“Probable Causes of the Abnormal Ridge Accompanying the 2013-2014 California Drought: ENSO Precursor and Anthropogenic Warming Footprint” by S.-Y. Wang, L. Hipps, R. R. Gillies, and Jin-Ho Yoon, AGU Publications: 3220 – 3226, 2014
CE481 or CE581
The paper analyses the causes of the California drought in 2011/12 and 2013/14 winter seasons. There were ensemble simulations and observations used for indicating the main forces that led to dry winters. Usually, they are associated with ridge off the west coast. SST forcing can be useful for explanation of one third of California winter precipitation variance. ENSO is another model that may explain even more severe variance. Both models demonstrated that dry winters of 2011/12 and 2013/14 were unusual. Rising greenhouse gases and a long-term warming trend definitely had their impact on the emergence of drought and surface moisture deficits.
The winter precipitation season in 2013/14 has been the sixth driest season since 1895, the year when records begin. The winter 2011/12 ranks the second. The California water resources were left in a depleted state after the third drought in 2013/14 year. For the leading agricultural state in the country such outcome was dramatic. Lack of irrigation was partly offset by groundwater pumping. However, there is ongoing concern regarding the future water reserves in the southwestern states. The combination of declining water resources and rising temperatures may dramatically affect water availability and food supply. The global warming is likely to increase the frequency of drought emergence in California. The decreasing precipitation is the essential cause of the recent drought. However, they were also subject to other causes such as SST forcing, anthropogenic impact, ENSO Precursor and random atmospheric variability that made them different from the previous droughts.
The main data source for the observational analysis was taken from NOAA because it comprises all data from 1895. Some model simulations are the examples of ensemble of various models. Different numbers of members allow observing the issue from different perspectives and under various conditions. The analysis also includes deep research on the atmosphere and ocean conditions at the time of droughts (Seager et al., 2015, p. 7002). Closer look was given to the ridge formation and anthropogenic influence on the weather conditions and climate in the region (Wang, Hipps, Gillies, and Yoon, 2014).
The results demonstrated that 2011-14 dry winters have been influenced by ocean forcing, while in general they are subject to internal atmospheric variability. The droughts were caused by the tropical Pacific SST anomalies. The sequence of weather events caused a ridge that depressed the precipitation in California. The role of SST abnormalities turned out to be the greatest component that influenced the circulation anomaly. The link to ENSO precursor is also obvious because it has a connection to all weather events in the region and is the dominant mode of variability (Seager et al., 2015, p. 7012).
The current drought is within the range of California hydro climate variability. Dry winters in the region are mostly caused by a ridge off the west coast, while the wet ones are the results of El Nino events. Despite the fact that dry California winters are subjected by the role of internal atmospheric variability, the precipitation deficit in 2011-14 was caused by a range of SST anomalies and was a component of ocean forcing. Different simulations project that California should be aware of inevitable, periodic and more severe droughts that will happen in the future, compared to those that constitute standard.
Seager, R., Hoerling, M., Schubert, S., Wang, H., Lyon, B., Kumar, A., Nakaruma, J., Henderson, N. Causes of the 2011-14 California Drought. Journal of Climate 28 (2015): 6997-7024.
Wang, S.-Y., Hipps, L., Gillies, R. R. and Yoon, Jin-Ho. Probable Causes of the Abnormal Ridge Accompanying the 2013-2014 California Drought: ENSO Precursor and Anthropogenic Warming Footprint. AGU Publications (2014): 3220 – 3226.