Goals

On completing this module, students are expected to be able to:

• describe surface and deep ocean properties;
• explain how surface and deep ocean circulation work;
• illustrate how ocean circulation affects weather and climate;
• predict changes in key ocean parameters and circulation patterns as a result of climate change;
• interpret deep sea circulation patterns from water column properties.

Learning Outcomes

After completing this module, students should be able to answer the following questions:

• What is the warmest temperature in the oceans today?
• What is the coldest temperature in the oceans today?
• What factors impact salinity of surface ocean waters?
• What is primary productivity and what controls it?
• What factors control dissolved oxygen contents of ocean waters?
• What factors control dissolved CO2 contents of ocean waters?
• What drives surface ocean circulation?
• What are the Coriolis effect, gyres, upwelling and downwelling?
• How do deep water masses form?
• What are the major deep water masses in the oceans and their properties?
• What is the significance of the Global Conveyor Belt, deep water aging, properties of the oldest waters in the ocean?
• What is ENSO?
• What are the impacts of ENSO on global weather patterns?

Below is an overview of your assignments for this module. The list is intended to prepare you for the module and help you to plan your time.

Assignments

1. Take Module 6 Quiz.
2. Yellowdig Entry and Reply
3. After completing all Lesson 6 assignments, take Midterm 1.

Humans have been involved in the scientific study of the ocean for hundreds of years. Most early ocean voyages were focused on exploration, colonization and economic gain. However, these early pioneers had to have an understanding of the ocean to survive their time at sea. It wasn’t until late in the 19th century that expeditions had the single goal of learning about the ocean. Charles Darwin’s famous five-year voyage on the HMS Beagle from 1831 to 1836 provided a literal treasure trove of information about the ocean and especially its geology and natural history. The HMS Challenger Expedition from 1872-1876 made even more important discoveries about oceanography.

The HMS Beagle off Tierra del Fuego, South America in 1833

Credit: Wikipedia CC BY-SA 3.0 (Creative Commons)

World War II from 1939-1945 ushered in a new era in ocean exploration. The importance of naval operations, and particularly submarines in combat, provided a major incentive for a much more accurate understanding of the bathymetry of the ocean as well as the acoustic properties of seawater. The urgency of acquiring this information intensified in the Cold War interval from 1946 to 1991 when submarines played a central role in covert operations. Around the world, federal investment in oceanographic research ballooned, and research ships were built with the sole purpose of mapping the oceans and studying their properties. This age brought major discoveries of the ocean and its geology, the life it supports, its chemistry, and its effect on climate.

The scientific discovery of the oceans was originally focused on ships, but more recently our technological capabilities have surged, and now submersibles and unmanned remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) provide a relatively low operating cost means of acquiring many different types of data.

Examples of Submersibles

This technology is changing the way that oceanography is being conducted, as demonstrated in the following video. Click play to watch the video.

Video: Tethys - A new breed of undersea robot (2:03)

At the same time, satellites provide continuous information about the ocean surface, while data such as temperature and surface current velocity is obtained from long-term deployments of instruments attached to buoys. In addition, seismological data are acquired by seismometers anchored on the sea bottom. Nevertheless, modern oceanography still relies heavily on sea-going vessels for sampling waters and biotas from plankton to fish, for deploying instruments that measure in-situ properties, and for taking cores. We have already learned about such coring techniques in Module 1.

Data Collection Technologies

In the future, seagoing research vessels are likely to be extremely high-tech. For example, the SeaOrbiter is due to launch in the near future. Part giant ship and part giant submarine, the vessel will enable study of the sea surface simultaneously with its depths. One of the goals of the SeaOrbiter is to allow scientists to remain at sea and study underwater for extended periods, a pursuit not possible in a traditional submarine.

The SeaOrbiter, a versatile seagoing research vessel

Credit: SeaOrbiter, Jacques Rougerie

We begin this module with an overview of the properties of seawater and follow with a thorough discussion of the circulation of different water masses.

Most of you have spent time at the beach or taken a boat trip from the coast. The realm that you have seen is the surface layer of the ocean. This is the warm, generally illuminated layer that is exchanging heat and gases with the atmosphere. The surface layer extends down usually a few hundred meters and accounts for only a small fraction of the total ocean volume. As such, it is very much like the skin of a piece of fruit. Yet this layer is home for much of the marine food chain. At the base of the surface ocean, is a sharp temperature drop at a layer known as the thermocline. Below the thermocline, ocean deep waters, by contrast, are generally cold, dark, and inhospitable. However, the deep ocean plays a vital role in heat transport, and thus we will spend considerable time considering deep-water masses.

Temperature

In terms of climate, the key property of seawater is its temperature. Temperature levels also help define the surface and deep ocean masses. Temperature of the surface ocean water varies seasonally, with warmer temperatures recorded in summer and colder temperatures in winter. When we study ocean temperatures, we usually consider the annual average. On this basis, surface ocean temperatures range from about 28^oC in the tropical western Pacific to –3^oC off the coast of Antarctica. Because seawater is salty, it does not freeze even though its temperature is below 0^oC. 

Map of average sea surface temperatures.

Credit: MyNASAData

The following video shows real sea surface temperature data as well as the distribution of ice from 1985 to 1997. Note the way temperature bands and ice sheets shift from south to north with the seasons.

Video: 40 Years of Sea Surface Temperatures (2:00) This video is not narrated.

Temperature has a major influence on the density of seawater and thus plays a large role in deep ocean circulation, as we will see. Warm water expands and thus is less dense (less mass per unit area) than cold water.

Salinity

Map of average sea surface salinity

Credit: n.a. “Annual Mean Surface Salinity Data.” Ocean Sciences. April 14, 2011.

