ANTISUBMARINE WARFARE

Global Antisubmarine Warfare Operations

There are no significant first-order effects from climate change on U.S. antisubmarine warfare capabilities. A robust infrastructure that collects, analyzes, and distributes oceanographic data essential to ASW effectiveness is in place and covers active submarine operating areas adequately. Climate change will, however, mandate that submarine and ASW operations become more robust in the Arctic Ocean, where essential data are sparse or nonexistent in both special and temporal senses. Moreover, as potential adversarial submarines have become acoustically more quiet, ASW operations have evolved away from a pure submarine-on-submarine mission to a cooperative, coordinated mission involving fixed and mobile sensors and surface, subsurface, and air platforms.

This extensive and deployable ASW infrastructure that supports the principal nuclear-powered attack submarine (SSN) hunter platforms is generally deployed in the temperate oceans, but it would be challenged to operate in the Arctic. As well, the supporting tactical oceanographic data collection, analysis, and distribution system does not extend to the Arctic. Additional support infrastructure must be established or restored to enable more effective ASW operations in that region, which will become an inevitable national imperative.

Ocean acoustics are fundamental to submarine operations and antisubmarine warfare. The speed of sound in seawater is a function of pressure (depth), temperature, and salinity. Acoustic waves reflect off the sea surface and seafloor boundaries, and seawater absorbs acoustical energy at a rate proportional to frequency squared. There are two net effects of these properties upon ocean acoustics, which can be summarized as follows: (1) the refractive properties can lead to a sound fixing and ranging (SOFAR) duct that traps energy and leads to very-long-range propagation of signals at low frequencies, and (2) the combination of boundary losses and absorption losses leads to an optimal frequency for efficient sound propagation.16 These effects, plus the ambient noise environment and capabilities of the sonar system, determine the performance (for example, detection range) of an ASW system. For concerns of this report, ocean climatology impacts both of these major effects as well as the ambient noise.

Refraction (and the resulting SOFAR duct) is a phenomenon that can lead to long-range detection of submarines. In temperate oceans, the SOFAR duct is typically at a depth of 1 km, but as one goes to higher latitudes and colder water, it gradually migrates to the surface, which is the case in the Arctic Ocean. The dominant trade-off for the depth of the SOFAR duct is between hydrostatic pres-

16

The sound speed in the oceans is a function of depth. In deep water the opposing effects of warm water at the surface and higher density at depth lead to a minimum in the sound speed. Since sound will always bend, or refract, toward a minimum, this leads to a duct trapping the acoustic power and very low loss propagation.



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