Relentless 3-year drought in California: What caused it?

     California's unprecedented drought, that persisted through three winters (2011 to 2014), appears to be relenting. More than half of the state was judged to be in “exceptional drought” as 2014 ended. The lack of water caused $2.2 billion in damages, dried up reservoirs, and even forced authorities to close the taps on all irrigation water for farmers in the prosperous Central Valley, for all of 2014. A report1 on its causes by the US National Oceanic and Atmospheric Administration (NOAA) immediately generated controversy within climate science. The drought was caused by natural climate variations, reinforced by unusually warm water across the North Pacific Ocean, NOAA affirmed. Warming of the Earth's climate did not bring on the recent drought; on the contrary, global warming should have boosted winter rainfall in California, but rainfall dramatically decreased during the three years mentioned. California has clearly warmed over the entire 120-year period of weather observations, just as the USA and the world have, but no clear trend toward a wetter or drier climate is evident in the state.

Severity and Extent.     The three-year period that just ended was the second driest such period in California since 1895. The bar graph in Figure 1, below, shows how unusually dry the recent period has been for California; only 1974–1977 was a bit drier. The extent of extreme drought is mapped in Figure 2 (right). The US Drought Monitor, source of this data, maps severity of drought in five categories from D0 (abnormally dry) to D4 (exceptional drought). Most of California was in exceptional drought on Dec. 2, 2014; while one year earlier, no part of the state had exceptional drought. Snowpack in the mountains covered only 24% of its normal extent in early 2014, after the rather warm and dry winters.

Figure 1: The unusual scarcity of rainfall in California in the period 2011—2014: the amount that fell in those three years has been observed only once in the last 120 years, and only one other 3-year period had even less rainfall. Negative numbers on the bottom indicate a deficit of rainfall, in millimeters per day, and the value zero is set to be the average rainfall.

Causes. NOAA asserted that natural variations of climate, though they can be extreme, were behind the intense drought. Two natural variations were intertwined: La Niña predisposed California to dryness, as it generally does, in the first winter, 2011-2012. By the third winter, 2013-2014, unusually warm water in the Pacific Ocean west of the US west coast and south of the Gulf of Alaska caused a strong “resilient ridge” of warm air to form in the atmosphere above the warm water. This ridge deflected the jet stream and storms steered by the jet stream far north into Alaska, and away from California.

    A few months before NOAA proclaimed this, the American Meteorological Society published reports from three groups of atmospheric scientists (citations 2, 3, and 4 below) in its new series of annual reports on extreme weather of the previous year “from a climate perspective.” Two groups affirmed the predominant role of natural causes and discounted global warming as the main cause of the drought; while the third group concluded that the climate variation that leads to a “resilient ridge” in the North Pacific is much more likely in today's climate when the effects of human greenhouse gas emissions are included.

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Figure 2: The intensity and extent of drought in December 2014. More than one-half of California was in exceptional drought.

      Daniel Swain and his team pointed out2 that the “resilient ridge” pattern caused dry periods in many California winters, but this time the ridge was more extensive and resilient than in previous dry winters. A resilient ridge forces the jet stream to flow far north over Alaska and the Gulf of Alaska, and from there, to proceed south toward California. This is great for transporting tropical moisture into Alaska, but not for bringing moisture into California. These authors also compared model simulations of climate in the 20th century with those in the pre-industrial era before greenhouse gases were emitted. They found that the likelihood of a strong resilient ridge was at least four times greater in the 20th century than in the pre-industrial era. That suggests a strong human influence.

      Funk and his team4 examined causes for the drought in two climate simulation models. In the first model, unusually high sea surface temperatures (SST) were reproduced in the North Pacific Ocean in the 2012-13 and 2013-14 winters. They concluded, “Anthropogenic climate change seems likely to have contributed to these extreme SST and the extreme upper-level height anomalies” that led to the resilient ridge.

      The second simulation model reproduced high water temperatures in the North Pacific, but not in the eastern Pacific near California where the warmest water was actually observed. Funk concluded that a long-term warming trend of Pacific Ocean water did not contribute substantially to the recent drought.

      Wang and Schubert3 agreed that the unusually warm Pacific waters led to a predilection for drought in California, but that long-term global warming did not increase the risk of extreme dry years. Two effects of the warming seem to counteract each other: the first effect is to establish a strong “resilient ridge” that indeed blocks many Pacific storms from entering California. The second effect is to boost the humidity in air over the warmer eastern Pacific; the added moisture is expected to increase the chances of rainfall over the state. But the combined effect is, as we have seen, no trend toward either dryer or wetter winters.

 CITATIONS:                                                                            Top

1. “Causes and Predictability of the 2011-2014 California Drought,” an Assessment Report by Richard Seager, M. Hoerling, S. Schubert, B. Lyon, A. Kumar, J. Nakamura, N. Henderson (2014). Climate Program Office, National Oceanic and Atmospheric Administration (NOAA), Washington DC, 40 p.

2. “The extraordinary California drought of 2013/2014: Character, context, and the role of climate change,” by Daniel Swain and 6 co-authors (2014) [in: Explaining Extreme Events of 2013 from a Climate Perspective, a special supplement], Bulletin of the Amer. Meteorological Society, vol. 95, no. 9, Sept. 2014, p. S3-S7.

3. “Causes of the Extreme Dry Conditions over California during early 2013,” by H. Wang and S. Schubert (2014), p. S7-S11 (in citation #2 above).

4. “Examining the contribution of the observed global warming trend to the California droughts of 2012/13 and 2013/14,” by C. Funk and 2 co-authors (2014), p. S11-S15 (in citation #2 above).


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