A key property of seawater that also has a significant effect on the behavior of water masses is salinity. This variable is a measure of the amount of dissolved salts such as sodium, potassium, and chlorine in the water, and, like temperature, it has a major effect on seawater density. The higher the salinity (the more salt), the denser the seawater. The major factor that controls salinity at a location is the balance of evaporation and precipitation. Where evaporation is higher than precipitation, salinity levels are higher; where precipitation exceeds evaporation, salinity levels are lower. The units of salinity are parts per thousand (ppt or o/oo), which refers to the weight of dissolved salt (in grams) in 1000 grams (or a kilogram) of water. The most saline surface waters are found in basins in highly arid regions with high rates of evaporation, such as the Persian Gulf, the Red Sea, and the eastern Mediterranean. Lowest salinities are found in locations with high levels of precipitation and runoff from land, typically near where large rivers enter the ocean. Highly saline water has more salt contained between the water molecules, and thus is denser than low salinity water.

The circulation of the surface ocean is driven primarily by surface winds. As we have seen, winds blow from areas of high atmospheric pressure to regions of low atmospheric pressure. These winds are generally transferring heat from areas where there is excess incoming radiation (the tropics and subtropics) to temperate and higher latitude regions, where there is a net loss of heat. Typically speaking, the distribution of pressure on the Earth’s surface is zonal or meridional, with high-pressure bands covering the subtropics and polar regions and low-pressure bands, the equatorial regions, and subpolar regions.

Where winds and surface currents are moving along a coastline, they draw the surface water away from the coast. The surface waters are replaced by waters from below by the upwelling described earlier. This is shown in the figure below.

Coastal currents, surface water, and upwelling

Click for a text description of the Coastal upwelling diagram.

The image is a diagram illustrating the process of upwelling along a coastline. It features a cross-sectional view of a coastal area with mountains on the left, transitioning into a body of water on the right. A wind arrow points parallel to the coast, driving surface water movement. The diagram highlights the upwelling process, where deeper, nutrient-rich water rises to the surface, depicted with labeled sections for the coast, wind, upwelling, and seafloor.

• Left Section
  • Feature: Mountains
  • Color: Green with white peaks
  • Position: Left side of the diagram
• Coastal Area
  • Label: Coast
  • Position: Adjacent to mountains, transitioning into water
  • Color: Yellowish-brown
• Water Body
  • Color: Blue, with darker blue indicating deeper water
  • Sections:
    • Surface water: Moved by wind
    • Deeper water: Source of upwelling
• Wind Indicator
  • Label: Wind
  • Representation: Arrow pointing parallel to the coast
  • Color: Black
  • Position: Above the coastal water surface
• Upwelling Process
  • Label: Upwelling
  • Description: Rising of deeper water to the surface
  • Position: Central water area, shown as an upward flow
  • Color: Dark blue transitioning to lighter blue
• Seafloor
  • Label: Seafloor
  • Position: Bottom of the water body
  • Color: Dark brown

Credit: Wikipedia / CC BY-SA 3.0 (Creative Commons)

 

Upwelling and downwelling

Click for a text description of the Upwelling and downwelling diagram.

The image is a diagram illustrating ocean water movement, highlighting convergence zones, divergence zones, upwelling, and downwelling processes. It features a cross-sectional view of the ocean, with the ocean surface at the top and the ocean floor at the bottom. The diagram shows water circulation patterns with arrows indicating movement: green arrows represent upwelling (water rising from the ocean floor) and downwelling (water sinking from the surface), while blue arrows indicate broader circulation. The convergence zones are marked where water flows meet and sink, and the divergence zone is where water spreads apart and rises.

• Top Section
  • Feature: Ocean Surface
  • Color: Light blue
  • Position: Top of the diagram
• Bottom Section
  • Feature: Ocean Floor
  • Color: Light brown
  • Position: Bottom of the diagram
• Water Circulation Patterns
  • Color: Blue arrows for general flow
  • Green arrows for specific movements:
    • Upwelling: Water rising from the ocean floor
    • Downwelling: Water sinking from the ocean surface
• Convergence Zones
  • Label: Convergence Zones
  • Description: Areas where water flows meet and sink
  • Position: Top center, marked with black arrows pointing inward
  • Color: Indicated by overlapping blue flow lines
• Divergence Zone
  • Label: Divergence Zone
  • Description: Area where water spreads apart and rises
  • Position: Central area, marked with green upwelling arrows
  • Color: Indicated by spreading blue flow lines

Credit: n.a. “Upwelling and Downwelling in the ocean.” redmap. Accessed April 22, 2025.

Upwelling also happens in parts of the ocean where winds cause surface currents to diverge or move away from one another. Downwelling is the opposite process to upwelling, where surface waters flow downwards and replace deep waters. This occurs in parts of the ocean where surface winds are converging. One place this happens is in the centers of gyres.

Winds generally blow out from the subtropics towards the equator and subpolar regions, and from the polar regions to the subpolar latitudes. Complicating matters is that the rotation of the Earth causes the winds to rotate as they move (the Coriolis effect). The rotation of the Earth causes an object to deflect towards the right (as viewed by a stationary observer) in the Northern Hemisphere, which results in a clockwise motion, and to deflect towards the left (as viewed by a stationary observer) in the Southern Hemisphere, which results in a counterclockwise motion. These rotations combined with the zonal distribution result in enormous, nearly ocean-scale major cells or gyres of surface winds.

Video: Coriolis Effect (1:00)

Major Surface Wind Maps

Video: Gyres (1:04)

As surface winds push the surface layer of the ocean with them, the surface wind gyres result in surface ocean current gyres. Along coastlines, the direction of movement of a gyre has a significant impact on continental climate. For example, a current moving from south to north in the Northern Hemisphere, or north to south in the Southern Hemisphere, will generally deliver warmer water to the coastal region, whereas a current moving from the north to south in the Northern Hemisphere or south to north in the Southern Hemisphere will generally deliver colder water. The flow of warm water will generally cause a larger moderating influence on coastal climate than will the flow of cold water. Take, for example, the Gulf Stream in the North Atlantic. This warm current has a major heating effect on the shores of Great Britain and other parts of Northern Europe, keeping these regions relatively balmy compared to locations at comparable latitudes. After it bathes the shores of Britain, the North Atlantic gyre bends towards the south, thus bringing relatively cold waters to the shores of Spain, Portugal, and Morocco further to the south, keeping these areas cooler than areas not influenced by the currents.

