PP44A-01 INVITED
The interhemispheric thermal gradient and tropical Pacific climate
We explore the impact of interhemispheric thermal gradients forcing on the tropical Pacific ocean-atmosphere climate in an intermediate coupled model. This forcing on the tropical Pacific is implicated in paleoclimate scenarios, and possibly 20th century climate changes. The equatorial zonal sea surface temperature (SST) gradient strengthens with an increased northward interhemispheric thermal gradient, the increase arising from earlier onset and later retreat of the seasonal cold tongue, and intensification during the peak cold season. When the mean interhemispheric thermal gradient is reversed, the central equatorial Pacific SST seasonal cycle abruptly reverses in phase, with its cold season in March-May rather than Sep-Nov. While startling, this response is consistent with a prevailing hypothesis that ties the cold tongue SST seasonal cycle phase to the hemispheric mean asymmetry of the Intertropical Convergence Zone. El Niño-Southern Oscillation activity is also sensitive to the interhemispheric thermal gradient, with peak activity occurring when the mean interhemispheric thermal gradient is small, reducing rapidly as the mean gradient increases in either direction.
PP44A-02
Northern and southern hemispheric climate records from the western tropical Pacific during MIS3
During MIS3 Mg/Ca-derived SSTs and planktonic δ18O changed by up to 0.6‰ and 2°C respectively in the western tropical Pacific (MD98-2181). This marine record located at 6.3°N correlates well with millennial-scale variations in the NGRIP ice core and Hulu Cave stalagmite records reflecting hydrographic changes in the western Pacific warm pool that are linked to Monsoon and Northern Hemisphere climate variability. Stronger summer monsoon and increased summer precipitation at Hulu Cave and warmer conditions in Greenland coincided with fresher and warmer conditions at MD98-2181. Due to the ideal core location, the benthic isotopic record from MD98-2181 is a record of upper Pacific Deep Water temperature and salinity variability. The benthic δ18O record documents large millennial-scale oscillations of between 0.3-0.5‰ that correlate closely with the Antarctic surface temperature history. This suggests deep water that reached the core site was changing temperature by up to ~1-1.5°C with the possibility that some of this variability reflects changes in salinity and minor glacial-eustatic changes. We interpret these changes of deepwater properties to reflect transient changes in the source water region where Upper Circumpolar Deep Water forms or as vertical migration of the water mass boundary between Upper and Lower Circumpolar Deep Water. The combined planktonic and benthic records from MD98-2181 thus provide a northern and southern hemispheric climate record and thus verify the anti-phased relationship associated with a bi-polar seesaw oceanographic behavior throughout MIS3.
PP44A-03 INVITED
Orbital- to Centennial-Scale Asian Monsoon Changes From Chinese Speleothem Records During the Late Pleistocene
We have pieced together 230Th dated and annual-layer counted stalagmite oxygen isotope records from several caves in Southeast China, which are separated by long distances. We show that the isotope records are very similar on orbital to decadal time scales, and therefore represent climate change over a large geographic area. Modern cave investigations indicate that dripwater isotopic values are close to that of summer rainfall and summer layers make up 2/3rds to 3/4ths of annual layer thickness. Three past 100-year isotope records from caves on a south-north transect generally follow the East Asian summer monsoon (EASM) index reconstructed from the meteorological data. These results confirm that Chinese speleothem isotope records can be regarded as a proxy of the EASM strength. The Late Pleistocene EASM is dominated by a ~23 ky cyclicity, synchronous within dating errors with summer (July 21) insolation at 65°N. Millennial-scale strong monsoon events in the last and penultimate glacial period show similar changes in duration and frequency over the ice age cycle, suggesting that ice volume changes affect the rhythm and pacing of monsoon events. However, the EASM events of the last glacial period are not identical to their Greenland counterparts, perhaps because of ties to South Hemisphere climate. Decadal-centennial fluctuations of the Holocene EASM have significant correlations with solar-induced 14C variations and temperature changes over Greenland.
