PP23C-01 INVITED 13:40h
A Review of Pacific Interdecadal Climate Variability: Possible Mechanisms and Surface Climate Signatures in the Pacific Sector
Many investigators have examined historical surface climate records from the Pacific sector and identified a relatively small number of spatial patterns varying at decadal to interdecadal time scales. "Pacific Decadal Variability" (PDV) is a label that has been used to describe this family of climate variations. Some patterns of PDV are contained completely within the northern extratropics, while others have signatures throughout the Pacific hemisphere on both sides of the equator. Mechanisms for observed patterns of PDV are not yet known, though a wide variety of hypotheses have been proposed. Various ocean-atmosphere mechanisms for PDV are contained within the extratropics, others within the tropics, while others involve tropical-extratropical interactions. Some investigators have proposed external forcing (solar, lunar, or volcanic) as potentially important for driving PDV. A relatively simple hypothesis couples ENSO forcing with upper ocean heat storage for extratropical PDV, and it suggests PDV predictability may be limited to ~2 year lead times. Paleo-PDV reconstructions have been based on materials throughout the Pacific sector using such things as extratropical tree-rings, tropical corals, extratropical clam shell growth rings, and ice cores. These different proxy records have generally provided different perspectives on paleo-PDV behavior.
http://jisao.washington.edu/pdo
PP23C-02 INVITED 13:55h
Northeast Pacific Summer Temperature Variability for the Last Two Millennia Inferred from Tree Ring and Glacier Expansion Proxy Records
Climate proxy records based on tree-rings series and tree-ring dated glacial histories have been shown to reflect multi-decadal to century-scale temperature variability at Alaskan, North Pacific coastal and near-coastal sites. New and extended records of summer temperature change for the past two millennia suggest multiple intervals of cooling during the first millennium AD (FMA), sustained warm intervals during Medieval times and well-defined pulses of Little Ice Age (LIA) cooling. Solar influences modulated by Pacific decadal variability, at least for the last 1000 years, appear to have had a role in the changes observed in the proxy records. Additional records are needed to better characterize the FMA, which according to glacier records, may be as cold as intervals during the LIA. This suggestion is consistent with the timing of cold periods inferred from glacial geologic data and lake records from the Canadian Coastal and Alaska Ranges.
PP23C-03 INVITED 14:10h
Abrupt and Cyclic Variations of Late-Quaternary Climate: High-Resolution Records From Alaskan Lake Sediments
In the sub-polar region of the North Pacific, there exist numerous published records of late-Quaternary climatic change based on lake-sediment analyses. However, these records typically do not have contiguous sampling, and many of them lack secure chronologies. These problems, along with the slow sediment rates of non-glacial high-latitude lakes, have made it impossible to compare the detailed structures of late-Quaternary climatic dynamics between the North Pacific and North Atlantic. We present here several continuous, multidecadal-resolution records of late-Quaternary environmental change in Alaska. Analyses of lake sediment for biogenic silica, organic carbon and nitrogen, pollen assemblages, oxygen isotopes, and compound-specific hydrogen isotopes reveal marked changes in temperature, effective moisture, aquatic productivity, and terrestrial vegetation during the last glacial- interglacial transition (LGIT) and Holocene. Our results reveal three abrupt reversals during the climatic warming of the last glacial-interglacial transition: the Younger Dryas, the Intra-Allerod Cold Period, and the Preboreal Oscillation. These data support the hypothesis that this region experienced the climatic reversals well documented in the North Atlantic region. The similarity of the triple climatic reversals between the two regions reflects large-scale climatic forcings, especially the strength of North-Atlantic thermohaline circulation, and their interactions with atmospheric processes that resulted in the propagation of cooling from the North Atlantic to the North Pacific. However, comparison of our data with the GISP2 \delta$^{18}$O and CH$_{4}$ profiles also unveils substantial discrepancies in the paleoclimatic patterns of the LGIT between these two regions. For example, the classic Bolling interstadial in the North Atlantic appears to be absent in our sediment records, and postglacial warming occurred $ > $1000 later in Alaska than in the circum- North Atlantic. Within the Holocene, high-resolution records from our sites show no clear signal of the 8.2-ka event, which has been well documented in the circum- North Atlantic. Nonetheless, evidence is emerging that centennial-scale shifts in the Holocene climate were similar between the subpolar regions of the North Atlantic and North Pacific possibly because of sun-ocean-climate linkages. In particular, Holocene climatic and ecosystem variations in Alaska appear to have occurred with periodicities of $\sim$200, $\sim$450, and $\sim$950 years. The $\sim$200-year period is similar to the well-known de Vries solar cycle, and periodicities of 400-500 and $\sim$950 years have been documented in residual atmospheric \Delta$^{14}$C data and a proxy record of North Atlantic Deep Water. Furthermore, our proxy records are generally coherent with time series of the cosmogenic nuclides $^{14}$C and $^{10}$Be as well as North-Atlantic drift ice. These results imply that small variations in solar irradiance have played a prominent role in the Holocene dynamics of the earth$^{,}$s climate system. They also illustrate that subtle solar variations can lead to pronounced changes in high-latitude terrestrial and freshwater ecosystems, which may in turn exert important feedbacks to climatic change.
