Kim M. Cobb

El Niño/Southern Oscillation and tropical Pacific climate during the last millennium

Any assessment of future climate change requires knowledge of the full range of natural variability in the El Niño/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30–150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.

North Pacific Gyre Oscillation links ocean climate and ecosystem change

Decadal fluctuations in salinity, nutrients, chlorophyll, a variety of zooplankton taxa, and fish stocks in the Northeast Pacific are often poorly correlated with the most widely-used index of large-scale climate variability in the region - the Pacific Decadal Oscillation (PDO). We define a new pattern of climate change, the North Pacific Gyre Oscillation (NPGO) and show that its variability is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll. Fluctuations in the NPGO are driven by regional and basin-scale variations in wind-driven upwelling and horizontal advection - the fundamental processes controlling salinity and nutrient concentrations. Nutrient fluctuations drive concomitant changes in phytoplankton concentrations, and may force similar variability in higher trophic levels. The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics. The NPGO pattern extends beyond the North Pacific and is part of a global-scale mode of climate variability that is evident in global sea level trends and sea surface temperature. Therefore the amplification of the NPGO variance found in observations and in global warming simulations implies that the NPGO may play an increasingly important role in forcing global-scale decadal changes in marine ecosystems.

Millennial-scale trends in west Pacific warm pool hydrology since the Last Glacial Maximum

Models and palaeoclimate data suggest that the tropical Pacific climate system plays a key part in the mechanisms underlying orbital-scale and abrupt climate change. Atmospheric convection over the western tropical Pacific is a major source of heat and moisture to extratropical regions, and may therefore influence the global climate response to a variety of forcing factors. The response of tropical Pacific convection to changes in global climate boundary conditions, abrupt climate changes and radiative forcing remains uncertain, however. Here we present three absolutely dated oxygen isotope records from stalagmites in northern Borneo that reflect changes in west Pacific warm pool hydrology over the past 27,000 years. Our results suggest that convection over the western tropical Pacific weakened 18,000–20,000 years ago, as tropical Pacific and Antarctic temperatures began to rise during the early stages of deglaciation. Convective activity, as inferred from oxygen isotopes, reached a minimum during Heinrich event 1 (ref. 10), when the Atlantic meridional overturning circulation was weak, pointing to feedbacks between the strength of the overturning circulation and tropical Pacific hydrology. There is no evidence of the Younger Dryas event in the stalagmite records, however, suggesting that different mechanisms operated during these two abrupt deglacial climate events. During the Holocene epoch, convective activity appears to track changes in spring and autumn insolation, highlighting the sensitivity of tropical Pacific convection to external radiative forcing. Together, these findings demonstrate that the tropical Pacific hydrological cycle is sensitive to high-latitude climate processes in both hemispheres, as well as to external radiative forcing, and that it may have a central role in abrupt climate change events.

Highly Variable El Niño–Southern Oscillation Throughout the Holocene

ENSO Variability The El Niño–Southern Oscillation (ENSO) is the most energetic, quasiperiodic climate oscillation in the world—every few years warming large expanses of the surface equatorial Pacific Ocean surface and impacting temperatures and rainfall patterns across the globe. A pressing question, in the context of global warming, is whether ENSO might be affected by the rising atmospheric temperatures caused by anthropogenic greenhouse gas emissions. Climate models do not agree on the answer to this question, but one place to look for data about how global temperatures might influence ENSO is the record of past ENSO variability. Cobb et al. (p. 67) present a record of ENSO variability spanning the past 7000 years, in an attempt better to define its response to insolation forcing over this same period. The findings reveal high variability in ENSO behavior that has no clear dependence on insolation, which implies that a link to warming, if it exists, may be difficult to detect. Coral records show that the El Niño–Southern Oscillation may be less sensitive to past climate forcing than previously thought. The El Niño–Southern Oscillation (ENSO) drives large changes in global climate patterns from year to year, yet its sensitivity to continued anthropogenic greenhouse forcing is uncertain. We analyzed fossil coral reconstructions of ENSO spanning the past 7000 years from the Northern Line Islands, located in the center of action for ENSO. The corals document highly variable ENSO activity, with no evidence for a systematic trend in ENSO variance, which is contrary to some models that exhibit a response to insolation forcing over this same period. Twentieth-century ENSO variance is significantly higher than average fossil coral ENSO variance but is not unprecedented. Our results suggest that forced changes in ENSO, whether natural or anthropogenic, may be difficult to detect against a background of large internal variability.

The Pacific Decadal Oscillation, Revisited

AbstractThe Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of ...

Origin of marine barite deposits: Sr and S isotope characterization

Barite can precipitate in microenvironments in the water column (marine barite), from supersaturated pore fluids at the oxic-anoxic boundary within marine sediments and where Ba-rich pore fluids are expelled and come into contact with sulfate-rich seawater (diagenetic barite), or from hydrothermal solutions (hydrothermal barite). Barite is relatively resistant to alteration after burial and has been used in paleoceanographic studies to reconstruct seawater chemistry and productivity through time. For such applications it is very important to determine the origin of the barite used, because both diagenetic and hydrothermal barite deposits may not accurately record the open-ocean contemporaneous seawater chemistry and productivity. We show here that it is possible to distinguish between the different types of barite by using Sr and S isotopes along with crystal morphology and size characteristics.

