PP21D-01 INVITED
Towards Understanding Patterns of Climate Change in Past Centuries
We review recent work aimed at establishing the nature of, and factors underlying, patterns of large-scale climate variability in past centuries. Evidence is compared from (1) recent proxy-based reconstructions of climate indices and spatial patterns of past surface temperature variability, (2) ensemble experiments in which proxy evidence is assimilated into coupled ocean-atmosphere model simulations to constrain the observed realization of internal variability, and (3) ensemble coupled model simulations of the response to changes in natural external radiative forcing. Implications for the roles of internal variability, external forcing, and specific climate modes such as ENSO and the NAO will be discussed.
PP21D-02 INVITED
Comparison of Indices of Volcanic and Solar Forcing over the last 2000 Years
A moderately healthy debate continues as to the relative importance of volcanism and solar variability for climate change over the last millennium. A new reconstruction of volcanism has now enabled that debate to be extended to the last two millennia. Preliminary results - which will be finalized by the time of AGU - indicate that there is a first-order shift in the intensity and frequency of global volcanism that began in the mid-13th century, almost at the same time as cooling events found in annual-scale reconstructions of temperature. The most puzzling aspect of a comparison of the bimillennial volcano and solar time series involves an almost eerie similarity in the timing of pulses of volcanism and C-14 and Be-10 inferred changes in 'solar' variability (note that C. Amman has independently discovered this phenomenon). Since there is little physical reason to expect co- variation of these two physical processes, it seems most likely that there is an unexpected contamination of one of these processes on the proxy representative of the other process. For example, pulses of volcanism could: (1) cause more changes in ocean and atmospheric C-14 variability than previously assumed; (2) change ice core Be-10 accumulation rates through effects on climate (e.g.,NAO); and/or (3) potentially change Be-10 deposition rates by increasing the number of stratospheric aerosols that are nucleation sites for cosmogenic Be-10. These results raise some questions about the reliability of 'solar' proxies as evidence of solar variability.
PP21D-03
Inter-Decadal to Multi-Decadal Sea Surface Temperature Variability in the Southwest Tropical Pacific Since AD 1648
The southwest tropical Pacific is a region with temporally and spatially sparse sea surface temperature (SST) records that limit investigations of climate variability on interannual to centennial time scales for this region. We present a monthly resolved coral Sr/Ca record from 1648 to 1999 from Amédée Island, New Caledonia (22.48°S, 166.47°E), and reconstruct SST variability in the southwest Pacific for the past 350 years. The coral Sr/Ca record was assembled from two 3-m long coeval cores from the same massive Porites lutea coral colony. The chronology is based on annual density-band counting, cross- correlation of the two intracolony coral Sr/Ca records, and 11 230Th dates with 2σ precision of ±1.1 to 16.5 years. The intracolony coral Sr/Ca variations are reproducible for more than three centuries (average monthly misfit error = ±0.015 mmol/mol; ~0.28°C), and the intracolony variations are coherent from interannual to centennial periodicities. The SST reconstructed from coral Sr/Ca shows a cooling trend from AD 1740 to 1815, a cold 19th century (~0.6°C with respect to AD 1967 to 1992), followed by a warming trend into the 20th century. Many of the cold events in the coral Sr/Ca record coincide with large volcanic eruptions (e.g., Tambora AD 1815 and Krakatau AD 1883). Spectral analysis reveals the record is dominated by modulating inter-decadal (14 to 21 years) periodicities and quasi-persistent multi-decadal (24 to 38 years) periodicities that do not exhibit coherence with the Pacific Decadal Oscillation (PDO) or the Inter-decadal Pacific Oscillation (IPO). Wavelet analysis reveals that the inter-decadal periodicities coincide with large volcanic eruptions, and the 55- to 70-year periodicities are coeval with volcanic cooling and warming trends in the 19th and 20th centuries. The multi-decadal periodicities may be a harmonic of the modulating inter-decadal periodicities or may represent an independent mode not previously recognized in the southwest Pacific.
