How Do Oceans Circulate? Crash Course Geography #9 - Free Educational videos for Students in K-12

How Do Oceans Circulate? Crash Course Geography #9 - By Math and Science

Transcript


00:0-1	Welcome to the North pacific garbage patch , the largest
00:02	human trash dump in the world spreading across the pacific
00:05	ocean . The North pacific garbage patch is really more
00:07	of a series of trash vortices and is the top
00:10	vacation destination for plastic chemical sludge and wood pulp ,
00:13	defining its size is difficult because it's always moving and
00:16	it's mostly a soup of tiny micro plastics , not
00:18	an island of grocery bags and drinking straws , But
00:21	some scientists estimate it's approximately 700,000 km2 , which is
00:25	roughly the size of Texas . Actually , it's one
00:27	of five giant collections of trash circulating in oceans all
00:31	over the earth . If you've spent time on beaches
00:32	, you might have noticed plastic trash like water bottles
00:35	and toys . But there's so much almost invisible junk
00:37	to like small beads or tiny slivers . Each beach
00:41	around the world has this plastic pollution because of how
00:43	trash moves from inland to the ocean and then gets
00:45	caught in ocean currents . Whether you live on a
00:47	coast or not , everyone is connected to global ocean
00:50	currents circulating tremendous amounts of energy and trash . I'm
00:53	al is a career and this is crash course geography
01:04	Somehow trash can show up on a beach in California
01:06	from a pot of trash 2500 nautical miles away east
01:09	of Hawaii . That's so far for one speck of
01:12	plastic or even a whole water bottle with or without
01:14	the secret message to travel to get there , it
01:17	all starts with the wind . In our last episode
01:19	, we explored how the horizontal movement of air ,
01:21	called wind moves in predictable directions and creates the general
01:24	patterns for global circulation in the oceans . We call
01:27	the horizontal movement of water and ocean current . Like
01:30	the winds , ocean currents are basically rivers of energy
01:33	moving in a persistent and predictable direction and also like
01:36	the winds . Ocean currents are driven by differences in
01:38	density and pressure in the air density , and pressure
01:40	changes come from differences in the amount of insulation or
01:43	incoming solar radiation that different parts of the atmosphere receive
01:46	. And ocean water is heated by insulation too ,
01:48	because of how much direct sunlight it gets . Water
01:51	closer to the equator absorbs more heat energy than water
01:54	at higher latitudes . This creates density differences within the
01:56	ocean because just like air , warm water is less
01:59	dense than cold water basically . When heated molecules like
02:02	to spread out , water density is also affected by
02:05	salinity or the salt content of the water . Saltier
02:07	water is more dense than less salty water because there
02:09	are more molecules of salt and water hanging around .
02:12	Warmer water in the ocean expands just like air ,
02:14	but because it can't expand sideways because , you know
02:17	, there's already water there . It expands up ,
02:19	elevating the surface just slightly like a hill and colder
02:22	or saltier water contracts , lowering the surface into a
02:25	depression so the ocean's surface isn't perfectly flat . It
02:28	contains sea surface height anomalies . A hill of water
02:31	exerts extra pressure compared to a dip in the sea
02:33	surface height and in general , whatever is in a
02:35	high pressure area , whether it's air or water or
02:38	a student and stressful class wants to move to a
02:40	low pressure area . So these pressure gradients force the
02:43	water to flow around the globe . Technically it's all
02:45	the same water but separate currents are defined because they
02:48	consistently move in the same way , kind of like
02:50	different highways of water . There are actually 30 major
02:53	named surface currents and dozens more smaller currents transporting ocean
02:56	water around the globe . So if our trash found
02:59	its way to the right spot , it could travel
03:01	the world . We can draw lots of comparisons between
03:03	ocean currents and wind patterns , but surface currents are
03:06	also driven by strong and steady streams of wind .
