Tuesday, 2 August 2011

The basics of ocean circulation - my rather rough first draft of section 1

This is my rather rough first draft of the first section of my EPQ. I realise that the phrasing is a bit clumsy in places and that, on a whole, it needs a lot of refinement and a great deal of condensing (I am currently in the process of doing this)! However I have a bit of a favour to ask; I have struggled to gauge how much scientific/academic content I can include and so what would be really useful to know is whether or not you understand it all? Are there any bits that are perhaps a bit too heavy? Anything that needs further explanation? Is there anything that is a bit to simple and you don't feel needs to be explained? Or do you want more scientific/academic content?!? Any feedback would be greatly appreciated! By the way, ignore all the little numbers, they are my footnotes and I didn't feel that you needed all my reference stuff! I hope your EPQ's are going well!

The circulation of the oceans can be defined as the “average movement of seawater, which, like the atmosphere follows a specific pattern” [1] and the general purpose of ocean circulation is to exchange waters of differing properties, principally temperature, salinity and density, throughout the interrelated and interdependent network of oceans that cover approximately 70.8%[2] of the world. In simplistic terms, the ocean circulation comprises of horizontal movements, known as currents and vertical movements, designated upwellings or downwellings.  
The disharmonious characteristics of water masses in the oceans are crucial to the oceanic stratification that divides the currents and allows for limited mixing only. This naturally occurring phenomenon is vital to not only the structure of the general ocean circulation but also the productivity of the oceans. Vertical stratification is provoked by the, sometimes only subtle, differences in density. Water density is regulated by temperature and salinity (with overlap between the two existing); where cold, salty water being the densest sinks and warm water, with a low salt content, thus more buoyant, floats upon the denser water.  The thermocline, the region in which the rate of decrease in temperature with increase in depth is the largest, forms a boundary between warm and cold currents across which water is prohibited from passively mixing and the halocline (the vertical zone in the oceanic water column within which salinity alters most rapidly with depth) essentially does the same, although linked to salinity. The depth of the thermocline, as with the halocline, varies seasonally, especially in mid-latitudes where an additional shallower thermocline often develops in the summer or in high latitudes where the thermocline may only become visible seasonally. The variations can extend over a larger temporal scale, especially in the Pacific, as the thermocline plays a fundamental role during ENSO. As the pycnocline is the area where density increases most abruptly with depth; both the thermocline and halocline can closely be associated with it and in fact substantial overlap between all three ‘layers’, which separate the mixed-layer from the deep ocean, exists. The idea of the close relationship between the thermocline, halocline and pycnocline is intrinsic to the stability of ocean stratification as a decrease in temperature harvests an increase in density and thus a stable stratification but a decrease in salinity provokes a decrease in density and thereby an unstable stratification. However, because the influence of temperature overrides that of salinity; the general stratification is stable. Although stable stratification represses passive mixing, due to the wind, upwellings, downwellings and storms, turbulent mixing does occur to varying extents. As this mixing allows for nutrients to be pushed to the surface and oxygen to the deeper layers, it is crucial for the productivity of oceans. Therefore the stratification is important but continual and intensified stratification can have a detrimental effect on ecosystems as, an increase in sea temperatures provokes an increase in strength of the thermocline boundary, thereby placing greater restraint on the movement on nutrient rich waters due to the further suppression placed on turbulent mixing; hence why during El Nino Peruvian fishermen suffer greatly as the fish move to the colder, deeper waters which have a higher oxygen and nutrient content.
Ocean currents are large-scale movements of water within oceans that are a critical mechanism in the Earth’s heat transfer system which conveys heat from areas of surplus to areas of deficit. There are essentially two varieties of currents: cold ocean currents which flow along the ocean floor from high latitudes to the tropical regions and warm currents that migrate polewards from the Equator, with close proximity to the surface.
