Eastern Equatorial Pacific Ocean Research Articles

Eastern equatorial Pacific benthic foraminiferal distribution and deep water temperature changes during the early to middle Miocene

Abstract We investigate changes in benthic foraminiferal assemblages and estimate bottom water temperatures (BWT), based on Mg/Ca ratios of Oridorsalis umbonatus , over three selected intervals of major climate change during the late early to middle Miocene at Integrated Ocean Drilling Program (IODP) Sites U1337 (4463 m water depth) and U1338 (4200 m water depth), eastern equatorial Pacific Ocean. The targeted intervals are: (1) the onset of the Miocene Climatic Optimum (MCO; at ~ 16.9 Ma) and associated carbon cycle perturbation (Site U1337); (2) the episode of peak warmth within the MCO centered at 15.6 Ma (Site U1338); (3) the major cooling step at ~ 13.8 Ma related to east Antarctic ice-sheet expansion (Site U1338). Assemblages from these three intervals are mainly composed of cosmopolitan, long ranging (e.g., Paleogene to recent), lower bathyal to abyssal species, including calcareous elongated forms that became extinct during the middle Pleistocene. The onset of the MCO and interval of peak warmth had little impact on benthic foraminiferal accumulation rates (BFAR), abundances and assemblage composition. A transient BWT warming of 2.6 °C at the onset of the interval of peak warmth concurred with a benthic stable oxygen isotope (δ 18 O) decrease of 1‰, suggesting that the δ 18 O decrease was largely controlled by BWT changes. A substantial increase in BFAR and in the abundances of Epistominella exigua and epifaunal taxa after ~ 13.83 Ma coincides with increased opal accumulation rates, reflecting enhanced and seasonally pulsed organic carbon (C org ) flux to the seafloor during the global cooling step. During this interval, BWT decrease by 1.7 °C concurrent with a 1.0‰ increase in the stable oxygen isotope composition of seawater (δ 18 O sw ). Comparable trends at Ocean Drilling Program (ODP) Site 1146 in the South China Sea, with a 2.2 °C cooling and 0.9‰ increase in δ 18 O sw at ~ 13.8 Ma, further support that the benthic δ 18 O increase during the major cooling step was largely controlled by ice volume changes.

The extinction of Chiloguembelina cubensis in the Pacific Ocean: implications for defining the base of the Chattian (upper Oligocene)

We conducted quantitative biostratigraphic and morphometric studies at two ocean drilling sites in the Pacific Ocean to investigate the extinction of the planktonic foraminifera genus Chiloguembelina, a distinctive biserial genus ranging in the Eocene and Oligocene. The extinction of the last species of the genus, C. cubensis, has historically been used as a correlation event for the base of the Chattian (upper Oligocene; ∼ 28 Ma). However, the bioevent has been controversial, as some authors have found a well-defined bio - horizon, while others have found a decline in abundance with a continuation of C. cubensis into the upper Oligocene. This has been reflected in the preference of using the Top Common occurrence (>5% of the foraminiferal assemblage) of the species as a correlation event for the boundary. We assessed the utility of C. cubensis as a reliable biohorizon at Ocean Drilling Program Leg 202 Site 1237 (south-east Pacific Ocean) and Integrated Ocean Drilling Program Expedition 320/321 Site U1334 (eastern equatorial Pacific Ocean). At both sites C. cubensis is a common and consistent component of early Oligocene planktonic foraminiferal assemblages. An abrupt termination of C. cubensis occurs at Site 1237 in magnetic polarity Subchron C10n.1n (27.89 Ma), while at Site U1334 peak abundances of C. cubensis are recorded in the upper Oligocene within Chron 9n with a younger datum for the extinction at ∼ 27.15 Ma. Therefore, the biostratigraphic utility of the species as a boundary marker for the base of the Chattian remains unclear. We propose that the phrase “C. cubensis crisis” is the most suitable for describing the extinction or decline in abundance of the species in the mid Oligocene, based upon the findings from these two sites and other age calibrated sites. Morphometric analysis of specimen size at the two sites shows a markedly smaller test size at Site 1237 compared with Site U1334, with specimens from the former being on average 1.37 × shorter and 1.45 × thinner. Differences in test size and extinction horizon are possibly controlled by ecological preferences, however, our biochronological compilation of ocean drilling sites does not reveal a latitudinal trend in extinction level.

