PP43C-01 INVITED
Pacific Climate Change and ENSO activity in the mid-Holocene
We advance the hypothesis that the Pacific climate was less 'noisy' (reduced transient eddy activity) during the mid-Holocene, and that mid-Holocene ENSO reduction comes as a direct consequence of the reduced noisiness. The basis of our argument comes from modeling studies that show mid-Holocene boreal wintertime North Pacific climate resembling 'midwinter suppression' conditions seen in present-day climate, with reduced transient eddy activity. If ENSO is viewed as an approximately linear, damped system requiring external forcing to keep going, the known reduction in ENSO activity during the mid-Holocene could then be interpreted as a straightforward consequence to the reduced level of external forcing. We thus present a view of the mid-Holocene Pacific climate change as a basinwide adjustment to the insolation changes, with ENSO changing as a consequence of this basinwide adjustment; rather than one where the tropical Pacific is seen as the primary responder and amplifier to mid-Holocene forcing.
PP43C-02 INVITED
Early, Middle and Late Holocene ENSO and the Role of Precession
Although ENSO remains one of the most poorly documented climate phenomena in the geologic record, accumulating evidence indicates it underwent pervasive changes during the Holocene, offering an opportunity to test prevailing theories given robust ENSO constraints from paleoclimate data. One of the more compelling hypotheses asserts that precession plays a major role in low-frequency ENSO modulation by altering the march of seasons and the annual temperature cycle of the equatorial Pacific. In this study we describe Holocene ENSO changes from a growing number (>1000) of foraminiferal oxygen isotopic analyses on individual G. ruber specimens from equatorial core V21-30 in the eastern Pacific upwelling zone. We use this dataset spanning the last 8,000 years to deduce changes in ENSO, the annual cycle, and the role of precession forcing. ENSO variance is estimated using various metrics, including total isotopic variance, number of extreme events, and the slope and shape of cumulative distributions of isotopic data. The results indicate unequivocal reductions in ENSO variance during the mid-Holocene and intensification between 4 and 5 ka, a time of widespread changes in the global climate. Although the mid-to-late Holocene ENSO transition is a robust result and conforms to the hypothesis of precessional forcing with summer insolation playing a key role, conditions during the early Holocene remain more enigmatic. At present the data suggest ENSO was active in the early Holocene (prior to 7 ka) even though this is not fully consistent with anomalous summer heating by precession. Additional constraints from ENSO variability during the LGM, a time when precession forcing was nearly opposite to the mid-Holocene, will be presented and discussed.
PP43C-03 INVITED
The Role of Tropical Pacific SSTs in Global Medieval Hydroclimate: A modeling study
The role of tropical Pacific sea surface temperatures (SST) in driving global medieval hydroclimate is addressed. Using fossil coral records from Palmyra Atoll, tropical Pacific SST boundary conditions are derived for the period 1320-1462AD. These boundary conditions describe La Nina like mean state conditions in the tropical Pacific conditions with inter-annual and decadal variability about that altered state. The reconstructed SSTs are used to force a 16 member ensemble of atmospheric general circulation model (AGCM) simulations, each with different initial conditions, coupled to a one layer ocean model outside of the tropical Pacific. The AGCM simulations are compared with paleo-proxy data from around the globe and show that forcing from tropical Pacific SSTs are consistent with much of the global hydroclimate variability seen in the paleo-records, though other oceans also play a role.
PP43C-04
The Holocene thermal maximum in transient coupled climate model simulations
Proxy records suggest that the timing of the Holocene thermal maximum (HTM) varied considerably from place to place. In some regions this timing coincided with the orbitally forced summer insolation maximum in the earliest Holocene, but in other areas (Northern Canada, Southern Greenland, Europe) the thermal maximum occurred several thousand years later. For instance, proxy records from Europe show widely varying timing of the HTM, reconstructed at about 8 ka in Iceland, at ~7.5 ka in northern Scandinavia, and between 7 and 6 ka in central Europe. This variation in timing is not expected by orbital forcing, suggesting that other forcings were responsible for the noted differences. It is likely that the remnant Laurentide Icesheet (LIS) played an important role. Two important effects of the LIS on the early Holocene climate can be distinguished. First, the actual presence of the ice, with its relatively high surface albedo and an additional elevation, potentially acting as a topographic barrier. Second, melt water of the LIS drained into the oceans, causing the surface ocean to be relatively fresh in some areas, with potential effects on the ocean circulation. Indeed, paleoceanographic evidence suggests that deep convection in the Labrador Sea only started after most of the LIS was gone at ~8 ka.To study this influence of the LIS deglaciation on the nature of the HTM, we have performed several transient experiments covering the last 9000 years using the coupled atmosphere-ocean-vegetation model LOVECLIM. Considering the LIS deglaciation, we quantified separately the impacts of the background melt-water fluxes and the changes in topography and surface albedo. These impacts are compared to the influence of the main long-term climate forcings, being variations in orbital parameters and changes in atmospheric greenhouse gas concentrations. We also compare the nature of the HTM in our simulations with the Holocene climate evolution as registered in proxy records around the globe.
