Every diver learns the basics of physical oceanography the first time a current takes hold of them. On a drift dive in Cozumel, the whole reef seems to slide past while you hang motionless in the water, carried by a current you cannot see. Physical oceanographers are the people who explain that current: where it comes from, why it speeds up and slows down, and how it connects to the circulation of an entire ocean basin. Their field is the physics of the sea, and it is the branch of oceanography most closely tied to weather and climate.

I have thought about water movement for most of my adult life for a more practical reason. In my reef tanks, flow was one of the hardest things to get right. Corals need water moving across them to bring food and oxygen and carry away waste, and I used wave makers and changes in rock layout to try to imitate the surge and current of a real reef. When I redesigned my 75-gallon system, a big part of the goal was simply better water flow. A physical oceanographer deals with the same problem on the scale of a planet.

What physical oceanographers study

Physical oceanography covers almost any motion of seawater, from tiny turbulent eddies to currents that span whole oceans.

Ocean currents include wind-driven surface currents such as the Gulf Stream and the Kuroshio, which carry warm water poleward along the western edges of ocean basins, and the slow, deep currents driven by differences in temperature and salinity that together make up the global overturning circulation. Waves range from ripples and wind waves to swell that crosses oceans, tsunamis and the huge internal waves that travel along layers of different density below the surface. Tides are driven by the Moon and Sun and shaped by the geometry of coastlines and basins. Mixing and upwelling bring cold, nutrient-rich water to the surface, feeding some of the most productive fisheries on Earth. And air–sea interaction covers the exchange of heat, moisture, momentum and gases that links ocean and atmosphere, which is at the heart of phenomena such as El Niño and hurricane intensification.

From Franklin to Munk: a short history

Physical oceanography grew out of practical seamanship long before it became a science.

In 1770, Benjamin Franklin, then deputy postmaster for the American colonies, worked with his cousin Timothy Folger, a Nantucket whaling captain, to publish a chart of the Gulf Stream after noticing that mail ships from England took longer than merchant ships that knew to avoid the current. In the 1850s, Matthew Fontaine Maury of the US Navy compiled thousands of ships’ logs to chart winds and currents, published as The Physical Geography of the Sea. The 20th century brought the mathematics. Scandinavian scientists such as Vilhelm Bjerknes and Harald Sverdrup developed the theory of ocean and atmosphere dynamics, Sverdrup and Walter Munk developed methods for forecasting waves that were used to plan Allied amphibious landings in the Second World War, and Henry Stommel explained in 1948 why currents like the Gulf Stream are concentrated on the western sides of oceans. Munk went on to become one of the most influential ocean scientists of the century, working on tides, waves and ocean acoustics.

Tools: from CTDs to satellites

Physical oceanographers measure the ocean from ships, from robots inside it and from space.

The CTD measures conductivity (salinity), temperature and depth, the basic properties that determine seawater density and therefore how it moves; see how oceanographers measure salinity. Current meters and acoustic Doppler current profilers on moorings record currents for months or years. The international Argo programme, begun in the early 2000s, maintains several thousand robotic floats that profile the upper 2,000 metres of the ocean every ten days or so, giving the first truly global picture of ocean temperature and salinity. Gliders and drifters add more data. Satellites measure sea surface temperature, winds, waves and sea level; altimeters such as TOPEX/Poseidon, launched in 1992, and its successors can detect sea-level differences of a few centimetres from orbit, revealing currents and eddies. Finally, numerical models, run on supercomputers, solve the equations of fluid motion to simulate the ocean and forecast its behaviour.

Physical oceanography and climate

The ocean is the climate system’s great reservoir of heat, and physical oceanographers are the people who measure it.

Assessments by the Intergovernmental Panel on Climate Change conclude that the ocean has absorbed the large majority of the extra heat trapped by greenhouse gases in recent decades, around 90 percent. That has slowed warming at the surface of the land but warmed the ocean, raised sea level through thermal expansion and contributed to marine heatwaves. Marine heatwaves are the main driver of mass coral bleaching, which is a subject close to my heart after years of growing corals; see coral reef bleaching. In a reef tank, temperature control is one of the first things you learn to take seriously. I ran a chiller on my multi-tier system and found that lowering the temperature helped me keep nuisance algae in check under intense lighting, which is a small-scale lesson in how sensitive reef life is to heat. On a real reef there is no chiller, and a few weeks of water just a degree or two above the usual summer maximum can be enough to bleach corals across hundreds of kilometres. Forecasting those heatwaves, which NOAA’s Coral Reef Watch does using satellite sea surface temperatures, is applied physical oceanography. Physical oceanographers also study whether the Atlantic overturning circulation is slowing, how warm water is reaching the undersides of Antarctic ice shelves, and how El Niño may change as the planet warms. These are among the most important questions in climate science, and the answers depend on careful measurement of the ocean’s physics.

Physical oceanography you can see on a dive

You do not need a research ship to see physical oceanography at work. Divers and snorkellers meet it constantly.

A thermocline is the boundary between warm surface water and colder water below. On many dives you can feel it as a sudden chill on your legs, and sometimes see it as a shimmering layer where water of different densities mixes. Currents are why drift diving exists: on the walls of Cozumel the boat follows the divers rather than the other way round, because the current running between the island and the mainland carries everyone along. Tides decide when it is safe to dive many channels and passes, such as the reef passes of French Polynesia, where incoming tides bring clear ocean water and outgoing tides can be strong enough to sweep divers out to sea. Upwelling explains why some tropical sites are suddenly cold and green with plankton for a few days. Surge, the back-and-forth motion of water under passing waves, is what makes shallow reef dives on a windy day so tiring.

Each of these is a small, local example of the processes physical oceanographers study on the scale of ocean basins. Learning to read them is one of the most useful skills a diver can develop.

How to become a physical oceanographer

Physical oceanography is the most mathematical branch of the field, and most practitioners come from physics, mathematics or engineering.

A typical path is an undergraduate degree in physics, applied mathematics, engineering or atmospheric science, with strong coursework in calculus, differential equations, fluid dynamics and programming. Graduate study in physical oceanography usually follows, at institutions such as Scripps, Woods Hole and MIT, Lamont-Doherty, Oregon State, the University of Washington and many others worldwide. A PhD is standard for research careers. Skills in scientific computing are essential, because much of the work involves analysing large data sets and running or developing models. Seagoing experience is valuable but not universal; some physical oceanographers are primarily observationalists, others almost entirely theorists or modellers.

Where physical oceanographers work

The skills are in demand well beyond universities.

Government agencies such as NOAA, NASA and its Jet Propulsion Laboratory, and national weather and climate services around the world employ physical oceanographers for forecasting and research. The US Navy relies on them for everything from sonar performance to ship routing; see the Oceanographer of the Navy. In industry, they work in offshore energy, including wave, tidal and offshore wind projects, in coastal engineering, in port design, in insurance and risk modelling, and in companies that build ocean instruments. Some move into data science, where their experience with complex, noisy data sets is highly valued.

For how physical oceanography fits with the other branches, see what oceanographers do, chemical oceanographers and geological oceanographers.