Physical oceanography

Physical oceanography is the study of the physical conditions and processes in the ocean, especially how the water moves and what its properties are.

It is one of several parts of oceanography, which also includes biological, chemical, and geological oceanography. Physical oceanography uses ideas from thermodynamics and fluid mechanics, just like the study of atmospheric physics.

It can be divided into two main areas. Descriptive physical oceanography looks at the ocean by using observations and computer models to describe things like temperature, saltiness, and currents. Dynamical physical oceanography focuses on understanding the processes that control how fluids move, using theory and models. This area is part of a larger field called Geophysical Fluid Dynamics, which is shared with meteorology and studies flows influenced by the Coriolis force.

Physical setting

Almost all the water on Earth is found in the oceans. This water turns into vapor and falls as rain or snow on the land. The oceans can hold a lot of heat, which helps control Earth's weather and temperature. They also take in gases that change the air around us. The oceans even affect rocks under the sea and the gases from volcanoes.

The oceans are much deeper than mountains are tall. Land usually rises only about 840 meters high, but the ocean often goes down about 3,800 meters deep. While these differences seem big, very tall mountains and very deep ocean trenches are not common.

Temperature, salinity and density

The ocean's temperature and saltiness change a lot depending on where you are and how deep you go. Most of the deep ocean is very cold, usually between 0° and 5°C. Surface temperatures can be much colder near the poles or very warm, up to 35°C, in tropical areas. Salinity, which measures how much salt is in the water, is usually between 34 and 35 parts per thousand, but it can range from 10 to 41 parts per thousand depending on location.

The ocean has three main layers based on temperature. The top layer, called the mixed layer, is stirred by wind and waves, so the temperature doesn't change much as you go deeper. Below that is the thermocline, where the temperature drops quickly the deeper you go. This layer is strong in warm areas but not in cold polar waters. The deepest layer, the abyss, stays very cold, just above freezing.

Salinity also changes with depth. In some places, like the Red Sea, the water is very salty because of lots of evaporation. In polar areas, melting ice makes the surface water less salty. The halocline is a layer where salinity changes quickly with depth.

The mix of temperature and salinity affects how dense the water is. Cold, salty water is denser than warm, less salty water. This creates layers in the ocean, with denser water at the bottom. These density differences help drive the ocean's slow, global flow, which helps control Earth's climate.

Circulation

Main article: Ocean current

The ocean moves because of energy from the Sun and the pull of the Moon and Sun on Earth. More sunlight hits areas near the equator than near the poles. This makes water and air move, spreading warmth from the equator toward the poles. Most of this warmth moves through the air, but some moves through the ocean too.

The ocean gets warmed from above by the Sun. Cold, salty water in polar areas sinks and starts a slow movement of deep water. This is called the thermohaline circulation.

Ocean currents are mostly pushed by winds on the surface. Big patterns of wind and air movement affect how the ocean flows. In the tropics, winds blow from the east, and in middle latitudes, winds blow from the west. This creates slow flow toward the equator in the middle of ocean basins and faster flows along western edges.

Coriolis effect

The Coriolis effect makes water move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This affects ocean currents, making them flow around high and low pressure areas. Small changes in water level can create noticeable currents. The Coriolis effect is stronger near the poles and weaker near the equator, which creates fast currents along western edges of ocean basins.

Ekman transport

Ekman transport is when wind moving over the ocean pushes surface water. The water moves at an angle to the wind because of the Coriolis effect. The top layer of water moves with the wind, but layers below move at increasing angles below the surface. Near the bottom, the water moves almost opposite to the wind direction.

Langmuir circulation

Langmuir circulation creates lines on the ocean surface called windrows when wind blows faster than 3 meters per second. These lines are made by small rotating water cells. Debris and foam collect in some areas, while tiny plants called plankton gather in other areas. Fish often come to eat the plankton.

Ocean–atmosphere interface

The ocean and atmosphere trade heat, moisture, and momentum at their meeting point.

Heat

The ocean gains and loses heat at the surface. Tropical oceans usually gain heat, while polar oceans lose heat. This movement of heat helps control Earth’s climate. Oceans store heat and release it, which keeps nearby areas milder. For example, Western Europe is warmer partly because of warm ocean currents.

Momentum

Winds move much faster than ocean currents. From the air’s point of view, the ocean seems still, but winds push on the ocean’s surface and create currents. Wind also creates waves on the ocean.

Moisture

The ocean can gain moisture from rain or lose it through evaporation. When water evaporates, the ocean becomes saltier. Some areas, like the Mediterranean and Persian Gulf, lose a lot of moisture through evaporation.

Planetary waves

Kelvin Waves

Main article: Kelvin wave

Kelvin waves are waves that move between boundaries, like coastlines or the equator. They keep their shape and direction for a long time. They are usually caused by sudden changes in wind, like shifts in the trade winds.

Coastal Kelvin waves follow shorelines and move counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Equatorial Kelvin waves move east along the equator in both hemispheres. These waves move fast, with speeds of about 2 to 3 meters per second, and have very long wavelengths.

