PP12A-01 10:20h
North Atlantic freshening and ice-age abrupt climate change--8k and older
Strong evidence shows that: 1) a large freshwater flood entered the North Atlantic very rapidly slightly more than 8000 years ago; 2) a widespread, abrupt climate event occurred immediately thereafter; and 3) reconstructed climate anomalies are very similar to those modeled as a response to abrupt North Atlantic freshening. Cause and effect are tightly linked; with high confidence, North Atlantic freshening and associated ocean-circulation changes caused the abrupt climate event. Several older abrupt climate events including the Younger Dryas immediately followed North Atlantic freshening and had anomaly patterns expected for North Atlantic freshening; sufficiently long events show deep-water as well as surface-water and atmospheric signals, as expected. The basic picture from W. Broecker of North Atlantic "conveyor" disruptions causing abrupt climate shifts is thus strongly supported. Differences in forcing and in mean climate state between the past events and future North Atlantic freshening motivate work to assess the possibility of future changes; the past anomalies provide limiting estimates of what might result in the future were a conveyor disruption to occur.
PP12A-02 10:35h
Reconstructing Arabian Sea Sea Surface Temperatures from the Last Glacial Maximum to the Present using foraminiferal Mg/Ca
The strong southwestly winds of the summer Indian Monsoon induce intense upwelling along the margin of the Arabian Peninsula. Paleoclimatological reconstructions of the Indian Monsoon have historically relied heavily on the assumption that sea surface temperature (SST) in the western Arabian Sea is dominantly controlled by summer monsoon strength. While this assumption does appear to hold true for today's climate, preliminary data, obtained by spatially mapping SST patterns across the Arabian Sea for three time horizons (8~kyr, 15~kyr, and 20~kyr B.P.) using a suite of sediment cores from throughout the basin, suggest that upwelling strength is only one of several factors controlling SST along the Arabian Peninsula on longer time scales. The 20~kyr time slice, for example, demonstrates that SSTs from both upwelling and non--upwelling regions of the Arabian Sea were significantly cooler ($\sim$3$^{\circ}$C) than today. Changes in SST as a result of changes in upwelling in the western Arabian Sea are therefore superimposed on a larger--scale, basin--wide cooling. Given this knowledge, we have produced a record of SST and salinity along the Oman Margin using paired Mg/Ca and $\delta^{18}$O analyses on two species of planktonic foraminifera, \emph{Globigerina bulloides} and \emph{Globigernoides ruber}. The record spans the time period from the Last Glacial Maximum to the present, during which large changes in the Indian Monsoon have been implied from other paleoclimatological reconstructions. We use the time--slice maps that we have produced to provide a broader context, albeit limited in the time domain, for the changes in SST we observe along the Oman Margin. Our record serves as a compliment to records from nearby cores of \emph{G. bulloides} abundance, assumed to reflect upwelling intensity, while providing a more quantitative view of SST changes over the past 20~kyr.
PP12A-03 10:50h
MILLENNIAL-SCALE VARIABILITY IN MEDITERRANEAN OUTFLOW IN THE GULF OF CADIZ: INSIGHTS FROM BENTHIC MG/CA AND BA/CA
The Mediterranean Outflow (MOW) is a prominent feature of the North Atlantic's hydrography, distinguishable in intermediate water depths by its high salinity and warm temperatures. Its spreading eastward across the Atlantic leads to accumulation of salt and heat in the North Atlantic's gyres and it has been implicated in pre-conditioning the North Atlantic ocean for the formation of North Atlantic Deep Water and thus could be a player in climate change beyond the Mediterranean region. Recent paleo-studies have shown that there is extremely high variability in the intensity of MOW on centennial and millennial timescales. In order to better understand changes in the contribution of MOW to the deep Atlantic and possible linkages with the abrupt climate changes during the last glacial, we are generating new high-resolution benthic foraminiferal Mg/Ca, Ba/Ca and oxygen isotope data from Gulf of Cadiz site MD99-2339 (35.883N, 7.528W; 1170 m water depth). This site is currently bathed in the lower core of Mediterranean Outflow. Our initial study focuses on the time period spanning 13,000 to 48,000 years ago. Grain size data from core MD99-2339 indicate systematic fluctuations in the intensity of MOW correlated with the well documented millennial-scale oscillations of the last glacial, with an increased intensity of MOW during cold periods such as the late phase of H1 and the older Dryas. Our preliminary benthic Mg/Ca data suggest that the increased MOW was generally associated with warmer temperatures in the lower core of MOW, with temperature oscillations of nearly 5C in the few thousand years surrounding H1. There is a strong correlation between bottom water temperatures at this site and SSTs derived from foraminiferal transfer functions. With additional data, we hope to use systematic variations observed in the benthic Ba/Ca data to sort out the relative contribution of Mediterranean Water and entrained North Atlantic Central Water in MOW. In combination with the benthic oxygen isotope data, Mg/Ca data will help to isolate salinity changes that will be important to identifying the mechanisms causing MOW variability.
