Ocean current complete description of oceans

related to ocean currents

1. Motion in the Ocean: Ocean Currents and Circulation 10 th October 2023 Dr. Samiran Mandal Centre for Atmospheric Sciences Indian Institute of Technology Delhi, New Delhi.

2. 2 Source : NOAA Worldwide SST Variability Knowing the role of SST is essential to predicting weather patterns and operational oceanography. Ocean: Source and Sink Abrupt warming of ocean has severe consequences for marine life and sea level rise by changing density. Marine Heatwave MHW can cause significant damage to ecosystems and socio-economic condition of fisheries. Global Warming

3. Oceans - The Regulator of Temperature Heat gained by the oceans in equatorial latitudes equals heat lost in polar latitudes. On average, these two balance each other, and the excess heat from equator is transferred to heat-deficient poles by both O ceanic and Atmospheric C irculations . How ??

4. 4 Carrier of Ocean Heat - Ocean Currents An idea to keep in mind is that warmer currents flowing (Western B oundary C urrents) from Equator to the Poles would carry warmth and colder currents flowing from Poles to the Equator (Eastern B oundary C urrents) also redistribute heat in terms of carrying cooler water out of the Polar regions and putting them in the equatorial regions to cool these regions. 

5. As you stand near the ocean and watch the waves come towards you, you want to understand how these waves might work. How are these waves generated? How do they propagate? Is there a pattern of propagation? Why do some of these waves break? Length Scale is a representative length by which you can describe the process or phenomenon. Time-Scale is the representative duration of time over which the phenomenon’s major changes can be described. Now, think about the spatial scales of these waves and how frequently you see the crests coming one after the other. Their typical spatial scales are meters and temporal scales are around seconds and minutes. And since these waves are generated and supported by gravity, they are called Surface Gravity Waves . Spatio -Temporal Scales

6. Few Gyres Round the World !!! There are six great current circuits in the world ocean, T wo in the Northern Hemisphere. Four in the Southern Hemisphere. Five of these circuits are large gyres. Five G yres: North Atlantic (Columbus) Gyre, South Atlantic (Navigator) Gyre , North Pacific (Turtle) Gyre, South Pacific (Heyerdahl) Gyre , Indian Ocean (Majid) Gyre . Sixth and the largest current circuit is the Antarctic Circumpolar Current (ACC) in Southern Hemisphere.

7. How are Gyres Formed ?? Three forces cause the circulation of a gyre: Global W ind P atterns, Earth’s Rotation, and Earth’s Landmasses. Wind drags on the ocean surface, causing water to move in the direction the wind is blowing. Earth’s rotation deflects, or changes the direction of, these wind-driven currents. This deflection is a part of the Coriolis effect. Northern Hemisphere – Ocean currents are deflected to Right – in clockwise motion. Southern Hemisphere – Ocean currents are deflected to Left – in counter-clockwise motion. An ocean gyre is a large system of circular ocean currents formed by global wind patterns and forces created by Earth’s rotation.

8. What are Boundary Currents !!

9. Western Boundary Currents When equatorial currents reach the western portion of an ocean basin, they must turn because they cannot cross land. The Coriolis Effect deflects these currents away from the equator as Western Boundary Currents ( red arrows ), which comprise the western boundaries of subtropical gyres. Western Boundary Currents are so named because they travel along the western boundary of their respective ocean basins. For Example, the Gulf Stream and the Brazil Current , are Western Boundary Currents. They come from equatorial regions, where water temperatures are warm, so they carry warm water to higher latitudes. These are narrow, warm, and strong currents . Canary

10. Eastern Boundary Currents When currents flow back across the ocean basin, the Coriolis effect and continental barriers turn them toward the equator, creating eastern boundary currents ( blue arrows ) of subtropical gyres along the eastern boundary of the ocean basins. Examples of eastern boundary currents include the Canary Current and the Benguela Current . They come from high latitude regions where water temperatures are cool, so they carry cool water to lower latitudes. Similarly, in the eastern branch of these geostrophic gyres, Eastern Boundary Currents , which are wider, colder and slower currents. Canary

11. Role of Gyres and Boundary Currents

12. Transverse Currents The poleward or equatorward currents that form the western and eastern boundaries , respectively, of the subtropical circulation gyres.  The east-west flowing currents that complete the gyres  are called Transverse Currents . Driven by Trade Winds, i.e., stronger the trade winds, so consistent are the transverse currents. ( Equatorial Currents are Stronger )

13. All-in-All Ocean Currents

14. Are Eddies and Gyres different ? Gyres are spiraling circulations thousands of miles in diameter and rimmed by large, permanent ocean currents. Eddies are smaller , temporary loops of swirling water that can travel long distances before dissipating . Eddies Current

15. Mesoscale Eddies Eddies are relatively small, contained pockets of moving water that break off from the main body of a current and travel independently of their parent. They can form in almost any part of a current.

