Captain LeMarchand’s “Under Ice Operations” in the October, 1985 SUBMARINE REVIEW was rocussed on the problems of warfare under the Arctic polar ice cap. In this environment, a sound velocity profile shows a steady increase with depth, producing in effect a good sound channel with the axis close to the surface. The transmission of sound in such a channel, consequently, is long-range and acoustic scattering is produced only by the irregularities in the lower surface of the ice-covering and particularly from the ice keels which extend downward. He also notes that the ambient noise is low under this ice cover. Overall, then, conditions for long range detections of enemy submarines are generally very good.
But Anthony Wells, in his January, 1986 SUBMARINE REVIEW article sounds a note of caution for u.s. submariners carrying out their ASW mission against Soviet submarines in their Arctic “bastions.” He suggests that u.s. submarines operating within the marginal ice zone (MIZ) where the polar ice is not solidly joined and consists of ice floes — might have a significant ASW problem against well-handled Soviet SSBNs, (in fact against enemy subs in general) which would be “like looking for a needle in a haystack in a hostile environment.”
Why then wouldn’t some Soviet submarines be operated in MIZ bastion areas which favor their survival — rather than under the polar ice cap where sound conditions make their detection by u.s. submarines a lot easier?
The Office of Naval Research has been conducting, since 1979, a series of basic science field investigations (along with other nations in an international program) in the unclassified MIZ, to better understand this environment relative to naval operations within such an area. The area chosen for the investigative MIZEX exercises is shown in Figure 1. and is generally between Svalbard and the east coast of Greenland. The marginal ice zone in this area has a changing geography as the ice edge moves hundreds of kilo-meters north and south on a seasonal cycle.
In Captain LeMarchand’s article, the sound velocity profile under the permanent ·ice cover of the Arctic ocean “is essentially all positive.” The sound velocity profiles taken in the Marginal Ice Zone of the area shown above indicate some-what different characteristics — with sound channeling unlikely and anomalies confusing the acoustic sound paths. See Figure 2.
The irregular nature of the sound velocity profiles in the Marginal Ice Zone is perhaps better shown by a plot of the sound velocities taken over a stretch or 45 miles within the area shown in Figure 1. The effect of surface warming or cooling in the ice floe areas produce greatly varying velocities in the first one hundred feet of depth, but below that there is an almost constant velocity. Thus, a submarine hiding near the surface might easily pose a problem “like hunting for a needle in a haystack.” See Figure 3.
The bathythermograph readings taken in the Marginal Ice Zone show considerable variance when taken at relatively close intervals of range or within a few days of each other. See Figure 4. An almost constant reevaluation of sound conditions appears necessary when operating within this area — plus an almost continual changing of submarine trim when moving rapidly through this zone.
It should be recognized that the relatively warm, saline Norwegian-Atlantic branch of the Gulf Stream moving toward the Pole, hugs the Svalbard side of the MIZ, while the far colder, ice-choked and fresher Arctic waters flow southward close to Greenland. This results in a pronounced frontal and current system called the East Greenland Polar Front. The tremendous interchange o~ energy between these cold and warm waters makes the area an extremely dynamic and unstable region characterized by complex oceanographic and atmospheric structures. In addition, fresh water derived from ice-melting creates additional instability due to density differences.
Unlike the low ambient noise enjoyed under the polar ice cap, the ambient noise is far higher in the MIZ. The ice floes become progressively smaller as one nears the edge of the “ice pack.” The first and multi-year ice floes in the inner zone or the MIZ tend to be a few hundred meters across and 2-5 meters thick. Leads through these floes are choked with pieces or thinner ice, with solar energy melting, for the most part, the first year ice. The ice floes in a transition zone or 5-15 kilometers in width, between the inner and outer zones, are uniformly broken and smaller, with an ice-concentration in this area or 70-90S and with the leads free or brash. The outer zone is a complex region or brash and tiny floes near the extreme edge of the Arctic ice. The ice floes in the MIZ are pushed together and pulled apart by surface winds, they drift into circular patterns where transient ocean eddy currents exist, they expand and contract with varying surface tempera-tures and they grind against each other, all or which results in a considerable production or noise. Also, surface gravity waves can break individual ice floes near the ice edge and ice-ocean eddies at the edge can cause high shear between adjacent bands or ice floes, each or which can radiate a significant amount or noise. It has been determined that ambient noise levels in the 6,000 Hz range can be attributed to thermal stress when ice drifts into warmer water, or from floe- floe crushing. The lower frequency 5 to 100 Hz noise results from ice “quakes” as the ice breaks in response to wind and current stress. Mid-frequency noise, 100 to 4,000 Hz, correlates with atmospheric cooling. In the range or 1,000 Hz, high frequency noise can be related to wind-driven snow impacting upon the ice. In addition, there is more animal life in this MIZ area (whales, seals, etc.), increasing the ambient noise level somewhat. On the plus side, this area is not often contaminated by any ship noise.
During the 1984 MIZEX operation, internal waves were observed in the marginal ice zone which could cause unpredictable fluctuations in a submarine’s trim while it is cruising well below the surface. A sample inner wave had a 20 minute period and a vertical displacement or 10 meters.
The propagation of acoustic signals through the highly variable oceanographic and ice conditions of the MIZ show a scale of acoustic fluctuation — as measured by the bandwidth-spreading over a range of 100 kilometers — which is much higher than that observed in the central Arctic, or even in the temperate oceans of the world. At the same time, the floe-bumping and shearing noise, the moment and gravity induced noise, and the atmospheric cooling-induced noise all contribute — with great variability — to the ambient noise level in this area of the ocean.
Added to these effects is the considerable variability in the sound velocity profile for any particular, relatively small area of the MIZ. Thus, the predictability of sonar range capability tends to be low and the actual acoustic ranges for detection of enemy submarines are likely to be low as well as extremely variable.[This discussion item is derived from numerous research reports on the Marginal Ice Zone submitted to the Office of Naval Research.]