NEWS | July 25, 2012

Navy Researchers Sail Down Under to Study Internal Tides with AUVs

By Donna McKinney

In November 2011, a team of ten Naval Research Laboratory scientists from the Oceanography Division led by Dr. Jeff Book began working with the Australian Institute for Marine Science (AIMS), the University of Western Australia and the University of Tasmania on an internal wave project off the northwestern coast of Australia, near one of the most active locations for internal tides in the world. Over the next six months, the researchers installed 30 moored buoys at 23 sites to collect real-world observations in four separate tidal areas and assigned an ocean glider to each area. The final dataset recovered in April 2012, is one of the most comprehensive studies of internal tides on a coastal shelf to date and has the potential for greatly expanding our knowledge of this phenomenon and operational capability in regions strongly influenced by internal tides.

In the ocean, most oscillating motions (waves) have gravity as a restoring force. Waves generated by wind at the air-sea interface are the surface gravity waves familiar to all who have sailed or stood on a shoreline. When the waves are at the interface between ocean waters of different densities (e.g., warm and cold layers) or internal to continuously varying vertical density structures they are called internal waves and generally have slower and larger oscillations than their counterparts at the ocean surface. Internal tides are internal waves with frequencies of the twice-daily and daily tides and are generated by surface tides interacting with sloping topography in the ocean. These wave packets generally have weak surface signatures despite their subsurface strength and thus are difficult to measure and study. They play an important role in ocean mixing, yet their exact contribution to ocean energy budgets remains relatively unknown and is a subject of current investigation.

Navy scientists, such as Dr. Book are studying internal tides to learn more about them and how their activity affects ocean prediction models. They're huge and they're strong in many places in the world ocean, said Book. Strong internal tides interfere with data assimilation. Without data assimilation, your ocean predictions are not very accurate. Data assimilation (combining real-world observations with computer model predictions) helps ensure a more accurate computer forecast.

Ocean gliders are becoming one of the Navy's main tools for collecting data on the internal structure of the ocean for assimilation into ocean models. A glider is a long-endurance autonomous underwater vehicle (AUV) used to collect ocean data; it surfaces periodically to transmit data via satellite. Gliders are capable of collecting numerous types of data, including currents, temperature, salinity, pressure, and optics. However, A glider collects so much data, so rapidly, that the model can't assimilate it all, said Book. It has to select a small subset of glider data for its use. This results in the aliasing of the relatively rapid changes in internal ocean structure caused by internal tides.

In March/April 2012, the NRL and Australian teams boarded AIMS' R/V Solander to collect glider and buoy data and initially compare the in situ data with the Navy Coastal Ocean Model (NCOM) output on scene. Now that they have returned to their offices at Stennis Space Center, Miss., Jeff and his team will re-run the model simulating March and April 2012 all over again, filtering the amount and types of data the model assimilates. They will then examine the new model output to see how well the model performs compared to the actual conditions in March and April 2012.

Ultimately, the researchers will develop standard techniques to improve the ability to forecast in areas of strong internal tides. We want to transition our techniques to the Naval Oceanographic Office for operational use and provide them with different ways they can use their models to provide accurate forecasts in the many places in the world ocean where internal tides are strong.

The NRL team from the Oceanography Division included Jeff Book, Derek Burrage, Mark Hulbert, Steve Sova, Justin Brodersen, Ana Rice and Andrew Quaid in the field, and Clark Rowley, Philip Chu, and Jim Richman at Stennis Space Center running the models. Sherwin Ladner, Joel Wesson, Courtney Kearney, and Ruhul Amin provided additional support by processing the real-time data coming from the satellites.