PP52A-01
Historical Chronologies of El Nino Events: Comparison With Instrumental and Proxy Data
Historical chronologies of El Nino events (by W.Quinn with collaborators and their recent revisions by L.Ortlieb) are trustworthy and very useful for climate research. However, the way they are produced sometimes resembles art, rather than hard quantitative science. W.Quinn presented a general scheme of converting the evidence for synchronous sets of El Nino-suggesting factors into a chronology of events occurrence and intensity. He provided references for these factors, but in most cases did not demonstrate how the intensity was derived from the evidence. His evidence set represents mostly coastal ocean and land impacts, plus a few teleconnection factors. These indicators are quite different from the typical El Nino indices (e.g. NINO3 or SOI) used today for monitoring or prediction. Nevertheless, consistency between Quinn's rating and instrumental indices like NINO3 or SOI is statistically significant; the outliers are usually excused on the grounds that historical and instrumental indices "measure different things". This work attempts to bring Quinn's ratings, Ortlieb's revisions, and the evidence on which they are based, into the context of the instrumental and high-resolution proxy data. New analyses of sea surface temperature, sea level pressure and surface winds based on the marine data, and analyses of air temperature and precipitation from land stations for the last two centuries, along with proxy-based El Nino reconstructions are compared with five centuries of historical chronologies. The instrumental data indices show a remarkable cross-variable agreement, particularly good for strong events. The difference between their consensus and historical chronologies are explained in the context of local coastal versus large-scale climate variability. Connection with proxy-based reconstructions is in general more loose, but seems to be affected by the same factors.
PP52A-02
Central American Rainfall Variations Over the Past 100,000 yr From Costa Rican Stalagmites: a Proxy for Cross-Isthmian Water Vapor Transport
We present results of oxygen isotope time series of several uranium-series dated stalagmites from Costa Rica that document decadal to multi-millennial scale variability in rainfall within the Intertropical Convergence Zone. Stalagmite samples recovered from caves along the Pacific Coast of Costa Rica largely reflect rainfall amount and moisture source on the Isthmus of Panama, and are direct proxies of the flux of moisture from the Caribbean to the Pacific Ocean. Such cross-isthmian transport is a key process involved in the thermohaline circulation of the North Atlantic Ocean and thus provides a direct link between low and high latitudes during the last glacial cycle. Our results indicate that Central American rainfall varied considerably over the past 100 ka, although our records do not cover marine isotope stage two. Rainfall was at a minimum at ca. 60 ka, with several rainfall maxima between 65-100 ka, at ca. 37 ka, and to a lesser extent during the Holocene. The data do not reveal any clear precessional insolation forcing of the Central American Monsoon. Rather, our data suggest an inverse correlation of Costa Rican rainfall with Galapagos sea surface temperatures, and wetter conditions associated with decreased salinity in the eastern equatorial Pacific Ocean.
PP52A-03
Meteorological and Paleoclimatological Studies of Eastern Pacific Tropical Cyclones Using Oxygen and Hydrogen Isotope Ratios
The East Pacific Ocean off the coast of Mexico encompasses the most concentrated region of tropical cyclone activity on the globe. This region offers a multitude of opportunities for scientific discovery concerning the meteorological characteristics of tropical cyclones and a long-term record of tropical cyclone activity. Aircraft flights into Eastern Pacific tropical cyclones by the Hurricane Research Division of NOAA have been conducted and will continue in the future. Understanding rapid changes in dangerous category 3-5 hurricanes has been a priority. A study was conducted in Hurricane Olivia (1994) as it peaked in intensity to a category 4 and then weakened. The combined use of stable analyses and meteorological data provided considerable insight into the dynamics of this transition. Sedimentary deposits in coastal regions may contain records of coastal storm activity, especially from tropical cyclones. A unique characteristic of tropical cyclones is that the oxygen and hydrogen isotopic composition of rain and water vapor from them is far below that from other types of storms. At least two specific opportunities exist. Ostracoda carapaces formed in modern tropical cyclone waters have been shown to record the oxygen isotope signal from floodwaters produced by tropical cyclones. Secondly, the potential exists that tree ring cellulose may record the uniquely low isotope values in the water vapor or rain produced by tropical cyclones. Near Puerto Escondido, Mexico in July 1998 a study was conducted that showed that the oxygen and hydrogen isotopic composition of the water vapor in the atmosphere reflected tropical cyclone activity off the coast. This low isotopic anomaly may control the isotopic composition of leaf water and thereby the isotopic composition of cellulose trees.
