GC51A-0150
Coral Geochemical Proxy Records Of The East Asian Winter Monsoon And Hydrological Conditions In The Central Vietnam From 1978-2004 AD
Monthly-resolution geochemical proxies, including δ18O, δ13C, Sr/Ca, and Ba/Ca, in a living Porites coral head, collected from Son Tra Island, a near-shore island located at the north tip of Vung Da Nang Bay, central Vietnam (16°12'59.4", 108°1'57.1"), was used to quantitatively reconstruct records of sea surface temperature (SST), sea surface salinity (SSS), seasonality of rainfall, and regional terrestrial input during a period of 1978-2004 AD. By comparing the 1/4-century geochemical data, five features are exhibited. (1) The coral Sr/Ca-inferred summer SSTs correspond well with the 1°x1° instrumental data to suggest that the regional SST record can be retrieved from this local coral head. (2) Interannual variation of coral winter SST data does not follow regional instrumental values. The harmonic phenomenon between coral inferred winter SST dynamic and the surface pressure difference, between the southern South China Sea (SCS) (0-10°N, 105-115°E) and the northern SCS (22.5-32.5°N, 112-122°E), indicating that the cold local SST induced by East Asian winter monsoon was addressed in the Son Tra coral. (3) 1‰ seasonal anomaly of δ18O residual (Δδ18O) suggests a 2-4-psu seasonal salinity change between dry and wet seasons. (4) The synchronous intra-annual changes of δ18O and Ba/Ca data suggest that the rainy season is from late summer to winter, which is consistent with the meteorological record. (5) The high Ba/Ca background level of 10 μmol/mol in 1992-2004, 2-3 times larger than the averaged value of 4 μmol/mol in 1978-1992, indicates an enhanced terrestrial sediment discharge into the bay over the past 10 years. Ba records probably reflect an impact of human activity on hydrological change since the Vietnam War.
GC51A-0151
Probing ENSO influences on subtropical humidity using tracers of last saturation
We have evaluated the impact of the 1997 El Nino and 1998 La Nina events on subtropical humidity dynamics using a tracer-based last-saturation method based on the NCEP/NCAR Reanalysis dataset and the NCAR MATCH tracer transport model. Reconstructions of relative humidity from last-saturation tracer fields are consistent with the reanalysis relative humidity, including enhanced subtropical aridity during the La Nina phase relative to the El Nino phase. Analysis of DJF-averaged North Pacific last-saturation pathways shows substantial extratropical influence on subtropical humidity during both the 1997 El Nino and 1998 La Nina, with a more poleward influence during the La Nina phase. This more poleward influence during La Nina is consistent with the reduced humidity in the subtropics during La Nina and with the poleward migration of the Pacific storm tracks during La Nina. These results suggest that the extratropical influence on subtropical humidity during ENSO is consistent with ENSO-driven variations in the Pacific storm track. The findings reported here further support the notion that the dryness of the subtropics is determined outside of the tropics, rather than directly from subsidence associated with Hadley circulation. Furthermore, the results suggest that interannual variability of subtropical humidity can be linked to last-saturation trajectories that vary coherently with ENSO modes.
GC51A-0152
Seasonal Isotope Records in a Galapagos Coral: Indications of Large Shifts in Circulation and Climate During the Early 1800s
We present isotope measurements of monthly coral samples that grew in Urvina Bay, Galapagos Islands in the eastern tropical Pacific Ocean. The purpose of this study is to understand the changes in circulation and climate that have occurred in this region of intense upwelling. This study builds on previous results that revealed low annual Δ14C values during the early 1600s and early 1800s. Measurements of Δ14C in monthly samples of coral that grew during the first three decades of the 1800s reveal distinct seasonal variability that generally ranges from summertime highs of -55 to -60 per mil to wintertime low values of -65 to -80 per mil. The seasonal range of Δ14C varies markedly over this period, from ~ 7 - 25 per mil. Low Δ14C water that arrives in the eastern tropical Pacific during the wintertime originates from high- latitude water (Subantarctic Mode Water) that is entrained into the Equatorial Undercurrent. Possible mechanisms that could cause the observed variability of the seasonal isotope records, including shifts of the westerlies, will be presented.
