Increasing precipitation whiplash in twenty-first century California

–A new study projects a 25% to 100% increase in extreme dry-to-wet precipitation events in CA, despite only modest changes in mean precipitation; CA will likely experience more frequent jarring transitions between dry and wet years, but also between dry and wet months within individual years.

–Most of California will likely experience a 100 – 200% increase in the frequency of very wet November-March “rainy seasons” similar to 2016-2017, which have historically occurred about four times per century.

California will likely experience an increase of anywhere from 50% to 150% (highest in the south) in the frequency of very dry November-March periods similar to 1976-1977(and only slightly drier than 2013-2014), which have historically occurred about once per century. …

–The core rainy season months of December-February will typically be wetter than their 20th century counterparts, but that precipitation during the “shoulder season” (i.e., the autumn months of September-November and the spring months of March-May) will typically be drier than in California’s historical past. … potentially leading to more fire -prone conditions later in the year

–There is a 300-400% increase in the risk of an extreme “sub-seasonal” 40-day precipitation event similar in magnitude to that which caused the 1862 flood; what is today considered to be the “200-year flood”—an event that would overwhelm the vast majority of California’s flood defenses and water infrastructure—will become the “40-50 year flood” in the coming decades.

Daniel Swain Read full Weather West article here and Nature Climate Change publication here

….The notion that California’s average precipitation might not change much in the future is actually somewhat surprising, as there is high confidence that other “mediterranean” climate regions on Earth will experience progressively less precipitation as the world warms and the region of stable subtropical influence expands. As we demonstrate in our new research, however, these small shifts in average precipitation mask profound changes in the character of California precipitation. We find that the occurrence of both extreme wet and extreme dry events in California—and of rapid transitions between the two—will likely increase with atmospheric greenhouse gas concentrations. The rising risk of historically unprecedented precipitation extremes will seriously test California’s existing water storage, distribution, and flood protection infrastructure…

…Interestingly, when we considered potential changes in the future frequency of multi-year dry spells (as experienced during the recent 2013-2016 drought), we didn’t find a significant change in either direction—essentially because 21st century droughts in California will have a greater propensity to be interrupted by brief but very wet interludes. On the other hand, there is already abundant evidence that rising temperatures themselves are increasing the likelihood and intensity of multi-year droughts in California through increased evaporation and snowpack loss, even in the absence of precipitation changes. …

….Our new analysis suggests that the risk of an extreme “sub-seasonal” 40-day precipitation event similar in magnitude to that which caused the 1862 flood will rise substantially as the climate warms. By the end of the 21st century, we find a 300 – 400+ % increase in the relative risk of such an event across the entire state. One specific statistic that my colleagues and I found particularly eyebrow-raising: on our current emissions trajectory, at least one occurrence of an 1862-level precipitation event is more likely than not over the next 40 years (between 2018 and 2060), with multiple occurrences plausible between now and the end of the century. In practical terms, this means that what is today considered to be the “200-year flood”—an event that would overwhelm the vast majority of California’s flood defenses and water infrastructure—will become the “40-50 year flood” in the coming decades….

…. An increase in the frequency of extremely both extremely wet and extremely dry years in California has recently begun to emerge in the observational record, and there is preliminary evidence that precipitation whiplash has started to increase across a broad swath of the western United States. Our findings regarding a 3- to 4-fold increase in the projected risk of an 1862-like flood event parallel similar insights into the rising risk of a Hurricane Harvey-likecatastrophic flood event along the Gulf Coast. While the physical mechanisms of extreme precipitation due to landfalling tropical cyclones (as occurred in Houston during Harvey) are quite different from that resulting from a persistent sequence of atmospheric river storms (as occurred in California during the 1862 flood), they both share a common factor: both processes act to locally concentrate atmospheric water vapor, which is increasing at an exponential rate as the atmosphere warms. …

…. This trend toward rain (at the expense of snow) will likely have a “double whammy” effect, amplifying the increased flood risks associated with more extreme precipitation events. And as noted above, expected large future increases in California drought risk will be driven primarily by increasing temperatures; the projected increase in extremely low precipitation years will only add to this risk. Together, these rather profound hydroclimatic changes will likely pose a progressively escalating series of tests upon California’s existing water infrastructure.

Swain, D. L., B. Langenbrunner, J. D. Neelin, and A. Hall, “Increasing precipitation volatility in 21st-century-California,” Nature Climate Change, doi:10.1038/s41558-018-0140-y, 2018.

Mediterranean climate regimes are particularly susceptible to rapid shifts between drought and flood—of which, California’s rapid transition from record multi-year dryness between 2012 and 2016 to extreme wetness during the 2016–2017 winter provides a dramatic example. Projected future changes in such dry-to-wet events, however, remain inadequately quantified, which we investigate here using the Community Earth System Model Large Ensemble of climate model simulations. Anthropogenic forcing is found to yield large twenty-first-century increases in the frequency of wet extremes, including a more than threefold increase in sub-seasonal events comparable to California’s ‘Great Flood of 1862’. Smaller but statistically robust increases in dry extremes are also apparent. As a consequence, a 25% to 100% increase in extreme dry-to-wet precipitation events is projected, despite only modest changes in mean precipitation. Such hydrological cycle intensification would seriously challenge California’s existing water storage, conveyance and flood control infrastructure.