PP52A-04
Circulation Patterns and Ventilation Variability From Thermocline Waters In The Northeast Pacific: Records For The Last 25 Ka
New records from benthic foraminiferal carbon isotopic time series of intermediate waters in the Northeastern Pacific suggest two patterns of ventilation variability at intermediate depths in the North Pacific ocean for the last 25 Ka, separating upper intermediate waters - the present realm of North Pacific Intermediate Water (NPIW)- from lower intermediate waters records -Pacific Intermediate Waters (PIW). The dissimilar behavior between organic carbon and biogenic opal records and ?13C time series rule out productivity as the dominant mechanism behind the observed changes and stresses the importance of changes in the upper thermocline circulation as the control of the carbon isotopic composition of dissolved inorganic carbon in the NPIW and PIW bathing these depths. We will use several time series for the last 25 Ka on a depth transect to show their variability patterns through time. These results support a shallow propagation of ventilation changes between the North Atlantic climatic variability and upper intermediate waters in the North Pacific, the present realm of NPIW, as the forcing behind them although the mechanism has remained obscure. Here we will present an alternative process to explain the North Pacific ventilation changes. Time series below the ventilated thermocline reveals a different source of variability for these lower intermediate waters in the North Pacific throughout the last 25 Ka.
PP52A-05
A Record of the Eastern Tropical Pacific of Water Column Structure Reorganization during the Rapid Climate Changes of Marine Isotope Stage 3.
Little is known about the details of paleoceanographic changes in the Eastern Tropical Pacific (ETP) during marine isotope stage 3. Here we present a high resolution record of climate change from core ME0005A 10JC (15.7°N; 95.3°E, 1040 m water depth) collected in the Gulf of Tehuantepec spanning 48 to 38 Ka. Planktonic and benthic stable isotope records have been generated alongside Corg, carbonate, δ15N and trace metal concentrations of bulk sediments. Seasonal intense wind forced upwelling produces high Corg flux in the Gulf. In winter, high atmospheric pressures in the Gulf of Mexico and low pressures in the ETP (associated with the ITCZ) create a strong pressure gradient generally blocked by high mountains along the isthmus. A gap near the Gulf of Tehuantepec allows air to spill over into the Pacific creating a hurricane force wind (the Tehuanos) that pushes water off the broad shelf, producing non-Ekman upwelling. Corg production increases from 48 to 38 Ka in association with increasing nitrate utilization as indicated by increasing δ15N values. Conservative trace metals increase relative to non-conservative between 45 and 43 Ka simultaneously with shift to more positive benthic δ13C, while non-conservative (nutrient- like) metals increase after 43 Ka. A prominent short ~1‰ negative shift in benthic δ18O occurs at 44.5 Ka with a 0.5‰ positive step occurring at 43.5 Ka. Globigerina ruber records δ18O values of ~-1‰ between 46 and 45 Ka, decreasing by ~1‰ at 45 Ka, while δ13C values vary between 0 and 1‰. Globigerina bulloides records δ18O values of ~0.5‰ and δ13C of 1‰ between 46 and 45 Ka, but records δ18O values of ~-1‰ and δ13C of -1‰ between 44 and 42 Ka. G. bulloides is associated with winter upwelling in the region, while G. ruber is a surface dweller associated with the Costa Rica Current that enters the Gulf in summer. Neogloboquadrina dutertrei and Globorotalia menardii generally record δ18O values of 0.5 to 0‰ and δ13C of 1‰, however, between 46.3 and 45.5 Ka these deep dwellers record δ18O values similar to G. ruber and δ13C values of -2‰. N. dutertrei and G. menardii are both deep dwelling foraminifera also associated with the Costa Rica Current. Clearly between 46 and 45 Ka, major changes occur in the surface waters that do not affect waters at 1000m water depth. The upper water column becomes isothermal between 46.3 and 45.5 Ka, yet a strong nutricline exists. Seasonal SST differences increase, while upwelling was apparently reduced. The timing of these changes between Interstadial 15 and 14 is suggestive of Heinrich 6 influencing atmospheric circulation in the ETP however, ~5 Ka calibration errors exist that make such a correlation difficult.