Remanence acquisition efficiency in biogenic and detrital magnetite and recording of geomagnetic paleointensity

AbstractRelative paleointensity (RPI) variations of Earth's magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences. RPI estimation is based on a series of assumptions. One key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner. In this paper, we test this assumption for sediments from the eastern equatorial Pacific Ocean that record well‐documented global RPI variations over the last ∼780 kyr. The magnetization is carried by two stable single domain magnetic components, which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles. By analyzing signals carried by the two components separately, we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals. The magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions. Variations in the concentration of both components are modulated climatically so that they record nearly identical RPI signals. In many sediment types, there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate RPI recording. Our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary RPI analysis.

Mechanism of seasonal eddy kinetic energy variability in the eastern equatorial Pacific Ocean

AbstractEnhanced mesoscale eddy activities or tropical instability waves (TIWs) exist along the northern front of the cold tongue in the eastern equatorial Pacific Ocean. In this study, we investigate seasonal variability of eddy kinetic energy (EKE) over this region and its associated dynamic mechanism using a global, eddy‐resolving ocean general circulation model (OGCM) simulation, the equatorial mooring data, and satellite altimeter observations. The seasonal‐varying enhanced EKE signals are found to expand westward from 100°W in June to 180°W in December between 0°N and 6°N. This westward expansion in EKE is closely connected to the barotropically‐baroclinically unstable zonal flows that are in thermal‐wind balance with the seasonal‐varying thermocline trough along 4°N. By adopting an 1½‐layer reduced‐gravity model, we confirm that the seasonal perturbation of the thermocline trough is dominated by the anticyclonic wind stress curl forcing, which develops due to southerly winds along 4°N from June to December.

Interdecadal Variability of the ENSO–North Pacific Atmospheric Circulation in Winter

ABSTRACTThe interdecadal change in the relationship between the El Niño–Southern Oscillation (ENSO) and atmospheric circulation over the North Pacific is investigated using both observational data and an atmospheric general circulation model. There are two prominent modes of winter mid-latitude atmospheric variability in the North Pacific: the West Pacific (WP) teleconnection and the Aleutian Low (AL). The relationship between ENSO and the WP-AL patterns changed notably around the late 1970s. From 1957 to 1975, during the mature phase of ENSO, significant sea surface temperature anomalies (SSTAs) occurred, mainly in the equatorial eastern Pacific Ocean; the associated atmospheric circulation anomaly pattern resembles the negative phase of a WP teleconnection pattern. In contrast, for the 1978–2011 period, significant negative SSTAs were observed in the western and extratropical Pacific in both hemispheres, with some significant positive SSTAs appearing over the eastern Pacific. This is in agreement with the defined regions of a mega-ENSO, the associated atmospheric circulation anomaly pattern resembles the AL mode. Further analysis suggests that a negative–positive anomaly pattern in the 500 hPa geopotential height throughout the entire North Pacific, possibly enhanced by the SSTAs in the extratropical North Pacific associated with the mature phase of ENSO, is responsible for modulating the relationship between ENSO and the North Pacific atmospheric circulation.

Understanding the Variations of the Length and the Seasonal Rainfall Anomalies of the Indian Summer Monsoon

AbstractThe canonical relationship between the length and the total seasonal rainfall anomalies of the Indian summer monsoon (ISM) is the association of the longer (shorter) season with wetter (drier) seasonal rainfall anomalies. This study shows that such canonical behavior is clearly associated with relatively strong ENSO SST anomalies in the eastern equatorial Pacific Ocean that appear in the boreal summer and fall seasons. The noncanonical relationship is caused by a longer (shorter) season associated with drier (wetter) ISM seasonal rainfall anomalies. A majority of these noncanonical seasons, with anomalously short season length but anomalously high seasonal mean rain, tend to occur under relatively weak La Niña forcing during the boreal summer season. Although the onset of such seasons occurs through canonical ENSO forcing of a large-scale meridional temperature gradient, the demise is dictated by the depletion of moist static energy from the underlying cooling of the upper ocean in the northern Indian Ocean. This is due to stronger meridional Ekman ocean heat transport forced by the stronger low-level atmospheric southwesterlies than those in the corresponding canonical wet ISM season.

