Ocean surface topography

Ocean surface topography, also known as sea surface topography or ocean dynamic topography, refers to the hills and valleys on the ocean's surface.

While the ocean might look flat from a boat, it actually has many highs and lows. Scientists measure these variations as the average sea surface height (SSH) relative to the Earth's geoid. The geoid is a surface that represents how water flows across the planet. Understanding these shapes is vital because they help us study large-scale ocean circulation. By mapping these features, researchers can see how the ocean moves heat around the entire globe.

To map these features, scientists use a process called satellite altimetry. A satellite sends a microwave pulse down toward the ocean surface. The pulse travels to the water and then bounces back up to the satellite. By measuring the round-trip time of this pulse, the satellite calculates the distance to the surface. This measurement is compared to the satellite's own orbit altitude. The satellite uses a reference shape called an ellipsoid to help with these calculations. Because the ellipsoid is not an equipotential surface of Earth's gravity, scientists must transform the data to the geoid. This ensures the measurements accurately reflect the actual water flow.

Ocean topography is directly linked to how water moves through currents. Currents flow around these surface hills and valleys in predictable patterns. This movement is influenced by the Coriolis effect, which is a force caused by Earth's rotation. In the northern hemisphere, water rotates in a clockwise direction around hills. It rotates in a counterclockwise direction around valleys. In the southern hemisphere, these patterns are reversed. These currents are essential for transporting heat and carbon throughout the world's oceans.

Many different factors cause the sea surface height to change over time. Short-term changes are often caused by the tidal forces of the Moon and the seasonal cycle of the Sun. Other factors include changes in temperature, salinity, waves, tides, and winds. On much longer timescales, the patterns in SSH are influenced by ocean circulation. Slower, larger variations can occur due to changes in Earth's gravitational field. This might happen because of melting ice, the rearrangement of continents, or the formation of sea mounts. Scientists can combine altimetry with satellite gravimetry from missions like GRACE to monitor sea level rise.

Our ability to map the ocean has improved greatly through space missions. For 100 years, scientists relied on observations collected by ships. The TOPEX/Poseidon mission changed this in 1992 by providing the first highly accurate maps. This mission allowed scientists to study large-scale current systems for the first time. Following TOPEX/Poseidon, the Jason series of satellites continued this important work. Jason-1 was launched in 2001 by a Boeing Delta II rocket. Jason-2 launched in 2008, and Jason-3 launched in 2016 using a SpaceX Falcon-9 rocket.

Today, a network of satellites works together to monitor our oceans. There are currently nine different satellites calculating ocean topography. These include Cryosat-2, SARAL, Jason-3, Sentinel-3A, Sentinel-3B, CFOSat, HY-2B, HY-2C, and Sentinel-6 Michael Freilich. Some of these satellites work in a tandem rotation. For example, Jason-3 and Sentinel-6 Michael Freilich orbit about 330 kilometers apart. These missions help scientists monitor phenomena like El Niño, La Niña, and the North Atlantic oscillation. They also track planetary waves that cross the oceans over several months.

This data is incredibly useful for many different parts of science. It helps researchers understand weather patterns and the long-term effects of Earth's climate. The information is also used for navigation, fisheries management, and offshore operations. By observing the ocean, we can better predict short-term and long-term weather changes. Scientists also use this data to study how much heat the ocean contains. This is a critical piece of information for understanding global warming. Looking forward, a new mission called the Surface Water Ocean Topography Mission has been proposed. It aims to provide a global survey of all surface water, including lakes and rivers.