GC51A-0153
The Tropical Atlantic Gradient in Global Climate Models
The Tropical Atlantic Gradient (TAG), the sea surface temperature difference between the Tropical North Atlantic and Tropical South Atlantic, strongly affects the latitude of the Atlantic Intertropical Convergence Zone, and precipitation in the Caribbean, Northeast Brazil, and West Africa. The TAG appears to have decreased in the second half of the 20th century until increasing in the last two decades, with aerosol forcing and the Atlantic meridional overturning circulation considered the most likely explaining mechanisms. Global climate models (GCMs) do not predict a consistent response in the TAG under climate change scenarios. We investigate the TAG in several GCMs from the World Climate Research Programme's (WCRP's) Coupled Model Intercomparison Project (CMIP) to understand the mechanisms that influence its twentieth-century behavior. We also examine these mechanisms in future climate simulations to better understand why different models project different TAG changes.
GC51A-0154
Tropical Lapse Rates From Radiosondes: Does a Model-Obs Discrepancy Exist?
Debate continues over tropical tropospheric temperature trends during the satellite era. In particular, over whether reported trend behaviour supports climate models which robustly show the tropical troposphere acting as a pseudo-moist adiabat. To date the structural uncertainty and potential systematic bias that results from the different choices that could be made during dataset construction are poorly quantified. We have developed an automated dataset homogenisation system for assessing these issues in HadAT, the Met Office Hadley Centre radiosonde temperature dataset (available at www.hadobs.org). Our system is able to produce an ensemble of dataset realisations. Here we assess our ability to recover the true long term tropical trends by analysing simulated data from an atmosphere only GCM run with prescribed SSTs and human and natural forcings with four very different sets of complex biases super-imposed. As we know the answer in each case these test case ensembles can be used to benchmark the system capabilities and then reinterpret the tropical trends that the system outputs when it is given the raw observations as input (where the truth is unknown). In this work we produce ensembles containing 100 members. Our results strongly imply that most current radiosonde and some satellite datasets exhibit too little warming aloft within the tropics. Although we can place a strong lower bound on the satellite era trends (0.05K/decade) this lower bound still allows for divergent behaviour between models (0.14-0.20K/decade) and the real world. That being said, we cannot place a defensible upper bound on trend estimates and therefore our uncertainty range does include this model expectation.
GC51A-0155
Simulation of Coupled Variability in the Tropical Indian Ocean
The coupled ocean-atmosphere variability in the tropical Indian Ocean is investigated by analysing three 100-year integrations of an Australian Bureau of Meteorology coupled seasonal forecast model. In its fully coupled (control) run, ENSO appears to be the leading mechanism that excites Indian Ocean coupled dipole/zonal mode. This involves a feedback between anomalous equatorial easterlies and zonal gradients in SST and rainfall, and is tightly tied to the seasonal cycle. The Indian Ocean zonal mode exhibits a dominant biennial periodicity, which is an amplification of the biennial ENSO mode in this model. In the second run, the local ocean - atmosphere coupling in the Indian Ocean is purposely suppressed by passing the climatological wind stresses derived from the control run to the ocean in the tropical Indian region. The dominant mechanism of SST variation in the Indian Ocean is investigated. A basin-scale surface warm anomaly is developed after the peak of El Nio in the Pacific. It is found that this warming is driven by surface heat flux anomalies that are remotely driven by SST anomalies in the equatorial Pacific. In this run, the biennial periodicity of Indian Ocean zonal mode is significant reduced. In the third run, the ENSO is artificially suppressed by applying climatological surface stresses to the tropical Pacific Ocean. In that case, the Indian Ocean zonal mode still develops in the absence of ENSO but its amplitude is about 20-30% weaker, supporting the notion that the Indian Ocean coupled mode is an intrinsic mode of the variability in the Indian Ocean. Furthermore, the biennial variation, mainly apparent the subsurface, is not amplified at the surface in the absence of ENSO, suggesting that biennial variation in the thermocline itself can not trigger the Indian Ocean zonal mode. Besides ENSO, the model results also suggest that the Indian Ocean coupled mode can be triggered by an equatorward shift of the extratropical ridge/jet, which is associated with a shift of the SAM into its low phase. The results are consistent with observational study by Thomson and Lorenz (2004)