03:09	Energy is transferred from the winds to the water through
03:11	friction as air blows across the surface . For example
03:14	, the winds that result from subtropical high pressure areas
03:16	around 30° latitude also helped create the ocean currents that
03:20	circulate there in patterns called Gyres . But even though
03:22	ocean currents generally follow the winds , the two aren't
03:25	mere images . Ocean currents come up against huge roadblocks
03:28	that air blows right over continents and large land masses
03:32	. These roadblocks give current irregular shapes , especially in
03:34	places like the indian ocean , the arctic ocean and
03:37	the northern atlantic ocean , where there's lots of land
03:39	currents are also curved by the Coriolis effect . Like
03:42	all fluids on the Earth's surface . Remember the Earth
03:45	is rotating fastest at the equator and slower as we
03:47	move towards the poles . So if something that's not
03:49	directly connected to land moves north or south , the
03:52	change in momentum causes its path to bend . The
03:54	Coriolis effect can actually deflect some surface currents depending on
03:57	where they are on the globe . Like as currents
04:00	move away from the equator , the Coriolis effect gets
04:02	stronger because the Earth's rotational speed rapidly slows down and
04:06	can break up currents into chains of lots of circular
04:08	vortices for Eddie's . These get smaller , the closer
04:11	you are to the polls as the coriolis effect bends
04:13	the currents ever more tightly . Then , as the
04:15	earth rotates faster as we move towards the equator ,
04:18	the Coriolis effect gets weakest until it's basically non existent
04:21	, exactly at the equator . So equatorial currents aren't
04:24	deflected right or left . Water can simply flow in
04:26	a straight direction , pushed by the winds . So
04:28	if we look at the circulation map , most of
04:30	the surface currents making up gyres don't cross the equator
04:33	but flow along at horizontally . Then it's the combined
04:36	forces of winds and the Coriolis effect that causes gyres
04:39	to flow in a clockwise direction in the northern hemisphere
04:41	and a counter clockwise direction in the southern hemisphere .
04:44	But let's go back to the idea that ocean currents
04:46	not only run into stuff like land , they can
04:48	also carry more stuff than wind . Maybe they're a
04:51	highway for eel migration or they play a role in
04:53	a water bottles journey from a store shelf to the
04:55	north pacific garbage patch . At some point , this
04:57	bottle got snagged in the north pacific gyre , which
04:59	is really four currents that follow how the air moves
05:02	around the northern subtropical high pressure area . To imagine
05:05	its journey , let's go to the thought bubble ,
05:06	let's say , our water bottle fell into a municipal
05:08	storm drain in Hawaii . They got flushed into the
05:10	pacific ocean starting at the equator . The trade winds
05:13	drive water west in a flow called the equatorial current
05:16	, This carries our water bottle to the eastern coastline
05:18	of Asia , where warm waters pile up against the
05:21	land , so the warm energy rich waters are deflected
05:23	towards the north pole , pushed by both pressure gradients
05:26	and the coriolis effect that balance each other out .
05:28	They flow into the Kuroshio current moving north along the
05:31	Asian east coast , where our bottle could wave at
05:33	the Philippines in japan . You know if water bottles
05:36	at arms as it reaches the latitude of the westerly
05:38	winds around 35° North , the current begins to wobble
05:41	more , and along with our bottle is pushed eastward
05:43	and separates from the coast . This forms the North
05:46	Pacific current , bringing warm waters to the southern coast
05:49	of Alaska and then land . Hope . Eventually ,
05:51	our water bottle encounters the west coast of North America
05:54	and is deflected back towards the equator . Moving from
05:56	british Columbia Canada to the baja peninsula in Mexico .
06:00	This California current is cold having released the warmth that
06:03	the water was holding in the equatorial and Kuroshio currents
06:06	. If the bottle manages to stay in the gyre
06:07	, it could float on through those warm and cold
06:09	currents for years , traveling through clear moonlit nights and
06:13	rough typhoons because our bottle is a processed plastic bacteria
06:16	don't eat away at it , but wind waves and
06:18	sunlight have broken it down into microscopic particles . So
06:21	all that's left is a bead that ends up floating
06:23	in the north pacific garbage patch . Thanks that bubble
06:26	wind current interactions are actually much more complicated than just
06:30	winds pushing water around and it's an area oceanographers are
06:33	still trying to understand for geographers were concerned with how
06:36	stuff gets moved around the globe . There are actually
06:38	five major gyres in the world , including this north
06:41	pacific gyre , each with its own garbage patch .
06:43	And they all follow similar patterns with warm currents bringing
06:46	warmth and humidity to the continental east coasts , and
06:48	cold currents , moderating temperatures and having a drying effect
06:51	on the West Coast . Ocean circulation in the southern
06:54	hemisphere is similar , except that the gyres flow in
06:56	a counterclockwise direction . And because there's very little land
06:59	polar to 40°, South the antarctic Circumpolar current , or
07:03	west wind drift circles around Antarctica as a cold current
07:06	almost without interruption or directly interacting with the warm equatorial
07:10	waters . Surface currents generally move warm waters , poll
07:12	word , and cold waters toward the equator . So
07:14	they're important regional air temperature regulators , in addition to
07:18	moving anything that happens to be in the ocean from
07:20	schools of fish to bits of plastic . But surface
07:22	currents don't make up all of the horizontal motion of
07:24	the ocean . To see the rest , we have
07:27	to go deep into the ocean . Deep currents travel
07:29	at slower speeds beneath the surface currents . They move
07:32	ocean waters both horizontally across the floors of the world's
07:34	oceans , but also vertically from the ocean floor to
07:37	the bottom of surface currents as part of earth's thermal
07:39	. Hey line circulation , just like surface currents ,
07:41	deep currents flow from high pressure to low pressure .
