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Physical Oceanography (PO)

PO Research at WHOI

Physical oceanography is the exploration and study of the physics and geography of the ocean currents and water properties. These complex oceanic motions occur over a wide variety of space and time scales including:

  • Grand persistent currents, like the Gulf Stream and the wind-driven ocean gyres
  • Transient eddies and waves of all sizes and speeds from surface gravity waves, to slower internal waves beneath the sea surface, to planetary-scale Rossby waves
  • Estuary and rivers outflows onto the continental shelf
  • Coupled ocean-atmosphere interactions, both local and global

Physical oceanography has important applications in global climate and coastal studies. It is also a key element in interdisciplinary studies of primary production, hydrothermal vents, and oceanic flux and storage of carbon dioxide.

Air and water

There are similarities in how the atmosphere and the ocean move and, as such, physical oceanography has much in common with meteorology. As in the atmosphere, relatively intense frontal systems exist in the oceans. Mixing and stirring occurs across these oceanic fronts, caused by a variety of physical processes ranging...

  • From instabilities of large-scale currents such as the Gulf Stream that generate long-lived "rings," much as atmospheric storm systems are spawned from the Jet Stream
  • To salt fingers, no bigger than a centimeter, which produce efficient vertical exchanges of salt and heat between different water masses

Understanding the interactions between the atmosphere and the ocean is also a key challenge for physical oceanographers. The ocean and atmosphere drive each other in as yet poorly understood ways, with a multitude of consequences.

From lab to field

Physical oceanographers employ a wide variety of scientific skills and approaches, from the most descriptive to the most theoretical, to explain oceanic motions. Want to be both a theorist and an experimentalist? This field is one of the few areas of physics in which you can achieve that goal. In fact, students in the Joint Program have the opportunity, and indeed are encouraged, to develop skills and experience in both approaches.

Observational work

To measure such oceanic properties as temperature, salinity, pressure, and velocity, observational oceanographers use a variety of sophisticated instruments, which may be shipborne, drifting, moored, on autonomous submersibles, or mounted on aircraft or satellites.

Research cruises collect water property and flow data in both nearby and remote regions of the globe. Submerged neutrally buoyant floats are tracked for hundreds of kilometers in studies of ocean currents. Instrumented moorings record currents, temperature, and salinity throughout the water column. These and other data provide the essential descriptive foundation for a conceptual understanding of the causes of oceanic motions.

But observational oceanography is much more than cataloging oceanic features. Modern observational work is designed as much to test theoretical hypotheses as it is to provide new data in unknown regions. Observational studies are sometimes limited to specific processes, such as deep overflows or air-sea interaction, and sometimes global in extent, such as efforts to map the distribution of eddy kinetic energy derived from satellite observations. Some observational programs are limited in duration but involve extensive spatial measurements; others are designed to give long-term time series of oceanic conditions at one specific location.

In almost all cases, observational work is done with the most modern of instruments and data analysis procedures to extract information from a dynamic and challenging environment.

Theoretical work

Theoreticians and modelers attempt to explain the observed flow and the structure of temperature and salinity distributions in the ocean. Another role of theory and numerical modeling is to uncover new dynamical concepts and processes that may not have been apparent, or were overlooked, in existing observations.

This work can involve the construction of the simplest mathemetical models as a means of illuminating some new, unexpected process. Or it can involve the analysis of sophisticated mathematical models with modern mathematical methods of considerable complexity.

Theory can be supplemented and extended by numerical modeling of dynamical processes that are just too complex to be treated with analytical methods. A good example is turbulent flows, both small-scale mixing and large-scale geostrophic turbulence. These numerical studies may produce solutions to specially derived sets of equations, or they may involve numerical models of the general equations governing fluid flow appropriate to oceanic conditions. This modeling ranges from models specific processes, such as western boundary currents, to global-scale simulations of oceanic flow.

Laboratory experimentation also plays a crucial role in the development and testing of basic theoretical ideas. Laboratory experiments can be used to explore new dynamical regions for which no theory exists, or to provide controlled, repeatable data (rarely obtainable from the ocean) to throughly test the predictions of a theory or numerical calculation.

Putting theory and observations together

Neither observations or theory alone are enough to solve the mysteries of oceanic circulation. Physical oceanography is mature enough to have a substantial body of theory to aid in the interpretation of observations, yet the ocean is continually offering new data to challenge existing ideas of how the ocean works. The deepest understanding and most interesting results almost always evolve from the interplay between these two approaches. This interaction is what makes physical oceanography such an exciting subject and leaves plenty of opportunity for someone entering the field.