Western-central Equatorial Pacific Research Articles

Limits on conservative behavior of Nd isotopes in seawater assessed from analysis of fish teeth from Pacific core tops

Neodymium (Nd) isotopes have become an accepted water mass tracer for paleoceanographic reconstructions; however, high rates of radiogenic Nd input from volcanic islands/arcs in the western equatorial Pacific (WEP) and the subarctic North Pacific (SANP) can significantly alter the isotopic composition of ambient seawaters, leading to questions about the conservative nature of Nd isotopes in the Pacific. Here we present the first comprehensive study of Nd isotopes preserved in core-top fossil fish teeth/debris from 31 sites in the Pacific to define the vertical and horizontal distributions of seawater Nd isotopes and to test the reliability of Nd isotopes as a water mass tracer in the Pacific. We found that Nd isotopic values of northward-flowing Lower Circumpolar Deep Water (LCDW) are significantly modified in the western–central equatorial Pacific and the SANP as a result of local radiogenic inputs from young circum-Pacific island arcs. Consequently, ‘modified LCDW’ with ~−5 ε Nd is created in the central equatorial Pacific, while recirculated North Pacific Deep Water (NPDW) acquires a radiogenic Nd isotope value (~−2 ε Nd) in the SANP. Nd isotopic ratios of fish teeth/debris in the central North Pacific plotted against salinity (or phosphate concentration) from the overlying bottom water produce a mixing envelope between modified LCDW and NPDW, indicating that these Nd isotopic values represent the southern- and northern-sourced end member values for the central North Pacific and that mixing of these water masses defines vertical profiles of Nd isotopes in this region. In the eastern equatorial Pacific (EEP) three water masses (modified LCDW, NPDW SANP, and NPDW western Pacific) combine to produce Nd isotopic values that reflect this conservative mixture. Therefore, Nd isotopes behave as a conservative water mass tracer in the central North Pacific and the EEP, but high rates of lithogenic inputs from young circum-Pacific island arcs compromise the conservative behavior of Nd isotopes in the WEP and SANP.

On the definition of El Niño and associated seasonal average U.S. weather anomalies

A new NOAA definition of El Niño identifies a number of additional El Niño seasons beyond those conventionally agreed. These additional seasons are characterized by SST anomalies primarily in the western central equatorial Pacific. We show here that the seasonal weather anomalies over the U.S. associated with these additional Dateline El Niño seasons are substantially different from those associated with conventional El Niño seasons. Although some regions have similar associated anomalies, most of the major regional anomalies are quite different. Treating the two as a single phenomenon yields weaker overall seasonal weather associations and does not take advantage of the stronger associations available when the two are treated separately.

On the impacts of the Indian summer monsoon on ENSO in a coupled GCM

AbstractThis study explores the impacts of the Indian summer monsoon (ISM) on the El Niño–Southern Oscillation (ENSO) evolution by utilizing two coupled general‐circulation model simulations. The first simulation uses the Center for Ocean–Land–Atmosphere Studies anomaly‐coupled model and the second simulation employs a new coupling strategy, interactive ensembles, that is designed to increase the signal‐to‐noise ratio of boundary forcing. The effects of monsoon variability related and unrelated to ENSO are separated through composites in terms of both sea surface temperature (SST) over the Niño‐3.4 area (170°W–120°W,5°S–5°N) and Indian summer monsoon rainfall (IMR) anomalies. It is found that ENSO‐related monsoon variability has significant impacts on warm events. In the interactive‐ensemble simulation, a weak (strong) monsoon enhances (weakens) an ongoing warm event. The monsoon impacts are manifested in the surface zonal wind‐stress anomalies in the western central equatorial Pacific. In the anomaly‐coupled simulation, the monsoon–ENSO relationship is difficult to detect. The ongoing cold events are only weakly affected by monsoon variability. Monsoon variability that is unrelated to ENSO also induces noticeable SST anomalies in the equatorial central Pacific in the following winter. In the interactive‐ensemble model, a weak (strong) monsoon induces noticeable warm (cold) SST anomalies.The long‐term change of the ISM–ENSO relationship is also analysed. Fluctuations of the IMR–Niño‐3.4 SST correlation are larger in the anomaly‐coupled model than in the interactive‐ensemble model. No apparent relation is found between the long‐term change of the correlation and that of the monsoon and ENSO anomalies or standard deviations. In the interactive‐ensemble simulation, the IMR and concurrent El Niño‐3.4 SST standard deviations show in‐phase long‐term changes. In the anomaly‐coupled simulation, the long‐term changes of the IMR and summer Niño‐3.4 SST standard deviations have an out‐of‐phase relation. Copyright © 2003 Royal Meteorological Society

87:6060 Trends in sea level in the western central equatorial Pacific during 1974–1975 to 1981: Inoue, Masamichi and J.J. O'Brien, 1987. J. geophys. Res., 92(C5):5045–5051

87:6060 Trends in sea level in the western central equatorial Pacific during 1974–1975 to 1981: Inoue, Masamichi and J.J. O'Brien, 1987. J. geophys. Res., 92(C5):5045–5051