PP23C-04 14:25h
Regional Atmospheric Circulation Change in the North Pacific During the Holocene Inferred from Lacustrine Carbonate Oxygen Isotopes, Yukon Territory, Canada
Analyses of sediment cores from Jellybean Lake, a small, hydrologically-open groundwater-fed lake, provide a record of changes in North Pacific atmospheric circulation for the last 7500 years at twenty to thirty-year resolution. A regional isotope hydrology study in the southern Yukon indicates that the oxygen isotope composition of water from Jellybean Lake reflects the oxygen isotope composition of mean annual precipitation. Thus, the oxygen isotope history of Jellybean Lake inferred from sedimentary carbonate oxygen isotope ratios suggests multi-decadal shifts in the oxygen isotope composition of mean annual precipitation superimposed on century and millennial trends. Recent fluctuations of Jellybean oxygen isotopes correlate well with changes in the North Pacific Index, a measure of the intensity and position of the Aleutian Low. We propose that oxygen isotope variability of precipitation in the interior of the Yukon is related to the degree of fractionation during moisture transport from the Gulf of Alaska across the St Elias Mountains that is ultimately controlled by the position and strength of the Aleutian Low. Following this model, Aleutian Low intensity during the early to middle Holocene was relatively reduced and increasing intensity coincided with the initial onset of Neoglacial advances. Rapid shifts during the last two millennia corresponds with glacial activity, changes in North Pacific salmon abundance, and shifts in atmospheric circulation over the Beaufort Sea.
PP23C-05 14:40h
1500 Years of Annual Climate and Environmental Variability as Recorded in Bona-Churchill (Alaska) Ice Cores
In 2003, six ice cores measuring 10.5, 11.5, 11.8, 12.4, 114 and 460 meters were recovered from the col between Mount Bona and Mount Churchill ($61\deg$24'N; $141\deg$42'W; 4420 m asl). These cores have been analyzed for stable isotopic ratios, insoluble dust content and concentrations of major chemical species. Total Beta radioactivity was measured in the upper sections. The 460-meter core, extending to bedrock, captured the entire depositional record at this site where ice temperatures ranged from $-24\deg$C at 10 meters to $-19.8\deg$C at the ice/bedrock contact. The shallow cores allow assessment of surface processes under modern meteorological conditions while the deep core offers a $\sim$1500-year climate and environmental perspective. The average annual net balance is $\sim$~1000 mm of water equivalent and distinct annual signals in dust and calcium concentrations along with $\delta$$^{18}$O allow annual resolution over most of the core. The excess sulfate record reflects many known large volcanic eruptions such as Katmai, Krakatau, Tambora, and Laki which allow validation of the time scale in the upper part of the core. The lower part of the core yields a history of earlier volcanic events. The 460-m Bona-Churchill ice core provides a detailed history of the `Little Ice Age' and medieval warm periods for southeastern Alaska. The source of the White River Ash will be discussed in light of the evidence from this core. The 460-m core also provides a long-term history of the dust fall that originates in north-central China. The annual ice core-derived climate records from southeastern Alaska will facilitate an investigation of the likelihood that the high resolution 1500-year record from the tropical Quelccaya Ice Cap (Peru) preserves a history of the variability of both the PDO and the Aleutian Low.