GC51A-0156
The weakening of the Indian Monsoon-ENSO relationship in the '80s and '90s and low frequency variability in the Tropical Atlantic
The Indian Monsoon-El Nino Southern Oscillation (ENSO) relationship, according to which a drier than normal monsoon season precedes peak El Nino conditions, weakened significantly during the last two decades of the 20th century. In this work an ensemble of integrations of an Atmospherical General Circulation Model (AGCM) coupled to an ocean model in the Indian basin and forced with observed sea surface temperatures (SSTs) elsewhere is used to investigate the causes of such a weakening. The observed interdecadal variability of the Monsoon-ENSO relationship during the period 1950-1999 is realistically simulated by the model and a dominant portion of the variability is associated to changes in the tropical Atlantic SSTs in boreal summer. In correspondance to ENSO, the tropical Atlantic SSTs display negative anomalies south of the Equator in the last quarter of the 20th century and weakly positive anomalies in the previous period. Those anomalies in turn produce heating anomalies which excite a Rossby wave response in the Indian Ocean in both the model and in reanalysis data, impacting the time-mean monsoon circulation. The proposed mechanism of remote response of the Indian rainfall to the tropical Atlantic SST anomalies induced by ENSO is further tested using the AGCM coupled to the ocean model in the Indian basin and forced by climatological SSTs in the Atlantic Ocean and observed anomalies SSTs elsewhere. In this second ensemble the ENSO-Monsoon relationship is characterized by a stable and strong anticorrelation through the whole second half of the XX century.
GC51A-0157
Constraining the shallow subtropical overturning circulation with archived radiocarbon records
Archived radiocarbon records in accretionary skeletons can be used to constrain the shallow overturning subtropical cells (STC's) that transport significant amounts of tropical heat poleward in the world's oceans. Radiocarbon values of DIC in the world's oceans reflect a continuum between waters residing on the surface over long periods (high Δ14C due to equilibration with "modern" atmosphere) and waters decoupled from the atmosphere in the abyss (low Δ14C due to radioisotope decay), as well as mixtures between water masses of different ages. Thus, measurements of radiocarbon have demonstrated utility in assessing convective heat tranport such as the Meridional Overturning Circulation that is central to global climate. A prominent radiocarbon gradient is also present between the subsiding subtropical surface waters and the upwelling equatorial surface waters in the world's oceans due to the presence of STC's. These convection cells transport a major proportion of tropical heat in the Pacific and a significant proportion of tropical heat in the Atlantic towards the poles. Archived radiocarbon records in surface corals and subsurface sclerosponges constrain the N. Atlantic STC's on a centennial time scale. Published short records from Cape Verde corals indicate significant changes in radiocarbon content; this is potentially related to migration of the front between upwelled tropical waters and downwelled subtropical waters. An approach is outlined to estimate the proportion of tropical to subtropical waters at Cape Verde using as endmembers high-resolution sclerosponge radiocarbon records from Bahamas subsurface waters and coral radiocarbon records from São Tome and Principe in the Gulf of Guinea. Preliminary data from Bahamas sclerosponges indicate the need for high-resolution subsampling of the skeletons. Initial novel AMS measurements from fine scale laser-decomposition of the skeletons are presented.
GC51A-0158
Bomb Radiocarbon and the Suess Effect in Palmyra Corals During the Past 100 Years
Annual samples from Palmyra coral cores that lived from 1900 to 2007 were analyzed for radiocarbon (D14C) using accelerator mass spectrometry. Palmyra atoll (6N, 162W) is located near the boundary of the North Equatorial Counter Current and the South Equatorial Current in the Pacific Ocean, making it an interesting location for observing changes in Pacific climate, e.g. ENSO. These results show the input of bomb radiocarbon to the tropical Pacific beginning in the mid-1950s, where D14C values rose from ~ -55 to -60 per mil to ~ +100 per mil by 1980. The Suess effect, which is the decrease in D14C noticed by 1955, will be compared to that observed at other locations in the Pacific and Atlantic Oceans. The usefulness of the D14C record at Palmyra as an indicator of climate will be explored.
