People often wonder if the ocean is a flat, uniform surface connecting all continents. The concept of “sea level” suggests a single, cohesive baseline, but this overlooks the planet’s dynamic nature. Comparing the Atlantic and Pacific Oceans reveals that their average water surfaces are not perfectly aligned. The question of a height difference introduces the complex reality of oceanography and geodesy.
Defining the Global Water Baseline
To compare ocean heights, scientists must establish a non-physical reference plane, as “sea level” is not a uniform measurement. The standard reference used is the Geoid, a theoretical surface representing mean sea level if water were influenced only by Earth’s gravity and rotation. The Geoid is an irregular, undulating surface that extends through the continents, acting as the true zero elevation for global height measurements.
This surface is an equipotential surface, meaning the force of gravity is perpendicular to it everywhere. The Geoid’s irregular shape reflects the uneven distribution of mass within the Earth’s interior, which influences local gravitational pull. Mean Sea Level (MSL) is the localized average of water elevations observed over a long-term tidal cycle. While MSL approximates the Geoid, the actual ocean surface is always higher or lower due to external physical forces.
The Measured Height Difference
When comparing the Atlantic and Pacific Oceans, particularly near the Panama Isthmus, the Pacific Ocean is consistently found to be slightly higher than the Atlantic. This difference in the average water surface is typically 20 to 25 centimeters (8 to 10 inches). This measurement is taken relative to the Geoid, which provides a gravitational baseline for comparison. The Pacific side also experiences a much greater tidal range, often exceeding 5 meters, whereas the Atlantic side’s tidal range is much smaller.
This small but measurable difference is not the reason the Panama Canal requires a system of locks to operate. A common misconception is that the locks are necessary to prevent a higher Pacific from rushing into the lower Atlantic. Instead, the canal’s locks serve the primary purpose of lifting ships over the continental divide. They raise vessels 26 meters (85 feet) to the elevation of Gatun Lake, a man-made freshwater reservoir, before lowering them back down to the opposite ocean.
If a sea-level canal had been constructed, the varying tidal ranges and the slight average height difference would have created persistent, powerful currents within the channel. The primary engineering decision to use locks was driven by the need to traverse the high central landmass and manage the freshwater supply. The resulting freshwater lake also acts as a biological barrier, preventing the mixing of marine life between the two oceans.
Physical Forces Driving Ocean Height Variation
The slight difference in ocean height is a result of a complex interplay of physical forces that continuously shape the ocean surface.
Water Density and Thermohaline Circulation
One significant factor is the variation in water density, a concept known as thermohaline circulation. Density is determined by temperature (thermo) and salinity (haline) levels. Warmer water is less dense and occupies a greater volume, causing the surface to stand slightly higher than surrounding colder water.
Lower salinity similarly results in less dense water and a higher surface elevation. The equatorial Pacific Ocean tends to have lower salinity and higher temperatures compared to the Atlantic, which contributes to its slightly elevated surface. This is because the Pacific receives more freshwater runoff and precipitation, making its water less dense.
Wind Stress and Ocean Currents
Global wind patterns and major ocean currents also force water against continental boundaries, a process called wind stress. Persistent trade winds blowing east to west along the equator push water toward the western side of ocean basins. This continuous pressure causes a localized “stacking” of water, contributing to higher sea levels along western coasts, such as the eastern Pacific. The Coriolis effect further deflects these moving water masses, influencing the direction and intensity of these currents and maintaining the height differential.
Gravitational Variation
Finally, the Earth’s non-uniform gravitational field, which defines the Geoid, is a permanent factor in ocean height. Areas with a higher concentration of mass exert a stronger gravitational pull, pulling the water toward it, causing a slight depression in the sea surface. Conversely, areas with less mass allow the water to bulge outward. This gravitational variation means the average height of the ocean surface is naturally uneven across the globe.