PP21D-04 INVITED
North Atlantic, ITCZ, and Monsoonal Climate Links
Major element chemistry and color data from sediment cores in the anoxic Cariaco Basin off Venezuela record with (sub)annual resolution large and abrupt shifts in the hydrologic cycle of the tropical Atlantic during the last 80 ka. These data suggest a direct connection between the position of the ITCZ over northern South America, the strength of trade winds, and the temperature gradient to the high northern latitudes, ENSO, and monsoonal climate in Asia. The mechanisms behind these decadal-scale ITCZ-monsoon swings can be further explored at major climate transitions such as the onset of Younger Dryas cooling at ~12.7 ka, one of the most abrupt climate changes observed in ice core, lake and marine records in the North Atlantic realm and much of the Northern Hemisphere. Annually laminated sediments from ideally record the dynamics of abrupt climate changes since seasonal deposition immediately responds to climate and varve counts accurately estimate the time of change. We compare sub-annual geochemical data from a lake in Western Germany, which provides one of the best-dated records currently available for this climate transition, with the new the Cariaco Basin record and a new and higher resolution record from Lake Huguang Maar in China, and the Greenland ice core record. The Lake Meerfelder Maar record indicates an abrupt increase in storminess, occurring from one year to the next at 12,678 ka BP, coincident with other observed climate changes in the region. We interpret this shift of the wintertime winds to signify an abrupt change in the North Atlantic westerlies to a stronger and more zonal jet. The observed wind shift provides the atmospheric mechanism for the strong temporal link between North Atlantic overturning and European climate during the last deglaciation, tightly coupled to ITCZ migrations observed in the Cariaco Basin sediments, and a stronger east Asian Monsoon winter monsoon as seen in lake Huguang Maar, when cave stalagmite oxygen isotope data indicate a weaker summer monsoon.
PP21D-05
Tropical-North Pacific Teleconnections: a mechanistic link between the strength of the Indian monsoon strength and the Aleutian Low.
Present day fluctuations in the intensity of the Indian monsoon, the position of the Intertropical Convergence Zone (ITCZ), El Niño/La Niña events, and the strength of the Aleutian Low (AL) pressure system, result in extreme weather events, large scale ecosystem shifts, and impact human life and livelihood. How these climate systems will interact as global climate changes occur is still unknown. Two new marine geochemical records of Holocene climate variability improve our understanding of past tropical – North Pacific teleconnections. The first, a Zr/Al record from the western Arabian Sea, demonstrates significant variability in soil moisture on the Horn of Africa. The second, a δ15N record from Effingham Inlet (Vancouver Island), indicates changes in the strength of the California Undercurrent and NE Pacific coastal upwelling and allows our understanding of climate change in the Pacific region to be expanded northward to include the Aleutian Low pressure system. Over the last 10 kyrs, decreased SW monsoon winds/southern ITCZ position are concurrent with an increase in El Niño events and a strengthening of the Aleutian Low. Climate models suggest that this relationship is modulated by changes in the sea surface temperature of the equatorial Pacific, which result in centennial-scale variability superimposed on the gradual Holocene decline in orbital forcing.
PP21D-06
Evidence for Changes in ENSO in the mid-late Holocene From Fossil Corals in Galápagos.
A variety of natural archives have been interpreted as indicating major changes in the strength of ENSO through the Holocene. However, most of the available proxy evidence comes from a few precipitation- sensitive regions, and uncertainty remains about the degree to which these records may be interpreted as reflecting the operation of the whole coupled ocean-atmosphere ENSO system. This question is highlighted by the fact that most modelling studies that have attempted to simulate early-mid-Holocene ENSO have failed to reproduce the magnitude of ENSO reduction inferred from the proxy data. Here we present preliminary results from the analysis of annually-banded fossil corals in the Galápagos that have the potential to help resolve some of these outstanding questions about the mechanisms and drivers of changes in ENSO and its atmospheric teleconnections. Well-preserved sub-fossil massive Porites corals were cored on the islands of Santa Fe and Floreana. U-Th dating reveals that these corals range in age up to ~4,500 years old. Each coral core contains a few decades of coral growth, up to a maximum length of about 50 years. Stable oxygen isotope analysis of ~monthly resolution increments reveals that the seasonal cycle is well preserved, and that it is possible to identify interannual variability consistent with the occurrence of palaeo-El Nino events. Analysis of one 50 year long record at about 3,200 years BP suggest that ENSO was similar to or slightly weaker than in recent decades. We are currently analysing further corals for δ18O and δ13C, as well as Sr/Ca and Ba/Ca with the aim of gaining independent estimates of changes in SST, upwelling and changes in water oxygen isotopic composition. Combining records from several corals should provide robust estimates of changes in ENSO variability and its relationship to seasonality in the cold tongue region.
