U42A-01 INVITED 10:20h
Multidecadal to Centennial-scale Climate Variability in the Holocene as Recorded in Northern Red Sea Sediments
Proxy data for past surface ocean properties and continental rainfall based on two high resolution sediment cores from the northernmost part of the Gulf of Aqaba (core GeoB 5804-4) and the northern Red Sea (core GeoB 5836-2) were used to infer hydrological changes at the southern margin of the Mediterranean climate zone during the Holocene. Past hydrographic conditions were deduced from stable oxygen isotope measurements on tests of shallow dwelling planktic and benthic foraminifera. A second, independent data-set from GeoB 5804-4 is the terrigenous sand accumulation record as an indicator of changes in the eolian input from the surrounding desert areas. Several multicentennial-scale variations can be observed in both the marine and continental proxies. An overwhelming correspondence of our records with e.g. North Atlantic proxy record of changes in drift ice and 14C tree ring data suggest a broad teleconnective response to Holocene variations in solar output probably involving long-term changes in AO/NAO-like anomaly pattern throughout the Holocene. One very special paleoclimate archive in the northern Red Sea is the anoxic sediment from the brine-filled Shaban Deep. The Shaban Deep is a small-scale basin filled with oxygen-depleted, highly saline brine. Sediments retrieved from this basin are partly laminated and most probably document annual deposition cycles. The laminated interval between 6400 and 4200 years BP documents multidecadal to centennial variations in the carbonaceous (coccoliths) versus siliceous (diatoms) sedimentation and generally lines up with variations in solar activity. At 4.2 ka the lamination in the sediment disappears and the Shaban Deep turns into an oxygenated environment, suggesting the temporary absence of the brine. Stable oxygen isotopes of planktonic foraminifera show a sharp increase (less than 100 years) pointing to a strong positive sea surface salinity anomaly. This "evaporation event" probably enhanced deep-water formation and ventilation of the small basin. The timing and strengths of the environmental changes ~4.2 ka, suggests that this event is the regional expression of the major drought event at 4.2 ka BP, which is widely observed in the neighbouring regions, and that strongly affected Middle East agricultural civilisations.
U42A-02 10:35h
Changes in Solar Irradiance as a Possible Mechanism for Century-Scale Droughts Reconstructed From a Holocene Pollen Record, Pyramid Lake, Nevada, USA
Pollen and algae microfossils preserved in sediments from Pyramid Lake, Nevada provide evidence for periods of persistent drought during the Holocene age. We analyzed one hundred nineteen 1-cm thick samples for pollen and algae from a set of cores that span the last 7630 years. The early middle Holocene, 7600 to 6300 cal yr B.P. was found to be the driest period, although it included one short but intense wet phase. We suggest that Lake Tahoe was below its rim for most of this period, greatly reducing the volume and depth of Pyramid Lake. Middle Holocene aridity eased between 5000 and 3500 cal yr B.P. and climate became variable with distinct wet and dry phases. Lake Tahoe probably spilled intermittently during this time. No core was recovered that represented the period between 3500 and 2600 cal yr B.P. The last 2500 years appear to have had recurrent persistent droughts. The timing and magnitude of droughts identified in the pollen record compares favorably with previously published oxygen isotope data from Pyramid Lake. The timing of these droughts also agrees with the ages of submerged rooted stumps in the Eastern Sierra Nevada and woodrat midden data from central Nevada. Prolonged drought episodes appear to correspond with the timing of ice drift minima (solar maxima) identified from North Atlantic marine sediments, suggesting that changes in solar irradiance may be a possible mechanism influencing century-scale drought in the western Great Basin.
U42A-03 INVITED 10:50h
Long-Term Extra-tropical Land Temperature Variability and Its Association with Solar and Volcanic Forcing
It has long been hypothesized that a significant fraction of observed Northern Hemisphere (NH) temperature variability at multi-decadal to centennial time scales can be attributed to variations in solar activity. This argument has been supported by some modeling experiments that used long-term changes in solar irradiance, explosive volcanism, and anthropogenic trace gases to model temperature changes over the past 1,000 years. The modeled temperatures based on these radiative forcings often compare reasonably well to proxy-based NH temperature reconstructions covering the past 1,000 years, with solar forcing being important up to the mid-20th century, after which anthropogenic forcing dominates. Volcanic forcing is also important, but it operates in a more impulsive, episodic fashion. Consequently, its impact on proxy estimates of NH temperatures over the past 1,000 years may episodically distort and even overwhelm the effects of solar forcing on temperatures, thus breaking down what would otherwise be the appearance of a strong solar influence on past temperatures. While this problem can be investigated with models, the level of radiative forcing attributed to both solar and volcanic effects is still poorly constrained. Here, we statistically investigate the combined influences of solar and volcanic forcing in a land-only, extra-tropical NH temperature reconstruction. This record has been shown to have a strong statistical association with instrumental annual temperatures over the same region of the NH. Using estimates of solar irradiance and volcanic forcing, we show evidence for a centennial time-scale influence of solar forcing on past temperatures on the order of 700-1,000 years ago during a period of relatively high solar activity and little explosive volcanism. After that time, the frequency of explosive volcanism increases substantially, during a period of generally reduced solar activity, which appears to overwhelm solar forcing. In an effort to better reveal the underlying solar signal in the temperature reconstruction, we try to factor out the volcanic forcing signal, with limited success. It appears, therefore, that the emergence of solar forcing as a long-term agent of temperature change over extra-tropical NH land areas is highly dependent both on the magnitude of its forcing and on the frequency and timing of explosive volcanism as a competing agent of temperature variability and change.