GC51A-0159
Influence of the Northern Hemisphere Annular Mode on El-Nino Southern Oscillation
The influence of the Northern Hemisphere annular mode (NAM) on the El-Nino Southern Oscillation (ENSO) was examined using 41 years of NCEP/NCAR reanalysis data set. The NAM index in spring has significant correlation with the Nino-3 index in the following winter. The NAMfs signature on an anomalous westerly in the tropical Pacific corresponds with the modulation of the westerly wind burst (WWB). The signature of the spring NAM on the tropical westerly anomalies follows the modulated WWB and continuously broadens eastward from summer to winter. The NAM-related SST anomalies in the tropical Pacific also broaden from summer to winter, and then, it consists with an anomaly pattern of ENSO in winter. An AGCM experiment, in which SSTs are fixed as a climatological-mean monthly distribution, also shows same result for the relationship between the NAM and tropical westerlies in spring. An agreement between the observational result and the model experiment indicates the possibility of that the NAM actively influences on the tropical circulations. A probable mechanism how the NAM affects tropics was examined using 26 years of NCEP/DOE reanalysis data set. The frequency of the cold and dry surge defined by northerly and temperature dropping increases associated with the positive phase of the NAM. The cold and dry air gains heat and moisture from the warmer ocean in the tropical Pacific. Since the tropical atmospheric heating induces a Rossby response through the Matsuno-Gill theory, surface pressure anomalies related to the spring NAM consists with the twin cyclone. The westerly between the cyclones intensifies the WWB. As a result, the NAM could be an indirect trigger of ENSO through the modulation of the WWB. http://www.agu.org/pubs/crossref/2006.../2005GL025432.shtml
GC51A-0160
The Trends of Pacific Shallow Overturning Circulation in Coupled Climate Models
The shallow overturning circulation in the Pacific links subtropical subduction with equatorial upwelling, and is referred to as the Subtropical Cells (STCs). Both observational and modeling studies have indicated the STCs exert a strong influence on equatorial Pacific SST and the decadal modulation of ENSO. We will show that this shallow overturning circulation is significantly correlated with tropical Pacific SST on decadal and multi-decadal time scales during the 20th century in a majority of 14 analyzed coupled climate model simulations. However, the observed slowdown of this circulation in the last 50 years of the 20th century is not well simulated, except in one model (the ocean eddy permitting MIROCH model). On the other hand, studies by Meehl et al. have linked weakening STCs to a change of ENSO amplitudes in global warming scenario runs in two NCAR models, even though the observed STC weakening is not seen in the models' 20th century simulation. More recently, Vecchi et al. have also reported the slow-down of tropical circulation in GFDL models' global warming runs. In this presentation, we will systematically examine how the STCs evolve in long coupled global warming runs and whether the signatures of changing STCs in these models are consistent with the observed in historical record.
GC51A-0161
Return Periods of Anomalous Sea-Surface Temperature Events Inferred From Wavelet Analysis of Thermal Proxy Records in Corals: Implications for Coral Bleaching
Coral populations are highly responsive to thermal stress, often resulting in considerable bleaching and subsequent mortality. Tolerance and adjustment to thermal stress are considered a consequence of the host- symbiont constitution and their thermal history. Since the frequency, severity and spatial scale of anomalously high sea-surface temperatures have increased in the past three decades, it is necessary to determine the return periods of anomalous temperatures in the past in order to ascertain the timescales to which corals have become accustomed. We test the hypothesis that more frequent thermal anomalies in the past have led to selective pressures culminating in more tolerant contemporary coral populations. Wavelet analysis was undertaken on stable oxygen isotope records extracted from modern corals at 17 locations to determine the significant return periods of anomalous sea-surface temperatures across time in the tropical to sub-tropical Pacific, Indian and Atlantic Oceans and Red Sea. Interpolation of the significant return periods was used to determine the spatial extent of ocean-wide modes of temperature variability in the Indian and Pacific Oceans. Considerable spatial and temporal variability was apparent in the strength of the interannual and decadal components dating back over 300 years. Interannual variability was high in the far western and central to eastern Pacific and at the two poles of the Indian Ocean Dipole, but weak throughout the remainder of the Indian Ocean. Decadal variability was considerable throughout most of the Pacific Ocean, but absent in the southwest Pacific, suggesting little decadal variability in the South Pacific Convergence Zone. Decadal variability was also apparent at the poles of the Indian Ocean Dipole and in the Agulhas outflow zone. Analysis of bleaching severity within the Indian and Pacific Oceans during the 1997-1998 bleaching event revealed that bleaching was less severe at sites with a high probability of interannual variability and a low probability of decadal variability. This suggests that corals at sites that have experienced frequent thermal events in the past may be more likely to resist bleaching during future global climate change-associated anomalies.
