East Greenland C.
c North Atlantic Drift
d Gulf Stream
e Canary C.
f North Equatorial C.
g Caribbean C.
h South Equatorial C.
i Benguela C.
j Brazil C.
k Falkland C.
l West Wind Drift
m West Australian C.
n South Equatorial C.
o Mozambique C.
p Agulhas C.
q Monsoon Drift
r Kamchatka C./Oya Shio
s Kuro Shio C.
t North Pacific Drift
u California C.
v North Equatorial C.
w Peru/ Humboldt C.
x South Equatorial C.
y East Australia C.
z East Auckland C.
img: ocean currents of the world

For details about currents around New Zealand, see Special NZ/Currents

Deep currents
Although ocean basins stretch from one pole to the other, a distance of 20,000 km, they are only 4-5 km deep on average. It is almost unimaginable that such a thin sheet of water can form layers that mix with difficulty. Deep ocean water may remain there for 1000 to 2000 years before returning to the surface. Although the deep ocean circulation is very slow, the amount of water and the temperature differences involved, play a major role in the heat distribution of the planet, the weather and human-induced global warming. We'll first look at the stratification (layering) of water in a lake.

img: Circulation in a lakeThe diagram shows a cross section through a typical freshwater lake. In summer the surface water warms up and becomes lighter than the bottom water. Although wind-induced surface currents stir the water around, they are not strong enough to mix the entire lake, if it is deep enough. A thermocline develops. Had the cooler autumn not arrived, the entire lake would eventually have warmed through by the conduction of heat downward. Organisms living above the thermocline use up all available nutrients there, which sink to the bottom layer with their deceased bodies. At one time, the surface water may become very clear. By contrast, the bottom layer may become anoxic (lacking oxygen) because oxygen is used for breaking down the dead organisms and returning their body salts to the water as soluble nutrients.

Autumn arrives, cooling the surface water until no thermocline remains, enabling surface currents to lift nutrient-rich bottom water to the surface. In winter an ice sheet forms and the top water becomes colder than the bottom water, due to a unique property of water, being heaviest at 4 degrees. Another thermocline develops. Fish hybernate in this layer under the thermocline. In spring the winter layering disappears and the whole lake is mixed, inviting spring blooms of phyto plankton and other organisms.
img: Warming/ cooling of North Sea; circulationThe way things happen in the oceans is much more complicated, partly because of the different properties of salt water (See salinity and temperature) and also because of the size of the oceans spanning a huge range of temperatures. Small seas like the North Sea may behave somewhat like a large lake, as shown in the temperature cross sections in this diagram, but the large Oceans tell a different story.

img: Atlantic Ocean deep circulationThe deep ocean circulation, which is still being studied extensively, has yielded an amazing complexity as shown in this diagram, a cross section through the Atlantic Ocean from the North Pole to the south Pole (90 to -90 degrees latitude), with the equator in the middle. The curves on the right show decreasing temperature and salinity with depth. Unlike atmospheric circulation where warm air rises at the tropics, forming Hadley Cells and Trade Winds, ocean water is reluctant to do the same. It is incompressible and doesn't have its water content change from gas to water to ice and back, all major factors in atmospheric circulation.

By about 1500m depth, both salinity and temperature assume constant values of 1-2 degrees Celsius and 3.5% salinity. In the previous chapter we've seen the complexity of the surface currents which are seldom more than 200m deep. One sees a band of about 300m deep where the water mixes sufficiently to give very gradual temperature and salinity readings. Underneath it extends a body of water where the temperature descends gradually to 2 degrees at 1500 m depth. Oceanographers call it the thermocline although it extends over 1500m. By about 45º north and south, one enters the sub-arctic and sub-antarctic waters where no thermocline exists.

In this diagram one sees zones of high evaporation (the desert band) and zones of high rainfall (the temperate zone). These influence the salinity of the surface water. But ice does so too. In winter it freezes, drawing fresh water from the sea, while leaving saltier water behind. In the summer, the reverse happens. In any case, the water is 1-2 degrees and dense enough to sink to the deepest layers of the ocean.