PP43C-05 INVITED
Rapid and Time-Transgressive Responses to Early Holocene Drying in the North American Mid-Continent
The early Holocene drying of the North American mid-continent is an important case study for understanding regional aridity, its drivers, and local-scale responses. This paper synthesizes eolian, fossil pollen, δ13C, lake-level, and other paleohydrological proxies to document the spatial and temporal patterns in the onset and rate of drying. Although most sites show first signs of drying between 10 and 8 ka, some sites begin to dry as early at 14ka and others as late as 6ka. There is a significant time-transgressive trend, with western sites beginning to dry before eastern sites. However, the mean squared error for this regression is 1.58ka, indicating that the timing of local responses can depart considerably from the regional trend. Reconstructions of the Holocene dynamics of the eastern prairie-forest ecotone agree with prior maps, showing eastward prairie advance between 10,000 and 8,000 calendar years before present (ka), a maximum eastward position of the ecotone from 7 to 6ka, and westward prairie retreat after 6ka. However, the reconstructed rate of early Holocene deforestation is more rapid than previous estimates. Sites vary in the rate of response, with approximately one-third of sites showing rapid (transitional periods lasting less than 300 years) responses to early Holocene drying. A cluster of rapid responses at ca. 8ka may be caused by accelerated rates of regional drying, in turn forced by the collapse of the Laurentide Ice Sheet and drainage of Lake Agassiz. Other rapid responses likely represent non-linear site responses to progressive drying. The 21st-century trajectory for the Great Plains is uncertain, because climate models differ over the direction of regional precipitation trends, but future drying likely would trigger threshold-type shifts in ecotone position. Local responses to future regional drying trends may vary widely in timing and rate, challenging detailed impact assessments.
PP43C-06
Statistical detection of mid-Holocene abrupt climate changes from proxy records
In many paleoclimate proxy records long-term climate trends through the Holocene, forced by gradually- varying orbital changes, appear to be punctuated by rapid transitions between about 6000 to 4000 calendar years ago. Previous syntheses of mid-Holocene proxy records have relied upon subjective methods of identifying these abrupt climate changes, a difficult task given the noise inherent to most Holocene records. To more objectively assess the evidence for abrupt climate change during the mid-Holocene, 292 previously- published proxy timeseries from 130 sites around the globe were analyzed statistically using established methods of changepoint detection. The records include all types of proxies (i.e., pollen, ice cores, lake sediments, marine sediments, loess, peat, speleothems). All records have a resolution of 150 years or better, well-defined age models, and clear proxy interpretations. The statistical analysis provides evidence for two abrupt changes centered at ~5.5 and ~4.1 cal ka with a broad, perhaps global, spatial distribution. The directions of climate change are regionally complex, but generally show a shift towards colder conditions around the North Atlantic, changes in mid-latitude continental aridity, a southward shift of the Intertropical Convergence Zone, and a northward shift of the Southern Hemisphere westerlies. The timing and direction of the abrupt changes suggest that minima in solar irradiance were an important trigger and that teleconnections between regions also played an important role.
PP43C-07
Holocene Atlantic Freshwater Redistribution and the Intensification of the South American Monsoon
The Atlantic-to-Pacific water vapor transport across Central America has been postulated to play an important role in the overturning circulation of the Atlantic Ocean (Schmittner et al., 2000). Paleoceanographic data suggest that the δ18Osw, and presumably salinity, of western Pacific surface waters has decreased over the course of the Holocene (Stott et al., 2004), and modeling work suggests a number of inter-related mechanisms - changes in water vapor transport in and out of the Pacific, weakening of the East Asian monsoon, and changes in surface ocean circulation (Schmidt et al., 2007; Oppo et al., 2007). Here, we examine whether the increase in water vapor transport from the Atlantic to the Pacific that occurred as the Intertropical Convergence Zone (ITCZ) migrated southward resulted in an increase in Atlantic salinities; in essence we test whether the Pacific Ocean freshened at the expense of the Atlantic Ocean freshwater budget. Our results from the North Atlantic confirm the previously documented southward migration of the Intertropical Convergence Zone, and results from the South Atlantic provide paleoceanographic evidence suggesting a Holocene intensification of the South American monsoon.
PP43C-08
Mid-Holocene to Present Climate Transition in Tropical South America
The classical illustration of Holocene climate changes in tropical South America is the huge rising of Titicaca lake level from 4400 to 4000 cal BP. Because the Amazon basin is the source of Andean rainfalls we have explored Amazonian data of climate changes during the Holocene to better understand the cause of this abrupt transition. Amazonian data confirm the existence of mid-Holocene dryness: (1) lacustrine level studies show a lower precipitation/evaporation budget than present, with the lowest lake levels between 8500 and 6800 cal BP; (2) although the dominant Holocene vegetation has always been the rainforest in the heart of Amazonia, this forest expanded towards the northwestern and southwestern regions from 6800 to 1550 cal BP, moreover, pioneer elements of the rainforest developed during the mid-Holocene and the best example is those of Cecropia, between 9000 and 5000 cal BP. (3) soil d13C indicates a forest expansion over savannas areas in Roraima (north), Mato Grosso and Rondonia (southwest), during the Holocene. (4) the mid-Holocene (8000- 4000 cal BP) is characterized by repeated occurrences of forest fires, marked by the presence of charcoals in soils and lacustrine sediments. However these different records are not characterized by abrupt transitions at the end of the Middle Holocene in Amazonia. In the Andean records there is a clear north-south shift in the timing of the transition. Analysis of coupled Ocean Atmosphere Model simulations suggest that convection in Amazon basin is directly controlled by insolation leading to an almost linear response of local climate to the global forcing. Differently, in the eastern and south-western regions where the rain is brought by the South American Monsoon, the climate transition appears more abrupt. It may be because the involved climate mechanisms are more complex and depend on Ocean/Atmosphere/Vegetation coupled process (ITCZ position, ZCAS formation, etc.). Tectonic movements or threshold links to lacustrine basin hydrology or to proxy responses to local climate changes must also be carefully taken into account in the identification of abrupt climate changes.