ENVR 2200 - Lecture 12: The Ocean

What is AMOC?

AMOC is a large overturning loop of water in the Atlantic

How does AMOC work broadly?

AMOC works via warm. salty surface water flowing north. It gives off heat to the atmosphere in the North Atlantic then it becomes cold and dense causing it to sink, and return to the south as deep water.

What causes water to sink in the ocean? What determines this level in water?

Sinking water is caused by the density of the water. The density of water depends on salinity of water and the temperature.

What is Thermohaline circulation?

Thermohaline circulation is water global circulation based on the temperature (thermo) and salinity (haline)

Is colder water more dense than warm water?

Yes cold water is more dense than warm water and sinks compared to warm water. Ice is less dense than water because the structure of water changes when it gets cold enough. Do not get it confused that colder water floats on warm water because of ice being cold. Ice is a different chemical structure. Cold water is denser than warm water because the molecules are moving less fast becoming closer together, increasing density.

Is salty water more dense than fresh water?

Yes, salty water is more dense than fresh water. Salty water has ions of Cl and Na salt taking up space within the water which increases the density. Salty water sinks below freshwater.

Temperature and salinity can vary in the ocean, but what is the important thing to understand with density, salinity, and temperature with AMOC?

As warm water flows northward and loses its heat, it can freeze leaving salt in super cold water. Super cold water and extra salt concentration causes water in the polar regions to be very dense and sink to the bottom of the ocean. This super dense water sinking is the engine of AMOC

Is AMOC a type of thermohaline circulation>

Yes, AMOC is a just a very large segment of the Earth’s total thermohaline circulation

What happens to ocean circulation as ice from Greenland melts and there is more precipitation into polar oceans?

Adding in freshwater from melting ice in Greenland and added freshwater precipitation decreases the salinity of polar oceans. Less salty Atlantic water means less dense water that is needed to be hyper-dense and sink. Less sinking of the cold-salty water weakens the engine of AMOC and weaknes the circulation of the atlantic ocean.

Is AMOC expected to collapse this century?

No, AMOC is expected to slow, but it is not expected to collapse this century. Warm surface water will still move poleward even if the AMOC weakens. Differential solar heating (heating in different parts of the world) drives the wind belts, and wind stress plus the Coriolis effect drives subtropical gyres.

What is Ekman transport?

Ekman transport is the process in which water blows 90 degrees to the right in the NH and to the left in the SH of the direction of the wind.

If you are standing on a beach in the Northern hemisphere and looking out to sea to the East and wind is blowing from your right from the south to the north, what direction is the water moving? What is happening to the water close to the beach?

The Ocean is moving 90 degrees to the right of the wind direction so moving the water out to sea in the direction you are facing. The water close to the beach is being pushed away and new deeper water is upwelling to the surface.

What creates rotating gyres?

Ekman transport piles water int gyre centers creating rotating gyres. These gyres also concentrate plastic

What direction do gyres rotate in the NH?

In the NH, gyres rotate clockwise

What is an example of a Western Boundary Current?

The Gulf Stream is an example of a Western Boundary Current

What are Western Boundary Currents like?

Western boundary currents are narrow, fast, and warm. Western boundary currents are currents on the western side of oceans. The Gulf Stream is on the western side of the Atlantic and is a western boundary current.

What are eastern boundary currents like?

Eastern boundary currents are wide, slow, and cool. Eastern boundary currents are on the eastern side of oceans. An eastern boundary current in the Atlantic is the Canary current. In the Pacific the eastern side of the Pacific has the California current.

How do you remember Eastern and Western boundary current attributes?

Eastern currents are on the east side of oceans and a good example is think East of pacific is California, and these currents are cool, wide, and slow just like California is a cool state with a slow vibe and is very wide.

Western currents are on the west side of the atlatic which is the East coast which is narrow fast paced, although remember Florida is our tropical paradise on the East so the Gulf Stream is warm and Western currents are warm.

What is happening to the Gulf Stream and how is it affecting the Gulf of Maine?

The Gulf of Maine is warming faster than almost all of the global ocean as the Gulf Stream shifts North.

What is upwelling?

Upwelling is when nutrient-rich deep water rise up to the sunlit zone and give nutrients to support life and major fisheries

What cells are moving weather and wind from the North to the South of the Planet?

Hadley cells, Ferrell Cells, and Polar Cells move wind and cause weather changes as you move north or south on Earth. Hadley Cells tell you why it rains at the equator and why there are subtropical deserts at 30 degree.

What circulation explains how air moves between different oceans and continents from East-to-West?

Walker Circulation looks at movement of air from East to West. Walker Circulation tells you why it is rainier in Indonesia/Northern Australia and drier in Peru. Walker Cells also explain how El Niño messes up that balance.

What direction do trade winds go?

Trade winds blow to the West. Trade winds come from the east and blow west.

What direction do Westerlies blow?

Westerlies blow to the east. They are called westerlies because they originate in the west and blow east.

What does the weather and wind circulation usually look like in the Pacific?

Typically trade winds pile warm water in the west of the pacific near Indonesia/Northern Australia. Typical walker circulation brings rain to the western pacific and dry air to the Eastern pacific like Peru and Chile

What does the weather in the pacific look like with El Niño?

In El Niño, the trade winds (the trade winds blow west) weaken. Warm water moves east to the americas and warms the surface layer of water pushing deep colder water down. Ekman transport continues to create upwelling at the coast but now the upwelling is nutrient poor warm water with deeper water inaccessible.
El Niño = bad fishing in Peru

What does the weather look like with La Niña?

La Niña is the opposite of El Niño. In La Niña trade winds blowing west intensify even more than typical. This causes warm water to move towards Indonesia and Australia. The eastern pacific is left with cold water and upwelling of Peru brings very nutrient rich deep water as the pull westward is very strong.

What is Bjerknes Feedback?

In normal weather on the pacific Easterly (blowing west) trade winds keep warm water piled up in Australia and Indonesia and upwelling in South America. If something disrupts the trade winds and blows the easterly trade winds back to the east, it weakens the system. The cold to warm gradient formed by the trade winds causes the upwelling. If the trade winds blow towards the East they bring warm water to the East and equalize the gradient of ocean temperature. The ocean temperature is what causes trade winds.

weaker trade winds = less ocean temperature gradient = weaker trade winds

Bjerknes Feedback is a positive feedback loop

What is ENSO tracked with?

ENSO is tracked with the Niño 3.4 Temperature anomaly. When the temperature in this zone reads above 0.5 degrees celcius it is El Niño. When the zone reads below -0.5 degrees celcius, it is La Niña.

Where does deep water form on the planet?

Deep water forms in the North Atlantic (Near Greenland, Iceland, Canada. The deep north pacific holds the oldest water

Where does AMOC carry heat?

AMOC carries heat north in the Atlantic towards Europe.

What are the base of marine food webs?

Phytoplankton are the base of marine food webs. Examples of phytoplankton are Diatoms, coccolithophores, and cyanobacteria)

Where are light and nutrients in the ocean? What needs to happen to make the ocean productive?

Light is located at the surface and nutrients are located at depth in the ocean. Life needs light and nutrients so where the ocean brings together light and nutrients is where the ocean is productive.

Where is oxygen in the ocean?

The surface is more oxygen rich than the deeper parts of the ocean

What limits growth day to day in the oceans? What limits growth in the ocean over geologic time?

Nitrogen limits growth day to day in the ocean. Phosphorous limits growth in the oceans over geologic time and iron sometimes limits growth over geologic time in some regions like the Southern Ocean.