Oceano Quiz 7

Which of the following statements about El Niño events is false?

Fishing along Peru and Ecuador is much better.

The California Current is a ________ current that flows ________.

cold; south

Origin of Thermohaline Circulation

Unlike circulation in the surface ocean that is driven by wind, deep ocean circulation is driven by differences in density that are caused by differences in temperature and salinity. Deep ocean circulation is therefore called thermohaline circulation (thermo = heat; haline = salt). Most water involved in thermohaline circulation originated at high latitudes at the surface. In these regions, surface water becomes denser as it gets colder (due to high latitude) and saltier (due to the formation of sea ice). This causes the surface water to sink. Once this water sinks, it doesn't change in characteristics very much. The processes that changed its temperature, salinity, and density all occurred on the surface that it is now removed from. For this reason, a water mass can be identified by its specific characteristics.

The atmospheric circulation cell located in the equatorial South Pacific and created by the movement of air between high- and low-pressure regions is called the __________.

Walker cell

What is one of the first indications that an El Niño event may be occurring?

Warm water originating near Australia migrates toward South America

Western Boundary Currents

Western boundary currents are the currents that are on the west side of each ocean basin. Due to "Western Intensification," western boundary currents are: -Faster (travel 100's of km/day) -Narrower (<100 km wide) -Deeper (~2 km deep) Additionally, western boundary currents are warm because they move warm water poleward in each gyre. In general, warm ocean currents (western boundary currents) warm the air at the coast. As a result, the adjoining landmasses will experience a humid climate

Converging Surface Water Causes Downwelling

When surface seawater moves towards an area, surface seawater piles up and seawater moves downward. This results in low biological productivity. Remember from Module 5: Pelagic Biogenous Sediment (Chapter 4 ) that cold, deep water is rich in nutrients. Phytoplankton like diatoms (siliceous microorganisms that create siliceous ooze) need nutrients to grow. Downwelling results in nutrient-poor surface water. Without nutrients, there are no phytoplankton. Without phytoplankton (the base of the food chain), overall biological productivity is low.

Upwelling and Downwelling

When the wind-driven horizontal movement of ocean waters induces vertical movement in surface water, upwelling or downwelling may occur. Upwelling involves the upward movement of water, and downwelling involves the downward movement of water in the water column. In this section, we will investigate the causes of upwelling and downwelling, and how upwelling impacts marine life.

Coastal Upwelling and Downwelling

Wind blowing parallel to shore can cause coastal upwelling or downwelling. The friction of the wind blowing along the ocean surface causes the water to move. The Coriolis Effect deflects it to the right (in the northern hemisphere) or to the left (in the southern hemisphere). Ekman Transport then moves the water offshore (causing upwelling) or onshore (causing downwelling).

Subtropical gyres flow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

false

Vertical mixing of surface and deep seawater takes place in the low latitudes.

false

Western intensification is ________.

caused by the rotational center of a gyre being located to the west of the ocean basin's geographic center, thus causing more water to flow within a narrower "channel"

South Atlantic

The four main boundary currents in the South Atlantic Gyre include: the South Equatorial Current (the equatorial current), the Brazil Current (the western boundary current), the Antarctic Circumpolar Current/West Wind Drift (the southern boundary current), and the Benguela Current (the eastern boundary current). The Brazil Current is weaker than the Gulf Stream (the western boundary current in the north) because the South Equatorial Current splits in two when it hits the coast of Brazil. Only some of the water continues southward as the Brazil Current, while the rest is pushed northwest along the northern coast of South America and into the Caribbean.

deep currents

-are driven by differences in density caused by differences in salinity and temperature; -involve vertical and horizontal motions; -are important because they provide oxygen to the deep sea and transfer heat around the world.

Ekman transport is an average path of motion that, under ideal conditions, is at ________ to the wind's direction.

90 degrees

Diverging Surface Water Causes Upwelling

Equatorial upwelling occurs between the westward-flowing equatorial surface currents due to Ekman Transport and the direction of the Trade Winds. In equatorial upwelling, water moving on either side of the equator is deflected slightly poleward due to Ekman Transport to the right of the wind in the northern hemisphere and to the left of the wind in the southern hemisphere. This surface water is replaced by deeper water. The upward movement of cold, nutrient-rich water is called upwelling. Because equatorial upwelling brings nutrient-rich waters to the surface, a long, thin band of high biological activity is found along the equator.

Seasonal Monsoons

Oceanic (and atmospheric) circulation in the Indian Ocean is characterized by seasonal monsoons. Differences in heat capacity of the land versus the ocean cause seasonal shifts in high and low pressure systems between the Asian continent and the Indian Ocean.

Other Causes of Coastal Upwelling

On the left, we see that an offshore wind would push surface water away from the land, and deeper water would move upwards to replace it (upwelling). On the right, we see that an obstruction on the seafloor, such as a tablemount, would block the flow of deep currents, forcing them upwards (upwelling).