Glacial–interglacial changes in equatorial Pacific surface-water structure during the Plio–Pleistocene intensification of Northern Hemisphere Glaciation

The late Pliocene/early Pleistocene intensification of Northern Hemisphere Glaciation (iNHG) represents the most recent major tipping point in Earth's climate history. In the equatorial Pacific, the iNHG has been suggested to mark the transition from a warm Pliocene climate state, in which the Pacific warm pool was greatly expanded relative to today giving rise to a surface ocean temperature field akin to modern El Niño conditions, to a climate state with strong upwelling in the east of the basin and stronger zonal temperature gradients. Here we examine this transition by presenting new high-resolution (∼800 yr) planktic foraminiferal Mg/Ca- and δ18O-based sea-surface temperature (SST) and salinity (SSS; based on δ18O residuals [δ18Oivc-sw]) records from Eastern Equatorial Pacific (EEP) Ocean Drilling Program Site 849 spanning ∼2.75–2.4 Ma. Our study interval contains the first large-amplitude (∼1‰ in benthic δ18O) early Pleistocene glacial–interglacial cycles (Marine Isotope Stages 100–96) representing the culmination of iNHG. In this study we investigate west-to-east equatorial Pacific SST and SSS gradients. We find that the zonal SST gradient increased by ∼1.5 °C from ∼2.75 to ∼2.4 Ma through EEP cooling. No glacial–interglacial pattern is seen in this gradient. On the other hand, we document obliquity-paced oscillations in the δ18Oivc-sw (SSS) gradient. We hypothesize that these oscillations originated from a response to a glacial southward and interglacial northward migration of the Intertropical Convergence Zone over the EEP.

Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D

The formation of new ocean crust at mid-ocean ridges is a fundamental component of the plate tectonic cycle and involves substantial transfer of heat and mass from the mantle. Hydrothermal circulation at mid-ocean ridges is critical for the advection of latent and sensible heat from the lower crust to enable the solidification of ocean crust near to the ridge axis. The sheeted dike complex (SDC) is the critical region between the eruptive lavas and the gabbros through which seawater-derived recharge fluids must transit to exchange heat with the magma chambers that form the lower ocean crust.ODP Hole 1256D in the eastern equatorial Pacific Ocean provides the only continuous sampling of in-situ intact upper ocean crust formed at a fast spreading rate, through the SDC into the dike–gabbro transition zone. Here we exploit a high sample density profile of the Sr-isotopic composition of Hole 1256D to quantify the time-integrated hydrothermal recharge fluid flux through the SDC. Assuming kinetically limited fluid–rock Sr exchange, a fluid flux of 1.5–3.2×106 kgm−2 is required to produce the observed Sr-isotopic shifts. Despite significant differences in the distribution and intensity of hydrothermal alteration and fluid/rock Sr-isotopic exchange between Hole 1256D and SDC sampled in other oceanic environments (ODP Hole 504B, Hess Deep and Pito Deep), the estimated recharge fluid fluxes at all sites are similar, suggesting that the heat flux extracted by the upper crustal axial hydrothermal system is relatively uniform at intermediate to fast spreading rates.The hydrothermal heat flux removed by fluid flow through the SDCs, is sufficient to remove only ∼20 to 60% of the available latent and sensible heat from the lower crust. Consequently, there must be additional thermal and chemical fluid–rock exchange deeper in the crust, at least of comparable size to the upper crustal hydrothermal system. Two scenarios are proposed for the potential geometry of this deeper hydrothermal system. The first requires the downward expansion of the upper crustal hydrothermal system ∼800 m into the lower crust in response to a downward migrating conductive boundary layer. The second scenario invokes a separate hydrothermal system in the lower crust for which fluid recharge bypasses reaction with the sheeted dikes, perhaps via flow down faults.

