Nathan D. Stansell

A 2,300-year-long annually resolved record of the South American summer monsoon from the Peruvian Andes

Decadal and centennial mean state changes in South American summer monsoon (SASM) precipitation during the last 2,300 years are detailed using an annually resolved authigenic calcite record of precipitation δ18O from a varved lake in the Central Peruvian Andes. This unique sediment record shows that δ18O peaked during the Medieval Climate Anomaly (MCA) from A.D. 900 to 1100, providing evidence that the SASM weakened considerably during this period. Minimum δ18O values occurred during the Little Ice Age (LIA) between A.D. 1400 and 1820, reflecting a prolonged intensification of the SASM that was regionally synchronous. After the LIA, δ18O increased rapidly, particularly during the current warm period (CWP; A.D. 1900 to present), indicating a return to reduced SASM precipitation that was more abrupt and sustained than the onset of the MCA. Diminished SASM precipitation during the MCA and CWP tracks reconstructed Northern Hemisphere and North Atlantic warming and a northward displacement of the Intertropical Convergence Zone (ITCZ) over the Atlantic, and likely the Pacific. Intensified SASM precipitation during the LIA follows reconstructed Northern Hemisphere and North Atlantic cooling, El Niño-like warming in the Pacific, and a southward displacement of the ITCZ over both oceans. These results suggest that SASM mean state changes are sensitive to ITCZ variability as mediated by Western Hemisphere tropical sea surface temperatures, particularly in the Atlantic. Continued Northern Hemisphere and North Atlantic warming may therefore help perpetuate the recent reductions in SASM precipitation that characterize the last 100 years, which would negatively impact Andean water resources.

1,500 year quantitative reconstruction of winter precipitation in the Pacific Northwest

Multiple paleoclimate proxies are required for robust assessment of past hydroclimatic conditions. Currently, estimates of drought variability over the past several thousand years are based largely on tree-ring records. We produced a 1,500-y record of winter precipitation in the Pacific Northwest using a physical model-based analysis of lake sediment oxygen isotope data. Our results indicate that during the Medieval Climate Anomaly (MCA) (900–1300 AD) the Pacific Northwest experienced exceptional wetness in winter and that during the Little Ice Age (LIA) (1450–1850 AD) conditions were drier, contrasting with hydroclimatic anomalies in the desert Southwest and consistent with climate dynamics related to the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). These findings are somewhat discordant with drought records from tree rings, suggesting that differences in seasonal sensitivity between the two proxies allow a more compete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.

Drought variability in the Pacific Northwest from a 6,000-yr lake sediment record

We present a 6,000-yr record of changing water balance in the Pacific Northwest inferred from measurements of carbonate δ18O and grayscale on a sediment core collected from Castor Lake, Washington. This subdecadally resolved drought record tracks the 1,500-yr tree-ring-based Palmer Drought Severity Index reconstructions of Cook et al. [Cook ER, Woodhouse CA, Eakin CM, Meko DM, Stahle DW (2004) Science 306:1015–1018] in the Pacific Northwest and extends our knowledge back to 6,000 yr B.P. The results demonstrate that low-frequency drought/pluvial cycles, with occasional long-duration, multidecadal events, are a persistent feature of regional climate. Furthermore, the average duration of multidecadal wet/dry cycles has increased since the middle Holocene, which has acted to increase the amplitude and impact of these events. This is especially apparent during the last 1,000 yr. We suggest these transitions were driven by changes in the tropical and extratropical Pacific and are related to apparent intensification of the El Niño Southern Oscillation over this interval and its related effects on the Pacific Decadal Oscillation. The Castor Lake record also corroborates the notion that the 20th century, prior to recent aridity, was a relatively wet period compared to the last 6,000 yr. Our findings suggest that the hydroclimate response in the Pacific Northwest to future warming will be intimately tied to the impact of warming on the El Niño Southern Oscillation.