This NASA video provides an excellent summary of surface ocean currents:

Video: Perpetual Ocean (3:02) This video is set to music with not narrated.

Schematic diagram illustrating the concept of upwelling and downwelling

Click for a text description of the upwelling and downwelling diagram.

The image is a diagram illustrating ocean water movement, highlighting convergence zones, divergence zones, upwelling, and downwelling processes. It features a cross-sectional view of the ocean, with the ocean surface at the top and the ocean floor at the bottom. The diagram shows water circulation patterns with arrows indicating movement: green arrows represent upwelling (water rising from the ocean floor) and downwelling (water sinking from the surface), while blue arrows indicate broader circulation. The convergence zones are marked where water flows meet and sink, and the divergence zone is where water spreads apart and rises.

• Top Section
  • Feature: Ocean Surface
  • Color: Light blue
  • Position: Top of the diagram
• Bottom Section
  • Feature: Ocean Floor
  • Color: Light brown
  • Position: Bottom of the diagram
• Water Circulation Patterns
  • Color: Blue arrows for general flow
  • Green arrows for specific movements:
    • Upwelling: Water rising from the ocean floor
    • Downwelling: Water sinking from the ocean surface
• Convergence Zones
  • Label: Convergence Zones
  • Description: Areas where water flows meet and sink
  • Position: Top center, marked with black arrows pointing inward
  • Color: Indicated by overlapping blue flow lines
• Divergence Zone
  • Label: Divergence Zone
  • Description: Area where water spreads apart and rises
  • Position: Central area, marked with green upwelling arrows
  • Color: Indicated by spreading blue flow lines

Credit: AE Nieblas, CSIRO (Anne.Neiblas@utas.edu.au)

The deep ocean is generally considered to include the ocean below a transition known as the thermocline. The thermocline is the sharp temperature decrease that lies at the base of the surface mixed layer, where waters are generally uniform in temperature as a result of convection. Deep-water masses are produced at the surface of the ocean and transported to depth via downwelling. Generally, downwelling occurs where the surface ocean is cool, or, rarely, unusually saline. Downwelling water travels along lines of equal density known as isopycnals and spreads out horizontally at the level where it is equal in density to the surrounding water mass.

The production of deep-water masses via downwelling occurs in high-latitude regions of the northern and southern hemispheres, where the surface ocean is cooled by winds. Wind moving over the water both cools it and causes an increase in evaporation. This evaporation targets just the water molecules, resulting in an increase in the salinity of the water. Falling temperature and increasing salinity render these surface water masses denser, allowing them to downwell. In certain locations, the formation of sea ice also causes an increase in salinity as the freezing removes fresh water, leaving the salt behind in a process known as brine exclusion. Pockets of salty water around the margins of the ice sink as a result of their higher density. Moreover, brine exclusion intensifies the cooling by wind.

Formation of North Atlantic Deep Water in the northern part of the North Atlantic.

Click for a text description of the Deep Water image.

A map depicting ocean circulation patterns in the North Atlantic Ocean, focusing on the movement of water masses. It shows the North Atlantic region, including parts of North America, Europe, Africa, Greenland, and Iceland. An orange path labeled "warm salty surface water" flows from the southwestern North Atlantic near North America, moving northward past Europe and looping around Iceland and Greenland. A blue arrow labeled "upper NADW" (North Atlantic Deep Water) and "lower NADW" indicates deeper water flowing southward, intersecting the orange path. Another blue arrow labeled "AABW" (Antarctic Bottom Water) flows northward at the deepest level. Numbers along the paths (e.g., 3.5, 8) likely represent salinity or temperature values.

• Ocean Circulation Paths
  • Warm Salty Surface Water
    • Label: Warm salty surface water
    • Path: Orange line starting near North America, moving northward past Europe, looping around Iceland and Greenland
    • Values: Marked with 8 near Iceland and Greenland
    • Color: Orange
  • North Atlantic Deep Water (NADW)
    • Labels: Upper NADW, Lower NADW
    • Path: Blue arrow flowing southward, intersecting the orange surface water path
    • Value: Marked with 3.5 near North America
    • Color: Blue
  • Antarctic Bottom Water (AABW)
    • Label: AABW
    • Path: Blue arrow flowing northward at the deepest level
    • Position: Southern part of the map, below NADW
    • Color: Blue
• Geographic Features
  • North America: Western boundary of the map
  • Europe: Eastern boundary of the map
  • Africa: Southeastern boundary of the map
  • Greenland: Northern region, west of Iceland
  • Iceland: Northern region, east of Greenland
• North Atlantic Ocean
  • Position: Central area of the map
  • Color: Light blue
• Latitude Markings
  • Range: 20° to 60° latitude
  • Position: Along the vertical edges of the map

Credit: Darin Toohey (Toohey@colorado.edu)

Today there are three major deep ocean masses. North Atlantic Deep Water or NADW is mainly produced where the surface ocean is cooled in the Norwegian Sea in the northern part of the North Atlantic on the north side of a ridge that runs between Greenland, Iceland, and Scotland. This cooled water seeps through the ridge and downwells. Portions of NADW are also produced in the Labrador Sea and in the Mediterranean. This water mass is 1-2.5oC and 35 ppt. NADW travels down the west side of the North Atlantic Ocean at a depth of 2000-4000m and through the west side of the South Atlantic. Much of NADW upwells in the Southern Ocean, but portions join the Antarctic Circumpolar Current and travel at depth into the Indian and Pacific Oceans.

Formation of Antarctic Bottom Water (AABW) in the southern part of the South Atlantic

Click for a text description of [the Antarctic Bottom Water diagram.

A cross-sectional diagram illustrating ocean water movement near Antarctica, focusing on the interaction between different water masses and the ocean floor. It depicts a layered view of the ocean, with the ocean surface at the top and the ocean floor at the bottom. The diagram shows the North Atlantic Deep Water flowing from the left, Antarctic Circumpolar Deep Water in the middle, and various water movements such as upwelling and downwelling. The Antarctic continent is on the right, with a mid-ocean ridge on the ocean floor. Labels indicate key features like the volume front and differences in water masses.