Influence of subtropical dipole pattern in Southern Indian Ocean on ENSO event

Subtropical Dipole Pattern (SDP) in southern Indian Ocean is another significant dipolelike sea surface temperature anomalies (SSTA) phenomena besides tropical Indian Ocean dipole (TIOD). It is very clear in interannual and interdecadal variations. But many recent studies have mainly aimed at the SST of equatorial Indian Ocean and less at southern Indian Ocean, especially less at the relationship between SSTA in southern Indian Ocean and El NinoSouthern Oscillation (ENSO). Present works investigated primarily the correlation between the SDP and ENSO. It is found that SDP event act as a crucial linkage between positive and negative ENSO phases. The SSTAs in equatorial eastern Pacific Ocean are completely reversed before and after SDP event. Further, we investigated the possible impact of SDP on ENSO event in interannual scale by researching the anomalous characteristics of sea temperature, wind, convergence and divergence, total cloud amount, radiation and so on. The results showed that the influence of SDP on ENSO involve not only the process of positive and negative air-sea feedback, but also the interaction between atmospheric circulation over tropical and subtropical, specially the eastward propagation of anomalous zonal wind from the equatorial Indian Ocean to the Pacific Ocean. Otherwise, it also relates, to some extent, to the transition of large-scale climatic mean flow over the Indian Ocean and Pacific Ocean.

Biostratigraphy and evolution of MioceneDiscoasterspp. from IODP Site U1338 in the equatorial Pacific Ocean

Assemblages of upper lower through upper Miocene Discoaster spp. have been quantified from Integrated Ocean Drilling Program (IODP) Site U1338 in the eastern equatorial Pacific Ocean. These assemblages can be grouped into five broad morphological categories: six-rayed with bifurcated ray tips, six-rayed with large central areas, six-rayed with pointed ray tips, five-rayed with bifurcated ray tips and five-rayed with pointed ray tips. Discoaster deflandrei dominates the assemblages prior to 15.8 Ma. The decline in abundance of D. deflandrei close to the early–middle Miocene boundary occurs together with the evolution of the D. variabilis group, including D. signus and D. exilis . Six-rayed discoasters having large central areas become a prominent member of the assemblages for a 400 ka interval in the late middle Miocene. Five- and six-rayed forms having pointed tips become prominent in the early late Miocene and show a strong antiphasing relationship with the D. variabilis group. Discoaster bellus completely dominates the Discoaster assemblages for a 400 ka interval in the middle late Miocene. Abundances of all discoasters, or discoasters at the species level, show only (surprisingly) weak correlations to carbonate contents or oxygen and carbon isotopes of bulk sediment when calculated over the entire sample interval.

Upper Irtysh River flow since AD 1500 as reconstructed by tree rings, reveals the hydroclimatic signal of inner Asia

In a warming world, water scarcity is one of the main concerns for sustainable development and human well-being in inner Asia. Due to the lack of instrumental streamflow records, the natural variability of the water supply from inner Asian rivers is not well understood from a long-term perspective. Here, we have reconstructed the streamflow of Upper Irtysh River from AD 1500 to 2010, based on the tree-ring width indices of spruce (Picea obovata) and larch (Larix sibirica) from the Altay Mountains. The reconstruction explains 48.4 % of the recorded streamflow variance over the common period 1958–2008. This streamflow reconstruction is representative of regional moisture conditions over the Irtysh River basin area. Some significant spectral peaks are identified, and suggest the influence of natural forcing on the streamflow of the Upper Irtysh River, such as ENSO and solar activity. The linkages of our reconstruction with sea surface temperature in the northern Indian Ocean, eastern equatorial Pacific Ocean, and equatorial Atlantic Ocean suggest the connection of regional streamflow variations to large-scale atmospheric circulation. We also find that there is the relationship between regional drought/streamflow variations in inner Asia and the interaction of the mid-latitude Westerlies and Asian summer monsoon. Our 511-year streamflow reconstruction provides a long-term perspective on current and twentieth century wet and dry events in the Irtysh River basin, is useful to guide predictions of future variability, and aids future water resource management.