Gravity, in relation to Sir Isaac Newton’s equation of motion[3], obviously has an effect on ocean currents, and their formation, within the general circulation, but pressure-gradient forces and frictional forces also play an influential role in their formation and motion. Hydrostatic pressure is important to the formation of currents as the Earth is not flat and completely evenly covered in water. Landmasses interact with currents firstly because land has different thermal properties to oceans[4] and secondly as landmasses form boundaries along which water tends to ‘pile up’, thereby creating a sloping horizontal sea surface, upon which the hydrostatic pressure will act on accordingly, depending on depth. In essence this generates horizontal pressure gradients and so, in the same way in which heat transfers from areas of surplus to deficit, water tends to flow so as to balance the lateral differences in pressure[5]. As the oceans are not homogeneous, the horizontal differences in density (as a result of fluctuations in temperature and salinity) provoke the hydrostatic pressure to vary along a geopotential surface and with depth thus favouring baroclinic conditions (where isobaric and isopycnic surfaces are inclined in relation to one another and isobaric surfaces follow the sea-surface less and less with increasing depth). It is the baroclinic conditions that are responsible for currents that vary with depth. Barotropic conditions (where isobaric surfaces are parallel to both sea-surface and isopycnic surfaces) form in well-mixed areas or below the permanent thermocline and so, to some extent, characterise the deep ocean.  Although vertical pressure gradients are greater than horizontal pressure gradients, the latter are a significant contributor to the occurrence of ocean currents. When the horizontal pressure gradient force is balanced by the Coriolis force, geostrophic currents are produced. The balance that allows for their occurrence dictates that the current direction has to be perpendicular to the horizontal pressure gradient as Coriolis always acts perpendicular to the motion. This results in high pressure always being situated to the right in the Northern Hemisphere and left in the Southern Hemisphere, in relation to the current direction.
The ocean circulation derives its energy initially from the Sun as, due to the fact that the intensity of insolation varies between the Equator and the Poles, a temperature gradient is generated which instigates the transfer of heat, in both the atmosphere and the oceans, to move from areas of surplus to deficit. Therefore it is this temperature gradient that essentially triggers the ocean circulation. The atmospheric and oceanic circulations are very closely coupled, principally due to the heat exchange that occurs between them, and so they play a dictatorial role in determining the general circulation patterns that occur. Basically, it is the Sun that drives all the ocean currents, although most directly the surface currents (found in the upper 100 metres of the ocean[6]), via either the prevailing winds generating friction with surface waters or temperature variations inducing density gradients.
When wind blows over the ocean, not all of the energy that is transferred is expended in the generation of surface gravity waves; some is used to drive currents and the pattern of these wind-driven currents is similar across all of the oceans. The prevailing winds drag on the sea-surface, provoking it to move and build up in the same direction as the wind is blowing. This effect of wind stress on the sea-surface is transmitted downwards as a result of internal friction within the upper ocean and starts to build up momentum and drive the currents. In general, the wind-driven circulation attenuates with depth but the exact penetration depth of the wind-driven circulation is regulated by the strength of water column stratification. Regions of strong stratification, like the tropics, will include surface currents that reach a maximum depth of 1000 metres[7], whereas in the poles, where stratification is low, these currents can extend to the sea floor. The reasoning behind why stratification is important in the extent to which wind-stress affects ocean currents is linked to the idea of eddy viscosity and its magnitude as the turbulent eddies, located in the upper layer, “act as a ‘gearing’ mechanism”[8] for translating motion from the surface to layers below.  
Winds are not always constant in strength or direction and so when a wind that has been driving a current ceases to provide sufficient energy to do so, inertia currents are created. Momentum will not leave the water immediately as, in open ocean, it takes a while to dissipate and whilst in motion, frictional forces and Coriolis force will continue to act upon them, thus resulting in a circular motion that characterises inertia currents. The extent of this circular motion is determined by the influence of Coriolis over other forces with the most circular inertia currents being generated when Coriolis is the only horizontally acting force on a current whose journey involves minimal latitudinal movement. The energy in the oceans is both kinetic, by virtue of its motion, and potential due to the displacement of isopycnic and isobaric surfaces and it is this huge store of potential energy in the oceans that ensures that the ocean circulation would take a few decades to completely cease if global winds stopped blowing.