07:44	Even at these crushing depths , slight pressure and density
07:47	differences are also caused by temperature and salinity changes .
07:50	For example , the more salt content the surface water
07:52	has , the more dense it is , the more
07:54	likely it will sink as it reaches the polls .
07:56	In some places , that water will sink up to
07:58	2000 m to where deep currents flow . A complete
08:01	circuit of a deep current may take up to 1000
08:04	years . Even still , deep currents are critical to
08:06	the movement of nutrients around the world . Many fisheries
08:09	, for example , depend on cyclical upwelling of nutrient
08:12	rich water moving from deep currents into local surface currents
08:14	, while nutrients released through decomposition near the ocean floor
08:17	are pulled up by upwelling oxygen cycles down from the
08:20	surface to the deep , Which keeps those d composers
08:22	and other deep sea organisms supplied with oxygen for respiration
08:25	. So the broad global circulation of deep currents is
08:28	like a vast conveyor belt of ocean water that brings
08:30	warmth from the equator to the poles , nutrients from
08:33	the floor to the surface and oxygen from the surface
08:35	to the floor . Marine habitats near upwelling only cover
08:38	about 1% of Earth's oceans , but account for up
08:40	to 50% of the global fish harvest . At least
08:43	a billion people rely on fish for their primary protein
08:46	. So deep ocean circulation not only moves energy around
08:48	the globe , but also helps create the conditions that
08:51	feed a large part of the world . Higher up
08:53	dominant winds and surface currents have helped move people around
08:55	the globe for thousands of years . With the movement
08:58	of ships has come the movement of people and the
08:59	things they deem most important . We can understand the
09:02	material culture of a people by the marine debris .
09:04	They create . Some of that trash will get swept
09:06	into regional surface currents like gyres , but some will
09:09	get caught by smaller local currents and wash up on
09:11	the shore without having traveled the world . Like ,
09:13	there have been outbreaks of whole toys washing up on
09:15	beaches from rubber ducks to lego dragons to Garfield phones
09:19	. What do all of these things have in common
09:21	? Well , like you've probably intuited marine debris in
09:24	the 21st century is mostly plastic . There are eight
09:26	million metric tons of plastic bits and debris that we
09:29	don't know the origin of , or non point source
09:31	pollution . That's estimated to be in the oceans ,
09:33	but there's also bigger garbage out there to take that
09:36	whole lego dragon . It didn't travel far . It
09:38	came from a wrecked shipping container that fell into the
09:40	ocean after a huge once in a 100 years wave
09:43	hit the cargo ship that was carrying it . In
09:45	fact , there are thousands of shipping containers each year
09:47	that fall off cargo ships due to rough weather or
09:50	other mishaps . This map shows the movement of the
09:52	50,000 ships each day moving goods around the globe .
09:55	Some estimates say 90% of global trade involves container ships
09:58	crossing the oceans , but his ships move , they're
10:01	emitting air pollution that rides on global air circulation currents
10:03	through the atmosphere or dropping stuff that contributes to ocean
10:06	pollution . These types of pollution can't be linked to
10:08	a particular ship because air and water across political boundaries
10:11	. In fact , those plastic beads and dragons on
10:13	the beach represent how we're all connected by the global
10:16	circulation of air and water and how garbage patches won't
10:19	clean themselves are . Global economies depend on the circulation
10:22	of goods moved by ships , and local economies depend
10:24	on the circulation of nutrients that create rich fisheries ,
10:27	all of which leveraged the dependability of ocean currents .
10:30	So who's responsible for cleaning international waters ? And how
10:33	do we balance our societal needs with protecting the planet
10:36	? There aren't easy answers . And there might be
10:38	rough seas ahead . In fact , I see some
10:40	clouds on the horizon . Thanks for watching this episode
10:43	of Crash Course Geography , which was made with the
10:45	help of all these nice people . If you want
10:47	to keep crash course free for everyone forever , you
10:49	can join our community on Patreon . Yeah .

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