Two Satellites Track Hurricanes During a Historic El Niño

NASA and its European partners are using two satellites flying 30 seconds apart to measure the ocean's response to what oceanographers expect to be a historic El Niño, data that is also feeding into hurricane forecasts this year. The newer satellite, Sentinel-6B, launched last November to improve hurricane forecasts, protect infrastructure and benefit industries such as shipping, and now flies behind its predecessor, Sentinel-6 Michael Freilich.

The two satellites make up the Copernicus Sentinel-6/Jason-CS mission, the latest in a series of ocean-observing radar altimetry missions that have tracked Earth's seas since the early 1990s. El Niño is a naturally occurring ocean pattern in which warmer-than-usual Pacific waters shift global weather. "This El Niño was a late-bloomer," said Josh Willis, Sentinel-6B's project scientist at NASA's Jet Propulsion Laboratory. "It didn't kick off until the middle of the year and is just now reaching a strength similar to what we've seen in the satellite record during significant El Niños in 1997 and 2015. We expect it to be big, and it's already having big impacts."

El Niño typically redistributes heat in the ocean, weakening winds that normally blow westward along the equator and letting warm water spread east toward South America — a shift that can move hurricane activity from the Atlantic toward the Pacific. On July 15, Sentinel-6B began delivering low-latency data that feeds into the hurricane tracking algorithms used by federal and state agencies, whose forecasts can trigger disaster response ranging from sandbag placement to evacuation orders.

A tropical storm can take a week or more to become a hurricane, but a hurricane can rapidly intensify in the 48 hours before landfall, leaving little time to prepare. "Hurricanes have been known to speed up quickly at the last moment, so the window in which to decide what to do is short," said Deirdre Byrne, an oceanographer with the National Oceanic and Atmospheric Administration who oversees one of the agency's key hurricane forecasting algorithms. Each Sentinel-6 satellite measures ocean height, wave size and marine wind speed with a radar altimeter, data Byrne says she plans to add to that algorithm by the end of the year because warmer water expands and raises sea level, helping forecast how fast a hurricane will grow.

The Sentinel-6 missions extend a precise record of sea level observations that traces back to the TOPEX/Poseidon mission, launched in 1992. Sentinel-6B will take over from Sentinel-6 Michael Freilich as the reference satellite for global sea level measurements later this year. "The key is consistency, measuring the same way, every time," said Severine Fournier, Sentinel-6B's deputy project scientist at NASA's Jet Propulsion Laboratory. "That's what lets us predict hurricanes, and, in turn, protect coastal communities and infrastructure."