What does the halocline refer to in oceanography?

In oceanography, the halocline is specifically defined as a layer of water in which there is a rapid change in salinity with depth. This phenomenon occurs due to various factors such as the mixing of freshwater from rivers and rain with saltwater from the ocean, creating a distinct boundary. The halocline is significant because it affects the density of seawater and, consequently, the ocean's stratification and circulation patterns.

The other options refer to different oceanographic concepts. For instance, a gradual change in temperature pertains to the thermocline, which is a layer characterized by a rapid decrease in temperature with depth. A steady salt concentration does not accurately reflect the dynamic nature of ocean water, as salt concentration varies in different regions and depths. Lastly, a stable temperature layer could describe a specific layer in the ocean where temperatures remain relatively uniform, but that does not pertain to salinity changes. Therefore, the halocline's defining characteristic is indeed the abrupt change in salinity, making the identification of option B as the correct choice.