Sea level isn't a uniform global surface because of two primary factors: ocean dynamics and variations in Earth's gravity field.
Ocean Dynamics
Ocean dynamics refer to the complex interplay of forces and processes that redistribute water mass throughout the world's oceans. These processes include:
- Wind Patterns: Winds exert force on the ocean surface, driving currents. Persistent wind patterns, such as trade winds, create large-scale ocean currents that pile up water in certain areas, raising sea level locally.
- Heating and Cooling: Water expands when heated and contracts when cooled. Uneven solar heating across the globe leads to variations in water temperature and, consequently, sea level height. Warmer regions tend to have higher sea levels than cooler regions.
- Evaporation and Precipitation: Evaporation removes water from the ocean, while precipitation adds water back. Regions with higher evaporation rates may experience lower sea levels, while regions with high precipitation may see higher sea levels.
- Ocean Currents: Currents like the Gulf Stream transport warm water northward, contributing to higher sea levels along the eastern coast of North America and lower sea levels in the North Atlantic. The slowdown or alteration of these currents can significantly impact regional sea level.
- Salinity: Differences in salinity (salt content) also affect water density. Saltier water is denser, leading to slightly lower sea levels compared to less salty water.
Earth's Uneven Gravity Field
The Earth's gravity field isn't uniform due to variations in the distribution of mass within the planet. This unevenness influences the shape of the ocean surface.
- Gravitational Pull: Areas with higher mass concentrations (e.g., due to mountains or dense rock formations) exert a stronger gravitational pull, attracting more water and causing localized increases in sea level. Conversely, areas with lower mass concentrations experience weaker gravitational pull and lower sea levels.
- Geoid: The geoid is a model of global mean sea level that represents the Earth's equipotential gravitational surface. It illustrates how sea level would appear if only influenced by gravity and Earth's rotation, without the effects of winds, tides, or salinity. The geoid is not a perfect sphere; it has bumps and dips that reflect the uneven distribution of mass within the Earth.
In summary, variations in sea level are a result of the complex interplay between ocean dynamics, driven by wind, temperature, evaporation, and precipitation, and the uneven distribution of mass within the Earth, which affects the gravitational pull across different regions. These factors combine to create a sea surface that is far from uniform.
