Source: Geophysical Research Letters
For more than a week in mid-February 2019, category 1 Tropical Cyclone Oma churned the Coral Sea east of Australia. An ocean desert, the region’s surface waters are typically devoid of life—until the cyclone passed, leaving a 250,000-square-kilometer (100,000-square-mile) phytoplankton bloom in its wake. The bloom persisted for several weeks.
In a new study, Russell and Horvat found that Oma’s unusually long tenure spurred the Coral Sea’s gargantuan algal bloom. The storm whipped up a vortex that lifted nutrients from the deep to the shallows, providing food for phytoplankton at the ocean’s surface.
Upwelling, the ascent of deep water, frequently happens along coasts, but it is uncommon in the open ocean of the South Pacific. According to the new study, to trigger a bloom, a cyclone needs to be in a hovering state, moving slower than 3 meters (9.8 feet) per second. Cyclones this sluggish are rare in the South Pacific: Only 16% meet that criterion.
In fact, by analyzing more than 2 decades of satellite data for the region (1997–2023), the researchers found just 14 additional instances of a phytoplankton bloom appearing in the wake of a tropical cyclone.
Compared with these other cyclones, Oma held a uniquely extended residence over the Coral Sea. The 14 other bloom-inducing cyclones passed by in 1–2 days, on average. Oma stayed for more than a week. That long of an occupancy is expected to occur only once every 1,500–2,000 years, the authors say.
Cyclone Oma instigated a bloom that introduced unprecedented quantities of food into the ecosystem, but even cyclones that trigger lesser algal blooms could have an impact on the food web and the ocean’s capacity to store carbon in the region, said study author Chris Horvat, a physical oceanographer at the University of Auckland and Brown University.
“We don’t currently know the impacts that these types of blooms can have on ecosystems,” Horvat said. “Seasonal blooms are crucial for ocean ecosystems everywhere, but whether the same is true for these rare events in the South Pacific is something we want to study.” (Geophysical Research Letters, https://doi.org/10.1029/2022GL100821, 2023)
—Kirsten Steinke (@Kirsten_Steinke), Science Writer