PP44A-04
Some aspects of the variability in the Indian Ocean Monsoon from marine sediment of the Bay of Bengal
We have reconstructed sea-surface temperatures (SST) by making paired δ18O and Mg/Ca measurements in Globigerinoides ruber (white) in core VM29-19 (14.71°N, 83.58°E; 3,182 mwd) from the western Bay of Bengal. Our initial results suggest that SST and seawater δ18O (δ18Osw) in the Bay of Bengal were ~3°C cooler and ~0.6‰ heavier, respectively, during the Last Glacial Maximum (LGM) relative to the early Holocene. The heaviest δ18Osw values were estimated in the "early deglacial interval". The enriched δ18Osw values during the LGM were interpreted as a result of less dilution from freshwater of the sea-surface by the reduced Ganges-Meghna-Brahmaputra outflow and less direct rainfall over the Bay of Bengal during the summer. The data suggest that the intensity of the monsoon was stronger in the Bρlling/Allerρd than in the Younger Dryas. The record further indicates that the intensity of the Indian Ocean Monsoon was stronger during the early Holocene and weakened significantly until after ~5 ka BP. Lighter δ18Osw values suggest that the early Holocene was warmer and wetter than the late Holocene. Our results suggest that the Indian Ocean Monsoon has undergone substantial hydrological changes during the last deglaciation and Holocene. The general pattern and timing of monsoon evolution in the Bay of Bengal from the LGM to the late Holocene parallels the records from the Arabian and Andaman seas, and northern Africa and Asian speleothem records.
PP44A-05 INVITED
Dansgaard-Oeschger events in stalagmites from Turkey, Eastern Mediterranean
The modern climate in Turkey and the eastern Mediterranean is strongly affected by two major climate systems; the North Atlantic/Siberian pressure system in winter and the Indian monsoon in summer. Turkey is ideally situated to study how and to what extent both systems were dynamically linked during the Holocene and late Pleistocene periods. Our current knowledge of continental climate variability in Turkey relies almost entirely on lake records, some of which even extend back to the Last Glacial Maximum. Another source of information on late Pleistocene and Holocene climate variability is speleothems, which can be found in caves throughout Turkey. Here we present a 50,300 year-long stalagmite oxygen (δ18O) and carbon (δ13C) isotope record from Sofular Cave at the Black Sea coast in north-western Turkey. A set of 99 230Th dates with unprecedented small age uncertainties of 0.25-2% and highly resolved (20.5 year resolution for the entire record) δ18O and δ13C profiles allow us to (1) identify Dansgaard-Oeschger events 1-13 (2) test the accuracy of the absolutely dated Hulu Cave record, (3) to further improve the absolute timing of D-O events 1 to 13, and (4) to fill a large spatial gap of paleoclimate reconstruction in the entire north eastern Mediterranean.
PP44A-06
The Wind-Evaporation-Sea Surface Temperature (WES) Feedback as a Thermodynamic Pathway for the Equator-ward Propagation of High Latitude Sea-Ice Induced Cold Anomalies
We study the role of the Wind-Evaporation-Sea Surface Temperature (WES) feedback in the propagation of high latitude cooling signal to the tropical oceans using the NCAR atmospheric Community Climate Model (CCM3) coupled thermodynamically to a Slab Ocean Model (SOM). Abruptly imposed additional northern hemispheric sea-ice cover equivalent to the Last Glacial Maximum (LGM, 18 kyr BP) in the model causes a northern hemisphere-wide cooling, and the generation and amplification of an anomalous cross equatorial meridional SST dipole associated with a southwards migration of the Inter-Tropical Convergence Zone (ITCZ) stabilizing within a period of five years. In experiments where the WES feedback is explicitly switched off by modifying the sensible and latent heat flux bulk aerodynamic formulations over the oceans in CCM3, imposed northern hemispheric sea-ice still results in wide-spread northern cooling at the same rate as the unmodified run, but generates a weaker cross-equatorial SST dipole that does not amplify with time. The southward movement of the ITCZ is also modest when the WES feedback is turned off. These results distinctly imply that the WES feedback may not be the dominant mechanism that leads to the propagation of high latitude cooling to the tropics as proposed. But, the WES feedback is found to be clearly responsible for amplifying the dipole-like equatorial eastern and central Pacific anomalies during transition to the new equilibrium state. The new steady state over the equatorial Atlantic ocean in response to high latitude sea-ice cooling in the absence of the WES feedback is similar to the unmodified run. The result indicates that unlike in the response to remote forcings like El-Niño Southern Oscillation and North Atlantic Oscillation, the WES feedback plays a lesser role in the response of the tropical Atlantic to high latitude cooling, and suggests the presence of other mechanisms in the region such as transient eddy transports.