Surface Currents

Surface currents involve about 10% of world ocean water, and are characterized by primarily horizontal motion in the uppermost 400 meters of the ocean. Surface currents are important because they transfer heat from warmer to cooler areas and affect coastal climates. Surface currents are wind-driven (the Easterlies and Westerlies drive circulation in the subtropical gyres). Energy in the moving air is transferred to the water in the form of waves by friction. surface currents are deflected to the right of the wind direction in the northern hemisphere, and to the left of the wind direction in the southern hemisphere. Additionally, continents and basin topography block continuous flow, and help to deflect the moving water into a circular pattern, with the help of gravity and friction. The circular flow around the periphery of each ocean basin is called a gyre.

Is the California Current a western boundary current or an eastern boundary current?

The east coast of North America has a warm current, called the Gulf Stream, that brings warm water up the coast. As a result, the air is warm and moist. This is why the east coast is so humid.

In relation to ocean circulation, a gyre is __________.

a large, circular-moving loop of surface water

The Peru Current is a _______ current that flows _______.

cold; north

A geostrophic current is one that (ideally) flows _______.

in a circular path around the central "hill" of a gyre

In subtropical gyres, western boundary currents have all of the following characteristics exceptbeing __________.

relatively wide

Surface ocean currents are caused by ___________, whereas deep currents are the result of __________.

surface wind belts; density differences

The ocean's deep water is initially formed at the _________ and in _________ regions.

surface; subpolar

Which of the following currents does not belong with the others as part of the same gyre?

the Agulhas Current

Subtropical Gyres

the closed-loop pattern of surface ocean currents around the margins of the major ocean basins; the flow is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere There are five subtropical gyres: two in the Pacific, two in the Atlantic, and one in the Indian Ocean (since the ocean is mostly in the southern hemisphere, and there is a continent to the north)

The East Australian Current is the western boundary current of the South Pacific subtropical gyre

true

The Gulf Stream is a ________ current that flows ________.

warm; north

The East Australian Current is a ________ current that flows ________.

warm; south

West Wind Drift

Antarctic circulation is unlike the other ocean basins because there are no subtropical gyres. Circulation around Antarctica encircles the continent. Circulation is dominated by the Antarctic Circumpolar Current (also called the West Wind Drift) that is driven clockwise by powerful prevailing westerlies (they are not slowed down by friction with land here). The Antarctic Circumpolar Current completely encircles the Earth, unlike any other current. In doing so, it transports more water than any other current.

Ekman Transport

Ekman transport describes the average movement of seawater under the influence of the wind. The average (or overall) movement of the surface current is 90o to right of wind in northern hemisphere and 90o to left of wind in the southern hemisphere. The Ekman spiral describes motion of the entire column of surface water. The uppermost surface current moves at a 45o angle to the wind, and deeper layers of water move progressively more to the right in the northern hemisphere and to the left in the southern hemisphere, until you reach the bottom of the surface current at a depth of about 100 meters.

Two Subtropical Gyres

North Atlantic and South Atlantic

One Subtropical Gyre

Since the Indian Ocean is primarily in the southern hemisphere, it has only one subtropical gyre, plus equatorial currents.The Indian Ocean Gyre is centered at 30oS during the winter, and shifts northward to the equator in the summer. The four main boundary currents in the Indian Ocean gyre are: South Equatorial Current (the equatorial current), the Agulhas Current (the western boundary current), the Antarctic Circumpolar Current /West Wind Drift (the southern boundary current), and the West Australian Current (the eastern boundary current). The shape of the Indian Ocean basin and its proximity to the high mountains of Asia cause it to have strong seasonal variations.

Is the Gulf Stream a western boundary current or an eastern boundary current?

The graph below shows another example of the effects of ocean currents on coastal climates. Rio de Janeiro and Arica are at about the same latitude, so we would expect them to have about the same temperatures over the course of the year. However, there is a cold current off the west coast of South America (the Peru Current) and a warm current off the east coast (the Brazil Current). As a result, temperatures in Rio are consistently ~10oF warmer throughout the entire year.

Upwelling Brings Nutrients to Primary Producers

-Photosynthetic primary producers (phytoplankton) are limited by access to nutrients and sunlight; -Primary producers (photosynthetic and chemosynthetic) are the base of the food chain; and -Where primary productivity is high, overall biological productivity is high.

Eastern Boundary Currents

Eastern boundary currents are the currents that are on the east side of each ocean basin. Eastern boundary currents tend to have the opposite properties of western boundary currents. Eastern boundary currents are: -Slower (travel 10's of km/day) -Wider (~1,000 km wide) -Shallower (<500 m wide) Eastern boundary currents are cold because they carry cold water toward the equator. Cool ocean currents (eastern boundary currents) the cool air at the coast. As a result, the adjoining landmasses will experience a dry climate.

