Ocean Circulation and Climate Change

Ocean circulation has always been thought to be driven by two main factors: the winds moving the sea  surface creating gyres and the deep ocean being moved by thermohaline, thus the icing of cold water in the poles resulting in salification and change in seawater density. 

It has now been corroborated instead, that most of the circulation in the ocean is driven by winds and tides which put the cold surface water in motion towards warmer climates where its density makes it sink. During the LGM (Last Glacial Maximum) deep sea water is known to have been very static as the water has been found very old in relation to the atmosphere since the poorly ventilated deep ocean experienced carbon sequestration from the terrestrial atmosphere. However, at the same time, the winter winds were blowing much stronger than now because of the thermal difference between atmosphere and hemisphere which is commonly acknowledged in the winter periods.

How can deep sea water be so stable with such strong winds?

The first climate models predicting the ocean circulation, forecast a weakening of the ocean overturning coming mostly from the warming of the atmosphere increasing the hydrological cycle, thus making surface water less dense and less able to sink.

At the time of LGM, the low levels of CO2 in the atmosphere prevented a strong warming of the lower longitudes, therefore inhibiting the thermal contrast in the middle of the atmosphere, which consequently as well weakened the westerlies winds, thus refuting the previous argument of having been strong winds during the LGM.

In order to better comprehend the concept of ocean circulation, it is highly beneficial to create visualisations in our minds in order to conceive such an interconnectedly global concept. Moreover, it is very valuable to be able to read topographic as well as satellite maps in order to translate the environmental events from image to notion, thus be able to also corroborate the oceanographic literature.

Referring to the events of ENSO, the below map can, for example, show us how the movement of the surface water towards west caused jointly by the wind in Eastern America and the rotation of the Earth allows deep seawater rich in nutrients to upwell, therefore creating a nourishing environment for plants and animals. In below image, the phytoplankton reflects predominantly green light back into space as opposed to the water itself that predominantly reflects blue wavelengths back to space. 

The Coriolis Effect is the natural phenomenon consisting in the effects which Earth's rotations have on ocean currents and general weather patterns.

In order to explain what this event consists in, visualisations help a lot to conceive the idea of the movement by bringing it from a global scale to a smaller more palpable one.

In a few words, the conceptualisation is simple: the rotation of the Earth from west to east causes ocean currents, storms, air travel to swirl clockwise in the Southern hemisphere and counterclockwise in the Northern Hemisphere. The Earth always takes 24h to rotate on itself, this means that if you would be standing next to the poles or on the equator it would take you the same amount of time to run a circle of 6 feet or a 25'000 miles long one. For this reason, some trains running at different altitudes will look like they're moving at the same speed if looked from above when in fact they are just traveling a different distance (on the equator a longer one whereas above or below a shorter one). 

How does this link to ocean currents?

When water is moved by the wind, it will follow the rotation of the Earth. According to the Coriolis effect above explained, it will do so in a circular way because of the continuous movement towards West which will cause a counter clockwise movement from North to South and clockwise from South to North.

In the specific case of a cycloned, the external high pressure air is attracted to move towards the internal low pressure one, however in its course, it is deflected so creating a swirl of wind.

As a conclusion, the Coriolis effect is a natural physical motion related to the motion of the object, the motion of Earth, and the latitude. 

References:

Millner, J. 'Ocean Circulation and Upwelling Visualizations", Oceanography Topical Resources (2021).  

Toggweiler, J.R. and J. Russell ‘Ocean circulation in a warming climate’, Nature 451(7176) 2008, pp.286–288. 

Julie Brown, Ocean Currents and Climate, National Geographic Society (2022)

https://education.nationalgeographic.org/resource/coriolis-effect

https://www.businessinsider.nl/why-hurricanes-spin-counterclockwise-2017-9?international=true&r=US