PP21D-07
Mid-Holocene onset of high-amplitude decadal to centennial scale variability along the Peru Chile Margin
Understanding the natural climate variations in the eastern tropical Pacific is crucial for predicting the evolution of the El Niño-Southern Oscillation (ENSO) system and for anticipating the ways in which increases in atmospheric CO2 will affect climate. Here we present the first continuous, high-resolution (11-12 yr) climate record across the mid-Holocene transition (10ka-1.4ka) from the Peru-Chile Margin near the epicenter of the modern ENSO system. Although the high productivity of the Peru margin should promote high deposition rates, and the anaerobic bottom water conditions should inhibit sediment mixing by benthic organisms, nearly all sediment cores recovered from this region suffer from major gaps in Holocene sedimentation. Our data comes from a ~5 meter piston core collected from the mid-Peruvian shelf (15° 15"S, 75° 58"W, ~250mwd) in the heart of the oxygen minimum/denitrification zone that provides the first uninterrupted archive of conditions along the Peru-Chile margin. A suite of geochemical proxies allow us to reconstruct sea surface temperature (SST- Uk'37), phytoplankton productivity (C37total and %BSi), and thermocline ventilation (δ15N), variables that are tightly correlated to ENSO events today. Despite the observation that the mean late Holocene state of all three variables did not change over the last 10,000 years, our data reveal a dramatic increase in climate variability after the mid Holocene (~5ka); represented by prolonged periods (50-200yrs) of climate extremes, which are absent in the early Holocene. To further investigate these climate extremes we examine benthic foraminiferal assemblages and oxygen isotopes in combination with our other proxy records in selected late Holocene sections. The roughly centennial-scale oscillations do not show typical El Niño-La Niña correlations between proxies. We therefore posit that a significant fraction of super-ENSO variance during the course of the Holocene may originate outside the tropics, through processes that ventilate the subsurface waters of the eastern tropical Pacific and modify its subsurface density structure and nutrient properties. Super-ENSO variability in SST may, however, have modulated the frequency of ENSO events over the Holocene.
PP21D-08 INVITED
Centennial- to millennial-scale changes in tropical Pacific climate from central tropical Pacific fossil corals: temperature and hydrological signals
Climate models predict a broad range of tropical Pacific climate responses to anthropogenic greenhouse forcing. One metric of interest is the frequency and intensity of El Nino-Southern Oscillation (ENSO) activity, and another tracks the evolution of the background climate state in the tropical Pacific. Several models suggest a dynamical relationship between these two, such that if radiative forcing impacts the mean state of the tropical Pacific, it will impact ENSO properties, and vice versa (Clement et al., 1999; Federov and Philander, 2001; Timmermann et al., 2006). Recently, there is a growing consensus that insolation forcing impacted tropical Pacific climate in the mid-Holocene, both with respect to the mean state and ENSO properties. More ambiguous are low-frequency changes to tropical Pacific climate potentially forced by solar and volcanic variability over the last millennium (Mann et al., 2005). Using a collection of U/Th-dated fossil corals from the central tropical Pacific, an area sensitive to both temperature and hydrological anomalies associated with ENSO variability, we reconstruct the centennial- and millennial-scale variability of tropical Pacific climate. Our approach focuses on the relationship between low-frequency changes in temperature and hydrology in the central tropical Pacific (using coral Sr/Ca and oxygen isotopic reconstructions, respectively). A detailed study of potential diagenesis in these fossil corals, the oldest of which dates back to 6kybp, allows us to assess the uncertainties associated with our geochemical proxy data. We interpret our results in the context of known changes in radiative forcing and global climate over the late Holocene.