The Coriolis force is a substantial dictating factor determining the direction of ocean currents and this effect is generated by the anti-clockwise rotation of the Earth; without the influence of the weak frictional coupling between moving water and the Earth surface[9]. If the Earth didn’t rotate then both the atmospheric and oceanic circulation would circulate to and from the polar regions (high pressure) to the Equator (low pressure) in a continual motion[10]. However, the incessant eastward rotation of the Earth causes a more complex pattern of movement. The circumference of the Earth is greatest along the Equator and so the eastward motion of the Earth’s surface is greatest here; whereas at the poles, where circumference is at is minimum, the velocity is zero. Therefore, if a volume of fluid (or any moving particle) flows north from the Equator; it will sustain its constant eastward momentum but, as it gets within a forever closer proximity to the Poles, the Earth beneath it will gradually slow, thereby provoking the water mass to move to the right, in relation to the Earth. Its general effect on currents is to cause then to turn to the right (east) as they progress north from the Equator and to the left (west) as they migrate south from the Equator. This is the Coriolis Effect and its strength increases as the water (or wind) moves further away from the Equator and thus, the distance that the currents flow away from the Equator governs how far, to the left or the right, they ‘bend’.  This stimulates the production of gyres – large circular flows that flow clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere – that flow around all of the major ocean expanses and divide the wind-driven circulation[11]. There are two types of gyres: subtropical gyres that extend from the equatorial current to the maximum westerlies (lies at latitude 50°) and subpolar gyres, which are cyclonic circulation features that extend polewards of the maximum westerlies and within which Ekman transport forces surface water divergence and upwellings. Subtropical gyres are anti-cyclonic circulation features where convergence occurs due to Ekman transport within them provoking downwellings. The central point of subtropical gyres is shifted to the west, thus intensifying the westward ocean currents, due to the strengthening of the horizontal Coriolis force with latitude (Stommel 1948)[12]. This produces faster flowing, western boundary currents that are warm but narrow. Winds that circulate around the subtropical gyre push cooler currents, which are slower moving broad but shallow eastern boundary currents[13], towards the Equator, affecting the western side of continents.
The Coriolis Effect also influences the Ekman layer (wind-driven layer) in other ways. As explained previously, wind exerts stress on the sea-surface proportional to the square of wind speed in the direction of the wind, thus initiating momentum in the surface water. It was Ekman that calculated that the influence of wind-stress attenuates exponentially with depth and therefore so does the velocity of the effected currents. However, the Coriolis force stays constant with varying depth, thereby provoking, in theory, the formation of an Ekman Spiral. This spiral current pattern is important but Ekman’s most significant theory was the fact that “the mean motion of the wind-driven layer is at right angles to the wind direction”[14] (to the right in the Northern Hemisphere and left in the Southern Hemisphere) and the volume transport in this direction, often referred to as Ekman transport, moves water in response to prevailing wind fields, therefore contributing greatly to the general circulation. Ekman transport that is wind-induced gives way to Sverdrup transport (Sverdrup 1947)[15] which produces the ocean current pattern. Ekman transport varies across the oceans and contributes to the generation of divergence and convergence. Ekman convergence forces downwellings that accumulate less dense surface water and forces it to sink whilst divergence leads to upwellings which replaces the less dense surface water with dense water drawn up from below. Variations in Ekman transport can be reflected in variations in sea-surface level and under divergence the sea-surface is lowered whilst the thermocline is raised; with this upward movement of water being known as Ekman Pumping; therefore also meaning that Ekman transport can have an impact on the horizontal pressure gradient force.
Landmasses, especially large continents, modify the pattern of ocean circulation. The Earth is not symmetrical and the Southern Hemisphere, of which only 19%[16] is land, contains most of the world’s oceans. As land heats up faster than water, the landmasses of the Northern Hemisphere make their surrounding seas warmer than those of the Southern Hemisphere – a factor that influences water density. Especially in relation to the thermohaline circulation, the presence and distribution of mountains in relation to prevailing winds has an impact due to their influence on the extent and location of evaporation and precipitation.  Also, the landmasses cause significant modification to the direction of both atmospheric and oceanic currents as they force masses to ‘pile-up’ along the boundaries they form. Seafloor topography also plays a role in determining the direction of movement as the shape of ocean basins impacts both surface and deep water currents by restricting the areas where water can move and funnel it into another.
