Before revision: The first term to describe this phenomenon is 18° water, which was used by Valentine Worthington to describe the isothermal layer in the northern Sargasso Sea cool to a temperature of about 18 °C each winter.
After revision: The first term to describe this phenomenon is 18° water, which was used by Valentine Worthington to describe the isothermal layer in the northern Sargasso Sea cooling to a temperature of about 18 °C each winter.