U42A-04 11:05h
Century-scale Variabioity in the Atlantic Intertropical Convergence Zone
The abundance of the planktic foraminifer Globigerinoides sacculifer in Gulf of Mexico (GOM) sediments is a proxy for the average position of the Intertropical Convergence Zone (ITCZ). Northward migration of the ITCZ results in increased presence of warm tropical waters in the northern and western GOM; southward migration of the ITCZ results in decreased presence of warm tropical waters in the northern and southern GOM. Abundance variations of G. sacculifer in sediment cores from the GOM show distinct century-scale cyclicity over the last 5,000 years. Spectral analyses of G. sacculifer abundance variations reveal century-scale periodicity that is similar to the century-scale periodicity observed in proxy records of solar activity. The GOM proxy records suggest that the average position of the ITCZ and thus Holocene century-scale variability in the Caribbean-GOM region are linked to solar variability.
U42A-05 11:20h
Solar Influence On ENSO And The Tropics?
Abstract We consider the possibility that variations in solar irradiance have a significant impact on the climate of the tropics, including the El Niño-Southern Oscillation (ENSO) cycle. It is well known that ENSO has a large impact on regional climates worldwide, with subsequent impacts on ecosystems and society. Very recent work (Schubert et al. 2004; Seager et al. 2004) has shown that a prime cause of the Dust Bowl droughts of the 1930s is a La Niña-like pattern of decadal mean sea surface temperature anomalies in the Pacific. These anomalies were very small amplitude (<0.5°C everywhere). Decadal anomalies of similar size in the tropical West Pacific and Indian Oceans have been shown to impact the North Atlantic sector (Hoerling et al., 2001); other small amplitude anomalies throughout the tropical Indian and Pacific Oceans appear to be responsible for widespread midlatitude droughts from 1998-2002 (Hoerling and Kumar, 2003). Our recent work (Mann et al. 2004) has shown that coupled ocean-atmosphere interactions in the tropical Pacific generate a significant response to solar and volcanic radiative forcing over the past 1000 years. Here we focus on solar variations. While they are unlikely to have much effect on individual El Niño-Southern Oscillation (ENSO) events, they may influence characteristics of the ENSO cycle by changing the background state. Shifts in the ENSO cycle or the background state of the tropical Pacific could help explain persistent climate anomalies around the world over the last 1000 years. Noise or chaos internal to the climate system clearly is responsible for much of the decade to decade variation in the ENSO cycle, which has caused droughts such as those of the 1930s. But does solar variability also play a role? What is the impact of solar variability on the tropical climate in general, on timescales from decades to millennia?
U42A-06 11:35h
Interactions Between Solar Forcing and Decadal to Centennial Timescale Climate Dynamics 1700-2000
Annually resolved ice core snow accumulation and coral $\delta^{18}$O records are shown to contain variability on decadal to centennial timescales resulting from internal atmospheric dynamics as well as solar forcing. The solar signal in the paleorecords was particularly large during the 19th century with a more muted response occurring during the 20th century. The magnitude of the correlation between the paleorecords and the solar forcing is largest at a lag of one to three years, suggesting an oceanic link in the response. The lagged climate response to the solar forcing involves the Hadley and Walker circulations as well as the Pacific North America teleconnection pattern and the Pacific Decadal Oscillation. This leads to variability in regional surface temperatures, atmospheric circulation and precipitation that occur at a lag with respect to the solar forcing. In addition, a reversal in the sign of the correlation between the ice core record, a proxy for surface and atmospheric temperatures in the North Pacific, and the solar forcing occurred around the start of the 20th century. A similar reversal also occurred in the correlation between sea surface temperatures over much of the Pacific Ocean and the solar forcing. We argue that this behaviour is the result of a non-linear resonant interaction between the solar forcing and internal dynamics of the climate system that results in phase reversals and/or amplitude variability in the response. Finally our results suggest that some of the regional warming that has occurred in western Canada over the past century may be associated with the current positive correlation between temperatures in the region and solar forcing.
U42A-07 11:50h
Holocene Sun-Monsoon Linkage Recorded in Stalagmites From Oman
In order to reveal a possible sun-climate connection well-dated and highly resolved climate records from key-localities are necessary. Such records can be gained from Uranium-series dated stalagmites from Oman, where climate is still strongly influenced by the Indian monsoon. Two high-resolution oxygen isotope profiles derived from Th-U-dated stalagmites from Hoti Cave ($23\deg$05N, $57\deg$21E; 800 masl) in Northern Oman and Qunf Cave ($17\deg$10N, $54\deg$18E; 650 masl) continuously cover the period between 10.1 and 6.2 kyr B.P. and 10.5 and 2.7 kyr B.P respectively. The temporal resolution of these oxygen isotope profiles averages 4 and 5 years for stalagmites H5 (Hoti Cave) and Q5 (Qunf Cave) respectively. We assume that oxygen isotope ratios (\delta$^{18}$O) of stalagmite calcite primarily reflect variations in the amount of monsoon rainfall (the so-called "amount effect"), with more negative \delta$^{18}$O values reflecting higher monsoon rainfall due to changes in monsoon intensity and/or convective activity within the ITCZ. The high temporal resolution of both speleothem \delta$^{18}$O records enables a precise comparison with atmospheric \Delta$^{14}$C measured in tree-rings. The similarity between the \delta$^{18}$O and \Delta$^{14}$C time series at multi-decadal timescales, both in their general pattern and in the number of peaks, suggests that higher monsoon rainfall in Oman coincides with higher solar activity and vice versa. Because much of the variation in \Delta$^{14}$C at these timescales is attributed to solar forcing through variations in solar activity, the data reveal a possible close sun-monsoon connection.