• Top Section
  • Feature: Ocean Surface
  • Color: Light blue with white patches indicating ice or foam
  • Position: Top of the diagram
• Bottom Section
  • Feature: Ocean Floor
  • Color: Brown
  • Position: Bottom of the diagram
  • Specific Feature: Mid-ocean Ridge
    • Label: Mid-ocean Ridge
    • Position: Central area of the ocean floor
    • Color: Dark brown
• Water Masses
  • North Atlantic Deep Water
    • Label: North Atlantic Deep Water
    • Position: Left side, flowing from the left
    • Color: Dark blue
  • Antarctic Circumpolar Deep Water
    • Label: Antarctic Circumpolar Deep Water
    • Position: Central area, above the mid-ocean ridge
    • Color: Medium blue
• Water Movement
  • Upwelling and Downwelling
    • Representation: Arrows indicating water flow
    • Color: White arrows
    • Position: Throughout the diagram, showing vertical movements
  • Volume Front
    • Label: Volume Front
    • Position: Left side, near the surface
    • Description: Boundary between water masses
  • Differences
    • Label: Differences
    • Position: Near the surface, above the volume front
    • Description: Indicates variations in water properties
• Antarctic Continent
  • Label: Antarctica
  • Position: Right side of the diagram
  • Color: Brown with a white top indicating ice
• Ocean Layers
  • Color: Gradient of blue, darker at the bottom, lighter at the top
  • Description: Represents depth and different water masses

Credit: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany, Creative Commons CC-BY-SA-2.5

Antarctic Bottom Water or AABW is produced by evaporative cooling off the coast of Antarctica and under the Ross ice shelf. With this source, AABW is amongst the coldest water in the ocean with a temperature of -0.4oC. This water is relatively fresh (average 34.6 ppt). AABW travels northward along the western side of the South Atlantic underneath NADW. Some of the water mass spills over into the eastern part of the South Atlantic, while the remainder travels into the equatorial channel between South America and Africa.

The third major source of deep water is called Antarctic Intermediate Water or AIW. AIW is produced near the Antarctic Convergence or Polar Front, where downwelling occurs as a result of the convergence of surface currents. AIW has a temperature of 3-7oC and a salinity of 34.3 ppt. It travels a considerable distance northward into the Atlantic, Indian and Pacific Ocean basins.

Cross-section of the Atlantic Ocean from north (right) to south (left) showing the major deep water masses.

Click for a text description of the Atlantic Meridional Overturning Circulation image.

A diagram titled "Atlantic Meridional Overturning Circulation," illustrating the movement of water masses in the Atlantic Ocean. It features a cross-sectional view with sea ice at both the left and right edges, a blue ocean layer above a brown ocean floor, and arrows showing water circulation. The diagram highlights the Gulf Stream carrying warm Atlantic intermediate water northward, with deeper flows of Atlantic deep water and bottom water moving southward. The circulation pattern forms a loop, indicating the overturning process.

• Top Section
  • Feature: Sea Ice
  • Position: Left and right edges
  • Color: White
• Bottom Section
  • Feature: Ocean Floor
  • Color: Brown
  • Position: Bottom of the diagram
• Water Masses and Circulation
  • Gulf Stream
    • Label: Gulf Stream
    • Description: Warm Atlantic intermediate water
    • Path: Northward flow near the surface, indicated by white arrows
    • Color: Light blue
  • Atlantic Deep Water
    • Label: Atlantic deep water
    • Path: Southward flow in the middle layer, indicated by white arrows
    • Color: Medium blue
  • Bottom Water
    • Label: Bottom water
    • Path: Southward flow at the deepest layer, indicated by white arrows
    • Color: Dark blue
• Circulation Pattern
  • Representation: White arrows forming a looping pattern
  • Description: Shows the northward surface flow and southward deep flow
  • Position: Throughout the ocean layer

Credit: The source of this material is the COMET® website of the University Corporation for Atmospheric Research (UCAR), sponsored in part through a cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2011 University Corporation for Atmospheric Research. All Rights Reserved

There are numerous other deep water masses, especially at intermediate depths, for example, North Pacific Intermediate water. As deep-water masses travel through the ocean, they gradually mix with surrounding water masses. For example, NADW mixes with AABW and AIW.

Downwelling supplies oxygen to the deep ocean and therefore ventilates this body of water. It does not bring nutrients. Deep water currents generally move very slowly, with a velocity of several cm per second. Typically, surface currents move 10-100 times faster than this. At these rates, deep water currents take thousands of years to encircle the globe. In fact, the oldest deep water in the ocean (in the North Pacific) is about 1500 years old. As deep waters circle the globe, their properties change. They mix with waters around them, and their chemistry changes as they acquire nutrients such as phosphate and CO₂ from decaying organic matter and lose oxygen.

The opposite process of downwelling is upwelling. Upwelling is where a deep-water mass that is lighter than waters around it rises to the level where it is no longer buoyant. This situation generally results when surface winds move the surface water masses away from a location, resulting in the upward movement of water from depth to fill the void. Upwelling is frequent in coastal regions, especially those in subtropical regions, where high pressure results in a dominant offshore wind flow. In addition, the ocean divergences where winds move surface current by Ekman transport are frequented by upwelling. Upwelling is crucial to the supply of nutrients to surface water masses, fueling high levels of productivity in the surface ocean. The most prolific fisheries of the world in coastal regions occur in nutrient-rich waters such as Peru and California and are supplied by upwelling.

As we have seen, the circulation of the deep ocean is driven by density differences that arise as a result of temperature and salinity of the different water masses. This type of circulation is known as thermohaline (temperature=thermo; haline=salt or salinity). Strictly speaking, since surface ocean currents are not driven by thermohaline mechanics but by winds and to a much lesser degree, tides, the circulation of the ocean as a whole is often called the meridional overturning circulation. However, we will continue to use the term thermohaline when addressing deep-water circulation.

Map of chlorophyll concentration.

