What happens when ocean currents meet
By Mia Tucker
Answer (Detailed Solution Below)
Detailed Solution
Warm currents: Those currents that flow from the Equator towards the poles are warmer than the surrounding water and so they are called warm currents . Examples- Kuroshio current, North Pacific current.
Cold currents: The ocean currents that flow from the polar areas towards the Equator are cooler compared to the surrounding water, so they are called cold currents . Examples- Oya shio current, California current.
Key Points
Formation of Fog:
When the warm and the cold currents meet , dense fog is created. This is because the air above the warm currents is warm which contains water vapor. When this warm current meets the cold current , the air above the cold current , causes the water vapor of the warm current to condense into tiny particles which form fog. This controls the temperature condition in the fishing regions of the meeting grounds.
There are five types of Fog:
- Radiation fog- They occur in the winter, aided by clear skies and calm conditions. The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fog usually dissipates soon after sunrise as the ground warms.
- Valley fog- They form where cold dense air settles into the lower parts of a valley, condensing and forming fog. It is often the result of a temperature inversion , with warmer air passing above the valley.
- Advection fog-They occur when moist, warm air passes over a colder surface and is cooled . A common example of this is when a warm front passes over an area with snow cover. It is also common at sea when moist tropical air moves over cooler waters. If the wind blows in the right direction, then sea fog can become transported over coastal land areas.
- Upslope fog- Upslope or hill fog forms when winds blow air up a slope (called orographic uplift). The air cools as it rises, allowing moisture in it to condense.
- Evaporation fog- It is caused by cold air passing over warmer water or moist land. It often causes freezing fog, or sometimes frost. When some of the relatively warm water evaporates into low air layers, it warms the air, causing it to rise and mix with the cooler air that has passed over the surface. The warm, moist air cools as it mixes with the colder air, allowing condensation and fog to occur.
Hence, the correct answer is Radiation fog.
In the North Atlantic, water heated near the equator travels north at the surface of the ocean into cold, high latitudes where it becomes cooler. As it cools, it becomes more dense and, because cold water is more dense than warm water, it sinks to the deep ocean where it travels south again. More warm surface water flows in to take its place, cools, sinks, and the pattern continues.
Worldwide, seawater moves in a pattern of currents known as thermohaline circulation, or the global ocean conveyor. The currents flow because of differences in water density and move between the deep and surface ocean.
However, melting Arctic sea ice and melting Greenland glaciers could change this pattern of ocean currents, or stop it altogether.
Recent research shows that Arctic sea ice is melting due to climate warming. The melting ice causes freshwater to be added to the seawater in the Arctic Ocean which flows into the North Atlantic. The added freshwater makes the seawater less dense. This has caused the North Atlantic to become fresher over the past several decades and has caused the currents to slow.
Water that is less dense will not be able to sink and flow through the deep ocean, which may disrupt or stop the pattern of ocean currents in the region. Scientists estimate that, given the current rate of change, these currents could stop within the next few decades.
What would happen if Atlantic ocean currents stopped?
Even though warming is causing the disruption to ocean currents, stopped or slowed currents in the North Atlantic would cause regional cooling in Western Europe and North America. The ocean currents carry warmth from the tropics up to these places, which would no longer happen. If the currents were to stop completely, the average temperature of Europe would cool 5 to 10 degrees Celsius. There would also be impacts on fisheries and hurricanes in the region.
The currents in the North Atlantic are part of a global pattern called thermohaline circulation, or the global ocean conveyor. If they were to stop, this would not be the first time that the global ocean conveyor was halted. There is evidence from sedimentary rocks and ice cores that it has shut down several times in the past which caused changes in climate. One of the most well-known, called the Younger Dryas Event, happened about 12,700 years ago and caused temperatures to cool about 5 degrees Celsius in the region.
© 2019 UCAR with portions adapted from Windows to the Universe (© 2011 NESTA)
Ocean Currents and Climate
There are two type of Ocean Currents:
1. Surface Currents — Surface Circulation
These waters make up about 10% of all the water in the ocean.
These waters are the upper 400 meters of the ocean.
2. Deep Water Currents — Thermohaline Circulation
These waters make up the other 90% of the ocean
These waters move around the ocean basins by density driven forces and gravity.
The density difference is a function of different temperatures and salinity
These deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.
Ocean Currents are influenced by two types of forces
1. Primary Forces –start the water moving
The primary forces are:
1. Solar Heating
2. Secondary Forces- -influence where the currents flow
1. Surface Circulation
Solar heating cause water to expand. Near the equator the water is about 8 centimeters high than in middle latitudes. This cause a very slight slope and water wants to flow down the slope.
