Like the global economy, where nations continually exchange goods and services with one another, the ocean constantly exchanges mass and energy with other components of the climate system. Winds drive mixing, waves, and currents that move water around in the ocean. Ocean temperatures rise and fall in response to warming and cooling at the surface. Rainfall freshens the ocean, while evaporation makes it more salty. Together, these exchanges cause differences in seawater density and mass between regions all across the global ocean. These regional differences in turn sustain major ocean currents, which transport incredible amounts of seawater. For example, the Gulf Stream, a strong northeastward-flowing ocean current off North Carolina on the U.S. East Coast, transports about 50 million cubic meters of water each second—or about 20,000 times the flow of Niagara Falls and equal to approximately 250 Amazon rivers.
Sterodynamic sea-level changes are those that arise from changes in the ocean’s circulation (currents) and its climate (temperature and saltiness). For example, easterly trade winds usually blow steadily over the equatorial Pacific Ocean, driving surface currents from east to west. As a result, seawater piles up to the west, so that sea level is normally 1-2 feet higher off Papua New Guinea compared to the coast of Peru. However, during strong El Niño events, such as in 1997-1998 and 2014-2016, the trade winds weaken or collapse, and sea level flattens out across the equatorial Pacific. Many such local and regional patterns of season-to-season and year-to-year changes in sea level observed by satellites over the past three decades represent sterodynamic sea-level changes. The West Coast of the United States provides an example of a region impacted by these patterns of sterodynamic sea-level change.
Sterodynamic sea-level changes are important not only locally and regionally, but also globally. The upper half of the global ocean warmed by about 0.05 degrees Celsius on average during the past 15 years. That amount of upper-ocean warming may sound small, but it requires as much heat as would be given off by roughly 3 trillion 100-watt light bulbs. Similar to mercury in a thermometer, which rises with temperature, this global-ocean warming caused seawater to expand thermally, increasing sea level globally at a rate of about 1.1 millimeters per year. This global sterodynamic sea-level rise due to global-ocean warming explains about one-third of the roughly 3.3 millimeters per year of total global-mean sea-level rise detected by satellites during the past decade and a half, with the other two-thirds coming from melting of glaciers and ice sheets.
It takes a dedicated global ocean observing system to measure sterodynamic sea-level changes. Satellite altimeters, which track the height of the sea surface, have orbited the planet for nearly three decades, whereas satellite gravimeters, which monitor changes in Earth’s gravity field, have been in orbit for almost two decades. For the past decade and a half, a fleet of thousands of robotic probes have patrolled the global ocean’s depths, from its surface to a mile or so down, all the while measuring its temperature and saltiness. Data from these satellites and probes together allow scientists to paint a portrait of sterodynamic sea-level changes locally, regionally, and globally—a tremendous achievement in observing the global climate system.
Vast tracts of the ocean remain largely unobserved by these probes and satellites, including the deep ocean and near the coasts. A more complete understanding of sterodynamic sea-level change requires next-generation observations. To track global climate in the deep ocean more than a mile below the surface, where few measurements exist, a new fleet of oceanic probes is being designed and deployed that can withstand the harsh conditions and great pressures of the abyss. New satellite systems will be launched in the near future that will monitor sea levels much closer to the coast than current satellite altimeters can. And recent studies suggest that the travel time of sound waves, triggered by earthquakes and moving through the ocean, can be used like a thermometer to take the ocean’s temperature, because the speed of sound in water depends on temperature. These and other technologies currently being developed promise a more complete understanding of global climate, ocean circulation, and sterodynamic sea-level change.