Eccentricity pacing of eastern equatorial Pacific carbonate dissolution cycles during the Miocene Climatic Optimum

AbstractThe Miocene Climatic Optimum (MCO; ~16.9 to 14.7 Ma) provides an outstanding opportunity to investigate climate‐carbon cycle dynamics during a geologically recent interval of global warmth. We present benthic stable oxygen (δ18O) and carbon (δ13C) isotope records (5–12 kyr time resolution) spanning the late early to middle Miocene interval (18 to 13 Ma) at Integrated Ocean Drilling Program (IODP) Site U1335 (eastern equatorial Pacific Ocean). The U1335 stable isotope series track the onset and development of the MCO as well as the transitional climatic phase culminating with global cooling and expansion of the East Antarctic Ice Sheet at ~13.8 Ma. We integrate these new data with published stable isotope, geomagnetic polarity, and X‐ray fluorescence (XRF) scanner‐derived carbonate records from IODP Sites U1335, U1336, U1337, and U1338 on a consistent, astronomically tuned timescale. Benthic isotope and XRF scanner‐derived CaCO3 records depict prominent 100 kyr variability with 400 kyr cyclicity additionally imprinted on δ13C and CaCO3 records, pointing to a tight coupling between the marine carbon cycle and climate variations. Our intersite comparison further indicates that the lysocline behaved in highly dynamic manner throughout the MCO, with >75% carbonate loss occurring at paleodepths ranging from ~3.4 to ~4 km in the eastern equatorial Pacific Ocean. Carbonate dissolution maxima coincide with warm phases (δ18O minima) and δ13C decreases, implying that climate‐carbon cycle feedbacks fundamentally differed from the late Pleistocene glacial‐interglacial pattern, where dissolution maxima correspond to δ13C maxima and δ18O minima. Carbonate dissolution cycles during the MCO were, thus, more similar to Paleogene hyperthermal patterns.

Abundant climatic information in water stable isotope record from a maritime glacier on southeastern Tibetan Plateau

Climatic significance of ice core stable isotope record in the Himalayas and southern Tibetan Plateau (TP), where the climate is alternately influenced by Indian summer monsoon and mid-latitude westerlies, is still debated. A newly drilled Zuoqiupu ice core from a temperate maritime glacier on the southeastern TP covering 1942–2011 is investigated in terms of the relationships between δ18O and climate parameters. Distinct seasonal variation of δ18O is observed due to high precipitation amount in this area. Thus the monsoon (June to September) and non-monsoon (October to May) δ18O records are reconstructed, respectively. The temperature effect is identified in the annual δ18O record, which is predominantly contributed by temperature control on the non-monsoon precipitation δ18O record. Conversely, the negative correlation between annual δ18O record and precipitation amount over part of Northeast India is mostly contributed by the monsoon precipitation δ18O record. The variation of monsoon δ18O record is greatly impacted by the Indian summer monsoon strength, while that of non-monsoon δ18O record is potentially associated with the mid-latitude westerly activity. The relationship between Zuoqiupu δ18O record and Sea Surface Temperature (SST) is found to be inconsistent before and after the climate shift of 1976/1977. In summer monsoon season, the role of SST in the monsoon δ18O record is more important in eastern equatorial Pacific Ocean and tropical Indian Ocean before and after the shift, respectively. In non-monsoon season, however, the Atlantic Multidecadal Oscillation has a negative impact before but positive impact after the climate shift on the non-monsoon δ18O record.

Climate variability and predictability associated with the Indo-Pacific Oceanic Channel Dynamics in the CCSM4 Coupled System Model

An experiment using the Community Climate System Model (CCSM4), a participant of the Coupled Model Intercomparison Project phase-5 (CMIP5), is analyzed to assess the skills of this model in simulating and predicting the climate variabilities associated with the oceanic channel dynamics across the Indo-Pacific Oceans. The results of these analyses suggest that the model is able to reproduce the observed lag correlation between the oceanic anomalies in the southeastern tropical Indian Ocean and those in the cold tongue in the eastern equatorial Pacific Ocean at a time lag of 1 year. This success may be largely attributed to the successful simulation of the interannual variations of the Indonesian Throughflow, which carries the anomalies of the Indian Ocean Dipole (IOD) into the western equatorial Pacific Ocean to produce subsurface temperature anomalies, which in turn propagate to the eastern equatorial Pacific to generate ENSO. This connection is termed the “oceanic channel dynamics” and is shown to be consistent with the observational analyses. However, the model simulates a weaker connection between the IOD and the interannual variability of the Indonesian Throughflow transport than found in the observations. In addition, the model overestimates the westerly wind anomalies in the western-central equatorial Pacific in the year following the IOD, which forces unrealistic upwelling Rossby waves in the western equatorial Pacific and downwelling Kelvin waves in the east. This assessment suggests that the CCSM4 coupled climate system has underestimated the oceanic channel dynamics and overestimated the atmospheric bridge processes.

