PP41E-01 INVITED
El Niño's ride through time
The El Niño-Southern Oscillation (ENSO) phenomenon is the leading oscillatory instabilility of the tropical Pacific climate background state. As such, changes of the background state are likely to affect ENSO's stability and statistical properties (period, amplitude, irregularity, onset characteristics). A second, somewhat overlooked, but similarly powerful coupled mode in the eastern equatorial Pacific is the westward propagating annual cycle mode. Whereas the sun "crosses" the equator twice a year, the eastern equatorial Pacific SST variability is dominated by an annual cycle, rather than a semi-annual harmonic. The annually varying off- equatorial diabatic forcing drives cross-equatorial winds that trigger an westward propagating annual SST mode. The amplitude of the annual cycle in the eastern equatorial Pacific is largely controlled by the meridional SST gradient. ENSO and the annual cycle are in constant competition with each other through the so-called frequency entrainment scenario. This talk discusses how past climate changes associated with millennial-scale, orbital and tectonic forcing affected the annual cycle strength in the eastern equatorial Pacific and hence ENSO. More accurate high- resolution reconstructions of the eastern tropical Pacific meridional SST gradient would help to further elucidate the details of El Niño's bumpy ride through time.
PP41E-02
Abrupt temperature and hydrologic changes during D-O and Heinrich Events at paleolake Les Echets, southeastern France
Paleoenvironmental proxy data from sediments of paleolake Les Echets record abrupt changes in terrestrial and aquatic ecosystems during the last Glacial period, from 36 to 16 kyr BP. Synchronous fluctuations in lake level, aquatic primary productivity, terrestrial vegetation and runoff occurred, consistent with the pattern of D-O and Heinrich events in Greenland ice cores and North Atlantic marine records. Although these events appear broadly consistent, differences in detail between millennial-scale oscillations in Les Echets and Greenland records have been interpreted as evidence for asynchroneity between Greenland temperatures and European ecosystem changes. Here we present new molecular isotopic data from Les Echets sediments providing independent evidence for temperature and hydrologic change in southwestern Europe during this interval. Carbon isotopes in aquatic biomarkers confirm increased primary productivity in Les Echets coincident with D-O interstadials in Greenland. Differences between hydrogen isotopic values in aquatic and terrestrial biomarkers record dramatic shifts to more arid conditions during D-O stadials. Crenarcheal membrane lipid distributions reveal abrupt shifts in runoff and temperature that are consistent with the timing of D-O and Heinrich events. These data provide a climatic context for the environmental changes previously reconstructed in this region, and suggest that it may be possible to identify the presence of lags between local climate changes and ecosystem responses.
PP41E-03
The Nature of Climate Change During the Last Glacial Termination as Revealed by Greenland High-Resolution Ice-Core Records
This abstract is submitted on behalf of the NGRIP isotope and chemistry working groups. The Greenland NGRIP ice core has been analyzed for water isotopic composition and for impurity content in sub-annual resolution across the last glacial termination. The records reveal a complex pattern for the fast climate transitions at the onset of Greenland Interstadial 1 (GI-1 or Bølling), the onset of Greenland Stadial 1 (GS-1 or Younger Dryas), and the onset of the Holocene. All three climate transitions are initiated by a significant 1-3 year change in the deuterium excess parameter, which is a proxy for the Greenland precipitation moisture source. The change in deuterium excess is followed by a more gradual warming/cooling in Greenland expressed by a change in δ18O. The warming events last up to 50 years whereas the onset of GS-1 takes longer and is less well defined. The temperature changes are accompanied by changes in accumulation rates and by important changes in the transport of dust to Greenland from the source areas in Eastern Asia. The abruptness in the change of the moisture source suggests that the triggering of the climate change is associated with a change in the atmospheric circulation pattern, possibly a shift in the location of the Intertropical Convergence Zone. The more gradual changes in Greenland temperature, accumulation, and dust proxies are, on the other hand, more likely to be a result of changes in the thermohaline circulation and sea ice cover.
PP41E-04 INVITED
South American Summer Monsoon history recorded in Brazilian speleothems
We have obtained three high-resolution oxygen isotopic records of cave calcites from Caverna Botuverá, southern Brazil, Gruta do Padre, central Brazil, and Caverna Paraíso, Amazonian Brazil. All three records have chronologies determined by U-Th dates and span the last 90, 20 and 50 thousand years, respectively. Tests for equilibrium conditions show that their oxygen isotopic variations are primarily caused by climate change. The three records thus can provide information about precipitation history and fluctuations of the South American Summer Monsoon along a latitudinal transect from 28° S to 4° S. During the last glacial period, the three oxygen isotopic profiles show abrupt millennial-scale variations, which are anti- correlated with the Chinese speleothem monsoon records and northern high-latitude ice core records. This is likely related to the displacement of the mean position of the intertropical convergence zone and associated asymmetry of Hadley cells, consistent with an oceanic meridional overturning circulation mechanism for driving the abrupt climate events. However, the three records show distinct isotopic patterns in Holocene epoch. The δ18O values in the Botuvera record decrease steadily throughout Holocene, while in the Padre record, the δ18O drops slightly until ~6-7 thousand years ago and then gradually increases until the present. The Paraiso Holocene record is similar to the Padre one, but with a much greater amplitude. Together with Andean ice core and lake records, our observations suggest asynchronous changes in Holocene monsoonal precipitation in South America, possibly related to strengthened zonal tropical air-sea interactions after the melting of the large northern ice sheets.
