Error Correction Means California’s Future Wetter Winters May Never Comehttps://www.pnnl.gov/news-media/error-correction-means-californias-future-wetter-winters-may-never-comeCalifornia 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.
... When new climate 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.
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.
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.
Winter precipitation includes more than just rain, encompassing snowpack in mountainous areas and other factors that influence climate processes throughout the year.
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.The double-ITCZ bias is prominent in all CMIP5 climate models, said Leung, as well as CMIP6 models, the most recent generation, though the latter were not considered in this work. "If you look at the whole ensemble of models," said Leung, "you see quite similar biases."
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."
The findings do not bode well for agricultural production, as over one third of the country's vegetables are grown in California soil, and two thirds of its fruits and nuts are grown on California farms, according to the California Department of Food and Agriculture. Almonds and grapes, two especially water-hungry crops, were among the state's top producing commodities, bringing in a combined $11.5 billion in 2019.