Long-term Atmospheric River History in California

Atmospheric rivers (ARs) are associated with some of the largest flood-producing extreme precipitation events in western North America. As the primary storm mechanism in California, the difference of a few large AR storms in a year dramatically changes precipitation totals and drives the state towards water abundance or drought. Current records of AR activity are limited to just 70 years of instrumental data. So, the key question motivating this work was: what is the long-term history of AR storms in California? 

We can get insight into past extreme precipitation by looking to the sediment record. Under the right circumstances, clues to past climate and extreme precipitation are preserved in layers of sediment and allow us to reconstruct their history going back centuries to millennia. Long-term data help water managers avoid underestimation of potential flood risks and aid future planning scenarios, particularly for water infrastructure. For example, many dams in California were designed over 50 years ago and rely on relatively time-limited climate datasets that do not capture the full range of hydroclimate variability. To help with planning for extreme events, understanding how large historic storms were (before our instrumental records) is useful because it provides a range of what is possible. 

The objective of this research was to produce long-term (i.e., multi-centennial/millennial) reconstructions of extreme precipitation in Northern California. The study sites – Leonard Lake and Wildcat Lake – were selected because they are located where atmospheric rivers make the most frequent landfall in California. At both sites, a team of USGS scientists found that lake sediment records can be used to reconstruct AR activity, a scientific first in paleo flood proxy reconstruction. The geochemical and physical properties of the lake sediments provide evidence of extreme precipitation deposits. In these studies, the ratios of elements (silicon/aluminum and titanium/aluminum) in clay layers or eroded sediments from the surrounding watershed were highly correlated with modern records of integrated vapor transport, a key metric of AR intensity. Essentially, the ratios are elevated when there is extreme rainfall because there is more soil erosion going into and being preserved in water bodies due to the large rain event. 

Using Leonard Lake sediments, the USGS team extended the record of AR activity over three thousand years. Findings show California has experienced periods of increased extreme rainfall that exceeded any in the meteorologic instrumental era, with the largest episodes occurring two and three thousand years ago. That is, the AR reconstruction shows the climate conditions in California are very capable of producing larger AR events than we have seen in our instrumental record. The AR reconstruction aligns with regional climate signals, showing decreased AR activity during the relatively dry Medieval Climate Anomaly (about 950 to 1250 AD) and an increase during the wetter Little Ice Age (about 1300 to 1850 AD). Sharp declines in AR activity were also coincident with known drought events like the 16^th century North American megadrought, further corroborating the novel AR proxy. Wildcat Lake, unlike Leonard Lake, has a long history of anthropogenic land-use that could have confounded the extreme precipitation signal. However, extreme precipitation events are still detectable, and a long-term reconstruction is forthcoming. 

These historic reconstructions of extreme precipitation will help water managers to plan for potential extreme events that are beyond what our short instrumental record has captured so far.