Ocean‐atmosphere forcing of centennial hydroclimate variability in the Pacific Northwest

Reconstructing centennial timescale hydroclimate variability during the late Holocene is critically important for understanding large-scale patterns of drought and their relationship with climate dynamics. We present sediment oxygen isotope records spanning the last two millennia from 10 lakes, as well as climate model simulations, indicating that the Little Ice Age was dry relative to the Medieval Climate Anomaly in much of the Pacific Northwest of North America. This pattern is consistent with observed associations between the El Nino–Southern Oscillation (ENSO), the Northern Annular Mode, and drought as well as with proxy-based reconstructions of Pacific and Atlantic ocean-atmosphere variations over the past 1000 years. The large amplitude of centennial variability indicated by the lake data suggests that regional hydroclimate is characterized by longer-term shifts in ENSO-like dynamics and that an improved understanding of the centennial timescale relationship between external forcing and drought is necessary for projecting future hydroclimatic conditions in western North America.

Lacustrine stable isotope record of precipitation changes in Nicaragua during the Little Ice Age and Medieval Climate Anomaly

Discerning the infl uences of the El Nino Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) on drought variability in the tropics during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) will help to improve our understanding of climate system responses to internal and external forcing. Sediments from Lago El Gancho, Nicaragua, provide an ~1400 yr record of water balance (precipitation and evaporation) changes from a region that is sensitive to teleconnected Pacifi c and Atlantic ocean-atmosphere dynamics. Oxygen isotope values of ostracod carapaces (! 18 O ostracod ) are consistently low in El Gancho sediments between ca. A.D. 950 and 1250, indicating that wetter conditions prevailed during the MCA, a period of La Nina‐like mean state conditions in the tropical Pacifi c, and a positive mean state of the NAO. The ~150 yr period between the MCA and LIA was marked by an abrupt shift to persistently drier conditions at a time of highly variable Pacifi c sea-surface temperatures, and a transition toward a more negative NAO phase. In sediment from ca. A.D. 1450 to the present, ! 18 O ostracod values increase, suggesting that drier conditions persisted through most of the LIA, a time of a relatively negative NAO phase and El Nino‐like mean state conditions in the tropical Pacifi c. The long-term precipitation trends inferred from the El Gancho data are not entirely consistent with modern associations between precipitation in the circum-Caribbean region and the NAO, suggesting that present-day hydroclimatic shifts resulting from variability in synoptic climate patterns are dissimilar to changes resulting from teleconnected ocean-atmosphere dynamics that operated during the MCA and LIA.

Late Glacial and Holocene glacier fluctuations at Nevado Huaguruncho in the Eastern Cordillera of the Peruvian Andes

Discerning the timing and pattern of late Quaternary glacier variability in the tropical Andes is important for our understanding of global climate change. Terrestrial cosmogenic nuclide (TCN) ages (48) on moraines and radiocarbon-dated clastic sediment records from a moraine-dammed lake at Nevado Huaguruncho, Peru, document the waxing and waning of alpine glaciers in the Eastern Cordillera during the past ∼15 k.y. The integrated moraine and lake records indicate that ice advanced at 14.1 ± 0.4 ka, during the first half of the Antarctic Cold Reversal, and began retreating by 13.7 ± 0.4 ka. Ice retreated and paraglacial sedimentation declined until ca. 12 ka, when proxy indicators of glacigenic sediment increased sharply, heralding an ice advance that culminated in multiple moraine positions from 11.6 ± 0.2 ka to 10.3 ± 0.2 ka. Proxy indicators of glacigenic sediment input suggest oscillating ice extents from ca. 10 to 4 ka, and somewhat more extensive ice cover from 4 to 2 ka, followed by ice retreat. The lack of TCN ages from these intervals suggests that glaciers were less extensive than during the late Holocene. A final Holocene advance occurred during the Little Ice Age (LIA, ca. 0.4 to 0.2 ka) under colder and wetter conditions as documented in regional proxy archives. The pattern of glacier variability at Huaguruncho during the Late Glacial and Holocene is similar to the pattern of tropical Atlantic sea-surface temperatures, and provides evidence that prior to the LIA, ice extent in the eastern tropical Andes was decoupled from temperatures in the high-latitude North Atlantic.