GC51A-0162
Can Stable Isotopes of Hydrogen and Oxygen Tell you Which way the Wind is Blowing or has Blown in the Past?
The region off the Pacific southwest coast of Mexico has greater tropical cyclone activity per unit area than any other region of the world. This is in part due to high sea surface temperatures (Elsberry et al., 1987). From May to November frequent periods of intense convection occur over the ocean near Puerto Escondido. The disturbances take the form of storms associated with the ITCZ, mesoscale convective complexes and tropical depressions, storms, and hurricanes (Zehnder et al., 1999). Water vapor samples were collected at Puerto Escondido around 0300, 1200 and 1900 UTC every day from 10 to 31 July 1998. They were analyzed for their hydrogen and oxygen isotope ratios. Over that time period there were large variations in isotope values. Trajectory analysis demonstrated that the large variations were directly related to the intensity of precipitation over the previous 48 hours upwind of the collection site. In addition a distinct contrast in the average isotope values existed before and after July 18. The isotope values were on average markedly lower during the first period. This shift was directly related to the shift in circulation regimes. In the first period air in Puerto Escondido originated to the south and west over the Pacific Ocean. In the second period air generally came from the east across the Mexican Isthmus or Guatemala from the Caribbean Sea (Lawrence et al, 2004). The above trajectory analysis has been extended to include June to September months for the years 1995 to 2006. Trajectories were calculated at 6-hourly hourly intervals. During these four-month periods the wind direction at Puerto Escondido for the most part shifted from the southwest quadrant to the northeast quadrant, lasting from a few days to about two months. It is hypothesized therefore that the isotope value of water vapor in Puerto Escondido undergoes a distinct shift in response to the shift in wind direction as described above. Also it is hypothesized that the magnitude of this shift will be related to the extent of exposure of the air parcel to rain. The stable isotopic composition of leaf water in trees in the tropics in theory is primarily controlled by the isotopic composition of water vapor in the atmosphere, and this isotopic signal in the leaf water is transferred to the tree cellulose as the tree grows. Therefore, the potential exists to document past circulation regimes along the southwest coast of Mexico by the isotopic analysis of tree cellulose. The long-range objective of such isotopic analysis of trees would be to correlate the mesoscale circulation changes recorded in trees of the southwest coast of Mexico to synoptic scale circulation regimes thereby allowing for a better understanding of climate variability and trends.
GC51A-0163
The observed widening of the tropics during the late 20th century and its mechanisms
Several lines of research indicate that the Tropics have been widening by a few degree latitudes over the past decades and that this widening may continue into the future. Associated with this widening are important shifts in precipitation and other climate elements, raising the question of what are the exact reasons behind the expansion. There is evidence that the increase in tropospheric static stability associated with global warming is driving this shift, but there may be other still unknown factors. Here, we investigate the relative roles of various stratospheric and tropospheric processes for the tropical expansion and conduct attribution experiments with stratosphere and non-stratosphere resolving versions of the latest GFDL general circulation model. The model is forced with different combinations of sea surface temperatures, carbon dioxide, halogen, and ozone concentrations. The impact of each of these factors and of stratospheric model resolution is isolated by comparing different pairs of simulations that differ by only one of these factors. The results highlight that both, tropospheric and stratospheric climate change, play a prominent role for the recent observed widening of the Tropics. We discuss the significance of these results for the future evolution of the tropical width and its realistic prediction.