But there's a difference in behaviour between the cold north pole water and that of the south pole. It is caused by the sill (barrier) between the Norwegian Sea and the Atlantic Ocean. Warm salty water from the Gulf Stream is cooled, becoming so dense that it overturns the Norwegian sea, while flowing back over its sill, descending as cold salt water, mixing turbulently (2-3 km/hr) with cool, less salty water, and finally sliding down over the Antarctic Bottom Water (at speeds of several metre per hour). Guided by mid-ocean ridges and continental margins, these waters do not swirl around by Coriolis forces that do act on them, but they remain flowing in one direction. At the South Pole, this north pole water re-surfaces, rich in nutrients, to give life to the most bountiful upwellings on Earth. There it joins the surface circulation of the West Wind Drift.
But some of it is cooled further to become antarctic bottom water, sinking to the bottom of both the Pacific, Indian and Atlantic Oceans, filling the deepest parts of their basins and flowing into the northern hemisphere. Some of this water passes close by New Zealand through the Hikurangi and Kermadec troughs. Inside these underwater canyons it can reach velocities of 0.5 km/hr.

Over the entire expanse of the ocean, mixing between cold nutrient-rich bottom water and warm, nutrient-depleted surface water, takes place, but very slowly. It takes 1000 to 2000 years for nutrients to resurface again.
img: ocean conveyor belt; deep circulationSomehow the deep circulation and surface circulations must join up, a puzzle that plagued scientists for many years, leading to several hypotheses. The map here shows how the great marine conveyor belt is thought to work now. The orange part is the surface component, which fits into the surface currents shown above. Only in one place does the surface component dip down into the cold depths of the deep sea: south of Greenland, as also shown in the deep sea circulation diagram above. The cold bottom water then flows above the ocean floor to antarctica, where some of it surfaces (not shown), but most of the cold, nutrient-rich water will surface in two places: in the northern Indian Ocean and northern Pacific Ocean. Although this ocean conveyor belt moves slowly, it is massive, and plays a major role in the heat transfer of the planet. It certainly plays a major role in the circulation of nutrients from the depths of the oceans, back to the surface. It is speculated, that during ice ages, the belt is interrupted over Indonesia, due to ocean levels dipping more than 100 metres.

(Joseph L Reid in Oceanography, the last frontier. 1974 and Ehrlich & Ehrlich  in Ecoscience. 1977)

El Niño
In the past fifteen years much has been published about the El Niño weather conditions to the extent that every year is pronounced as either an El Niño or a La Niña event. On this web site a whole chapter will be devoted to it, in order to unravel the confusions. From the current chapter we have seen that the oceans have a major influence on climate. Countries alongside a cold current may experience good fishing, but their climate will always be colder than normal. The warming water draws heat from the atmosphere, causing a dry climate with little rainfall but with sunny weather. Where warm currents cool off on their way to the poles, the climate will be warm and moist. When the ocean currents stagnate, it will have profound effects on not just weather, but on the climate for a certain duration. 

It can be expected that the largest ocean will also have the largest influence on the world's climate, and that is the South Pacific. In order of importance (size), the oceans line up as: 1= South Pacific, 2= North Pacific, 3= Indian Ocean, 4= North Atlantic, 5= South Atlantic. They all play a role in the world's climate but the South Pacific is their cheerleader.

Where confusion begins, is when people try to find a cause for the El Niño weather oscillations. There is no cause for the phenomenon but perhaps there is one for its frequency. Let me explain.
The south Pacific currents form a massive gyre rotating anti-clockwise, but all oceans have a gyre. Its currents are propelled mainly by the equatorial easterlies and the subantarctic westerlies, but contributions from the south Indian Ocean and winds in the west and east Pacific, cannot be ignored. Where cold water surfaces, as explained above, an area with dry climate and high barometric pressure is formed. On the other side of the ocean, the reverse happens. The difference in barometric pressure produces the very winds that made the currents, that made the highs and lows. So it is principally an unstable system where effect makes its cause. When circulation reaches a maximum, only a small perturbation suffices to wind the motor back: decreasing currents causing decreasing barometric difference, causing decreasing winds and so on. It is a cycle, but its pendulum, the inertia of the enormous water mass, is rather large. Water masses of all oceans interact, so we are talking about a very large system.

Scientists measure the barometric difference between east and west side of the South Pacific and call it the ENSO index (El Niño Southern Oscillation). It is only a very small symptom of a very large system. From this index, many, often too far reaching, conclusions are drawn. The El Niño cycle used to be strong, in twenty year intervals, but recently it has become weak with more frequent intervals. Such change may have been caused by human influence on the atmosphere. In a warming world, strong ocean currents are needed to spread the extra heat evenly.

For further reading, see the chapter on global warming. Many resources exist on the Web. Visit NASA and NOAA.

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