Correcting for the double-ITCZ bias, a persistent error in many climate models, reveals that future U.S. Southwest winters will be drier than expected
Brendan Bane, PNNL
RICHLAND, Wash.—California and other areas of the U.S. Southwest may see less future winter precipitation than previously projected by climate models. After probing a persistent error in widely used models, researchers at the Department of Energy’s Pacific Northwest National Laboratory estimate that California will likely experience drier winters in the future than projected by some climate models, meaning residents may see less spring runoff, higher spring temperatures, and an increased risk of wildfire in coming years.
Earth scientist Lu Dong, who led the study alongside atmospheric scientist Ruby Leung, presented her findings at the American Geophysical Union’s fall meeting on Tuesday, Dec. 1, and will answer questions virtually on Wednesday, Dec. 16.
As imperfect simulations of vastly complex systems, today’s climate models have biases and errors. When new model generations are refined and grow increasingly accurate, some biases are reduced while others linger. One such long-lived bias in many models is the misrepresentation of an important circulation feature called the intertropical convergence zone, commonly known as the ITCZ.
The ITCZ marks an area just north of the Earth’s equator where northeast trade winds from the northern hemisphere clash with southeast trade winds from the southern hemisphere. Strong sunlight and warm water heat the air here, energizing it along with the moisture it holds to move upward.
As the air rises, it expands and cools. Condensing moisture provides more energy to produce thunderstorms with intense rainfall. From space, one can even see a thick band of clouds, unbroken for hundreds of miles as they move about the region.
“The ITCZ produces the strongest, long line of persistent convection in the world,” said Dong. “It can influence the global water cycle and climate over much of the Earth,” including, she added, California’s climate.
Doubling down on climate model bias
Many climate models mistakenly depict a double ITCZ: two bands appearing in both hemispheres instead of one, which imbues uncertainty in model projections. Scientists refer to this as the double-ITCZ bias. Variations in the wind and pressure systems that influence the ITCZ add to that uncertainty.
“There’s a lot of uncertainty in California’s future precipitation,” said Dong, who described climate models that project a range of winter wetness in the state averaged over multiple years, from high increases to small decreases. “We want to know where this uncertainty comes from so we can better project future changes in precipitation.”
To peer through the effect of the double-ITCZ bias and create more accurate projections, Dong and atmospheric scientist Ruby Leung analyzed data from nearly 40 climate models, uncovering statistical and mechanistic links between the bias and the models’ outputs. The lion’s share of the models they analyzed projected a sharpening of California’s seasonal precipitation cycle, bringing wetter winters and drier fall and spring seasons.
Less water, more fire
Those uncovered relationships, Dong said, now cast doubt on estimations from CMIP5 models that projected wetter winters in the future. Models saddled with a larger double-ITCZ bias, it turns out, tend to exaggerate the U.S. Southwest’s wetter winters. They also understate the drier winters in the Mediterranean Basin, which also features pronounced wet winters and dry summers similar to California, under warming climate scenarios.
Correcting for the bias reduces winter precipitation projections to a level that’s roughly equal to California’s current winters, amounting to little change and no future wetter winters. In the Mediterranean Basin, said Dong, the correction means winter drying will be intensified by 32 percent.
“An important implication of this work,” said Dong, “is that a reduction in estimated winter precipitation will likely mean a reduction in spring runoff and an increase in spring temperature, and both increase the likelihood of wildfire risk in California…”
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