Since early 2013, the state of California has been plagued by a multi-year drought that is unprecedented in the state’s 100-year observational record. The critically important Sierra Nevada snowpack reached its lowest level in 500 years in 2015. The loss of groundwater in the state’s aquifers has relocated mountains, decreased all water availability, elevated fire risk, dramatically increased dying trees, badly affected river and marine ecosystems and caused infrastructure damage to pipelines and roads.
Over the past several years, California has experienced atmospheric high pressure which redirects winter storms over the Pacific, creating record-breaking warmth and dryness during the typically wettest time of year between December and March. California’s entire water supply depends on just a few atmospheric river events each winter which means that a surplus or deficit of just one or two such storms can quickly increase the risk of flood or drought in any given year. As a result, seasonal-scale shifts in the Pacific storm track associated with unusually persistent winter ridges are the most common cause of California droughts—since there is little opportunity to make up for accumulated winter precipitation deficits during the rest of the year.
Recent studies indicate that certain unusual atmospheric patterns are occurring more often. Most notably, patterns similar to those during California’s extremely warm and dry years of 2013-2014 and 2014-2015 occurred more frequently over the past three decades. These years were characterized by unusually low pressure over the Pacific north of Hawaii and a very strong ridge of high pressure along the entire West Coast of North America, extending from southern California north to the Alaskan arctic.
However, there is no evidence that large-scale pressure patterns associated with California’s wettest years have become less common. In fact, several wet patterns have actually increased in recent decades, while some studies suggested little change. Therefore, while there is high confidence that patterns conducive to extreme warmth and dryness in California are occurring more frequently, this increase in patterns conducive to drought is not occurring at the expense of those associated with California’s wettest years.
There is also little change in the occurrence of atmospheric patterns leading to California’s most extremely cool years, despite a large increase in actual temperatures across the state. This suggests that the majority of California’s observed warming trend arises from a more uniform warming trend across all years, adding to the effect of extremely warm seasons like 2014-2015.
Middle atmospheric pressures are also increasing at a greater rate along the West Coast than elsewhere over the Pacific or North America. Warmer layers of the atmosphere take up more space than cooler ones, which occurs because air expands when temperatures increase. The particular spatial pattern of warming turns out to be critically important. Because temperatures along the West Coast warmed much more than those in adjacent regions, the overall increase in middle atmospheric pressure reached a local maximum in the region.
It is worth noting that climate model projections for 21st century California depict a warmer future characterized by larger swings between dry and wet conditions. New findings reveal that the atmospheric patterns conducive to extreme California drought are indeed increasing, but that patterns conducive to very wet years may also be increasing despite little or no long-term change in average precipitation—which highlights the critical importance of considering changes in the most extreme years when planning for the future.
Unfortunately, the outlook for the coming winter is not yet clear. La Niña appears to be fizzling in the tropical Pacific, and the latest multi-model ensemble forecast suggests very inconsistent precipitation for the same period–varying from rather wet over California to quite dry during the first half of the rainy season (forecasts for the latter half of the rainy season, though, have generally been on the dry side–especially in Southern California).
Multi-month precipitation outlooks are challenging in California because it only takes a small handful of powerful atmospheric river storms to “make or break” annual precipitation totals across much of the state. It is possible to have very dry conditions punctuated by a few brief but intense periods of heavy precipitation that heavily sway the overall average. Recent evidence suggests that wider swings in California precipitation are likely as the climate warms.
Nevertheless, along with the computer models, another feature that is lending confidence to the forecast for a La Niña is the amount of cooler-than-average water under the surface of the Pacific. This large pool of cool water stretches across the entire Pacific, along the Equator, and extends down from just below the surface to around 500 feet.
During this past March–April, the average temperature in this part of the subsurface ocean was the second-coolest on record (records start in 1979). The coldest on record? March–April 1998, immediately following the strong El Niño event of 1997-98. Colder subsurface ocean water during the spring has a strong association with La Niña. La Niña winters followed all of the six springs with the coolest temperature anomalies.
So, in short there continues to be a debate about which factors contribute the most to a California drought—whether it is a gradual atmospheric warming, tropical Pacific Ocean warmth, random atmospheric variations, or even the loss of Arctic sea ice, it remains unclear which factors most contribute to the California drought. Meanwhile, October is expected to be warmer and drier than usual. To confuse the outlook even more, there is an equal probability that rain will be above, below or near normal levels, making the outlook clearly a ‘mixed bag’ of guesses!
Meanwhile, the CA Water Resources Board is closely monitoring how suppliers conserve water despite a modest reprieve from mandated usage reductions in 2015. New standards set include permanent prohibition on wasteful water use, improved drought planning and enhanced leak detection and repair requirements which clearly means more regulations and higher costs to produce food supplies. The Water Board will reevaluate their position in February, 2017.