Credit: Wikimedia by Plumbago (CC BY-SA 3.0)

Probably the most important property of seawater in terms of its effect on life in the oceans is the concentration of dissolved nutrients. The most critical of these nutrients are nitrogen and phosphorus because they play a major role in stimulating primary production by plankton in the oceans. These elements are known as limiting because plants cannot grow without them. However, there are a number of other nutrients that also play a role, including silicon, iron, and zinc. Nutrients in the ocean are cycled by a process known as biological pumping, whereby plankton extract the nutrients out of the surface water and combine them in their organic matrix. Then, when the plants die, sink and decay, the nutrients are returned to their dissolved state at deeper levels of the ocean. The abundance of nutrients determines how fertile the oceans are. A measure of this fertility is the primary production, which is the rate of fixation of carbon per unit of water per unit time.  Primary production is often mapped by satellites using the distribution of chlorophyll, which is a pigment produced by plants that absorbs energy during photosynthesis. The distribution of chlorophyll is shown in the figure above. You can see the highest abundance close to the coastlines, where nutrients from the land are fed in by rivers. The other location where chlorophyll levels are high is in upwelling zones, where nutrients are brought to the surface ocean from depth by the upwelling process.

Upwelling of deep waters off Antarctica

Click for a text description of the Antarctica ocean water movement diagram.

The image is a cross-sectional diagram illustrating ocean water movement near Antarctica, focusing on the interaction between different water masses and the ocean floor. It depicts a layered view of the ocean, with the ocean surface at the top and the ocean floor at the bottom. The diagram shows the North Atlantic Deep Water flowing from the left, Antarctic Circumpolar Deep Water in the middle, and various water movements such as upwelling and downwelling. The Antarctic continent is on the right, with a mid-ocean ridge on the ocean floor. Labels indicate key features like the volume front and differences in water masses.

• Top Section
  • Feature: Ocean Surface
  • Color: Light blue with white patches indicating ice or foam
  • Position: Top of the diagram
• Bottom Section
  • Feature: Ocean Floor
  • Color: Brown
  • Position: Bottom of the diagram
  • Specific Feature: Mid-ocean Ridge
    • Label: Mid-ocean Ridge
    • Position: Central area of the ocean floor
    • Color: Dark brown
• Water Masses
  • North Atlantic Deep Water
    • Label: North Atlantic Deep Water
    • Position: Left side, flowing from the left
    • Color: Dark blue
  • Antarctic Circumpolar Deep Water
    • Label: Antarctic Circumpolar Deep Water
    • Position: Central area, above the mid-ocean ridge
    • Color: Medium blue
• Water Movement
  • Upwelling and Downwelling
    • Representation: Arrows indicating water flow
    • Color: White arrows
    • Position: Throughout the diagram, showing vertical movements
  • Volume Front
    • Label: Volume Front
    • Position: Left side, near the surface
    • Description: Boundary between water masses
  • Differences
    • Label: Differences
    • Position: Near the surface, above the volume front
    • Description: Indicates variations in water properties
• Antarctic Continent
  • Label: Antarctica
  • Position: Right side of the diagram
  • Color: Brown with a white top indicating ice
• Ocean Layers
  • Color: Gradient of blue, darker at the bottom, lighter at the top
  • Description: Represents depth and different water masses

Credit: Wikipedia (CC BY-SA 3.0)

Another critical element for the health of the oceans is the dissolved oxygen content. Oxygen in the surface ocean is continuously added across the air-sea interface as well as by photosynthesis; it is used up in respiration by marine organisms and during the decay or oxidation of organic material that rains down in the ocean and is deposited on the ocean bottom. Most organisms require oxygen, thus its depletion has adverse effects for marine populations.  Temperature also affects oxygen levels, as warm waters can hold less dissolved oxygen than cold waters. This relationship will have major implications for future oceans, as we will see.

The final seawater property we will consider is the content of dissolved CO_². CO_² is nearly opposite to oxygen in many chemical and biological processes; it is used up by plankton during photosynthesis and replenished during respiration as well as during the oxidation of organic matter. As we will see later, CO_² content has importance for the study of deep-water aging. 

World map of surface ocean dissolved oxygen levels.

Credit: Astrobiology at NASA

Dissolved CO₂

Present day Sea-Surface DIC

Credit: Wikimedia by Plumbago (CC BY-SA 3.0)

Diagram illustrating the major surface and deep water circulation components of the ocean that combine to form the Global Conveyor Belt

Click for a text description of the Global Ocean Circulation diagram.

a world map illustrating global ocean circulation patterns, highlighting the movement of warm and cold water currents. The map shows continents including North America, South America, Europe, Africa, Asia, Australia, and Antarctica, with the Arctic Ocean covered by ice at the top and the Southern Ocean at the bottom. Red arrows indicate warm currents: the Gulf Stream and North Atlantic Deep Water flow northward from the Atlantic Ocean, while the Indian Warm Current and Pacific Current circulate through their respective oceans. Blue arrows represent cold currents: the Deep Cold Current and Antarctic Bottom Water flow southward from the Arctic and around Antarctica. Labels also note "Sinking Cold, Salty Water" and "Sea to Air Heat Transfer" in the Arctic region.

• Continents
  • North America: Northwest region
  • South America: Southwest region
  • Europe: Western region
  • Africa: Central region
  • Asia: Eastern region
  • Australia: Southeast region
  • Antarctica: Southern region
• Oceans
  • Arctic Ocean: Top, covered by ice
  • Pacific Ocean: Central-east
  • Atlantic Ocean: Central-west
  • Indian Ocean: Central-south
  • Southern Ocean: Bottom
• Warm Currents (Red Arrows)
  • Gulf Stream
    • Path: Northward from the Atlantic Ocean near North America
  • North Atlantic Deep Water
    • Path: Northward continuation from the Gulf Stream
  • Indian Warm Current
    • Path: Circulates through the Indian Ocean
  • Pacific Current
    • Path: Loops through the Pacific Ocean
• Cold Currents (Blue Arrows)
  • Deep Cold Current
    • Path: Southward from the Arctic Ocean
  • Antarctic Bottom Water
    • Path: Circulates around Antarctica
• Arctic Region Features
  • Sinking Cold, Salty Water
    • Position: Near the Arctic Ocean
  • Sea to Air Heat Transfer
    • Position: Near the Arctic Ocean
• Additional Notes
  • Arctic Circle: Marked near the top
  • Color Coding: Red for warm currents, blue for cold currents

Credit: The M Factory © Smithsonian Institution

As we have seen, surface ocean currents are the dominant sources of deep water masses. In fact, it is a little more complicated than this, as other deep water masses also feed one another. However, in a generalized sense, the surface and deep ocean currents can be viewed as an integrated system known as the Global Conveyor Belt, a concept conceived by the brilliant Geoscientist Wally Broecker of Columbia University. Diagrams of the Global Conveyor Belt (GCB) are two-dimensional and therefore simplified and do not, for example, include all of the intermediate water masses or surface water currents. However, the key of the Global Conveyor Belt concept is that it explains the general systems of heat transport as well as bottom water aging and nutrient supply in the oceans.