Rossby Waves

Main article: Rossby wave

Rossby waves, or planetary waves, are huge, slow waves created by temperature differences between the ocean and land. They are found at low to middle latitudes. There are two types: barotropic and baroclinic. Barotropic Rossby waves move fastest and do not change with depth. Baroclinic Rossby waves are slower.

Rossby waves always move west overall, but their groups can move in different directions. Shorter Rossby waves usually move east as a group, while longer ones move west.

Climate variability

The way the ocean moves helps balance heat around the planet. This, together with things like carbon dioxide, can cause changes in Earth’s climate over many years. Some well-known patterns include the Pacific decadal oscillation, North Atlantic oscillation, and Arctic oscillation. Changes in the thermohaline circulation can greatly affect the climate.

La Niña–El Niño

Main articles: El Niño and La Niña

Antarctic circumpolar wave

Main article: Antarctic Circumpolar Wave

This is a wave in the ocean and atmosphere that circles the Southern Ocean roughly every eight years. It moves east with the Antarctic Circumpolar Current.

Ocean currents

Among the most important ocean currents are the:

• Antarctic Circumpolar Current
• Deep ocean (density-driven)
• Western boundary currents
  • Gulf Stream
  • Kuroshio Current
  • Labrador Current
  • Oyashio Current
  • Agulhas Current
  • Brazil Current
  • East Australia Current
• Eastern Boundary currents
  • California Current
  • Canary Current
  • Peru Current
  • Benguela Current

Further information: Ocean gyre

Antarctic circumpolar

The ocean around Antarctica is the only continuous stretch of water that links the Atlantic, Pacific, and Indian oceans. Strong winds help create a strong current that circles Antarctica.

Deep ocean

In areas like the Norwegian Sea, cold, salty water sinks and flows south through gaps between islands. This deep water moves through the Atlantic, Indian, and Pacific oceans. Flow from the Arctic Ocean into the Pacific is blocked by shallow water in the Bering Strait.

Also see marine geology about that explores the geology of the ocean floor including plate tectonics that create deep ocean trenches.

Western boundary

In an ideal ocean, winds create a large spinning pattern of water flow. Near the western edge, a narrow, fast current flows north. Real oceans are more complex, but currents like the Gulf Stream, Agulhas, and Kuroshio are examples. They are about 100 kilometers wide and move about 1.5 meters per second.

Equatorward western boundary currents occur in tropical and polar regions, like the East Greenland and Labrador currents in the Atlantic, and the Oyashio current near Japan. They are pushed by winds around areas of low pressure.

Gulf Stream

The Gulf Stream is a fast, warm current that starts in the Gulf of Mexico, flows past the east coast of the United States and Britain, and crosses the Atlantic Ocean.

Kuroshio

The Kuroshio Current is a warm current off the east coast of Taiwan that flows northeast past Japan. It is similar to the Gulf Stream, moving warm water north in the Pacific Ocean.

Heat flux

Heat storage

Main article: ocean heat content

Ocean heat flux is a complicated system that scientists study using special tools to measure how heat moves. Heat flux is the amount of energy that moves in a certain area over time. Most of Earth’s heat is stored in the oceans, with smaller amounts moving through processes like evaporation or radiation. The main way heat moves in the ocean is through currents. For example, warm water in the south Atlantic likely comes from the Indian Ocean. Scientists are worried about recent changes in very cold deep water in the Southern Ocean because these changes can affect currents and plants and animals elsewhere. Since 1988, when the Intergovernmental Panel on Climate Change was created, more research has been done on how heat affects the world.

Sea level change

Main article: Sea level rise

Tools like tide gauges and satellites show that sea levels have been rising by about 1.5 to 3 millimeters each year for the past 100 years.

The IPCC predicts that by the years 2081 to 2100, global warming could cause sea levels to rise by 260 to 820 millimeters.

Rapid variations

Tides

Main article: Tides

The rising and falling of ocean water, called tides, greatly affect areas near the coast. Tides happen because of the pull of gravity from the Sun and the Moon. Both the Sun and Moon create tides, but the Moon’s movement makes the tides change over a month.

Tides create currents along the coast, and in narrow areas, these currents can become very strong. Sometimes, tides can create a big wave called a tidal bore when water pushes against a river’s flow.

Tsunamis

Main article: Tsunami

Big waves called tsunamis can happen when a lot of ocean water is moved suddenly. This can be caused by underwater landslides, shaking of the ocean floor from earthquakes, or a large meteorite hitting the ocean.

These waves can move very fast across the ocean, but in the open water, they are hard to notice because they are very wide. Tsunamis can cause big changes to the land near the coast when they reach shore.

Surface waves

Main article: Ocean surface waves

Wind creates waves on the ocean surface. These waves affect things like buildings far from the shore, boats, the land near the ocean, and places where ships stop. After wind makes waves, they can travel far across the water.