Millennial-scale tree-ring isotope chronologies from coast redwoods provide insights on controls over California hydroclimate variability

To understand drivers of hydroclimate variability in north-coastal California, we obtained tree cross-sections from eleven coastal redwoods (mean age of 1232 years old) from the northern half of the species range. Tree rings from eight trees were cross-dated and sampled at sub-annual resolution for carbon isotope discrimination (Δ13C) and oxygen isotope composition (δ18O). Tree-ring Δ13C and δ18O, compared to modern climate data, demonstrate these signals primarily record summertime hydroclimate variability—primarily through variables associated with evaporative conditions and/or precipitation. Our 1100-year stable isotope chronologies showed that north-coastal California did not undergo the megadroughts observed elsewhere in California and the western United States. This result implicates extended periods of low winter precipitation, rather than growing season evaporation, as the primary driver of previous megadroughts across California and neighboring regions. Compared to cool conditions prevailing over the Northern Hemisphere during the Little Ice age (1301–1875 of the common era, CE), the frequency of isotopic events of a certain magnitude was greater during periods with warmer Northern Hemisphere temperatures such as the Medieval Climate Anomaly (900–1300 CE) and the modern period (1876 to present). This association between tree-ring isotopic variability and long-term shifts in temperatures is consistent with the expected patterns in mid-latitude hydroclimate variability expected from arctic amplification (i.e., shifts in equator-to-pole temperature differences that modify jet stream speed and amplitude) or amplified quasi-resonant wave activity (i.e., wave-patterns in high-altitude winds that become “trapped” within a certain pattern, thereby producing a longer-duration periods of drought or wetness) across mid-latitudes during the boreal summer.