The Gulf Stream is one of the strongest western boundary ocean currents in the world and is driven by surface wind-stress. It is only one of many currents that construct the oceanic circulation but, with reference to its impact on the UK, is one of the most influential. It is paired with the eastern boundary Canary current and flanks the North Atlantic gyre. The waters feeding into the Gulf Stream begin flowing off the west coast of Northern Africa where this water is moved across the Atlantic Ocean by the Atlantic North Equatorial Current that flows from African, across the Atlantic. When this current reaches the eastern side of South America it splits into two currents. One of these currents is the Antilles current, which is a branch of the Atlantic North Equatorial Current, and forms part of the clockwise-setting ocean current system in the North Atlantic. The Antilles current flows along the north side of the Greater Antilles Islands before merging with the Florida current, which emerges from the Gulf of Mexico through the Straits of Florida to form the initial portion of the Gulf Stream. The other branch of the original current is funnelled through the Caribbean Islands and the Yucatan Channel where, because it is narrow, it is able to compress and thus gather strength. As the strengthened current enters the warm waters of the Gulf of Mexico is start to circulate, gaining further strength, before exiting, via the Straits of Florida, and re-joining the Antilles current. The characteristics of the Gulf Stream at this point, in relation to temperature and salinity, represent the fact that it has been supplemented by waters from both the Antilles current as water that has recirculated in the Sargasso Sea. The Gulf Stream becomes visible on satellite images in the Gulf of Mexico and is therefore said to originate here. After exiting via the Straits of Florida, the Gulf Stream flows parallel to the east coast of the USA until it reaches the open ocean at the Cape of Hatteras where it then proceeds to move north. Between the Straits of Florida and Cape Hatteras, the current flows along the Blake Plateau, following the continental slope. This keeps the current well defined, very narrow and limits it to a depth of about 800m[17]. Beyond Cape Hatteras it moves into deeper water (4000-5000m)[18] because it has left the continental slope and any topographic constraints. This means that the current can now meander, which is exactly what it does, and it is these meanders that give rise to the Gulf Stream ‘rings; or eddies. As the Gulf Stream moves towards the Grand Banks off Newfoundland it broadens and starts to be referred to as the North Atlantic current. At this point the current branches off into two directions: some turns south-eastward to contribute to the Canary current where it recirculates in the subtropical gyre whilst the rest progresses north-eastward between the UK and Iceland where it joins the subpolar gyre.
Initially the Gulf Stream is considered to be a wind-driven current but, because during winter it is forced to sink at subpolar latitudes, thus forming dense deep water which flows equatorwards and contributing to the deep recirculatory flow, whilst also provoking more of the Gulf Stream polewards to replace that forced to sink; it is also considered to be driven by the thermohaline circulation. 
Not all parts of the ocean circulation pattern are driven by wind-stress. Deep water currents are driven by the thermohaline circulation, which is initiated by density differences, and are found below 400m[19] where they make up approximately 90%[20] of the ocean. Although gravity, frictional forces and Coriolis force also affect the deep water currents, it is the density differences that are the real driving forces behind them.
This deep circulation is caused by density changes in oceanic water resulting from changes in temperature and salinity, hence its name ‘thermohaline circulation’, which are caused by cold winds cooling surface waters, the input of freshwater from either precipitation or melting ice, the cooling and freezing of seawater into sea ice or the evaporation of sea water. The basic thermohaline circulation is initiated when denser water (predominately the cooler, saltier water) sinks below the more buoyant water (warm, with a low salt content). Convection penetrates to a level where the density of the sinking water matches that of the surrounding water. When this maximum penetration level has been reached, it will gradually spread into the rest of the ocean. Once the dense water masses have spread into the full extent of the ocean, they will slowly upwell to supply the slow return flow to the sinking regions and replace the surface waters lost.
The fluctuations in density are responsible for the formation of intermediate, deep and bottom water masses, all of which are crucial to the idea of the Global Conveyor Belt. Intermediate waters are defined as being relatively dense and therefore sink part of the way. Deep water is very dense so sinks and navigates along the ocean floor and bottom waters which are the very densest water (overlap between deep and bottom water exists). The depth that the water sinks to is determined by its density and that of the surrounding water masses as each water mass sinks from the surface until it reaches a depth where it has less dense water on top and denser water below[21].
This thermohaline circulation is best developed in the Atlantic due to high evaporative enrichment of salt which produces a high salinity. However, it cannot be initiated in other oceans as conditions prohibit the crucial sinking of the denser water. For example, the Pacific sea-surface waters have too low a salt content to allow for sinking into the interior and the Indian Oceans are too warm for sinking to occur.