PP12A-04 11:05h
Evidence for Decadal and Centennial Variability of Southwest Indian Monsoon Precipitation During the B$\o$lling-$\AA$ller$\o$d
Nearly one quarter of all people living today are affected by the seasonal rainfall of the Southwest Indian Summer Monsoon. Meteorological records and historical accounts reveal sub-decadal intervals of reduced monsoon rainfall that resulted in droughts, crop failures, and extensive famines. However, little is known about how monsoon precipitation varied before the short instrumental record began. Furthermore, there are few terrestrial records of SW Indian Monsoon variability from within India. Here we present our first SW Indian Monsoon precipitation record from stalagmites collected from caves located throughout India. The oxygen isotopic composition of speleothem calcite is a proxy for monsoon precipitation variability that we use to extend the instrumental record back to the last deglaciation. We generated a thorium-230-dated ($\sim$11.6 to 15.6 ka) high-resolution stalagmite oxygen isotope record from Timta Cave in the western Himalayas that documents precipitation changes throughout the B$\o$lling-$\AA$ller$\o$d. While the lower and upper portions of the stalagmite grew at a rate such that our isotope record has a temporal resolution of $\sim$20 yr during the transitions into and out of the B$\o$lling-$\AA$ller$\o$d, the middle portion grew much more rapidly. This portion of the precipitation record has a resolution of $\sim$2.5 yr allowing us to assess decadal-to-interannual variability of the SW Indian Monsoon during the late B$\o$lling and early $\AA$ller$\o$d. Compared to the glacial and Younger Dryas, the SW Indian Monsoon was enhanced during the B$\o$lling-$\AA$ller$\o$d and apparently coupled to the East Asian Monsoon and North Atlantic climate on millennial time scales during the deglaciation. Additionally, multi-centennial precipitation variations within the B$\o$lling-$\AA$ller$\o$d may be correlative to the climate variations recorded in the Greenland ice cores. This work suggests that the high and low latitude climate systems are closely coupled on millennial and possibly centennial time scales. Significant decadal variability was recorded in the middle high-growth-rate interval. Periodicities of this precipitation variability are similar to those found in the modern instrumental record suggesting that climate dynamics similar to today may have been responsible.
PP12A-05 11:20h
Vegetation Response to ENSO-tied Precipitation and Human Induced Hydrological Changes in the Fakahatchee Strand Preserve State Park, Florida
In southern Florida wetlands, hydrological factors control the composition and growth conditions of the vegetation. The amount of winter precipitation, which is strongly linked to the El Ni\~{n}o - Southern Oscillation (ENSO) system, determines water availability during the growth period. The low topography creates an integrated wetland system were local hydrological conditions follow regional patterns. The high water-sensitivity, ENSO teleconnection and low topography make the southern Florida wetlands highly suitable for climate variability studies. Peat sequences accumulated in {\it Taxodium} old growth stands from the Fakahatchee Strand Preserve State Park provide an unique natural archive for studying paleo-precipitation changes by means of pollen-, leaf macro remain- and tree-ring analysis. In annual-resolved studies spanning the 20th century all proxies indicate significant responses of the wetland vegetation to the hydrological changes provoked by urban development and drainage activities. Superimposed natural dynamics reveal changes in pollen production rates, leaf- and wood-growth parameters, indicative for high frequency El Ni\~{n}o - La Ni\~{n}a dynamics. The information gathered by studying the response range of the FSPSP wetland to environmental perturbations is used for interpretation of a 5ka paleo-vegetation record from the same locality. Changes in vegetation units document the long-term shifts in regional hydrological conditions and point to significant variability in the system during the early part of the record. These variations are likely caused by short regional wet phases and, through the strong ENSO teleconnections, are of prime importance to past climate studies.
PP12A-06 11:35h
Quasi-periodic Climate Teleconnections via the North Atlantic Oscillation: A New Perspective From Tree Rings
Internal modes of climate variability such as the North Atlantic Oscillation (NAO) and the El Nino-Southern Oscillation (ENSO) significantly contribute to regional weather patterns on an inter-annual basis. Changes in the behavior of these modes over decadal and/or centennial timescales may represent an important driver of past climate events and future climate change. Importantly, if the internal modes express band-limited (periodic to quasi-periodic) variability, they provide a useful template for climate forecasting. Unfortunately, our ability to directly quantify the periodic/quasi-periodic nature of climate response to the internal modes is constrained by the limited temporal extent of instrumental records. In this study we present a novel approach toward recognition of band-limited climatic effects of the NAO in proxy records that span the past 400 years. The spatial climatic response of the NAO between northern and southern Europe provides a framework for detecting the influence of the NAO in proxy climate records. Specifically, if the NAO-forced climate signal is present it should be strongly correlated and anti-phased between the northern and southern regions of western Europe. To prospect for the NAO signal in paleoclimate data we employ independent networks of tree ring width series from Scandinavia and the Mediterranean. These locations were selected because modern instrumental records of the NAO and precipitation are significantly correlated in these regions, and tree ring width sensitivity to climate variability is maximized. The tree-ring width data from western Europe reveals a distinct 25-year quasi-periodic synchronization of climate change between Scandinavia and the Mediterranean during the 17th-20th centuries. Based on the dipole character of this signal, we propose that it is representative of climate forcing via the NAO. On this timescale of climate variability, dry/cold climate events in northern Europe are closely tied to wet events in southern Europe, and vice versa. Further, the 25-year quasi-period is modulated by a 200 year cycle, with the most pronounced climate cycles in Europe during the early 19th century and the weakest cycles during the late 17th century-early 18th century. The interval of weakest 25-year cycle expression (centered on 1700 AD) occurs within the Little Ice Age during the Maunder Minimum, and is coincident with a maximum in cosmogenic Delta 14C. These results, in conjunction with the 208-year and 23-26 year cycles observed in the cosmogenic Delta 14C record, suggest that the NAO embodies a mechanism for amplifying climate response to minor changes in solar irradiance.
PP12A-07 INVITED 11:50h
Abrupt Climate Changes Revisited
A number of TV documentaries, a report commissioned by the U.S. Pentagon, and last summer's movie all push the theme that were global warming to lead to a halt in the Atlantic's conveyor circulation, Europe would be plunged into another ice age. It seems to me that the only way this could happen is if the northern Atlantic were to freeze over. As models suggest that in order to shut down the conveyor global temperatures would have to rise 4 to $6\deg$C such a freeze over is highly unlikely. The context for this line of thought will be presented.