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Henry F. Diaz and Eugene R. Wahl

Abstract

An analysis of the October 2013–September 2014 precipitation in the western United States and in particular over the California–Nevada region suggests this anomalously dry season, while extreme, is not unprecedented in comparison with the approximately 120-yr-long instrumental record of water year (WY; October–September) totals and in comparison with a 407-yr WY precipitation reconstruction dating back to 1571. Over this longer period, nine other years are known or estimated to have been nearly as dry or drier than WY 2014. The 3-yr deficit for WYs 2012–14, which in California exceeded the annual mean precipitation, is more extreme but also not unprecedented, occurring three other times over the past approximate 440 years in the reconstruction. WY precipitation has also been deficient on average for the past 14 years, and such a run of predominantly dry WYs is also a rare occurrence in the authors’ merged reconstructed plus instrumental period record.

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Scott D. Rutherford, Michael E. Mann, Caspar M. Ammann, and Eugene R. Wahl

Abstract

In a recent paper, Christiansen et al. compared climate reconstruction methods using surrogate ensembles from a coupled general circulation model and pseudoproxies. Their results using the regularized expectation maximization method with truncated total least squares (RegEM-TTLS) appear inconsistent with previous studies. Results presented here show that the poor performance of RegEM-TTLS in Christiansen et al. is due to 1) their use of the nonhybrid method compared to the hybrid method; 2) a stagnation tolerance that is too large and does not permit the solution to stabilize, which is compounded in another paper by Christiansen et al. by the introduction of an inappropriate measure of stagnation; and 3) their use of a truncation parameter that is too large. Thus, the poor performance of RegEM-TTLS in both Christiansen et al. papers is due to poor implementation of the method rather than to shortcomings inherent to the method.

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Eugene R. Wahl, Andrew Hoell, Eduardo Zorita, Edward Gille, and Henry F. Diaz

Abstract

Year-to-year extreme alterations in California (CA) precipitation, denoted here as flips, present significant challenges to resource managers, emergency management officials, and the state’s economy and ecosystems generally. We evaluate regional (north, central, and south) and statewide flip behavior since 1571 CE utilizing instrumental data and paleoclimate reconstructions. Flips, defined as dry-to-wet and wet-to-dry consecutive alterations between the tailward 30th percentiles of the precipitation distribution, have occurred throughout this period without indication of systematic change through the recent time of modern anthropogenic forcing. Statewide “grand flips” are notably absent between 1892 and 1957; bootstrap Monte Carlo analysis indicates that this feature is consistent with random behavior. Composites for northeastern Pacific Ocean winter sea level pressure and jet-stream winds associated with flip events indicate anomalous high or low pressure during the core precipitation delivery season for dry or wet flip years, respectively, and jet-stream conditions that are also like those associated with individual dry or wet years. Equatorial Pacific sea surface temperatures play a partial role in both dry-to-wet and wet-to-dry events in central and southern CA in the longer-period reconstruction data, with response restricted primarily to southern CA in the smaller sample-size instrumental data. Knowledge of a prior year extreme, potentially representing initiation of a flip, provides no enhancement of prediction quality for the second year beyond that achievable from skillful seasonal prediction of equatorial Pacific sea surface temperatures. Overall, results indicate that the first-order nature of flip behavior from the later 1500s reflects the quasi–white noise nature of precipitation variability in CA, influenced secondarily by equatorial Pacific sea surface conditions, particularly in southern CA.

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Henry F. Diaz, Eugene R. Wahl, Eduardo Zorita, Thomas W. Giambelluca, and Jon K. Eischeid