El Niño Conditions

El Niño is the warm phase of ENSO. During El Niño, all normal conditions reverse. During El Niño: The pressure difference across the equatorial Pacific switches. It is now higher in the west and lower in the east. This leads to weaker trade winds (that sometimes even switch in direction). As the winds slow, the Pacific Warm Pool migrates eastward across the equatorial Pacific. The thermocline deepens in the eastern Pacific, becoming more horizontal. As warm water stacks up agains the coast of Peru, upwelling stops and downwelling occurs. Due to the lack of upwelling, there is lower biological productivity off the coast of Peru (no fish). The Walker Circulation Cell is counterclockwise, with air rising in the eastern Pacific (creating the low pressure system) and air sinking in the western Pacific (creating the high pressure system).

Subpolar Gyres

In addition to the West Wind Drift and East Wind Drift that dominate circulation around Antarctica, there are also a couple of small subpolar gyres around Antarctica. These are created by the interaction of the Prevailing Westerlies (pushing the currents eastward) and the Polar Easterlies (pushing the currents westward). Subpolar gyres rotate in the opposite direction than their adjacent subtropical gyres.

Walker Circulation Cell

In the equatorial Pacific, the Walker Circulation Cell is created by air pressure differences across the region. The Walker Circulation Cell is just like the Hadley Cell (a closed loop of rising and sinking air) except that it operates across the equatorial region from east to west and not from north to south like the Hadley Cell. The Walker Circulation Cell is created by sinking air over the cold water in the eastern Pacific (creating a high pressure system in the east), surface trade winds blowing from east to west (from high to low pressure), warm air rising over the warm water in the western Pacific (creating a low pressure system in the west), and air moving eastward in the upper atmosphere.

La Niña Conditions

La Niña is the cool phase of ENSO. You can think of La Niña as an intensified "Normal" phase. Everything follows "normal" patterns, but is more intensified. During La Niña: An increased pressure difference across equatorial Pacific creates stronger trade winds. There is stronger upwelling in the eastern Pacific and a shallower thermocline. Cooler than normal seawater extends across the central and eastern Pacific. Higher biological productivity occurs off the coast of South America due to stronger upwelling

El Niño-Southern Oscillation: ENSO

Pacific Ocean circulation is characterized by annual variability known as ENSO (El Niño Southern Oscillation). There are three phases of ENSO: Normal, El Niño, and La Niña.

Boundary Currents in the Subtropical Gyres

The five subtropical gyres each have four main boundary currents: -Equatorial: the current that runs close to the equator; -Western: on the western side of the ocean basin; -Eastern: on the eastern side of the ocean basin; and -Northern or Southern: on the northern or southern end of the ocean basin (that is not the equator). We also have Equatorial Countercurrents that travel in the opposite direction from the equatorial currents. This return of water to the east replaces water that moves westward due to the Trade Winds. Subpolar gyres also operate at higher latitudes, with circulation patterns that are more controlled by surrounding land.

North Atlantic

The four main boundary currents in the North Atlantic Gyre include: the North Equatorial Current (the equatorial current), the Gulf Stream (the western boundary current), the North Atlantic Current (shown as part of the Gulf Stream here; the northern boundary current), and the Canary Current (the eastern boundary current). The North Atlantic Gyre has many loops and complexities, including water that splits off in the North Atlantic where it becomes so dense and cold that it sinks (creating North Atlantic Deep Water that will be discussed later).

"Normal" Conditions

The main components to notice are: -Air pressure across the equatorial Pacific is higher in the eastern Pacific, leading to strong southeast trade winds. -The strong trade winds push the Pacific Warm Pool to the western side of the Pacific Ocean basin. -Due to the position of the Pacific Warm Pool, sea surface temperatures are warmer in the west, and cooler in the east. The thermocline (recall from Chapter 5 that this is the depth where there is a rapid change in temperature with depth) is deeper on the western side, and intersects the surface on the eastern side. Coastal winds blow parallel to the coast of South America, so there is upwelling off coast of Peru (and therefore plentiful fish). The Walker Circulation Cell is clockwise, with air rising in the western Pacific (creating the low pressure system) and air sinking in the eastern Pacific (creating the high pressure system).

Sources of Deep Water

The sinking of water in the North Atlantic starts the conveyor-like flow of deep ocean currents. The lack of a thermocline and pycnocline in high latitudes (covered in Chapter 5) allows this vertical mixing to occur. This creates North Atlantic Deep Water (NADW). NADW continues to travel southward through the global ocean at a middle depth due to its intermediate density. The bottom current in the deep ocean is Antarctic Bottom Water. This water mass is the densest in the world due to its low temperature and high salinity. Antarctic Intermediate Water flows from Antarctica where it slides above NADW.

Gulf Stream

The world's best studied ocean current is the Gulf Stream. This is because it plays such a large role in warming the east coast of the United States (which is where Europeans first colonized North America) and Europe. Due to the interaction between the Gulf Stream traveling north and the Labrador Current traveling south, eddies often form that trap cold or warm waters in their centers and then separate from the main flow. These eddies host unique biological populations, as the cold water holds nutrients that are otherwise scarce in the warmer Gulf Stream waters.

Thermohaline flow is a term that describes ________.

deep-ocean current movement

Downwelling

the downward movement of fluid, especially in the sea, the atmosphere, or deep in the earth.