PP52A-06
The role of tropics in climatic change: Variations in productivity and intermediate waters ventilation to resolve propagation of millennial time-scales in the Eastern Tropical Pacific
A 40 metre Calypso piston core (MD02-2519) was raised from the Mazatlán margin off NW Mexico (22° 30.89N / 106° 39.00W; 995 m water depth) is used to reconstruct millennial scale oscillation in eastern Tropical North Pacific over the last 130Ka. We have reconstructed proxy records of productivity (% organic carbon, reflectivity, δ13C in OC & opal) and denitrification (δ15N) in very high resolution. These records show that the accumulation of biogenic components and denitrification declined during Dansgaard-Oescheger (D/O) stadials and glacial periods, but are higher during warm and interglacial periods. Thus, millenial scale records in this area are similar to records off Southern California and Santa Barbara Basin. The underlying cause for changes in denitrification in this area will be scrutinized using additional proxy records off benthic foraminiferal δ13C and assemblage changes recording intermediate water circulation/ventilation changes. These new records will be compared with existing records from the ETP to address (1) whether millennial scale oscillations in the Pacific originate at low or high latitudes and (2) whether these oscillations first propagated in the study site through the ocean (intermediate water circulation/ventilation) or through the atmosphere (upwelling/productivity).
PP52A-07
Upwelling Cycles During the Last 40 Ka in the Mexican Tropical Pacific.
Planktonic foraminiferal records of IMAGES core MD02 2520 collected on the MONA campaign in the Gulf of Tehuantepec, Mexico, exhibit variations in the dominant assemblages during the last 40ka. Q-mode factor analysis shows that during warm periods, planktonic foraminiferal populations are dominated by Globigerinita glutinata and Globigerina bulloides indicating upwelling conditions. During cold periods, assemblages are dominated by Globorotalia menardii and Neogloboquadrina dutertrei suggesting warm stratified oceans and absence of upwelling. Previous studies off the Mazatlan Pacific margin and other places in the ENTP indicate decrease in ä15N and accumulation of biogenic sedimentary components related to reduced upwelling. The correlation between the ä15N and planktonic foraminiferal assemblages in core MD02-2520 supports this hypothesis. Since upwelling in the Gulf of Tehuantepec is driven by wind, the succession of planktonic foraminiferal faunas during the last 40ka indicates changes in the ocean-atmosphere system. At present, strong northerly winds impact the gulf during winter-spring resulting in upwelling. It has been suggested that the presence and topography of the Laurentide Ice Sheet modified the wind patterns in the western northern hemisphere during the last glacial maximum, and caused the lack of upwelling in the Mexican Pacific margin. The data presented here, suggest a similar mechanism was present throughout the last 40ka.
PP52A-08
Equatorial Moisture Transport in the Asia-Australia Region During the Last Glacial Maximum - Evidence from Paleoceanographic Observations and Modelling Results
The Last Glacial Maximum (LGM), about 20,000 years ago, was on average cooler and also drier across the globe. Northern Australia, however, has few appropriate sites from which to gain proxy evidence of the climatic conditions during the culmination of the last glacial. The sediment blown off the continent and deposited in marine cores can begin to provide such evidence. In Australia there are two preferred off-shore dust paths - one of which is to the north-west. Sediment cores from the Timor Trough show higher elemental ratios of Mg/Al, K/Al, and Ti/Al and the presence of an inorganic nitrogen fraction during the late glacial compared to the Holocene (the last 10,000 years). These changes indicate both greater amounts of material off the continent during glacial times and a weathering regime more common in arid, cold climates at the source region of the sediment. To complement the proxy evidence, the results from GCMs forced with LGM sea surface temperatures, ice-sheets and lower atmospheric CO2, were examined. They show cold and dry conditions across northern Australia and stronger off-shore winds in the north-west, conditions that match the climate reconstructions based on the sediment in the marine cores. Cross-equatorial moisture transport during this time does not extend as far south as in the present day, contributing to these dry conditions. However, further north there was limited change in that transport. The LGM sea surface temperature grids used to force these GCMs were based upon temperature reconstructions from cores such as the ones examined here. The fact that the proxy reconstructions of the continental climate from the marine cores matches the GCM simulations based on the surface temperatures reconstructed from such cores provides a consistent picture of the climate of northern Australia during glacial times. This provides evidence where there are few appropriate sites for direct proxy evidence to be obtained.