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Few if any high-resolution (annually resolved) paleoclimate records are available for the Hawaiian Islands prior to ~1850 CE, after which some instrumental records start to become available. This paper shows how atmospheric teleconnection patterns between North America and the northeastern North Pacific (NNP) allow for reconstruction of Hawaiian Islands rainfall using remote proxy information from North America. Based on a newly available precipitation dataset for the state of Hawaii and observed and reconstructed December–February (DJF) sea level pressures (SLPs) in the North Pacific Ocean, the authors make use of a strong relationship between winter SLP variability in the northeast Pacific and corresponding DJF Hawaii rainfall variations to reconstruct and evaluate that season’s rainfall over the period 1500–2012 CE. A general drying trend, though with substantial decadal and longer-term variability, is evident, particularly during the last ~160 years. Hawaiian Islands rainfall exhibits strong modulation by El Niño–Southern Oscillation (ENSO), as well as in relation to Pacific decadal oscillation (PDO)-like variability. For significant periods of time, the reconstructed large-scale changes in the North Pacific SLP field described here and by construction the long-term decline in Hawaiian winter rainfall are broadly consistent with long-term changes in tropical Pacific sea surface temperature (SST) based on ENSO reconstructions documented in several other studies, particularly over the last two centuries. Also noted are some rather large multidecadal fluctuations in rainfall (and hence in NNP SLP) in the eighteenth century of undetermined provenance.

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Eugene. R. Wahl, Henry F. Diaz, Russell S. Vose, and Wendy S. Gross

Abstract

The recent dryness in California was unprecedented in the instrumental record. This article employs spatially explicit precipitation reconstructions for California in combination with instrumental data to provide perspective on this event since 1571. The period 2012–15 stands out as particularly extreme in the southern Central Valley and south coast regions. which likely experienced unprecedented precipitation deficits over this time, apart from considerations of increasing temperatures and drought metrics that combine temperature and moisture information. Some areas lost more than two years’ average moisture delivery during these four years, and full recovery to long-term average moisture delivery could typically take up to several decades in the hardest-hit areas. These results highlight the value of the additional centuries of information provided by the paleo record, which indicates the shorter instrumental record may underestimate the statewide recovery time by over 30%. The extreme El Niño that occurred in 2015/16 ameliorated recovery in much of the northern half of the state, and since 1571 very-strong-to-extreme El Niños during the first year after a 2012–15-type event reduce statewide recovery times by approximately half. The southern part of California did not experience the high precipitation anticipated, and the multicentury analysis suggests the north-wet–south-dry pattern for such an El Niño was a low-likelihood anomaly. Recent wetness in California motivated evaluation of recovery times when the first two years are relatively wet, suggesting the state is benefiting from a one-in-five (or lower) likelihood situation: the likelihood of full recovery within two years is ~1% in the instrumental data and even lower in the reconstruction data.

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Laura Landrum, Bette L. Otto-Bliesner, Eugene R. Wahl, Andrew Conley, Peter J. Lawrence, Nan Rosenbloom, and Haiyan Teng

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An overview of a simulation referred to as the “Last Millennium” (LM) simulation of the Community Climate System Model, version 4 (CCSM4), is presented. The CCSM4 LM simulation reproduces many large-scale climate patterns suggested by historical and proxy-data records, with Northern Hemisphere (NH) and Southern Hemisphere (SH) surface temperatures cooling to the early 1800s Common Era by ~0.5°C (NH) and ~0.3°C (SH), followed by warming to the present. High latitudes of both hemispheres show polar amplification of the cooling from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA) associated with sea ice increases. The LM simulation does not reproduce La Niña–like cooling in the eastern Pacific Ocean during the MCA relative to the LIA, as has been suggested by proxy reconstructions. Still, dry medieval conditions over the southwestern and central United States are simulated in agreement with proxy indicators for these regions. Strong global cooling is associated with large volcanic eruptions, with indications of multidecadal colder climate in response to larger eruptions. The CCSM4’s response to large volcanic eruptions captures some reconstructed patterns of temperature changes over Europe and North America, but not those of precipitation in the Asian monsoon region. The Atlantic multidecadal oscillation (AMO) has higher variance at centennial periods in the LM simulation compared to the 1850 nontransient run, suggesting a long-term Atlantic Ocean response to natural forcings. The North Atlantic Oscillation (NAO), Pacific decadal oscillation (PDO), and El Niño–Southern Oscillation (ENSO) variability modes show little or no change. CCSM4 does not simulate a persistent positive NAO or a prolonged period of negative PDO during the MCA, as suggested by some proxy reconstructions.

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