A convergence zone is a phenomenon
of sound transmissivity
beneath the surface of the ocean
, where the local sound paths come together in such a manner that sound energy generated or reflected within the convergence zone (CZ) can be detected and interpreted at extremely long range - much greater range than that at which a straight-line path between the zone and the listener/emitter
can be drawn. In other words, it is a way to make sonar (active or passive) work even around the curvature of the earth
The ocean is an immensely complex environment, varying in all manner of ways as one changes depths or locations. Salinity varies, temperature varies, the direction and speed of currents vary, the population of micro- or macroorganisms varies, etcetera etcetera. This matters a great deal to various professions who deal with the ocean; this writeup deals with how it matters to a small subset of those folks - sonar operators and those who make decisions based on their reports.
Let's ground it (haha, just kidding) in one specific area - that of military submarines and surface vessels. These two groups spend a great deal of time trying to find each other, ideally without being themselves located, as they play at the Great Game. To do this, they will most often use sonar, either active or passive. Said sonar depends on the transmission of sound energy from the sonar to the target (in the case of active) and from the target back to the sonar (in both cases). The aforementioned ocean conditions mean that not only does the sound energy not travel consistently, it doesn't even travel in a straight line!
One reason for this is that sound travels at different speed in water of different temperature, different salinity, and different pressure. Hence, it will refract as it passes from water of one condition to water of a different condition. More specifically, it will always refract away from the water in which it travels faster. This will typically cause two 'normal' types of sound propagation - the surface reflection effect and deeper sound channeling.
The surface reflection effect is relatively simple - since sound refracts away from higher-speed water, and reflects off the surface, sound waves can travel long distances by simply bouncing off the surface and then refracting back upwards again when they reach a depth with a higher speed of sound (called a Sound Speed Profile, or SSP, by the U.S. Navy). In effect, it ricochets back between the surface and the colder depths until it is completely attenuated or until it hits something solid in that region of water.
Sound channels are layers of water in which the sound speed profile first decreases as depth increases, then increases. Essentially, it's a layer of water with a slower SSP trapped between layers of faster SSP water. If sound is emitted inside this layer, or if it angles into it, it will tend to refract away from the boundaries at the top and bottom, and can travel quite long distances, trapped in the middle. It operates much like fiberoptics, in this respect. Sound channels are typically present in the ocean; their depth and height will tend to vary depending on local conditions.
In any case, sometimes, the sound speed at depth will be greater than that at the depth of the sound source (sonar). When this happens, sound waves that are initially reflected downward due to the surface reflection effect can travel a great distance in sound channels before hitting an area where sound speed at that depth increases enough to refract the sound waves back up towards the surface. When this happens, sound from this deep channel path meets up with sound travelling in the surface reflection effect, and an area of high sound energy is created. This area is the convergence zone, as varying pathways of sound meet up - converge - to create a high-energy area.
This is important in sonar because the effect of a CZ is to create a return which appears to come from much farther away than the sound waves should be capable of. This is extremely useful to those who wish to increase the range of their sonar systems. The U.S. Navy SURTASS (Surface Towed-Array Sonar System) is designed specifically to hunt up convergence zones and use them to detect distant targets.
There are ups and downs, of course. CZ contacts will have much rougher range data, and rough directional data - it's not possible to measure their distance through the simple expedient of timing the pulses. Not only have the pulses travelled a great deal farther than the straight-line path (bouncing around in the sound channels) but pulses from different emission times are combining to make up the contact (different paths to the CZ take differing times to traverse) so in fact the contact may 'look' like a big, smeary layer rather than a nice locational dot. As a result, reading CZ contacts is a science that has been elevated to an art; sonar ops the world around attempt to refine their abilities on a constant basis. Despite the magic of computers, there is still a great deal that the trained human ear and brain can detect and process that computers can't, yet. In the meantime, however, high-tech navies spend scads of money and resources on mapping the ocean's varying acoustic environment, the better to not only predict these things but to read conditions when using sonar in an attempt to 'fine down' the results.
The computer naval wargame Harpoon depends a great deal on convergence zone simulation; hence, the newsletter of the Harpoon community is titled Convergence Zone.