Global Conveyor Belt

The following animation traces the path of water through the surface and deep ocean, showing the dominant features of the GCB, including formation of NADW in the North Atlantic.

Video: The Thermohaline Circulation - The Great Ocean Conveyor Belt (2:46) This video is not narrated.

The GCB shows the dominant source of deep water in the oceans as North Atlantic Deep Water, and how this splits in two to flow into the Indian and Pacific Oceans. In these locations, upwelling of the deep water mass produces surface water currents that generally flow back towards the original source of deep water in the North Atlantic. For heat supply, the conveyor belt involves the transport of heat and moisture to northwest Europe by the Gulf Stream; this accounts for about 30% of the heat budget for the Arctic region, making the GCB extremely important for climate in the Arctic.

Because deep-water masses circulate very slowly, the GCB takes about 1500 years to complete, meaning that the oldest water in the oceans is about this age. In addition, because oxygen is gradually depleted in deep waters as they age, and because CO2 contents and nutrients conversely increase, the oldest water masses of the ocean in the North Pacific are among the most nutrient-rich, CO2 rich, and oxygen-depleted waters in the ocean. Conversely, the newly produced NADW waters are among the most nutrient-depleted, CO2 depleted, and well-oxygenated waters in the world.

As it turns out, recent research on the detailed configuration of surface and deep currents shows that circulation is much more complex than the GCB. Floats deployed in the ocean don’t always follow expected pathways in the GCB model. Wind actually plays a more significant role in causing downwelling than previously thought. Moreover, mixing by small systems or eddies plays a large role in driving surface currents.

Overview

El Niño, the most powerful control on weather across the planet, is rearing its ugly head as we speak. The telltale signs are building, and atmospheric scientists are predicting one of the most powerful El Niño events in history, certainly the strongest since 1998. This actually could be very good news for drought-stricken California, as El Niño typically delivers a lot of rain and snow to the state. In the case of California, the drought has been so severe that rains last winter caused massive mudslides. And the rain caused so much new vegetation to grow, and this served as fuel for the fires in Summer 2017. Southern Australia becomes very dry in El Niño years, and the drought causes great hardship for farmers and can often lead to very dangerous bushfires. And in Peru, El Niño comes with changing ocean currents that lead to the collapse of fisheries and great financial hardships. In this section, we learn about ocean circulation and how it impacts climate on a global scale.

One of the clearest examples of the influence of the oceans on climate is the El Niño Southern Oscillation (ENSO). This is the strongest inter-annual (i.e. periodic) variation in the modern climate system. The ENSO cycle lasts between two to seven years (average of 4 years) and corresponds to sea surface and thermocline temperatures and atmospheric pressure changes in the Pacific Ocean that ultimately have effects on weather and surface ocean conditions across the globe. The ENSO cycle consists of two main climatic intervals, the El Niño interval and the La Niña interval. The El Niño stage begins with an increase in sea surface temperatures across the eastern Pacific as warm waters spread from the west.

Ocean temperature distribution (top panel) that characterize El Niño (left) and La Niña right episodes in the Pacific Ocean. Changes from mean temperatures are shown in the bottom panel.

Credit: El Niña and La Niña Ocean Temperature Patterns. NOAA Center for Weather and Climate Prediction (NOAA) (Public Domain). 

Typically, there is a very large east to west SST gradient across the Pacific, but El Niño virtually erases this gradient. For reasons that are not fully understood, the El Niño stage is characterized by high atmospheric pressure in the western Pacific. The opposing La Niña stage is characterized by relatively cool waters in the eastern tropical Pacific and low atmospheric pressure in the western Pacific. The westward trade wind flow piles up warm water in the western Pacific and this causes sea level to be higher by about 20 cm in the western compared to the eastern Pacific (so sea level is, relatively speaking, higher in the eastern Pacific during El Niño). To compensate for this wind flow, the thermocline slopes down upwards towards the eastern Pacific, resulting in greater stratification in the east. The slope of the thermocline shallowing changes markedly during ENSO. As El Niño piles up warm water in the eastern Pacific, the thermocline deepens in that location, whereas the La Niña part of the cycle corresponds to a very shallow thermocline in the eastern Pacific.

The following animation shows how an El Niño event in 1997 developed in the tropical Pacific Ocean:

Video: Visualizing El Niño From NASA Scientific Visualization Studio (2:28)

Visualizing how three key datasets differ from normal conditions reveals the magnitude of the 1997-98 event and gives new insights into how the ocean and atmosphere couple to produce El Niño. First, we look at sea surface height. The gray sheet rises and dips as much as 30 centimeters from normal. Next, we map sea surface temperature. Red is 5 degrees Celsius above normal and blue 5 degrees below normal. Finally, black arrows are added to mark sea surface winds.

A weakening of the trade winds in the far western Pacific fuels an eastward moving wave. Sea level is raised as the windswept wave progresses. The waves arrival at the South American coast reduces the normal upwelling of cold water and warms the surface by as much as five degrees Celsius. Intense atmospheric convection associated with equatorial ocean warming causes local winds to converge, abating the trade winds strength and extending the warm pool of water into the eastern Pacific. Beneath the ocean surface, warm and cold waters are separated by the thermocline, a boundary at 20 degrees Celsius. El Niño flattens the thermocline, squeezing the warm bulge of water eastward into a long, shallow pool. As scientists collect more detailed data through efforts such as NASA's Earth observing system, visualization will be crucial in probing the elaborate interactions and far-flung effects of future El Niños.

