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Oceanography
Lecture 2: Ocean Circulation
Ocean Currents: Surface Currents
• The surface ocean is the water above the pycnocline (a mixed layer which varies in
depth throughout the year, and from place to place, meaning the depth of surface
currents also varies by time and place)
• Heavily affected by the continents, blocking water and forcing it move
• Primarily horizontal motion
• Driven by prevailing winds in the atmosphere, causing frictional drag - ocean currents
therefore follow the major wind belt pattern in the atmosphere
• Ocean currents are mostly constant, however monsoons as well as the ITCZ cause
the wind belt to change slightly monthly, altering ocean current circulation somewhat
Global atmospheric circulation
• Incoming sunlight reaches the Earth’s surface, warming the surface up which in turn
warms the air around it
• At the Equator, the Earth receives much more solar radiation than at the Polar
regions, meaning it is a much warmer area
• The differences in temperature between the Equator and the Poles results in the
formation of convection cells (warm, moist air that is less dense rises while cool, dry
air that is more dense sinks)
• Convection cells occur in the Earth’s atmosphere, between the surface of the Earth
and the upper troposphere
• There are a 3 different circulation cells:
• Hadley Cell - 0-30° latitude
• Ferrel Cell - 30-60° latitude
• Polar Cell - 60-90° latitude
• Low pressure zones have rising air
• Equatorial low - Equator
• Subpolar lows - 60° latitude
• High pressure zones have sinking air
• Subtropical highs - 30° latitude
• Polar highs - 90° latitude
• The boundaries between these wind belts
(or convection cells have various names:
• At the Equator: Doldrums or the
Intertropical Convergence Zone
(ITCZ)
• At 30° latitude: Horse Latitudes
• At 60° latitude: Polar Fronts
The Coriolis Effect
• The surface winds are not totally straight, but curved due to the Coriolis effect
• At the Equator, the Earth’s radius is far greater, so it spins at a higher velocity (has to
rotate quicker than at the poles to complete one orbit in a day), more than 1,600 km/h
• At the Poles, the Earth spins at a velocity of almost 0 km/h, as it has a far smaller
radius so must rotate much slower to complete an orbit in one full day
• The velocity with which the Earth rotates decreases with increasing latitude

• This causes wind to be deflected as it moves to an area of greater or lesser velocity
• Winds are deflected to the right in the Northern Hemisphere
• Winds are deflected to the left in the Southern Hemisphere
Global Wind Belts
• The winds that operate on the surface of the ocean in the Hadley cells are known as
the Trade winds, which blow from the Subtropical highs to the Equator
• In the Northern Hemisphere, Trade winds blow from the Northeast to the
Southwest - called NE Trade Winds
• In the Southern Hemisphere, Trade winds blow from the Southeast to the
Northwest - called SE Trade Winds
• The winds that operate on the surface of the ocean in the Ferrel cells are known as
the Prevailing Westerlies, which blow at a latitude of 30°-60°
• The winds that operate on the surface of ocean in the Polar cells are known as the
Polar Easterlies, which blow at a latitude of 60°-90°
Subtropical Gyres
• These winds push the circulation of the oceans
• The trade winds and prevailing westerlies act to spin the water round in a circular
motion
• These circular loops of water are called subtropical gyres
• There are 5 subtropical gyres in the world centred around 30°
• They are found in the:
• North Atlantic
• South Atlantic
• North Pacific
• South Pacific
• Indian Ocean
• There are 1 of 4 types of ocean
current in any given gyre
• Equatorial Currents
• Western Boundary
Currents
• Northern/Southern
Boundary Currents
• Eastern Boundary Current
• After the 2011 Japanese Tsunami, debris was taken off the land by the waves and
into the ocean and, as Japan is located in the corner of the subtropical gyre found in
the North Pacific Ocean, was carried by the currents around the gyre towards
America
• As well as much of the debris ending up on the American Coastline, the gyre also
took small organisms and fish historically only found in Japan across America, the
populations of which are now gaining a foothold in the US, raising all sorts of
questions on the impact this will have
Ocean Currents and Climate
• Warm ocean currents (warm, humid air above) lead to a humid climate on their
adjoining landmass
• Cool ocean currents (cool, dry air above) lead to a dry climate on their adjoining
landmass
• For example:
• South Atlantic Ocean subtropical gyre