PP41E-05
An oceanic mechanism for glacial greenhouse gas fluctuations
Earth's climate and atmospheric greenhouse gases varied strongly on millennial time scales during past
glacial periods, providing an important test bed for understanding the dynamical links that will control the
response of the Earth system to ongoing and future anthropogenic perturbations. The Dansgaard-Oeschger
(D-O) oscillations were characterized by coupled, but contrasting, temperature changes in Greenland and
Antarctica, accompanied by fluctuations in the concentrations of atmospheric carbon dioxide (CO2) and
nitrous oxide (N2O) as recorded in ice cores. Abrupt changes in the Atlantic Meridional Overturning
Circulation (AMOC) have often been invoked to explain the physical characteristics of the D-O oscillations,
but the mechanisms for the greenhouse gas variations and their linkage to the AMOC have remained
unclear. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles,
forced only with AMOC changes. The model simultaneously reproduces characteristic features of the D-O
temperature, CO2 and N2O fluctuations. Despite significant changes in land carbon, CO2
variations on millennial time scales are dominated by slow changes in the deep ocean inventory of
biologically-sequestered carbon and are correlated to Antarctic temperature and Southern Ocean
stratification. In contrast, N2O co-varies more rapidly with Greenland temperatures due to fast
adjustments of the thermocline oxygen budget. The results suggest that ocean circulation changes were the
primary mechanism that drove glacial CO2 and N2O fluctuations on millennial time scales,
underscoring the importance of ocean biogeochemistry for projections of future climate change.
http://mgg.coas.oregonstate.edu/~andreas/
PP41E-06
Modes of climate variability identified in MIS-3 climate records using singular spectral analysis.
Climate records from MIS-5 (~25 to 60 ka) have been characterized as containing two modes of climate variability. The "northern hemisphere" mode is exemplified by the Dansgaard–Oeschger (DO) cycles in Greenland ice core records, whereby abrupt warmings occurred throughout the mid- to high-latitude North Atlantic region initiating the relatively warm DO interstadial which is then followed by a gradual cooling trend that eventually culminated in rapid 'collapse' to the next cold DO stadial. This DO-style of variability was widespread throughout the northern hemisphere and beyond. The "southern hemisphere" mode of variability is exemplified by the Antarctic ice core records, with fewer and more symmetrical climate cycles than seen in Greenland records. Our working hypothesis is that both of these records reflect variability of the global climate system and thus must contain similar scales of variability. We use singular spectral analysis to resolve the "northern" and "southern" modes into their first four components that account for 90 and 95% of their respective variances. In order of importance, these four components have wavelengths of variability of 17ky, 5.6ky, 3.4ky and 1.6ky. Comparison of each of these individual components from the "northern" and "southern" modes shows that they are essentially identical with the exception of phase: component one from the "northern" mode is highly coherent with component one from the "southern" mode at all frequencies but is out of phase. This is true for components two and three. The fourth component is not phase shifted. Mapping the phase observed between a global array of climate records with respect to the first two singular spectral components gives insights to their origin. The long-wavelength component one shows zero phase in the low latitudes followed by the Southern Ocean and then the North Atlantic. While these records are not long enough to precisely identify effects of orbital precession, this pattern indicates a response of the tropical ocean to precession that is then transmitted to higher latitudes. The phase pattern of the second component shows an in-phase relationship with the North Atlantic and Indian monsoon region. The southeast Asian monsoon shows a very different response. Thus, SSA analysis supports our working hypothesis that the "modes" of MIS-3 climate variability reflect a response to common components of the climate system. We speculate that the different phases of these records reflect different response times of various components of the climate system.
PP41E-07 INVITED
The Nature of Paleoclimate Signals from a Modern Perspective
Because climate changes span time scales from a few to a billion years,it is unsurprising that they have a
huge diversity of physical characteristics. The most basic labels include those described in the time-domain,
including seemingly event-like phenomena, (e.g., the Younger Dryas), and trend-like (e.g.,the last
deglaciation). In contrast,
are those phenomena commonly depicted as isolated in frequency space, including those that are periodic
(probably non-existent), narrow-band deterministic (e.g., obliquity and
precessional band changes), narrow-band stochastic (e.g., ENSO), broad-band stochastic (e.g., variations
between decadal and millennial), and those that are likely a combination
of all elements (e.g., the late Pleistocene glaciations). Modern analogues to most of these signal types exist
(e.g., the near-pure periodicities of tides, the coupled modes of ENSO,
decadal droughts, sea level change). At least some of the modern experience with them suggests
methodologies useful for understanding the much more data sparse paleo signals.
http://ocean.mit.edu/~cwunsch