PP23C-06 14:55h
Forest Fire Signals Recorded in Ice Cores from Eclipse Icefield, Yukon Territory, Canada
A 500+ year record of forest fire frequency has been developed using two annually- dated ice cores from Eclipse Icefield, Yukon Territory, Canada. An empirical orthogonal function (EOF) analysis was used to identify biomass- burning signatures, with the third EOF describing a positive association between high ammonium concentrations and enhancements of potassium and oxalate. Similar chemical enhancements have been reported in forest fire plumes. Large ammonium concentrations spikes, without concurrent enhancements in potassium or oxalate, are also considered, after robust smoothing to remove the seasonal ammonium signal and estimate background ammonium concentrations. Comparison of forest fire signals preserved in two ice cores drilled 1 m apart at Eclipse Icefield demonstrates good agreement between the two records over their 60+ years of overlap, although the peak ammonium concentrations for a given event can vary by up to a factor of three. Several periods of increased forest fire activity are observed in the Eclipse record, most notably during the late 1700's and again during the period 1850- 1900. The most likely source of forest fires recorded in the Eclipse ice cores is Alaska and the Yukon Territory.
PP23C-07 15:10h
Mount Logan Ice Cores : the Water Cycle of the North Pacific in the Holocene
The three ice cores recovered on , or near Mt Logan at 3 , 4.3 and 5.4 km above sea level (kasl), together with a high resolution lake record at 0.8 km asl cover variously 500 to 30000 years . The stable isotopic (18O/16 O, and D/H) records in particular are compared for the most recent 500 years that they all share. This suite of records offers a unique view of the lapse rate in stable isotopes from the lower to upper troposphere. The region is climatologically important , being beside the Cordilleran-pinning-point of the Rossby wave (jet stream) system and the biggest low pressure centre in the North Pacific , the Aleutian low. Comparison of the various sites' stable isotope series (delta(18O) , delta(D)) and model simulations suggest sudden and persistent shifts between meridional and zonal flow of water vapor . The last such shift was in 1845 AD. Prior to 1845 , the delta's at 5.4 kasl were 3.5 o/oo more positive than after. At the 0.8 kasl site the shift was 1.6 o/oo in the same direction and at the 3 kasl site there was virtually no shift . There are also shifts in deuterium excess and accumulation rate at 1845 AD. Model simulations for `pure' meridional and zonal flow , suggest that these shifts are consistent regime changes between these flow types, with predominantly zonal flow prior to 1845 and meridional after. The changes in predicted delta(18O) , deuterium excess and accumulation are altitude sensitive . The 5.4 and 0.8 kasl records show a shift at 1845 and another at 800 AD. One can speculate that the 1845 regime shift coincided with the end of the Little Ice Age and that the 800 AD shift the beginning of the Medieval Warm Period. The 5.4 kasl delta(18O) record shows a Holocene that contains many such regime shifts. These shifts are also recorded in lower resolution Pacific paleo-records from ocean and lake cores. The climate-ocean system must have a natural tendency to "ring" in the scale time of ~1 ka that these shifts suggest and there is probably an external forcing to excite this tendency. It is now up to the ocean-climate models and external forcing candidates to simulate these changes.
PP23C-08 15:25h
ENSO-Influenced Upwelling off of Southern Baja California Since the Last Glaciation
During modern El Nino events, upwelled nutrients off of southern Baja California are sharply reduced due to a regionally deeper nutricline. We use two rapidly accumulating sediment cores from this region to reconstruct upwelling and productivity during the past 52 kyr. By doing so, we test hypotheses regarding ENSO system behavior on both orbital and millennial time scales. One of our cores, recovered near the core of the regional oxygen minimum zone (OMZ, 705 m), exhibits fluctuations in organic matter content and sediment color that bear a striking resemblance to Greenland oxygen isotope records. Specifically, organic matter was lower (suggesting a weaker OMZ) during Dansgaard-Oeschger stadial events. As noted previously at Santa Barbara Basin, OMZ weakening could have been due to reduced productivity or to increased ventilation from the North Pacific. Several lines of evidence point to the dominance of productivity over ventilation. First, benthic foraminiferal Mg/Ca and d18O are inconsistent with cooler (North Pacific derived) intermediate waters during stadials. Second, benthic radiocarbon ventilation ages suggest significant changes in intermediate depth circulation that appear to be unrelated to the OMZ fluctuations. Finally, benthic foraminiferal fluxes decrease dramatically during stadials, indicative of lower productivity. The productivity record is consistent with El Nino-like conditions during stadials, though other mechanisms cannot be ruled out. Our second core is from nearby Soledad Basin, which has an effective sill depth of 290 m. In contrast to the deeper core, planktonic foraminiferal preservation is excellent in Soledad Basin. We are currently analyzing planktonic Mg/Ca, Cd/Ca, and Zn/Ca in an attempt to reconstruct the upwelling history of this site since the Bolling/Allerod. We will test model results that suggest ENSO has varied with orbital forcing, and search for millennial-scale variability within the Holocene.