16. Mesoscale Eddies in Bay of Bengal

17. What exactly are Ocean Currents ? Ocean currents are the continuous, predictable, directional movement of seawater. It is a massive movement of ocean water that is caused and influenced by various forces.  Two types of Major C urrents, – Surface Currents , which are horizontally flowing water in the uppermost ocean ( above the pycnocline ) driven by thermal expansion and contraction, and wind friction. – Deep Ocean Currents/ Thermohaline Circulation which are comparatively much slower, deeper circulation ( below the pycnocline ) due to the action of gravity on water masses of different densities.

18. Forces driving the Surface Currents !! Factors Related to the Earth’s R otation : Gravitational Force and Coriolis Effect. Atmospheric Factors : Surface Winds, Evaporation, Pressure and Radiation. Oceanic Factors : Pressure Gradient, Temperature and Salinity Gradients. Factors Modifying the Ocean C urrents : Direction and Shape of the Coast, Seasonal V ariations and Shapes of Ocean Basin.

19. Coriolis Force The rotation of the earth affects any moving object due to the object’s motion, or in other words deflects the moving object from its original path. This deflection can then be thought of as being caused by an “ Apparent Force ” acting on the moving object! If we can quantify this deflection of a moving object in terms of a “rate of change of velocity” as “force per unit mass” or the Coriolis Acceleration .

20. Wind-Driven Currents Nansen spearheaded a cross-Greenland expedition between 1881 and 1896 to understand the polar currents and Greenland ice coverage . Nansen and his fellow explorer, Johanesen were on a trek in their vessel Fram in September of 1895, when the Fram was frozen into the Arctic ice drifting slowly in an eastward direction as the ice was being pushed by the surface currents. Nansen noticed that the direction of the drift was not following the wind ; instead, was about 20–40 to the right of wind ! Later, Ekman developed the first theory of the wind-driven currents and described the veering of the currents from surface to layers below in the so-called “ Ekman Spiral ”.

21. How the Wind moves the Water ? When surface waters are moved by the wind, they drag deeper layers of water molecules below them. Like surface water, the deeper water is deflected by the Coriolis effect– to the right in the Northern Hemisphere and to the left in the Southern Hemisphere . As a result, each successively deeper layer of water moves more slowly to the right or left, creating a Spiral Effect, known as Ekman Spiral. Ekman Depth is functionally defined as the depth at which the current moves in the opposite direction of the wind stress .

22. How the Wind moves the Water ? Northern Hemisphere : Net Transport is 90° to the Right of Wind Direction. Southern Hemisphere : Net Transport is 90° to the Left of Wind Direction. The transport of water in this layer is called Ekman Transport .

23. Ekman Pumping/ Ekman Suction Another means by which Ekman Transports can generate vertical motion is either through the Ekman Pumping (Downwelling) or Ekman Suction (Upwelling) .

24. Geostrophic Currents Ekman Transport piles water toward the Centre of the Gyre . Force of Gravity on the piled-up water creates pressur e gradient away from centre of the Gyre . The balance between these forces is known as Geostrophic Balance. Gravity and Coriolis Effect balance to create an ideal geostrophic current that flows in equilibrium around the hill. However, friction makes the current gradually run downslope (actual geostrophic flow).

25. Why Western Boundary Currents ?? Western Intensification is a result of Earth’s rotation ( the change of Coriolis Parameter with latitude ) and causes the western boundary currents of all subtropical gyres to be fast, narrow, and deep .

26. Ocean Currents

27. Deep currents occur below the pycnocline , so they influence about 90% of all ocean water. The  thermohaline  term is derived from thermo-  referring to  temperature  and  - haline  referring to  salt content , factors which together determine the  density of seawater . Density differences create deep currents . Although these density differences are small, but they are large enough to cause denser waters. Deep-water currents move larger volumes of w ater and are much slower than surface currents . Typical speeds of deep currents range from 10 to 20 kms per year. There are five common water masses: Surface Water, Central Water, Intermediate W ater, Deep Water, Bottom Water . What Deep-Ocean Currents Exist?

28. Thermohaline Circulation

29. Wind-driven surface currents (such as the Gulf Stream) travel  polewards  from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water). Cold water is usually denser than warm water (4°C is where water is densest). The thermohaline circulation plays an important role in supplying heat to the polar region. Great Ocean Conveyor-Belt

30. The More you Observe the Ocean, the More you are Building an Oceanographer in You !!!