PP44A-07 INVITED
Abrupt Climate Changes During the Last Glacial Cycle and the Deglaciation: a Southern Hemisphere Mid-latitudes Perspective
The synchronization of ice cores from Antarctica and Greenland using methane concentrations provides strong evidence for thermal seesaw pattern in the Northern and Southern Hemisphere during abrupt climate changes in the last glacial period and the last deglaciation. Modeling studies show that changes in temperature, sea ice extent and/or salinity around Antarctica could influence the strength of the Atlantic Meridional Overturning Circulation (AMOC) and possibly trigger abrupt events in the North Atlantic region. Here, we focus on results based on ODP Site 1233, located at the upper continental slope off southern Chile (41°S) at the northern margin of the Antarctic Circumpolar Current (ACC). This site has received particular attention because the ~70-kyr-old sequence extends over ~135 m composite core depth, resulting in high sedimentation rates, unprecedented in the South Pacific Ocean. Based on a zonal comparison with other paleoceanographic records from the Southern Hemisphere mid-latitudes and Antarctic ice-cores, we will show that the main pattern is consistent with the bipolar see-saw mechanism and changes in the AMOC. However, as suggested by a detailed focus on the early deglaciation, the see-saw mechanism induced changes in the coupled oceanatmosphere system of the ACC and the Southern Westerly Wind (SWW) belt. Changes in the strength and/or latitudinal position of the SWW may have in turn substantially affected the upwelling of deepwater masses in the Southern Ocean high latitudes and the release of CO2 to the atmosphere as suggested by recent modeling studies.
PP44A-08
Synchronous Western North American and Polar Abrupt Climate Events
The period since the last interglacial is characterized by dramatic abrupt changes in climate in the polar regions. The most dramatic of these in the North Atlantic region are Heinrich (cooling) and Dansgaard- Oeschger [DO] (warming) events. Continental response to these changes, although inherently interesting, may also provide insights into why and how these changes occur. Here we present a high resolution δ18O speleothem record of precipitation from 11 to 60 ky from the western interior of North America, Fort Stanton Cave (FS2), New Mexico, that shows distinctive Heinrich and DO events of similar amplitude and structure as those seen in ice core records from Greenland (GISP2) and speleothems from China (Hulu Cave). δ18O values in the FS2 speleothem vary from about -11 to -5.45 ‰. As in GISP2, FS2 Heinrich events are characterized by negative excursions of the δ18O values, while DO events are marked by abrupt and dramatic rises in δ18O values. Regionally there are two seasons of precipitation, summer precipitation, related to the North American monsoon derived mainly from the Gulf of Mexico with a mean δ18O value of about -2 ‰ and Pacific winter precipitation with a mean value of about -11 ‰. Our preliminary interpretation is that the variations in δ18O in FS2 reflect changes in the contribution of winter precipitation. Accordingly, during Heinrich events the winter contribution increases, while during DO events it decreases. These changes are likely to be modulated by changes in the position of the Polar Jet Stream. Cooler northern hemisphere pushes the Polar Jet stream further south closer to the study site. Conversely, warmer northern hemisphere drives the Polar Jet stream further north, away from the study area. Changes in δ18O in FS2 are similar to but opposite in sign to those from the Hulu Cave samples. Concurrent shift in the ITCZ and Polar Jet Stream during both Heinrich and DO events would result in the sense of changes observed in Hulu and FS2. Based on Brazilian speleothems it seems that in the southern hemisphere although Heinrich and DO events are also observed the DO signals seem diminished compared to the Fort Stanton and Hulu records. Such trans-hemispheric pattern changes demonstrate that DO events are likely to have been caused by Northern Hemisphere polar drivers.