PP34B-01 INVITED
Tropical-North Pacific Climate Linkages derived from Tree Rings and Corals
We use tree-ring analysis to extend the understanding of North Pacific climate variability and the tropical-high latitude climate connection over recent centuries. A tree-ring reconstruction of the December-May North Pacific Index (NPI) – an atmospheric circulation index related to the Aleutian Low pressure cell – spans from 1600-1983 and shows evidence for the three regime shifts seen in the instrumental NPI data (including the noteworthy 1976 transition in Pacific climate) and additional events in prior centuries. A 1300-year tree-ring based record for the Gulf of Alaska (GOA) provides additional extended information on North Pacific multi-decadal temperature variability. These records correlate significantly with both instrumental tropical climate indices and a coral-based reconstruction of an optimal tropical Indo-Pacific climate index, supporting evidence for a tropical-North Pacific climate link extending as far west as the Indian Ocean. The coral-based reconstruction (1781-1993) shows the twentieth century regime shifts evident in the instrumental NPI and tropical Indo-Pacific climate index, as well as additional shifts. Changes in the strength of correlation between the reconstructions over time, and differences in identified shifts in the tropical vs North Pacific series prior to the instrumental period suggest a varying tropical influence on North Pacific climate, with greater influence in the twentieth century.
PP34B-02
North Pacific Atmospheric Circulation and Tropical Teleconnections Over the Past 1500 Years From a Mt. Logan Ice Core
A calibrated 1500 year-long glaciochemical proxy record for the strength of the wintertime (November-March) Aleutian Low (ALOW) from the Mt. Logan summit (PR Col; 5300 m a.s.l.) ice core [Na+] time series reveals a strong ALOW from ca. 650-900 A.D., ca. 1300-1550 A.D., and ca. 1700-1998 A.D., and a weaker ALOW from ca. 900-1300 A.D. and ca. 1550-1700 A.D. The proxy record was calibrated to instrumental sea-level pressure data using standard regression techniques, and verified using statistical, spectral, and spatial correlation analyses. The Mt. Logan ALOW proxy record shows strong similarities with sea surface temperature, precipitation, and glacier extent proxy records from the Pacific basin sensitive to the El Niño-Southern Oscillation, indicating a robust coupling between high-latitude and tropical Pacific climate during the late Holocene. The Medieval Warm Period was characterized by more persistent La Niña-like conditions (weak ALOW), while the Little Ice Age (LIA) was characterized by at least two intervals of more persistent El Niño-like conditions (strong ALOW; 1300-1550 A.D., 1700-1998 A.D.). The LIA intensification of the ALOW is similar to the LIA intensification of the Icelandic Low in the North Atlantic (based on the GISP2, Greenland ice core [Na+] calibrated proxy record) and the Amundsen Sea Low in the high-latitude South Pacific (based on the Siple Dome, West Antarctica ice core [Na+] calibrated proxy record).
PP34B-03
Paleoclimatic Inferences From a High Resolution Bristlecone Pine δ18O Chronology
This study presents a comprehensive high resolution 400-year chronology of tree-ring α cellulose δ18O values of Pinus longaeva (Bristlecone Pine) from the White Mountains of California. The δ18OVSMOW stratigraphy exhibits a distinctive bidecadal oscillation during the 20th century with peak excursions of 4‰. The cellulosic δ18O values appear to correlate both with growing season temperatures and the isotopic composition of regional precipitation. Because there is not a good instrumental record of δ18O in precipitation for this region, the latter statistic was estimated by calculating the percentage of each year's precipitation that had a subtropical origin by the use of daily NCEP Reanalysis data and a recently developed catalog of Pineapple Express storms. The subtropical influence on the region exhibits large interannual variability, ranging from years where no such storm occurs to years where close to 15% of the total water budget has a subtropical origin. Precipitation in this region falls predominately during the winter months and the growing season is restricted to late spring through early fall, so it can be stated that the average annual δ18O value integrates a distinct summer and winter signal. The δ18O variability during the instrumental period is dwarfed by a dramatic enrichment (~10‰) in cellulosic δ18O values between 1905 and 1855 AD. This mid 19th century isotopic shift correlates with major climatic changes across the Northern Hemisphere that have been documented in a wide-range of proxy records. Both the magnitude and direction of the Bristlecone Pine isotopic excursion suggest it is not likely the result of post-Little Ice Age warming but rather a major change in the dominant storm tracks striking this region. We hypothesize that the large isotopic shift in the mid-19th century is evidence for a change in mean storm trajectories brought about by a more southerly position of the mid-latitude jet and changes in the strength and zonality of Pacific wind fields. Dramatic and coincident changes in atmospheric circulation have been noted in both ice cores (i.e. Mt. Logan) and marine sediment records (i.e. Santa Barbara and Santa Monica Basins) and these findings thus offer insight onto the dramatic hydroclimatic changes that occurred in western North America at the terminus of the Little Ice Age.