Winds blowing on the surface of the ocean push the water. Friction is the coupling between the wind and the water’s surface.
A wind blowing for 10 hours across the ocean will cause the surface waters to flow at about 2% of the wind speed.
Water will pile up in the direction the wind is blowing.
Gravity will tend to pull the water down the “hill” or pile of water against the pressure gradient.
But the Coriolis Force intervenes and cause the water to move to the right (in the northern hemisphere) around the mound of water.
These large mounds of water and the flow around them are called Gyres . The produce large circular currents in all the ocean basins.
North Atlantic Gyre
Note how the North Atlantic Gyre is separated into four distinct
Currents , The North Equatorial Current, the Gulf Stream, the North Atlantic Current, and the Canary Current.
But why doesn’t the water spin towards the center of the ocean? Why does it flow around the hill in this circular motion.
Remember the hill of water– This hill is formed by the inward push of water through a process call Ekman Transport
Remember the Coriolis Force move objects to the right in the northern hemisphere
Wind blowing on the surface of the ocean has the greatest effect on the surface. However, for the lower layers of the ocean to move they must be pushed by the friction between the layers of water above. Consequently, the lower layer moves slower than the layer above. With each successive layer down in the water column the speed is reduce. This leads to the spiral affect seen in the above diagram.
The net movement of water (averaged over the entire upper 330 meters of the ocean) is 90o to the right of the wind direction (in the northern hemisphere).
When the water is pushed to the right it forms the hill we described above. So, when water is pushed along by the wind it wants to be turned to the right by the Coriolis force (in the northern hemisphere) but it must fight against gravity (trying to move up the hill of water formed by Ekman transport). A balance is met between the Coriolis and the gravity (pressure gradient force). This balance produces a balanced flow called a Geostrophic current.
Eastern and Western Boundary Currents
Boundary Currents are the major geostrophic currents around the gyre
Note the difference is strength (Sv) between the western and eastern boundary currents. This is caused by the effect of the rotating Earth which tends to move the “hill” of water to the western sides of the ocean basins
The Gulf Stream is an example of a Western Boundary Current
The effect of winds on the vertical movement of water
Upwelling along the coast caused by Ekman transport of waters (waters move to the right of the wind).
The waters moved offshore are replaced by waters from below. This brings cold, nutrient rich waters to the surface.
Downwelling caused by Ekman transport onshore (movement of water to the right of the wind direction).
The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that carry warm water from the tropics northwards into the North Atlantic.
How does the AMOC work?
The AMOC is a large system of ocean currents, like a conveyor belt, driven by differences in temperature and salt content – the water’s density. As warm water flows northwards it cools and some evaporation occurs, which increases the amount of salt. Low temperature and a high salt content make the water denser, and this dense water sinks deep into the ocean. The cold, dense water slowly spreads southwards, several kilometres below the surface. Eventually, it gets pulled back to the surface and warms in a process called “upwelling” and the circulation is complete.
This global process makes sure that the world’s oceans are continually mixed, and that heat and energy are distributed around the earth. This, in turn, contributes to the climate we experience today.
Has the AMOC been changing?
Oceanographers have been measuring the AMOC continuously since 2004. The measurements have shown that the AMOC varies from year to year, and it is likely that these variations have an impact on the weather in the UK. However it is too early to say for sure whether there are any long term trends. Before 2004 the AMOC was only measured a few times, and to go back further into the past we need to look at indirect evidence (for example from sediments on the sea floor). The indirect evidence doesn’t always agree on the details, but it seems likely that there have been some large, rapid changes in the AMOC in the past (for example around the end of the last ice age).
What will be the effect of climate change on the AMOC?
Climate models suggest that the AMOC will weaken over the 21 st Century as greenhouse gases increase. This is because as the atmosphere warms, the surface ocean beneath it retains more of its heat. Meanwhile increases in rainfall and ice melt mean it gets fresher too. All these changes make the ocean water lighter and so reduce the sinking in the ‘conveyor belt’, leading to a weaker AMOC. So the AMOC is very likely to weaken, but it’s considered very unlikely that large, rapid changes in the AMOC, as seen in past times, will happen in the 21 st Century.
The effect of a weaker AMOC is included when making projections of future climate change for the UK. A weaker AMOC will bring less warm water northwards, and this will partly offset the warming effect of the greenhouse gases over western Europe. For the gradual weakening that is likely over the 21 st Century, the overall effect is still a warming.
For more on AMOC tipping point, see our briefing doc (pdf. 1mb)
