Remote Sensing of Ocean Internal Waves: An Overview

KLEMAS, V., 2012. Remote sensing of ocean internal waves: an overview. Journal of Coastal Research, 28(3), 540–546. West Palm Beach (Florida), ISSN 0749-0208. The oceans are density stratified because of vertical variations in temperature and salinity. Oceanic internal waves can form at the interface (pycnocline) between layers of different water density and propagate long distances along the pycnocline. Internal waves on continental shelves are important because they can attain large amplitudes and affect acoustic wave propagation, submarine navigation, nutrient mixing in the euphotic zone, sediment resuspension, crossshore pollutant transport, coastal engineering, and oil exploration. Internal waves induce local currents that modulate surface wavelets and slicks, causing patterns of alternating brighter and darker bands to appear on the surface. The surface patterns can be mapped by satellites using synthetic aperture radar (SAR) or visible imagers. The objectives of this article are to discuss methods for remotely studying and mapping ocean internal waves and to present examples illustrating the application of satellite remote sensing. ADDITIONAL INDEX WORDS: Ocean internal waves, remote sensing, satellite oceanography. INTRODUCTION AND BACKGROUND The water column in the ocean is frequently not homogeneous, but stratified, with low-density, warmer water residing on top of high-density, colder water. The boundaries that separate water layers of different density and temperature are called pycnoclines and thermoclines, respectively. Internal waves (IWs) or packets of IWs can form at the interface between layers of different water density and travel long distances along the pycnocline. The IWs are especially common over the continental shelf regions of the oceans and where brackish water overlies salt water at the outlets of large rivers. The period of IWs approximates the period of the tides, suggesting that the IWs may be generated by strong tidal currents flowing over sharply varying bottom topography, such as shelf breaks and shallow sills. Internal waves in the open ocean and on continental shelves can attain amplitudes in excess of 50 m and strongly influence acoustic wave propagation, submarine navigation, nutrient mixing in the euphotic zone, sediment resuspension, crossshore pollutant transport, coastal engineering, and oil exploration (Duda and Preisig, 1999; Ebbesmeyer, Coons, and Hamilton, 1991; Shanks, 1987). Studies of IWs are of particular interest to companies using oil-drilling rigs and other vulnerable structures in the ocean. The U.S. Navy has been investigating the nature and causes of internal waves and their effect on acoustic waves used for detecting subsurface objects; IWs have also been suspected of causing the sinking of several submarines (Pinet, 2009). Since ancient times, mariners sailing the oceans have observed internal waves in the form of alternating darker and brighter bands, with increased sea-surface roughness. The IWs cause local currents that modulate surface wavelets and slicks. Because subsurface internal waves produce this strong surface signature, they can be mapped with satellite and airborne sensors. Satellite synthetic aperture radars (SARs) have become the most important sensors for observing internal waves. The IWs have also been studied using multispectral satellite images and space-shuttle photographs. The objectives of this article are to discuss the most-effective methods for remotely observing ocean IWs and to present examples illustrating the application of remote sensing to studies of IW generation and propagation. OCEANIC INTERNAL WAVES The oceans are density stratified because of vertical variations in temperature and salinity. As the temperature decreases toward the bottom of the ocean, it causes the density of the water to increase toward the bottom. Ocean stratification is primarily determined by the action of turbulent mixing of water masses because of wind stress and heat exchange at the air–sea interface. Stratification retards vertical mixing of water masses, reduces vertical transport of nutrients, and affects propagation of acoustic signals in the ocean. The pycnocline is a zone that vertically separates water masses having different densities (Mirie and Pennell, 1989; Pinet, 2009). The IWs can form at the interface between water layers of different density and propagate along the pycnocline. Oscillations are more easily set up at an internal interface than at the sea surface because the difference in density between two water layers is much smaller than between water DOI: 10.2112/JCOASTRES-D-11-00156.1 received 26 August 2011; accepted in revision 20 September 2011. ’ Coastal Education & Research Foundation 2012 Journal of Coastal Research 28 3 540–546 West Palm Beach, Florida May 2012