As you may recall from Module 2, there is a good correlation between El Niño events and warmer temperatures on a global scale; in fact, a good part of the temperature variation superimposed on the general warming trend of the last 60 years can be explained by ENSO variability. This can be seen in the figure below (from Module 2).

Global Temperature Anomaly

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

During an El Niño event (red parts of the ENSO curve), more water vapor is added to the atmosphere, making it warmer, and during a La Niña event (blue parts of the ENSO curve), less water vapor makes it into the atmosphere, so the global temperature drops. Remember that water vapor is a greenhouse gas, and it also transports heat to the atmosphere when the vapor condenses to form water droplets (clouds and rain).

ENSO has major implications on weather across the globe. For example, drought conditions in 2012 in the Midwest are a direct result of the ENSO cycle. El Niño intervals correspond to increased precipitation in California and eastern South America, due largely to the ability of warm water to hold more moisture than cold water. The warm surface water in the eastern Pacific also increases surface ocean stratification and decreases upwelling offshore Peru and Ecuador. The implications of diminished upwelling are severe for coastal fisheries in these countries. Warm water also increases the occurrence of bleaching in the Caribbean and eastern Pacific corals. The position of the high-pressure region in the Pacific affects the tracks of Pacific cyclones. In the Atlantic, increased wind shear during El Niño intervals inhibits the formation of hurricanes. El Niño intervals also correspond to cooler and wetter winters in Florida and the southeastern US, and warmer and drier winters across the northern tier of the US and in Canada. The La Niña phase corresponds to the opposite conditions from El Niño, most notably an increase in the frequency in Atlantic hurricanes as conditions become more favorable for their formation (i.e., less shear in the tropical Atlantic).

Overview

Since records have been kept, the average temperature of the global surface ocean has warmed by more than 1^oC. Models suggest that all areas of the surface ocean will warm significantly more than this by 2100, especially in the high latitudes. This warming will have major implications for a number of key processes in the oceans, some of which will impact climate and weather patterns on land. In this section, we discuss the impact of climate change on the intensity and frequency of hurricanes, the potential shutdown of the global conveyor belt, the intensity of the ENSO cycle and the occurrence of oceanic hypoxia.

Hurricanes

Hurricane Isabel washes ashore in North Carolina, September 2003.

Credit: NASA

Many of you learned about the devastating force of hurricanes when Katrina hit New Orleans in August 2005. In fact, there have been a number of devastating hurricanes over the last few decades with Hurricane Hugo (Charleston) in 1989, Hurricane Andrew (South Florida) in 1992, and Typhoon Yasi in the Pacific (Australia) in 2011. Atlantic Hurricanes and Cyclones in the Pacific Ocean get their energy from the warm tropical ocean, so wouldn’t it follow that a warmer ocean will fuel more deadly hurricanes? Well, it’s not quite that simple and is more than a little controversial.

Early in spring, Bill Gray from Colorado State University puts out an annual hurricane forecast for the late summer and fall Atlantic Hurricane season. Gray’s forecast is based on a number of factors including sea surface temperatures in the tropical Atlantic, a number of atmospheric parameters, and the stage of the ENSO cycle. While hurricane experts agree about the feasibility of season forecasting, there is much less consensus among them about the effect of climate change on storms. Part of the problem is that coupled climate-ocean models do not have the resolution to forecast storms with a great deal of accuracy.

Since 1980 there has been a marked increase in the SSTs in the tropical Atlantic from August to October, the interval when hurricanes are formed, and the maximum power dissipation of hurricanes, which is the product of the summed maximum wind speed cubed and its duration. In fact, the agreement between these parameters is exceptional. Yet over the same time period, there is no observed increase in the frequency of hurricanes. In fact, when climate modelers downscale models of future climate to attempt to observe predictions for smaller regions than models typically yield, the result is a prediction of increased hurricane intensity but actually fewer storms in the future.

Global Conveyor

As we have seen earlier, the North Atlantic Ocean is the central driver of the global conveyor. Changes in density of surface waters in the Norwegian Sea as a result of warming, increased precipitation and melting of Greenland Ice can impact the flow of water downwelling as NADW. Sufficient changes in density could potentially shut down the production of NADW and literally halt the global conveyor. This, in turn, would have major implications for global climate.

You might find it hard to believe that such a massive engine of water and heat transport could possibly shut down. However, it has happened numerous times in the past, and this is perhaps the strongest evidence that it could happen in the future. Moreover, the past shutdowns have occurred over very short time intervals. Here, we will summarize our current understanding of the likelihood of GCB shutdown.

Approximately 8200 years ago, in the midst of the gradually warming climate of the current interglacial, a very abrupt cool period occurred that was likely the result of rapid melting of the Northern Hemisphere ice sheet and the flow of massive amounts of freshwater into the North Atlantic Ocean. The event was about 10-15 years long---a geologic heartbeat---and was likely due to a massive breach of the meltwater over a barrier in the St Lawrence Seaway. The impact of the flood of freshwater across the surface of the North Atlantic Ocean was to decrease the density of the water to such an extent that downwelling virtually stalled. The switching off of thermohaline circulation drastically slowed oceanic heat transport, which in turn led to rapid cooling in the Northern Hemisphere, especially in Northern Europe.

Moving on to the present day, flow of NADW is likely weakening due to increasing SSTs and freshening of the North Atlantic. Freshening is the result of melting of Greenland ice, as well as increased moisture transport in a warmer atmosphere. However, detection of a slowdown is very difficult as the meridional overturning circulation is highly variable, with a yearly average of 18.7 Sverdrup (a Sverdrup is one million m3 per second), but a range of 4.4 to 35.3 Sverdrup. Weakening is likely to continue into the future, also as a result of increased precipitation in northern high latitudes. However, most coupled ocean-models suggest that under realistic emission scenarios, thermohaline circulation will continue to slow but not shut down. The consensus of model runs suggests reductions of 10-50% in meridional overturning circulation intensity in the year 2100. However, these predictions are somewhat difficult because the relationships between many of the variables are not linear, rather there appear to be thresholds or tipping points that make prediction difficult.

