Revamped tropical Pacific buoys could aid atmospheric river forecasts

For 3 years in a row, cool La Nina conditions have reigned in the tropical Pacific Ocean, suppressing the steady march of global warming. But warm waters are now rolling east and gathering off the west coast of South America, signaling the likely arrival of El Niño later this year and, next year, a surge in heat that could push the planet past 1.5°C of warming. These fluctuations in the Pacific—the greatest short-term control on global climate—once caught the world off guard. But they are now predictable months in advance, largely because of the Tropical Atmosphere Ocean (TAO) array, a series of 55 U.S. buoys, moored to the sea floor, that stretch some 13,000 kilometers along the equator.

Now, the TAO array is getting a $23 million overhaul, the first since it was set up in the mid-1990s, the National Oceanic and Atmospheric Administration (NOAA) says. The revamped buoys, the first of which was deployed on 13 April, will be more robust and able to monitor the ocean below in more detail, potentially allowing earlier and more accurate El Niño forecasts. Some will be moved into locations north of the equator, to enable better forecasts of cyclones and atmospheric rivers, the parades of storms that can inundate coastal regions such as California. “This is the most significant change the array has undergone,” says Karen Grissom, an oceanographer at NOAA’s National Data Buoy Center, which runs TAO. “It’s going to be completely different.”

During El Niño, the Pacific trade winds that blow along the equator from east to west weaken, allowing warm water from the western Pacific to flow east; during La Nina, the trade winds strengthen, trapping the warm water in the west and dragging cool water up from the depths in the east. The shifts not only change global ocean and land temperatures, but also reshape weather patterns, fueling flooding and drought as far away as India and Australia. “It’s like an anchor of the global climate system,” says Susan Wijffels, a physical oceanographer at the Woods Hole Oceanographic Institution.

Predicting these conditions is more than an academic exercise: The unprecedented “triple-dip” La Nina between 2020 and 2022 led to severe drought and food insecurity in eastern Africa. But by feeding TAO data into long-range weather models, researchers were able to forecast the event, allowing aid agencies to prepare while the Kenya Meteorological Department advised farmers to plant drought-tolerant crops, says Chris Funk, a climate scientist at the University of California, Santa Barbara. “That all leads back to the TAO array.”

The TAO overhaul stems from a crisis a decade ago, when NOAA decommissioned the ship that serviced the 5-meter-tall buoys and had trouble financing the replacement of broken ones. Although funding was restored, it prompted a reassessment of what the tropical Pacific observing system, or TPOS, should look like with the advent of satellite observations and the Argo array, a fleet of nearly 4000 drifting, autonomous floats that profile ocean temperatures down to 2000 meters.

Each part of the observing system should focus on what it does best, says William Kessler, an oceanographer at NOAA’s Pacific Marine Environmental Laboratory who co-chaired the international TPOS 2020 committee. For satellites, that’s a broad view of the ocean surface temperatures and height. For Argo, it’s the deep vertical profiles. And for the TAO moorings, which measure temperatures to a depth of 500 meters, it’s providing a high-resolution view of fast-changing tropical ocean flows.

NOAA expects to finish deploying the new moorings by 2027, Grissom says. “As far as government time, that’s just right around the corner.” Each will beam back data every 10 minutes rather than once an hour. They’ll be less power hungry and more resilient, allowing engineers to remotely access and reset instrumentation.

They will also collect much more data. Each will have up to seven new sensors for measuring the “mixed layer,” the shallow waters just below the surface, in efforts to understand how the deep heat of El Niño rises through this layer to cause weather at the surface. Meanwhile, other instruments suspended from the buoys will measure currents down to 315 meters to better understand the propagation of El Niño’s heat. And several will also be relocated about 1000 kilometers north of the equator, to a cloud-covered region where cyclones and atmospheric rivers form.

Overall, NOAA’s plans are “very exciting,” and follow the TPOS 2020 recommendations, says Sophie Cravatte, a physical oceanographer at France’s National Research Institute for Sustained Development. But she has a few concerns. Because the array is being reduced by seven moorings, the committee recommended compensating by doubling the number of Argo floats released in the tropical Pacific. But so far, NOAA and its partners have only upped those releases by 20%, Kessler says.

Perhaps the biggest hole is in the western Pacific, where Japan used to operate the Triangle Trans-Ocean Buoy Network (Triton), which completed TAO’s equatorial line of sensors. Only one Triton buoy remains operational, Kessler says. China has promised to fill that gap, but the first buoys in its Ding array failed soon after deployment. “This western part is key for weather and [El Niño] forecasting,” Cravatte says, “and it is missing.”