Internal Tides of the Oceans
Harper Simmons
International Arctic Research Center
Story by Jenn Wagaman


The ocean's interior has its own weather and climate, much like some of the Earth's highest mountain peaks. The weather of the ocean results from fluctuating currents (wind) and from waves similar to the waves on the surface of the ocean. The rhythmic movement of the ocean, caused by the tidal cycle, creates internal waves. One important type of internal wave is called an internal tide. As the ocean sloshes back and forth, it flows over geographic variations in the bottom depths, such as seamounts. As a result, waves that are excited in the interior of the ocean radiate away. When these waves are generated by the tides, they are called internal tides. Internal tides fill the ocean's interior with waves, carrying energy from one part of the ocean to another.


As tides slosh back and forth over the ocean bottom, internal waves are excited. At day 2 of the simulation, internal waves have just begun to radiate from topographic features.

Harper Simmons and his colleagues at the International Arctic Research Center (IARC) at the University of Alaska Fairbanks (UAF) are interested in the internal tides of the ocean for reasons as diverse as research logistics and climate change. With the help of his fellow researchers, Simmons is modeling these tides on the global scale at a higher resolution than ever before accomplished.

The Ocean and Climate

Internal waves eventually run out of energy and break, just like surface waves. When they break in the deep ocean, they create turbulence. Regions where there is a great deal of turbulence are regions where heat can be transferred from the upper ocean and stored in the deep ocean. But the Arctic Ocean is unique because it is actually warmer at intermediate depths (150-900 meters) than on the surface. In the Arctic, turbulence transfers heat from the deep ocean to the surface. Heat transfer between the surface and the ocean interior is important for a better understanding of the ocean's role in climate. The amount of heat transferred vertically from the warm interior of the Arctic Ocean, affects the amount of sea ice cover in the Arctic. The amount of sea ice in the Arctic, in turn, can affect climate on the broadest scales: the amount of sea ice can affect the rate of ice export into the Greenland-Iceland-Norwegian (GIN) seas and ultimately the Labrador Sea. If a large amount of sea ice moves into these areas, then deep convection can be slowed down or even halted, causing the Ocean Conveyor Belt to slow down and alter global climate. The Ocean Conveyor Belt is thermohaline circulation driven mostly by the formation of sinking deep water in the Norwegian Sea.

By day 6, waves are beginning to fill the oceans.


By day 20, the tide model has energetically equilibrated and internal tides have saturated the oceans. The evidence of "beams" of internal waves extending across entire ocean basins is intriguing.

Protecting and Understanding Data from the Field

Simmons is also interested in internal tides because the fluctuations in temperature and speed that occur as a result of these waves look like "noise" in oceanographic instrument records. Knowing where internal wave energy might be high and where it might be low will help researchers distinguish between fluctuations in the data record originating from ocean currents or fluctuations resulting from internal waves/tides that happen to be passing by.

Oceanographers are interested in the energy transfer ("conversion rate") from surface tides to internal tides. The simulated conversion rate is clearly associated with the major bathymetric features.


A simulation of internal wave activity focusing on the Arctic Ocean.

Currently, IARC researchers are working to establish a network of oceanographic monitoring instruments in the Arctic. Simmons and his colleagues are supporting this effort by working to create a system for predicting where internal wave energy is low or high, so scientists can decide where to place future deployments. Thus, oceanographic instruments can be deployed in oceanographically interesting locations where scientists can quantify the vertical redistribution of heat in the Arctic Ocean.

Three-dimensional Tide Model

Simmons and fellow researchers recently reported on the first global three-dimensional model of tides. In one report, special focus was given to the physics of the ocean's internal waves and the computational feasibility of simulating internal waves in a global domain. Another report, led by a colleague at Princeton University focused on the fidelity of the tidal simulation itself. This kind of work requires extremely high resolution when compared to conventional ocean models. In addition, the model output is very large, causing the post-processing to present almost as many challenges as the computations themselves. The researchers are working on adding internal waves excited by surface winds as well as Arctic Ocean and South China Sea regional models with even higher resolutions. Simmons is also working with ARSC specialists Ed Kornkven and Jeff McAllister to refine his ten-layer model to a 36-layer model, in an effort to increase the detail of the model's predictions. "ARSC has a knowledgeable staff who greatly improve my ability to make progress on this research. In porting my model, we have found that ARSC supercomputers are able to make the required computations two to three times faster than previously possible," says Simmons.