A central tropical Pacific coral demonstrates Pacific, Indian, and Atlantic decadal climate connections

While instrumental and proxy-based climate records describe significant decadal-scale climate variabil- ity throughout the tropical Pacific, Indian, and Atlantic Oceans, the processes responsible for these variations and their interactionsare not readily apparent from the obs er- vations. A new 112-yr coral-based sea surface temperature (SST) reconstruction from Palmyra Island in the central tropical Pacific (CTP) exhibitss trong decadal variability with an amplitude of roughly 0.3 ◦ C. A 12-13yr-period sig- nal in thiscoral record ishighly coherent with long equa- torial Atlantic and Indian Ocean climate records, implying a unified phenomenon. The Atlantic pattern suggests that it may fall under direct influence of anomalousSST in the CTP, as it does over interannual timescales, while the In- dian Ocean pattern exhibits maximum response during the switch between warm/cold states in the tropical Pacific. The results demonstrate that the CTP has played a significant role in determining the expression of global decadal climate variability over the twentieth century.

Monsoon–tropical ocean interaction in a network of coral records spanning the 20th century

Abstract The 20th century evolution of basin-wide gradients in surface ocean properties provides one essential test for recent models of the interaction between the Asian monsoon and the tropical ocean, because various feedback mechanisms should result in characteristic regional patterns of variability. Although the instrumental record of climate variability in the tropics is essentially limited to the last few decades, the stable isotopic composition of living corals provides an effective means for extending the instrumental observations. Here we present two coral isotopic records from the Indonesian Maritime Continent, and we use these records with other previously published records to describe: (i) the relationship between western Pacific and central Pacific climate variability over the past century, with special emphasis on the biennial band; and (ii) the strength of the west–east ‘Indian Ocean Dipole’. We find that the amplitude of the biennial cycle in the Pacific did not vary inversely with the strength of ENSO (El Nino Southern Oscillation), as might be expected from some models of monsoonal feedback on the central Pacific. Instead, the biennial variability was modulated on decadal timescales throughout much of the Pacific. We also show that the zonal oxygen isotopic gradient in the Indian Ocean coral records was significantly correlated with central Pacific sea surface temperature on a variety of timescales. Thus, it is likely that this ‘coral dipole’ was a product of strong ENSO-like teleconnections over the Indian Ocean, as opposed to being the result of unique Indian Ocean or monsoonal dynamics.

U/Th-dating living and young fossil corals from the central tropical Pacific

This study evaluates the accuracy of U/Th dates for young (6 a few thousand years old) reef corals, both living and fossil, and explores strategies for refining those dates. The high precision of the U/Th method ( 0 1^2%) for dating young corals is well-established. Earlier studies have demonstrated the method’s accuracy for select samples of known age. However, these studies have focused on typical samples that have extremely low 232 Th concentrations (tens of pg/ g). Here we study the dating systematics of young corals that have low but significant amounts of 232 Th (up to 1000 pg/g), indicating the presence of small fractions of non-radiogenic 230 Th (i.e. 230 Th not generated by in situ U decay). We report U/Th ages for living and subaerially exposed fossil corals from Palmyra Island, located in the central tropical Pacific, that range from 50 to 700 yr old. The Palmyra corals contain varying amounts of 232 Th and small fractions of associated non-radiogenic 230 Th. Uncertainty associated with the correction for non-radiogenic 230 Th can lead to significant errors in U/Th dates. We have characterized non-radiogenic 230 Th/ 232 Th values, ( 230 Th/ 232 Th)nr ,a s a means of minimizing this source of error. We calculate ( 230 Th/ 232 Th)nr values ranging from 0 to 2U10 35 for the Palmyra living corals by comparing measured U/Th dates to absolute dates for the living coral, whose chronology is well-established. For the fossil corals, we employ three different approaches to arrive at ( 230 Th/ 232 Th)nr estimates. First, we compare measured U/Th dates to absolute dates in samples from a young fossil coral that overlaps the living coral. Next, we use the firm relative dating constraints imposed by five overlapping fossil corals from the 14th^15th centuries to calculate ( 230 Th/ 232 Th)nr values. Finally, we attempt to anchor the 14th^15th century floating coral chronology to an absolute chronology by correlating the climate signals in the coral records to those in absolutely dated climate proxy records. All lines of evidence point to a range of ( 230 Th/ 232 Th)nr for fossil corals that overlaps the range determined for the living coral, suggesting that most of the thorium is primary or is added while the coral is still alive. Our work also demonstrates the utility of multiple ( 230 Th/ 232 Th)nr estimates. Most importantly, we demonstrate a method by which accurate ( 0 5 yr) U/Th-based chronologies can be obtained for young fossil corals with significant 232 Th concentrations.

Decadal-Scale SST and Salinity Variations in the Central Tropical Pacific: Signatures of Natural and Anthropogenic Climate Change

Accurate projections of future temperature and precipitation patterns in many regions of the world depend on quantifying anthropogenic signatures in tropical Pacific climate against its rich background of natural variability. However, the detection of anthropogenic signatures in the region is hampered by the lack of continuous, century-long instrumental climate records. This study presents coral-based sea surface temperature (SST) and salinity proxy records from Palmyra Island in the central tropical Pacific over the twentieth century, based on coral strontium/calcium and the oxygen isotopic composition of seawater (d 18 OSW), respectively. On interannual time scales, the Sr/Ca-based SST record captures both eastern and central Pacific