Daily precipitation in California has been projected to become less frequent even as precipitation extremes intensify, leading to uncertainty in the overall response to climate warming. Precipitation extremes are historically associated with Atmospheric Rivers (ARs). Sixteen global climate models are evaluated for realism in modeled historical AR behavior and contribution of the resulting daily precipitation to annual total precipitation over Western North America. The five most realistic models display consistent changes in future AR behavior, constraining the spread of the full ensemble. They, moreover, project increasing year-to-year variability of total annual precipitation, particularly over California, where change in total annual precipitation is not projected with confidence. Focusing on three representative river basins along the West Coast, we show that, while the decrease in precipitation frequency is mostly due to non-AR events, the increase in heavy and extreme precipitation is almost entirely due to ARs. This research demonstrates that examining meteorological causes of precipitation regime change can lead to better and more nuanced understanding of climate projections. It highlights the critical role of future changes in ARs to Western water resources, especially over California.

It is difficult to untangle the mixed influences of high-and low-latitude climate forcing in the eastern equatorial Pacific (EEP). Here we test the hypothesis that the Southern Ocean drove change in the EEP via subsurface intermediate waters during the last deglaciation. We use the δ 18 O signature of benthic foraminifera to reconstruct water density changes during the last 25 kyr at three intermediate water depths (370 m, 600 m, and 1000 m) in the EEP. Carbonate δ 18 O records a combined signature of temperature and salinity and is therefore more closely related to density than temperature or salinity alone. We find that benthic foraminiferal δ 18 O values decreased first in the subsurface, simultaneously with rising temperatures over Antarctica, and propagated up to the surface within~3 kyr. The early subsurface response initiated a rapid decrease in density stratification over the upper water column as indicated by reduced δ 18 O gradients between surface and intermediate depths. Stratification of the upper water column remained low through the termination, with stratification minima reached during Heinrich Stadial 1 and the Younger Dryas (YD), synchronous with the two-part deglacial rise in atmospheric CO 2 . Centennial-scale shifts toward heavier δ 18 O signatures at 370 and 600 m during the YD indicate short-lived shifts in the Subantarctic Mode Water/Antarctic Intermediate Water boundary to shallower intermediate depths. We suggest that decreased density gradients during the deglaciation accelerated vertical mixing across the EEP, and potentially the entire South Pacific subtropical gyre, which enhanced CO 2 delivery from depth to the surface ocean and atmosphere.

The ocean constitutes the largest heat reservoir in the Earth's energy budget and thus exerts a major influence on its climate. Instrumental observations show an increase in ocean heat content (OHC) associated with the increase in greenhouse emissions. Here we review proxy records of intermediate water temperatures from sediment cores in the equatorial Pacific and northeastern Atlantic Oceans, spanning 10,000 years beyond the instrumental record. These records suggests that intermediate waters were 1.5-2°C warmer during the Holocene Thermal Maximum than in the last century. Intermediate water masses cooled by 0.9°C from the Medieval Climate Anomaly to the Little Ice Age. These changes are significantly larger than the temperature anomalies documented in the instrumental record. The implied large perturbations in OHC and Earth's energy budget may seem at odds with very small radiative forcing anomalies throughout the Holocene and Common Era. We suggest that even very small radiative perturbations can change the latitudinal temperature gradient and strongly affect prevailing atmospheric wind systems and hence air-sea heat exchange. These dynamic processes provide an efficient mechanism to amplify small changes in insolation into relatively large changes in OHC. Over long time periods the ocean's interior acts like a capacitor and builds up large (positive and negative) heat anomalies that can mitigate or amplify small radiative perturbations as seen in the Holocene trend and Common Era anomalies, respectively. Evidently the ocean's interior is more sensitive to small external forcings than the global surface ocean because of the high sensitivity of heat exchange in the high-latitudes to climate variations.