PP34B-04
Identifying Controls on the Stable Water Isotope Composition of Precipitation in the Southwestern Yukon Using GCMs
The goal of our work is to better understand what controls the stable water isotope (SWI) composition of precipitation in the southwestern Yukon, and in particular, to better-interpret the SWI signal from the Mount Logan ice core. To this end, we are conducting experiments with the GISS ModelE general circulation model, which is equipped with SWI diagnostics. One feature of interest in the Mt. Logan ice core record is a significant drop in d18O in the 1850's towards more depleted values. The current explanation for this shift is a transition in the North Pacific circulation towards a deeper Aleutian Low, with the stronger meridional flow bringing moisture from more southerly sources. Because of their greater arrival times, these air masses would have undergone a greater isotopic depletion than moisture from closer, colder sources under a more zonal flow regime. Although physically plausible, it is possible that the d18O drop caused by this proposed shift in circulation might be offset by warmer source evaporation conditions and integrated air mass trajectories, both of which would be associated with less depleted precipitation. To test the physical plausibility of the meridional hypothesis, we conducted numerical experiments with the NASA GISS ModelE isotopically-equipped general circulation model. In the Yukon, SWI variability is influenced, via the regional temperature, by the Pacific North America pattern and ENSO. We found that positive d18O anomalies in the SW Yukon region were in fact associated with a deeper Aleutian Low; it would appear that the effect of a longer transit time is offset by a warmer moisture transport pathway, in disagreement with the current moisture shift explanation. Our results are in agreement, however, with recent tree-ring reconstructions of the North Pacific Index, which suggest an 1850's shift towards a weaker Aleutian Low. We also found that the degree of Pacific control on the SW Yukon isotope signal is highly dependant on seasonality. During the summer months, both the local temperature control and underlying circulation controls on d18O are largely absent. During the winter months, when most precipitation in the SW Yukon occurs, the circulation features become much more enhanced. This suggests that in addition to the annual resolution at which ice-core reconstructions are carried out, much information can potentially be gained by separating the winter from summer signals. To determine the relative importance of moisture source conditions and transport pathway characteristics, we also conducted a sensitivity test with the GCM to isolate the effects of isotopic fractionation at the ocean and land surface from that in the atmosphere. The absence of isotopic fractionation during surface evaporation resulted in a near-constant 12 permil enrichment in precipitation in the SW Yukon. The absence of fractionation during atmospheric transport, however, had a significant impact on the d18O variability in the SW Yukon region.
PP34B-05 INVITED
The North Pacific Gyre Mode
Discussion of North Pacific Decadal decadal variability has focused primarily on the Pacific Decadal Oscillation, the leading mode of sea surface temperature anomalies north of the tropics. The PDO appears to result from a superposition of SST pattern forced by the North Pacific atmosphere due to its intrinsic dynamics and teleconnected from the tropics, with a regional impact of the ocean circulation in the frontal regions associated with the Kuroshio/Oyashio and their extensions into the interior. Recent modeling, however, suggest that previously unexplained decadal changes of salinity, nutrient upwelling and chlorophyl in the California Current are not dominated by the PDO. Rather, these are associated with a mode of variability associated with wind driven changes of the North Pacific Gyre. Consideration of this mode variability may thus be important to understand present and future variations of the North Pacific ecosystem, and in the interpretation of climate proxies.
PP34B-06
Biennial Record of Northeast Pacific Ventilation Over the Past two Centuries: Radiocarbon and Benthic Foraminferal Evidence From the Santa Barbara Basin
By volume considerations alone, the ventilation of the interior North Pacific Ocean must play a crucial role in global climate dynamics on timescales of decades to centuries. However, the sub-thermocline dynamics of the North Pacific remain largely inaccessible beyond sparse instrumental observations spanning the last twenty years. The annually laminated sediments of the Santa Barbara Basin (SBB) provide a unique opportunity to document subsurface variability over the last few centuries. The radiocarbon age of benthic foraminifera from a box core removed at the basin bottom (578 m) was measured at biennial resolution through 1819 to create a continuous benthic Δ14C record. In addition to Δ14C analysis, benthic foraminiferal species assemblages were quantified at the same resolution through 1736. Comparison of radiocarbon results with instrumental measurements reveals that benthic foraminfera faithfully capture the Δ14C of bottom water dissolved inorganic carbon (DIC). We observe substantial Δ14C variability on interannual to decadal timescales with excursions of as much as 40‰. Significant correlations of the radiocarbon record with SST records and PDO reconstructions suggest for the influence of decadal scale North Pacific climate on the vertical density structure of the northeast Pacific margin as the driver of Δ14C variability, although pore-water chemistry and advection of intermediate water masses may also contribute. The cosmopolitan benthic foraminifera of the SBB exhibit a pattern of decadal variability similar to that observed in the Δ14C time series with the exception of one chloroplast bearing species, ( Nonionella stella), which experiences unprecedented growth over the late 20th century. While the mechanism driving this growth and behavioral deviation from the other species remains unclear, the sensitivity of benthic foraminifera to their environment hints at subtle chemical and/or physical changes in the SBB benthos occurring over the last three decades. These records provide a means for discriminating the mechanisms of natural sub-surface variability in the North Pacific and to resolve the interaction between bottom water chemistry and benthic ecosystems.