GC51A-0164
The evolving influence of local and remote sea-surface temperature variability on the Asian monsoon during the last century
During the last century, the warm pools of the tropical oceans have increased substantially in association with global warming. Waters warmer than 28C have increased by almost 70% globally and large changes have occurred in the Indian Ocean. We consider whether these changes to the warm pool are associated with secular changes in precipitation over the monsoon regions. The Simplified Parameterization Primitive Equation Dynamics (SPEEDY) model is used to explore how these changes in sea surface temperature (SST) influence the Asian monsoon dynamics. To gain a better understanding of the interaction between SST and monsoon dynamics, five experiments are designed using different sets of prescribed sea surface temperature. An analysis of the runs reveals that warmer SST in the Indian Ocean and Pacific Ocean basins produce a decrease in precipitation in the eastern Indian Ocean and south eastern China. We also explore how anomalous SST affects the dynamics indexes of the monsoon. Results indicate that the zonal monsoon index (a measure of monsoon intensity) decreases under the warmer SST scenario, implying a weaker monsoon circulation. Additionally, surface winds weaken in both the summer and winter monsoon. The Asian monsoon is found not only to be influenced by local variations of SST but by changes in Pacific Ocean SST as well. Regions most affected by Pacific Ocean SST are the Bay of Bengal, the southern Indian Peninsula, which exhibit stronger monsoon characteristics for cold Pacific Ocean SST anomalies, and the Western Indian Ocean, which shows an increase in the monsoonal circulation and precipitation for warmer Pacific Ocean SST conditions.
GC51A-0165
Role of Wind-Evaporation-Sea Surface Temperature (WES) Feedback in Internal and Forced Tropical Variability
Tropical climate variability is known to be governed by atmosphere-ocean coupled interactions. The Wind- Evaporation-Sea surface temperature (WES) feedback is one such mechanism that has been hypothesized to play a significant role in the deep tropics. The WES feedback is conjectured to amplify the inter-hemispheric gradient mode variability in the deep tropics. While the WES feedback amplifies the internal atmospheric variability, such as that associated with the North Atlantic Oscillation (NAO), it also potentially enhances the response to remote forcings. Observational studies backed by modeling studies reveal a strong influence of El- Nino Southern Oscillation (ENSO) on tropical Atlantic variability. The response of the tropical Atlantic to ENSO has been hypothesized to be enhanced by local air-sea interactions, possibly via the WES feedback and air-sea temperature and humidity differences. It is challenging to identify individual impacts of the role of individual impacts of these phenomena in observations or even in coupled models, as their effects are super-imposed on each other. Presented here is a report on experiments using a modified version of an atmospheric GCM (AGCM), where the WES feedback is deliberately suppressed in the bulk aerodynamic formulation for surface fluxes. A comparison of coupled integrations using the modified AGCM to those carried out using the control AGCM could isolate the role of the WES feedback. Preliminary analysis suggests that WES feedback intensifies the natural inter-hemispheric gradient mode in the tropics. Experiments with forced ENSO cycle also suggest that WES feedback amplifies the surface heat flux anomalies related to ENSO in the deep tropical Atlantic, probably through the influence of local winds. It is also found that WES feedback reinforces the tendency of the ITCZ to stay north of the equator over the Atlantic during El-Nino events.
GC51A-0166
On the predictability and dynamics of westerly wind bursts in the equatorial Pacific
Present-day ENSO's variability is strongly affected by westerly wind bursts (WWBs) in the equatorial Pacific. Because of their short time scale, the seemingly stochastic WWBs were thought to be purely external to the equatorial Pacific system. However, more recent analysis shows that these events are, in fact, strongly modulated by the large scale SST, and ENSO in particular. Thus these events seem a part of the ENSO cycle rather than external forcing and it is important to understand their relation to ENSO in order to be able to understand both future and past ENSO's behavior and its dependence on the general climate state. Although the WWB wind stress at a given location is not well correlated linearly with the SST, the characteristics of WWBs, such as duration, wind strength, location, fetch, and probability of occurrence, are well described as a linear function of SST. A prognostic WWB model is developed using a multiple linear regression to a record of satellite-derived winds. Given the observed large-scale tropical Pacific SST, the model reproduces the majority of observed variance in seven designated WWB characteristics and it shows the potential to be used for prediction purposes. In order to further understand the dynamics of these WWBs, we use the analysis of wind and OLR observations as well as linear and nonlinear shallow water atmospheric model to investigate the link between tropical atmospheric convection and westerly wind bursts. It is found that convection may force the observed westerly wind burst signal, but that the momentum dynamics of these wind bursts may involve more than the linear Gill- type atmospheric model equations.
