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How can ocean circulation influence whether a planet has liquid surface water?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

Ocean circulation plays a critical role in determining whether a planet can maintain liquid surface water by regulating the distribution of heat across its surface. While a planet's position relative to its star is a primary factor in its habitability, atmospheric and oceanic circulation are essential mechanisms that redistribute energy, preventing extreme temperature variations that could otherwise lead to the freezing or boiling of surface water.1

On a global scale, ocean currents act as a heat-transport system, moving thermal energy from warmer tropical regions toward colder high-latitude areas. This process is driven by wind friction and variations in water density, such as in thermohaline circulation, which involves the large-scale overturning of ocean waters. By transferring heat to polar regions, these currents can maintain surface temperatures above the freezing point of water in areas that might otherwise be too cold to support it. Conversely, in the context of planets orbiting faint stars, efficient circulation—whether atmospheric or oceanic—is necessary to redistribute heat to the dark hemisphere, preventing the atmosphere from freezing onto the surface.23451