Vertical Ocean Circulation
What does thermohaline circulation mean?
Which ways to gyres rotate in the northern hemisphere?
Clockwise
Oxygen minima zone
Area usually 300-800 m deep where oxygen is depleted due to high levels of organic matter decomposition (and therefore respiration)
What controls ocean pH?
Dissolved Inorganic Carbon - as more carbon dissolves, the acidity increases (think CO2)
& total alkalinity - increases pH/makes it less acidic by accepting protons
How deep are the oceans?
On average, about 3.5 km
How long does it take for water to make it through from the North Atlantic to North Pacific?
1-2 thousand years
Ekman transport
The process by which water is deflected through the top 100m of the ocean. At the surface, water is deflected 45˚ away from the direction of wind. Each layer has slower and more diverted water (friction) so that at 100m, water is flowing slowly in the opposite direction of the wind. Net transport is to 90˚ of the wind.
Nutricline
The depth at which nutrient concentrations increase most (because they are depleted at the surface by phytoplankton)
Oxygen is more soluble at cold temperatures, so as the climate warms, less will dissolve into the surface. Increasing organic matter will also use up more oxygen through respiration. And the slowing thermohaline circulation (waters are warmer and ice melting reduces salinity in the Arctic, reducing the formation of AABW), means that the bottoms of the ocean will be less well ventilated.
What are the two main mechanisms for mixing in the ocean?
Chemical diffusion & turbulent mixing
Why do the ocean and atmosphere circulate?
Where does upwelling occur?
Where there are patterns of divergence! Reverse gyres (such as the Norwegian current), around the equator (along the ITCZ), and around Antarctica between the East and West Wind Drifts
What are the three main element behaviours in the ocean, and what do their depth profiles look like?
Nutrient type - used in biological processes, NOT particle reactive: depleted at the surface, increases with depth (P, N, Si, Fe, Zn)
Conservative elements - NOT used in biological processes, NOT particle reactive: concentration is scaled with salinity (Na, Cl)
Scavenged elements - particle reactive, NOT used in biological processes: concentration is highest at the surface and decreases with depth as they are adsorbed onto particles (Al, Pb, Cr)
Which is more acidic: the Pacific or the Atlantic?
The Pacific.
Both DIC and total alkalinity increase with depth because of marine snow. As particles fall in the snow, CaCO3 dissolves and OM reprecipitates. Both contribute DIC, and CaCO3 dissolution also increases the total alkalinity
The increased DIC outweighs increasing alkalinity, so pH is driven down as water ages. pH in the Pacific is close to 7.2. pH in the Atlantic is close to 8.0
Salt fingering
An effect of double diffusion. When overlying water is saltier and warmer, temperature diffuses faster than salinity, so once temperature has homogenised with depth, the overlying water is more dense because it is still more saline. It will sink, sometimes as "salt fingers" which are units of salty water ~5cm wide and ~10 cm long
Where does deep water form?
In the North Atlantic (NADW - very salty, pretty cold) & in polynyas around Antarctica (AABW - very cold, pretty salty)
What is the geostrophic current?
When the pressure gradient force (PGF) - which acts away from the high pressure center of a gyre - and the Coriolis force are equal and opposite, geostrophic balance is reached. This causes water to flow ALONG rather than across lines of equal pressure (isobars). This flow is the geostrophic current.
How can we work out temperatures of past oceans?
Sr/Ca (strontium to calcium) ratio in skeletal corals - less strontium substitutes for calcium at higher temperatures
How is stratification expected to change with climate change? What are the consequences of this?
The surface layer is becoming warmer, so oceans are becoming more stratified. This reduces vertical mixing and the upwelling of nutrients, limiting primary productivity? However, there's some evidence that stratification is weakening in the Southern hemisphere, so who really knows
Which parts of the ocean are most stratified?
The tropics because the surface is warmest and therefore least dense
The gulf stream (surface current) moves North across the Atlantic and mixes with cold fresh water from the Fran Strait. The salty water cools and sinks in chimneys of the Labrador & GIN seas. This deep water then overflows and spreads out across the ocean floor
What is westward intensification?
The building up of water to the west side of ocean basins - The highest pressure in all gyres is offset to the west because the Coriolis force is stronger at higher latitudes, causing the water to turn more. Near the equator, the water does not turn as much unless it runs into a land mass. Therefore, water builds up against the land mass to the west.
How can we work out nutrient concentrations of past oceans?
Hard to figure out N and P... but we can approximately estimate the Cd/Ca ratio by looking at this ratio in calcium carbonates. When this ratio is high, this reflects zones of upwelling
Where do we find calceous ooze?
Calceous ooze forms only when CaCO3 is still being preserved at depth. This occurs when the carbonate compensation depth (CCD) is deeper than the ocean floor. The CCD depends on how much Ca and CO3 has already been dissolved in the water - the more that is already dissolved, the less soluble CaCO3 will be (the higher the saturation state). Since the Pacific is more acidic, there is a lower concentration of carbonate ions (CO3-) (think of the Bjerrum plot). This means that CaCO3 is more soluble in the ocean, so the CCD is higher and CaCO3 is less readily preserved.
Therefore, calceous ooze forms more readily in the Atlantic and in shallower areas (above the CCD). As such, red clays are more common in the Pacific, especially in the North Pacific
Isopycnal
A line joining points of equal density