• A warm ocean current moves along the Equator to Brazil, leading to warm air
filled with moisture arriving here, causing lots of rainfall and partly
contributing to the Amazon rainforest
• As the water moves back parallel to Antartica it becomes very cold, meaning
the air above it is cold so cannot hold moisture, causing a very dry, arid
climate where it meets Africa, leading to the Kalahari desert found here
Ocean Currents: Deep Ocean Circulation
• Deep ocean currents are driven by differences in density caused by differences in
temperatures and salinity
• Deep ocean currents have both vertical and horizontal motions
• The density of the ocean is calculated by the Equation of State: ρ = density
s = salinity
• ρ (s, t, p) kg/m³
t = temperature
• Freshwater density = 1,000 kg/m³
p = pressure
• Ocean surface water density = 1,022 - 1,030 kg/m³
• Desity anomaly (σ) = Density - 1,000
• So freshwater = 0 kg/m³
• Ocean surface water = 22-30 kg/m³
• Generally speaking, the water gets more with increasing distance towards the Poles
Density
• Density increases with:
• Decreasing temperature
• Except for that from 0-4°C density decreases as temperature decreases
• Increasing salinity
• Increasing pressure
Sea Ice and Brine Rejection
• The highest density ocean water is found where sea ice forms, which then sinks
down into the deep ocean, this happens both in the North and South Poles
• When sea ice forms, all the salt in the water is rejected into the ocean, called brine
rejection
• The remaining ocean water therefore is much more concentrated with salt
• Increased salinity leads to increased water density
• Therefore, the density of the water here is very high due to this process
• This happens in the GIN Seas (Greenland, Iceland and Norway)
• The water here becomes so dense that it sinks down, forming the North Atlantic deep
water (NADW)
• The same thing happens off the coast of Antarctica, forming Antarctic Bottom water
(AABW)
• As the two ocean currents, one in the North Pole and the other in the South, reach
the sea floor they are pushed towards the Equator and eventually meet
• The AABW then flows underneath the NADW because it has a far greater density
• Antarctic Intermediate Water (AAIW) is also formed slightly further away from
Antarctica, but as there is less sea ice here the water is less dense and so sinks to a
depth above the NADW
• The NADW is very important as it forms the beginning of a global ‘conveyor belt’ of
deep ocean currents, often referred to as the AMOC (Atlantic Meridional Overturning
Circulation), or the Global Thermohaline Circulation (THC)
• As water travels around the AMOC, it slowly warms up and begins to rise, eventually
becoming surface water

Link between surface and deep ocean currents
• In the North Atlantic subtropical gyre, not all the water circulates round, with some
branching up and flowing towards Britain and Europe - it is this water that cools and
forms the NADW in the GIN Seas, and is called the Gulf Stream
• The Gulf Stream current is driven by two things, the winds and also by the formation
of this deep water - as water is sinking in the GIN Seas, the water in the Gulf Stream
is pulled Northwards to replace the lost water
Gulf Stream
• The Gulf Stream is a warm North-moving current
• It therefore warms the East Coast of the US and Northern Europe
• The Gulf Stream then extends into the North Atlantic and Norwegian currents,
warming Northwest Europe
• This means our climate is far more warm and mild than would otherwise be expected,
particularly in winter months
• Britain is the same latitude as NYC and Washington DC in America, which in
winter are marked by extreme snow and cold conditions, whereas
temperatures in Britain remain fairly mild, because those areas of America
don’t enjoy the benefits of the Gulf Stream as we do
Current Debates
• Questions have now been raised about how climate change will impact on our
climate, seeing as the Gulf Stream is so important in maintaining our mild
temperatures
• Climate change is causing the world’s oceans to warm,
• This is affecting the density of the oceans - warmer water is less dense
• It is also altering the salinity of the oceans - the melting of large areas of ice
sheets across Antartica and Greenland is injecting huge volumes of water
with no salt in it into the ocean
• This has raised the issue of whether the AMOC and the Gulf Stream is slowing down
• The temperature in the GIN Seas is increasing due to climate change
• This is decreasing the density of the water, so it is less likely to sink down
• Simultaneously, an increase in freshwater from melting ice sheets in
Greenland is decreasing the salinity of the water, further preventing the
formation in NADW
• Therefore the global conveyor belt of deep ocean water AMOC may be
slowing - in turn slowing down the Gulf Stream
Is the AMOC slowing?
• A slowing of the Gulf Stream would have a drastic impact on the climate
• Computer models have shown that if there was no formation of NADW, slowing the
Gulf Stream, temperatures in Britain and Europe would drop by about 4°C
• Winters would be far similar to those seen in NY, Canada, etc
...
has shown there is spatial
variability, with some parts of the Gulf Stream actually getting faster, with others
slowing



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