6.2 Ocean Water Properties and Movements

Ocean water's properties and movements shape Earth's climate and ecosystems. Salinity, temperature, and density variations create layers in the ocean, influencing global circulation patterns. These factors drive the thermohaline circulation, a crucial system for heat distribution.

El Niño and La Niña, opposite phases of a climate pattern, dramatically affect ocean conditions. These events alter sea surface temperatures, wind patterns, and marine life, causing widespread impacts on weather and ecosystems worldwide.

Ocean water properties

Salinity and its influencing factors

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• Salinity is the measure of dissolved salts in ocean water, with an average salinity of 35 parts per thousand (ppt) or 3.5% salt content
• The two main factors affecting ocean water salinity are:
  • Evaporation, which increases salinity by removing fresh water and leaving behind dissolved salts (Mediterranean Sea)
  • Precipitation and freshwater input from rivers, which decrease salinity by adding fresh water to the ocean (Baltic Sea)
• Other factors influencing salinity include sea ice formation and melting, as well as the mixing of water masses with different salinities

Temperature and density variations

• Ocean water temperature varies with depth and latitude
  • Warmer temperatures occur near the surface and equator due to increased solar radiation (tropical oceans)
  • Colder temperatures are found in deeper waters and near the poles, where there is less solar radiation and more heat loss to the atmosphere (Arctic Ocean)
• Density of ocean water is determined by its salinity and temperature
  • Higher salinity and lower temperature result in higher density
  • Colder, saltier water is denser than warmer, fresher water, leading to layering of ocean water (Antarctic Bottom Water)
• The relationship between temperature, salinity, and density is described by the equation of state for seawater, which is used to calculate water density based on these properties

Ocean water layering

Stratification based on temperature and salinity

• Ocean water is stratified into layers based on differences in temperature and salinity, which affect water density
• The uppermost layer is the surface mixed layer, which is well-mixed by wind and waves and has relatively uniform temperature and salinity
  • The depth of the mixed layer varies seasonally and geographically, typically ranging from 20 to 200 meters (shallower in summer and deeper in winter)
• Below the mixed layer is the thermocline, a region of rapid temperature change with depth, separating the warm surface waters from the colder, denser deep waters
  • The depth and strength of the thermocline vary with latitude and season, being shallower and more pronounced in the tropics and during summer (equatorial Pacific)
• The halocline is a layer of rapid salinity change with depth, often found in regions with high freshwater input, such as near river mouths or in polar regions with sea ice melt (Amazon River plume)

Deep ocean characteristics

• Beneath the thermocline and halocline lies the deep ocean, characterized by cold, dense, and relatively homogeneous water
• Deep ocean water has a relatively constant temperature of around 2-4°C and a salinity of about 34.6-35.0 ppt
• The deep ocean is divided into several distinct water masses based on their formation regions and properties, such as North Atlantic Deep Water and Antarctic Bottom Water
• These water masses slowly circulate throughout the global ocean, taking hundreds to thousands of years to complete a full circuit (radiocarbon dating of deep water)

Thermohaline circulation

Driving forces and circulation patterns

• Thermohaline circulation, also known as the global ocean conveyor belt, is the large-scale movement of ocean water driven by differences in temperature and salinity
• The circulation pattern is initiated in the North Atlantic, where cold, dense water sinks and flows southward along the ocean bottom, eventually upwelling in the Indian and Pacific Oceans (North Atlantic Deep Water formation)
• The sinking of cold, dense water in the North Atlantic is a key component of the Atlantic Meridional Overturning Circulation (AMOC), which transports warm, salty water northward from the tropics (Gulf Stream)
• In the Southern Ocean, the Antarctic Circumpolar Current (ACC) connects the Atlantic, Indian, and Pacific Oceans, allowing for the global transport of water masses (Drake Passage)

Role in global heat distribution and climate

• Thermohaline circulation plays a crucial role in redistributing heat globally
  • Warm surface currents carry heat from the equator towards the poles (North Atlantic Current)
  • Cold deep currents return cooler water to the tropics (Antarctic Bottom Water)
• The heat transport by ocean currents helps to moderate global climate by reducing the temperature gradient between the equator and the poles
• Changes in thermohaline circulation, such as a slowdown of the AMOC, can have significant impacts on global climate
  • A weakening of the AMOC could lead to regional cooling in the North Atlantic and Europe (Younger Dryas event)
  • Alterations in thermohaline circulation patterns may also affect the distribution of nutrients and marine productivity (upwelling regions)

El Niño vs La Niña

El Niño characteristics and impacts

• El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO), characterized by weakening of the easterly trade winds and a shift of warm surface water towards the eastern Pacific
• During an El Niño event, sea surface temperatures in the eastern Pacific increase, leading to a decrease in upwelling of cold, nutrient-rich water along the coast (Peruvian anchovy fishery collapse)
• The increased sea surface temperatures during El Niño can lead to changes in global weather patterns
  • Increased rainfall in the eastern Pacific and along the west coast of South America (flooding in Peru and Ecuador)
  • Drought in the western Pacific, including Australia and Indonesia (bush fires)
• El Niño events can also cause coral bleaching due to increased water temperatures, and alter the distribution and abundance of marine species (Great Barrier Reef bleaching events)

La Niña characteristics and impacts

• La Niña is the cool phase of ENSO, characterized by stronger-than-normal easterly trade winds and a shift of warm surface water towards the western Pacific
• During a La Niña event, sea surface temperatures in the eastern Pacific decrease, leading to increased upwelling of cold, nutrient-rich water along the coast (enhanced Peruvian anchovy fishery)
• The cooler sea surface temperatures during La Niña can result in opposite weather patterns compared to El Niño
  • Increased rainfall in the western Pacific, including Australia and Indonesia (flooding)
  • Drier conditions in the eastern Pacific and along the west coast of South America (drought in Peru and Ecuador)
• La Niña events can also enhance the formation of tropical cyclones in the Atlantic basin due to reduced wind shear and cooler upper-ocean temperatures (active Atlantic hurricane seasons)