What does this weakening of the NADW mean for the climate? Most ocean models show that as it weakens, less of the Gulf Stream warmth travels to high latitudes. Today, this warmth accounts for something like 30% of all the heat supplied to the Arctic region, so as the NADW weakens, models predict less heat moving into the Arctic. This might lessen the expected dramatic warming in the Arctic, but if it slows too much, there is a chance that the Arctic could become much colder — and very rapidly — as it did during the Younger Dryas event about 11 thousand years ago.

ENSO

The connection between climate change and changes in the duration and intensity of ENSO events has been hotly debated as the relationship of extreme weather events (often connected to ENSO) and climate change has been debated. Are extreme events getting more extreme as a result of climate change or is this just a part of the normal ENSO variability? Examples include major blizzards in the northeastern US during La Niña years and intense “Pineapple Express” rainfall in California during the El Niño stage of the ENSO cycle. As easy as it is to attribute such extreme events to climate change, reality is not nearly that simple, as these events fall under weather and not climate and much longer-term changes are required before an empirical connection can be made with certainty.

Models do not agree on the effects of warming on the ENSO cycle and so we will have to wait to see what happens in the future.

Occurrence of aquatic dead zones and their relationship to global population

Credit: NASA

Ocean Dead Zones or Hypoxia

Another great threat to marine environments is hypoxia, a condition in which seawater or freshwater becomes deficient in oxygen. As with harmful algal blooms, the main culprit for hypoxia is nitrification from agriculture or pollution. This oxygen deficiency can cause massive fish kills that can devastate coastal economies. Since warm waters can hold less oxygen than cold waters, climate change will render coastal regions more prone to hypoxia in the future. Hypoxia tends to occur when waters become increasingly stratified, which is gradually occurring in many coastal areas.

Processes that control the formation of hypoxic conditions in coastal environments such as the Gulf of Mexico.

Click for a text description of the Ecological Impact of Nutrient-Rich Water diagram.

A diagram illustrating the ecological impact of nutrient-rich water on marine life near a coast, focusing on oxygen depletion and its effects. It presents a cross-sectional view with the coast on the left, extending 25 miles into the ocean on the x-axis, and depth from 0 to 80 feet on the y-axis. The diagram is divided into numbered steps: (1) nutrient-rich water near the coast, (2) algae growth feeding on nutrients, (3) zooplankton consuming algae, (4) bacteria feeding on fecal pellets and dead algae, (5) bacteria depleting oxygen in the water, and (6) marine life dying due to low oxygen levels (2.0 mg/l or 1.0 mg/l). Green and brown shading indicates oxygen-deficient zones at different depths.

• Axes
  • X-axis: Distance from the coast (0 to 25 miles)
  • Y-axis: Water depth (0 to 80 feet)
• Coastal Area
  • Position: Left side
  • Color: Brown
• Steps of Ecological Process
  • Step 1: Nutrient-rich water
    • Position: Near the coast
    • Color: Green shading
  • Step 2: Algae grow, feed
    • Position: Near the surface, 0-20 feet depth
    • Representation: Small green dots
  • Step 3: Zooplankton eat algae
    • Position: Near the surface, 0-20 feet depth
    • Representation: Small black dots
  • Step 4: Bacteria feed on fecal pellets and dead algae
    • Position: Mid-depth, 20-60 feet
    • Representation: Small brown dots
  • Step 5: Bacteria deplete the water of oxygen
    • Position: Mid-depth to bottom, 20-80 feet
    • Color: Brown shading
    • Oxygen Levels: 2.0 mg/l and 1.0 mg/l zones marked
  • Step 6: Marine life dies (2.0 mg/l or 1.0 mg/l)
    • Position: Bottom layer, 60-80 feet
    • Representation: Text indicating impact
• Oxygen-Deficient Zones
  • 2.0 mg/l Zone
    • Color: Light brown
    • Position: 20-60 feet depth
  • 1.0 mg/l Zone
    • Color: Dark brown
    • Position: 60-80 feet depth
• Water Body
  • Color: Blue, with green and brown shading for nutrient and oxygen levels
  • Position: Central and right side of the diagram

Credit: Nancy Rabalais, Louisiana Universities Marine Consortium (LUMCON)

The northern Gulf of Mexico is a region where a seasonal dead zone is now well established. This dead zone was first noted in the 1970s and has gradually grown in size and intensity since. The critical driver for hypoxia is the outflow of nutrients from the Mississippi River. The Mississippi drains an enormous agricultural region of the mid-continent and thus acquires a very high level of nutrients. These spur production by algae in the surface ocean, which in turn rains down and utilizes oxygen in the deeper water column. The dead zone appears in spring as nutrient loads increase and peaks in late summer as oxygen becomes steadily depleted, exacerbated by the fact that warmer waters hold less oxygen than colder waters. Other regions with prominent dead zones include the Chesapeake Bay and Long Island Sound in the US and the Baltic Sea in Northern Europe. These areas will be more prone to hypoxia in the future.

The following video gives an overview of how hypoxia develops in the Gulf of Mexico.

Video: Gulf of Mexico Dead Zone (3:50) This video is not narrated.

End of Module Recap:

In this module, you should have grasped the following concepts:

• techniques used to study the oceans and their properties;
• distribution of the major properties (temperature, salinity, oxygen, nutrients) of surface and deep water and the processes that control them;
• the drivers of surface ocean circulation and how it impacts climate;
• the drivers of deep ocean circulation and the dominant deep water masses;
• upwelling and downwelling and how they impact ocean properties;
• El Niño Southern Oscillation and how it impacts global weather patterns;
• the global conveyor belt;
• likely changes in the ocean circulation in the future and how they will impact climate.

Assignments

You should have read the contents of this module carefully, completed and submitted any labs, the Yellowdig Entry and Reply, and taken the Module Quiz. If you have not done so already, please do so before moving on to the next module. Incomplete assignments will negatively impact your final grade.