Physical oceanography

Physical oceanography is the study of the ocean’s conditions and how water moves. It looks at things like temperature and currents.

Oceanography has many parts, including biological, chemical, and geological studies. Physical oceanography uses ideas from thermodynamics and fluid mechanics, similar to atmospheric physics.

It has two main areas. Descriptive physical oceanography uses observations and computer models to describe the ocean. Dynamical physical oceanography studies why and how fluids move, using theory and models. This area is part of Geophysical Fluid Dynamics, which is also used in meteorology and looks at flows affected by the Coriolis force.

Physical setting

Almost all the water on Earth is 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 affect rocks under the sea and 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. The Coriolis effect is stronger near the poles and weaker near the equator.

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.

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.

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. 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 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.

Coastal Kelvin waves follow shorelines. 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.

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.

Rossby waves always move west overall, but their groups can move in different directions.

Climate variability

The way the ocean moves helps balance heat around the planet. This 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.

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.

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.

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.

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

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.

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.

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 complex system that scientists study. 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. Heat moves mainly through ocean currents. For example, warm water in the south Atlantic comes from the Indian Ocean. Scientists watch 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, change areas near the coast. Tides happen because of the pull of gravity from the Sun and the Moon. The Moon's movement makes the tides change over a month.

Tides create currents along the coast. 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 moves 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. In open water, they are hard to notice because they are very wide. Tsunamis can change 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 buildings far from the shore, boats, land near the ocean, and places where ships stop. After wind makes waves, they can travel far across the water.