Spring “Predictability Barrier” and Indian Summer Monsoon

Monthly correlation of Indian monsoon rainfall (IMR) index with various monsoon indices changes predominantly and peaks within summer season (Ailikun and Yasunari, 2001). There is transition in this lead and lagged correlation during spring (March, April and May). This spring transition could not be captured in many general circulation models (GCMs) and is termed as “predictability barrier” (Latif and Graham, 1991). It has been less investigated and hence reasons for dampening/reversal of associated large scale fields during spring are intriguing. Present study illustrates dynamics of spring predictability barrier with Indian summer (June, July, August and September) monsoon (ISM). In this study, based on data from 1979 to 2008, an internal dynamics between various monsoon indices and corresponding IMR anomaly is diagnosed. It is found that secular behaviour of El Nino– Southern Oscillation (ENSO) during preceding spring of monsoon months (June, July, August and September) gets reversed in succeeding spring. Though correlation between various monsoon indices and ISM index remains similar during preceding and succeeding springs with peaks during monsoon (June, July, August and September), corresponding large scale fields show that strong significant convection dominates over the southern equatorial eastern Pacific Ocean, northern equatorial western Pacific Ocean, Indonesian throughflow and weaker significant convection dominates over the Head Bay of Arabian Sea and western to central Indian sub-continent during preceding spring. Such patterns reverse during succeeding spring.

Plio‐Pleistocene glacial‐interglacial productivity changes in the eastern equatorial Pacific upwelling system

AbstractThe eastern equatorial Pacific Ocean (EEP) upwelling system supports >10% of the present‐day global ocean primary production, making it an important component in Earth's atmospheric and marine carbon budget. Traditionally, it has been argued that since intensification of Northern Hemisphere glaciation (iNHG, ~2.7 Ma), changes in EEP productivity have predominantly depended on trade wind strength‐controlled upwelling intensity. An alternative hypothesis suggests that EEP productivity is primarily controlled by nutrient supply from the high southern latitudes via mode waters. Here we present new high‐resolution data for the latest Pliocene/early Pleistocene from Ocean Drilling Program Site 849, located within the equatorial divergence system in the heart of the EEP upwelling regime. We use carbon isotopes in benthic and planktic foraminiferal calcite and sand accumulation rates to investigate glacial‐interglacial (G‐IG) productivity fluctuations between 2.65 and 2.4 Ma (marine isotope stages (MIS) G1 to 94). This interval includes MIS 100, 98, and 96, three large‐amplitude glacials (~1‰ in benthic δ18O) representing the culmination of iNHG. Our results suggest that latest Pliocene/early Pleistocene G‐IG productivity changes in the EEP were strongly controlled by nutrient supply from Southern Ocean‐sourced mode waters. Our records show a clear G‐IG cyclicity from MIS 100 onward with productivity levels increasing from full glacial conditions and peaking at glacial terminations. We conclude that enhanced nutrient delivery from high southern latitudes during full glacial conditions together with superimposed intensified regional upwelling toward glacial terminations strongly regulated primary productivity rates in the EEP from MIS 100 onward.

Shiyang River streamflow since AD 1765, reconstructed by tree rings, contains far‐reaching hydro‐climatic signals over and beyond the mid‐latitude Asian continent

AbstractIn this study, the regional tree‐ring chronology of Picea crassifolia was used to estimate annual (September to August) streamflow of the Shiyang River for the period from AD 1765 to 2010. The linear regression model was stable and could explain 41.5% of the variance for the calibration period of 1955–2005. According to the streamflow reconstruction, dry periods with below average streamflow occurred in AD 1775–1804, 1814–1823, 1831–1856, 1862–1867, 1877–1885, 1905–1910, 1926–1932, 1948–1951, 1960–1963 and 1989–2002. Periods of relatively wet years are identified for AD 1765–1774, 1805–1813, 1824–1830, 1857–1861, 1868–1876, 1886–1904, 1911–1925, 1933–1947, 1952–1959, 1964–1988 and 2003–2010. Comparisons with the precipitation reconstructions from surrounding areas supplied a high degree of confidence in our reconstruction. Our reconstructed streamflow is significantly correlated with sea surface temperature in the eastern equatorial Pacific Ocean and the North Atlantic Ocean. The Multitaper spectral and correlation analyses also suggested that the reconstructed streamflow variation in the Shiyang River could be associated with large‐scale atmospheric‐oceanic variability, such as El Niño‐Southern Oscillation (ENSO). The linkages among the streamflow reconstruction, NAO and ENSO suggest the connection of regional streamflow variations to the Asian monsoon and westerlies circulations. Copyright © 2016 John Wiley & Sons, Ltd.

