What factor greatly influences the seasonal temperature variation of seawater?

The significant influence of latitude on the seasonal temperature variation of seawater stems from its relationship with the angle of solar insolation. As one moves from the equator toward the poles, the angle at which sunlight strikes the Earth's surface changes, leading to variations in the intensity of solar energy received.

Near the equator, sunlight is more direct, resulting in warmer temperatures throughout the year. However, as latitude increases, particularly in higher latitudes, sunlight becomes more oblique, decreasing the amount of solar energy received during the winter months. This results in pronounced seasonal temperature variations, where coastal regions at higher latitudes can experience colder water temperatures in the winter and comparatively warmer temperatures in the summer.

While factors like longitude, depth, and salinity do affect seawater temperatures, they do not exert as significant an impact on seasonal variation as latitude does. Longitude mainly affects ocean current patterns and time zones, depth influences temperature but is more related to vertical gradients rather than seasonal shifts, and salinity can affect density and stratification but does not directly dictate seasonal temperature swings.