GC51A-0167
Radiocarbon Variability in the Tropical Pacific During the Last Millennium
The strength of upwelling in the tropical Pacific strongly influences global climate, as demonstrated during El Niño Southern Oscillation (ENSO) extremes. Understanding the causes of past variability in tropical Pacific upwelling will help to predict how future climate may evolve under radiative forcing caused by the increase in greenhouse gases. We measured radiocarbon (14C) records in corals collected from Palmyra (6°°N, 162°°W) and Christmas (2°°N, 157°°W) Islands, located in the central tropical Pacific, to reconstruct high-resolution variations in tropical Pacific upwelling over the last millennium. Corals incorporate 14C/12C ratios equal to those in the dissolved inorganic carbon of surrounding seawater, and thus provide monthly-resolved records of surface ocean radiocarbon concentrations back through time. Surface ocean waters are more enriched with radiocarbon than deep waters, owing to the atmospheric source for radiocarbon. Upwelled waters are depleted in radiocarbon because significant radioactive decay has occurred during their isolation in the deeper ocean. Previous work with central tropical Pacific corals has determined that upwelled waters are approximately -55‰ to -65‰ in this region, while currents flowing from the west Pacific to the east Pacific carry waters with a higher 14C signature, roughly -38‰, and waters from the east have a value of -72‰ (Konishi et. al, 1981, Druffel, 1981). Therefore, variations in 14C in the Palmyra and Christmas corals reflect the mixing of the cool, radiocarbon-depleted, waters associated with equatorial upwelling and the warm, radiocarbon-enriched waters advected from the western tropical Pacific. Oxygen isotopic (δ18O) analyses of the Palmyra and Christmas fossil corals reveal a rich climatic history of interannual to centennial variability (Cobb et al., 2003). Our approach targets specific time intervals associated with strong interannual to centennial δ18O anomalies for high-resolution radiocarbon analysis. We compare seasonally-resolved radiocarbon measurements from the Palmyra and Christmas modern corals to their fossil counterparts. The 1941 El Niño event is not associated with a significant radiocarbon departure, despite significant changes in upwelling and horizontal advection inferred from sea-surface temperature records and coral δ18O data. Indeed, high-resolution radiocarbon sampling of the 10th, 15th, and 17th century fossil corals reveals no significant radiocarbon variations associated with ENSO activity. Decadal to centennial-scale variations in radiocarbon variability are resolved by annually-averaged samples during the 10%th, 15th and 17th centuries. These analyses indicate that during active ENSO periods, the corals are more enriched in 14C than during suppressed ENSO activity. Coral radiocarbon from the 17th century, when ENSO was very prominent, averaged -51‰, compared to -57‰ and -60‰ during the 15th and 10th centuries, respectively. Discussion of the mechanisms that would allow low variability in 14C, and large changes in δ18O, including the variations in response times of the corals to incorporating temperature differences versus radiocarbon will be presented.
GC51A-0168
Corals at the Flower Garden Banks: Monitors of Environmental Change and North American Climate Variability
The Pacific/North American (PNA) pattern is a dominant atmospheric pattern of climate variability in the extratropical Northern Hemisphere and strongly influences the winter climate of the southeast United States. The instrumental record used to characterize the PNA pattern does not exist prior to the mid-1940s. However, information about past variability in the PNA pattern is preserved in the skeletons of long-lived corals at the reefs of the Flower Garden Banks National Marine Sanctuary (NMS). The Flower Garden Banks NMS is located approximately 180 km south of the Texas/Louisiana border in the Gulf of Mexico and is the northernmost hermatypic reef on the United States continental shelf. It has previously been shown that linear coral extension rates at the Flower Garden Banks are highly correlated with winter air and sea surface temperatures. In addition, average winter temperatures in the southeastern United States are negatively correlated with the phase of the PNA pattern. During a positive phase of the PNA pattern, the southeast US experiences stronger and more frequent winter storms while a negative phase of the PNA pattern brings milder winters to the region. Thus, past coral extension rates at the Flower Garden Banks provide a means to reconstruct the history of temporal variations in the PNA pattern. We collected several long cores of skeletal material from long-lived Montastrea faveolata and Siderastrea siderea coral heads from the Flower Garden Banks NMS. Annual extension rates have been determined based on X- radiographic analysis of high/low density growth bands and are used to characterize interdecadal variability associated with changes in the PNA pattern. In addition, the presence of winter stress bands due to below average water temperatures indicate winters with more severe and/or frequent storms. Analysis of these results will contribute directly to our understanding of the temporal character of interannual and interdecadal variations of North American winter climate.