PP34B-07
Southern California Hydrographic Shifts During the Medieval Climate Anomaly and the Little Ice Age.
We present records of the marine environmental change that occurred as regional climate shifted from the Medieval Climate Anomaly (MCA) into the Little Ice Age (LIA) based on planktonic foraminiferal and diatom assemblages alongside geochemical evidence of surface water temperature and salinity change from Santa Barbara Basin (SBB). The present climate reconstructions available in SBB clearly demonstrate that ocean and atmospheric circulation changes are occurring during the MCA and LIA. Records of sinistral Neogloboquadrina pachyderma absolute and relative abundance from ODP Site 893 suggest that surface waters cooled at ~600 years BP (varve chronology) or ~800 years BP (radiocarbon chronology). SSTs must have dropped below 8-10°C during winter through these intervals to provide favorable environmental conditions as sinistral N. pachyderma does not inhabit the modern SBB where annual SSTs range from 13-17°C. These results conflict with assemblage records of planktonic diatoms from the same core where increased relative abundance of Fragilariopsis doliolus occurred during the LIA. F. doliolus is a subtropical diatom associated with warm gyral waters that has been used extensively on the northern California Margin as an indicator of surface water warming and weak California Current (CC) flow. However, 20th century records suggest a close relationship of F. doliolus relative abundance to spring SSTs in the basin. Preliminary planktonic foraminiferal Mg/Ca results ( Globigerina bulloides) from core MV0508-32TC indicate that during the MCA, SBB surface waters were between 15 and 16°C, similar to modern warm season temperatures in the region, and cooled to 14°C during the LIA. This is consistent with the planktonic foraminiferal and radiolarian assemblage records from the region, which suggest a warm CC during the MCA, but stands in contrast with the δ18O and diatom assemblage records. δ18O records from ODP Site 893 indicate a cooling of surface waters during the MCA and warming during the LIA (demonstrated by a δ18O decrease during the LIA of ~0.4‰). Mg/Ca and δ18O records can only be reconciled if surface salinity decreased dramatically during the LIA. The following scenario may explain the record. As upwelling cools surface waters, warming of surface waters indicated by the presence of F. doliolus suggests reduced spring upwelling during the LIA. The presence of sinistral N. pachyderma suggests that during a portion of the year during the LIA surface waters in the basin were extremely cool perhaps cooled by the advection of subpolar water into the region via the CC. Reduced salinity during the LIA may be the result of either increased upwelling of low salinity water (although this conflicts with the diatom evidence), or advection of subpolar water from the NE Pacific. In contrast the MCA appeared to be associated with more saline, warmer surface waters and spring upwelling. These surface water shifts maybe related to ocean and atmospheric circulation changes associated with the PDO.
PP34B-08
Was the North Pacific wintertime climate less stormy during the mid-Holocene?
We present intriguing model evidence that North Pacific wintertime activity in the storm track region was significantly weaker during the mid-Holocene (~6,000 bp), though a mechanism that has an analog in today's climate. This decreased activity is shown across several PMIP2 coupled model simulations of the mid-Holocene climate, and in our coupled ocean atmosphere model. The reduced activity is co-incident with basinwide climate changes over the North and tropical Pacific, including a deepening of the Aleutian low, colder SSTs in the western and central North Pacific, a strengthening and southward shift of the subtropical jet, and a strengthened South Pacific Convergence Zone (SPCZ). Northward heat transport by transient eddies is reduced, but is more than compensated for by an increase in the stationary eddy transport. The climate changes show striking parallels to similar changes observed in the modern climate during times of reduced storm track activity associated with the ‘mid-winter suppression' first documented by H. Nakamura. The linkage suggests that dynamically the mid-Holocene and present-day share common origins. We postulate that the change in the North Pacific storminess originates from a strengthened SPCZ and weakened north branch of the ITCZ induced as a simple consequence of the mid-Holocene insolation changes. The changed convection alters the Hadley circulation and increases the northward angular momentum transport that strengthens the north Pacific subtropical jet, creating the ‘midwinter suppression' conditions that reduce activity over the storm track region. A similar reduction in the storm track activity also occurs over the North Atlantic during the mid-Holocene as well, with similar changes in the large-scale North and tropical Atlantic climates (deeper Icelandic low; colder midlatitude north Atlantic SST; strengthened north Atlantic subtropical jet; and southward shifted Atlantic ITCZ). If the reduced mid-Holocene atmospheric activity turns out to be correct, then it offers potentially a straightforward explanation for why mid-Holocene ENSO activity was significantly reduced as well (as noted in several paleoproxies), as the atmosphere provides less stochastic forcing to the ENSO system.