Pacific upwelling much faster at the equator than scientists thought
New discovery shows why global climate models overestimate warming in the tropical Pacific Ocean
Winds blow east to west along the equator in the Pacific Ocean. Because the Earth spins, water on the surface rips like a seam and is pushed in two directions toward the northern and southern hemispheres. Deep, cold water then rises to the surface, filling the void. This process is called upwelling, and is why waters along the coast of California are colder than swimmers might expect.
Since the 1960s, oceanographers have relied on sparse observations that found the speed of upwelling on the equator in the Pacific to be about one meter per day. But new research from CIRES Fellow Kris Karnauskas found that those estimates are far off — the water is rising much faster.
“It turns out that equatorial upwelling in the Pacific is about 10 times faster than we previously thought,” Karnauskas said. “And this could be really important because that water rising toward the surface in the Pacific covers a huge fraction of the ocean surface, and it affects things like temperature and nutrients needed for photosynthesis.”
His work, published today in the Journal of Climate, reveals the faster rate of upwelling and determines why older estimates were off. Karnauskas combed through old observations and analyzed vast amounts of new data from state-of-the-art measurement tools to get a more accurate estimate.
The findings point to a key discrepancy in global climate models, which currently predict significant warming along the equator in the Pacific. This new rate may help researchers understand why they have struggled to capture key climate trends in the region.
How to effectively measure upwelling and its cooling effects
Upwelling occurs in all of Earth’s oceans. But in the Pacific, particularly along the equator, the effects are felt around the world. As cold water moves to the surface, it brings with it high concentrations of nutrients, which are integral to ecosystem health. Upwelling also plays a role in the global carbon cycle, delivering carbon from deep in the ocean up to the surface where it can enter the atmosphere. Changes in upwelling from year to year are the underlying cause of major global temperature events like El Niño and La Niña. One theory even argues that upwelling may serve as a buffer to the effects of global warming.
Research on upwelling in the Pacific dates back to the 1950s and 60s when scientists dropped their first instruments deep below the surface while on research cruises. Gathering these observations required travel to remote locations, and taking continuous measurements close to the equator proved difficult. Everything changed in the 1980s, when nearly a thousand drifting buoys and tracking satellites from NOAA’s Global Drifter Program were deployed into the ocean, making it possible to analyze countless measurements without leaving home.
When Karnauskas dove into historical records, he found large variations in upwelling predictions: from less than one meter per day up to 14 meters per day. To this day, researchers rely on these varied observations. The large range makes it difficult to validate climate models, and for the most part, the high-end estimates have largely been forgotten or discounted.
“One meter per day has become something of a rule of thumb in the physical oceanography community,” Karnauskas said. “It’s certainly what I’ve been quoting in the classroom when teaching students about upwelling in the open ocean.”
Karnauskas didn’t need to embark on a months-long research cruise to gather observations; instead, he crunched numbers at his desk using data from the satellite-tracked drifters. Since 1979, NOAA has deployed over 8,000 satellite-tracked drifters to track the speed of currents in the tropical Pacific Ocean. After using simple geometry, he landed on the new rate of 10 meters per day.
Were the historical observations “wrong?”
Older, one-off measurements weren’t wrong per se, but they were taken in less-than-ideal locations — further away from the equator. Instruments lowered from boats or attached to moorings farther away from the equator to measure the rate of upwelling generated a fuzzy view of the pipeline of water flowing toward the surface, compared to NOAA’s satellite-tracked drifters. With over one million high-density observations, their precision allows researchers to observe the diverging water that causes upwelling within about 70 miles of the equator.
Karnauskas believes the study’s results should encourage the oceanography community to reexamine some of the finer details of global climate models, especially those that control the speed of upwelling in the equatorial Pacific. Models currently predict more warming on the equator in the Pacific Ocean, but faster upwelling brings more cold water to the surface, buffering some global warming impacts. How to incorporate this finding into climate models, and their projections of the future, is the true question moving forward.
Equatorial upwelling is near the top of the list of fundamental features of the global ocean circulation.“It’s like the jet stream for oceanographers. So it was kind of shocking to realize how little we knew about its average value. It would be like if atmospheric scientists figured the jet stream was on average between 10 and 100 mph,” Karnauskas said. “Error bars that big wouldn’t last for long.”