The Gulf Stream transports warm, salty water to the north-east of the Atlantic where this warm water cools and mixes with the cold water originating from the Arctic Ocean. This causes it to become dense enough to sink, both to the south and east of Greenland. The resulting current is part of a larger system that connects the North Atlantic to the rest of the Atlantic, the Indian and Pacific Ocean and the Southern Ocean, where the two main sinking regions spread out in the subsurface ocean where it is able to affect all the world’s oceans from depths of 1000m[22] and below.  The cold, dense water gradually warms and returns to the surface, throughout the world’s oceans. The surface and subsurface currents, the sinking regions, and the return of the water to the surface form a closed loop, commonly referred to as the global thermohaline conveyor belt. This conveyor belt starts in the Atlantic where the salty upper Atlantic water proceeds northwards to the vicinity of Iceland. Here it is cooled and thus thermally densified, allowing it to sink through the interior before it flows south where it forms the conveyors lower limb. It passes the tip of Africa before joining the Southern Ocean Runway, which transports this dense water around the Antarctic continent. Whilst flowing around Antarctica, mixing with the brine-densified winter runoff waters from the surrounding ice shelves occurs. The resulting denser water then goes on to enter both the Pacific and Indian Oceans as bottom water which forms the lower limbs of the anti-conveyor circulation. Branches of intermediate water, which is formed along the northern boundary of the Southern Ocean, infiltrates into all the three main oceans before further horizontal mixing occurs, deep into the  oceans.
There are many important deep water formations that have great influence on many factors that affect our lives but explaining them all is beyond the scope of this project; instead focus is going to be placed on the North Atlantic Deep Water (NADW). NADW is initially formed in Greenland and the Norwegian seas due to salty water introduced by the cold Norwegian current and the increase in salinity, as a result of sea ice formation which causes the NADW to descend with close proximity to Iceland where it is further densified by both additional evaporation of the waters within the NADW and subtropical brines. The NADW extends southwards, in the form of a convective plume, from its formation site in high latitudes and therefore has to be replaced by a northwards counterflow of surface water. This is where a link between the Gulf Stream and NADW is developed as the deep-water movement amplifies the Gulf Stream flow; therefore making the two currents interdependent. Further deep water is added by both the Labrador seas, via the convective feature known as chimneys, and as a result of the net evaporation that occurs in the Mediterranean Sea. This incredibly salty water soon ventilates the Atlantic Ocean and spreads rapidly away from its source. Upon reaching the Antarctic Circumpolar current, it spreads into the Indian and Pacific Oceans. The sinking of the NADW is counter-acted by an upwelling in the Southern Ocean as the NADW exported to the other oceans must be balanced by an inflow of upper-layer water into the Atlantic. Some of this water returns in the form of the Antarctic Intermediate Water whilst the rest returns as warm salty thermocline water from the Indian Ocean. At this point, the remainder of the NADW combines with the water in the Southern Ocean where it spreads into the northern Pacific and starts to gradually upwell along the Equator, becoming shallower as it does so. It eventually reaches the surface in the northern Pacific before returning to the North Atlantic as part of the upper warm water circulation.
This general circulation pattern is very important to us as its close coupling with the atmospheric circulation allows it to play a dictatorial role in determining our climate and thus other factors such as the global distribution of biomes. The Gulf Stream is responsible for the temperate climate we experience in the UK – a climate that is much milder than countries of similar latitude. The upwellings and downwellings that occur within the circulation are linked to the productivity of the oceans and the currents themselves allowed for the early navigation of the oceans, thus aided development. However, the current pattern has not always existed and is vulnerable to many external factors and cyclic atmospheric changes in the future. The cessation of the thermohaline circulation and possible alterations to the current pattern could be provoked in the future by many external forcing factors and the outcome of such an event would have a huge, although in the long run not irreversible, impact on us.

I realise that it is rather lengthy but I hope it didn't bore you too much! This is kind of what I would class as the basics of ocean circulation and there is lots more (especially a lot of maths!) and so if you have any questions feel free to ask them and I will try to answer them. Some posts on some of the more scientific stuff that I have been learning with the Met Office, like things linked with vorticity and its importance etc, will be appearing soon! Like I said before any comments would be greatly appreciated!

No comments:

Post a Comment