Potential predictability of Indian summer monsoon rainfall in NCEP CFSv2

AbstractThe potential predictability of the Indian summer monsoon rainfall (ISMR), soil moisture, and sea surface temperature (SST) is explored in the latest version of the NCEP Climate Forecast System (CFSv2) retrospective forecast at five different lead times. The focus of this study is to find out the sensitivity of the potential predictability of the ISMR to the initial condition through analysis of variance technique (ANOVA), information‐based measure, including relative entropy (RE), mutual information (MI), and classical perfect model correlation. In general, the all methods show an increase in potential predictability with a decrease in lead time. Predictability is large over the Pacific Ocean basin as compared to that of the Indian Ocean basin. However, over the Indian land region the potential predictability increases from lead‐4 to lead‐2 and then decreases at lead‐1 followed by again increase at lead‐0. While the actual ISMR prediction skill is highest at lead‐3 forecast (second highest at lead‐1), the potential predictability is highest at lead‐2. It is found that highest and second highest actual prediction skill of the ISMR in CFSv2 is due to the combined effects of initial Eurasian snow and SST over Indian, west Pacific and eastern equatorial Pacific Ocean region. While the teleconnection between the ISMR and El Niño‐Southern Oscillation is too strong, the ISMR and Indian Ocean dipole have completely out of phase relation in the model as compared to the observation. Furthermore, the actual prediction skill of the ISMR is now very close to the potential predictability limit. Therefore, in order to improve the ISMR prediction skill further, development of model physics as well as improvements in the initial conditions is required.

Global change across the Oligocene-Miocene transition: High-resolution stable isotope records from IODP Site U1334 (equatorial Pacific Ocean)

The Oligocene-Miocene transition (OMT) (~23?Ma) is interpreted as a transient global cooling event, associated with a large-scale Antarctic ice sheet expansion. Here we present a 2.23?Myr long high-resolution (~3?kyr) benthic foraminiferal oxygen and carbon isotope (?18O and ?13C) record from Integrated Ocean Drilling Program Site U1334 (eastern equatorial Pacific Ocean), covering the interval from 21.91 to 24.14?Ma. To date, five other high-resolution benthic foraminiferal stable isotope stratigraphies across this time interval have been published, showing a ~1‰ increase in benthic foraminiferal ?18O across the OMT. However, these records are still few and spatially limited and no clear understanding exists of the global versus local imprints. We show that trends and the amplitudes of change are similar at Site U1334 as in other high-resolution stable isotope records, suggesting that these represent global deep water signals. We create a benthic foraminiferal stable isotope stack across the OMT by combining Site U1334 with records from ODP Sites 926, 929, 1090, 1264, and 1218 to best approximate the global signal. We find that isotopic gradients between sites indicate interbasinal and intrabasinal variabilities in deep water masses and, in particular, note an offset between the equatorial Atlantic and the equatorial Pacific, suggesting that a distinct temperature gradient was present during the OMT between these deep water masses at low latitudes. A convergence in the ?18O values between infaunal and epifaunal species occurs between 22.8 and 23.2?Ma, associated with the maximum ?18O excursion at the OMT, suggesting climatic changes associated with the OMT had an effect on interspecies offsets of benthic foraminifera. Our data indicate a maximum glacioeustatic sea level change of ~50?m across the OMT.

Phylogenetic characterization of marine benthic archaea in organic-poor sediments of the eastern equatorial pacific ocean (Odp site 1225)

Phylogenetic characterization of marine benthic archaea in organic-poor sediments of the eastern equatorial pacific ocean (Odp site 1225)