skip to main content


Title: Investigating Past Thermocline Conditions in the Eastern Equatorial Pacific Since the Last Glacial Interval Using Oxygen Isotope Gradients from Shallow- and Deep-dwelling Planktonic Foraminifera
The El Niño Southern Oscillation (ENSO) is a climate variation that occurs in the Eastern Equatorial Pacific (EEP) ocean, which influences the position of the thermocline today. The ENSO cycle is split into three states: a normal state, an El Niño state, and a La Niña state. Each one is marked by different sea surface temperatures (SST). Under normal conditions, the trade winds push warm water westward, away from the coast of South America. This allows cool water to upwell the east and creates a zonal SST gradient. Every few years, the trade winds slow, preventing the flow of warm water. This increases the SST in the EEP and produces an El Niño. The winds can also strengthen and move more warm water westward. The heightened zonal SST gradient forms a La Niña. This project investigates past conditions in the EEP by reconstructing the mean position of the thermocline: a layer in the ocean where temperature rapidly changes with depth. In the modern ocean, the thermocline is shallower in the east, where SSTs are cool, and deeper in the west, where SSTs are warm. The more uniform SST gradient during an El Niño event flattens the zonal slope thermocline; the stronger gradient during a La Niña steepens it. The temperature proxy used to determine past thermocline positions is the isotopic composition of oxygen in foraminifera (δ18O). Foraminiferal δ18O increases with depth in the water column, as temperature decreases and density increases. Two species with contrasting depth-habitats were analyzed; Globigerinoides ruber, which lives near the surface above the thermocline, and Neogloboquadrina dutertrei, which lives in the lower thermocline. When the thermocline shifts, it changes their difference in δ18O. A smaller difference indicates a deeper thermocline and an El Niño-like state; a greater difference indicates a shallower thermocline and a La Niña-like state. The forams were collected from two deep-sea sediment cores. The first was Ocean Drilling Program (ODP) Leg 202 Site 1239 (0.67˚S, 82.08˚W, 1414 m) drilled in the east near the coast of Ecuador. The second was ODP Leg 138 Site 849 (0.10˚S, 110.31˚W, 3858 m) toward the west. Rather than identifying specific ENSO events, this method provides insight into the position of the thermocline and therefore the mean state of the EEP during the Holocene and last glacial period.  more » « less
Award ID(s):
2050923
PAR ID:
10514882
Author(s) / Creator(s):
; ;
Editor(s):
McManus, J
Publisher / Repository:
American Geophysical Union
Date Published:
Journal Name:
Proceedings American Geophysical Union
Edition / Version:
Final
Volume:
0
Issue:
0
Page Range / eLocation ID:
PP51D-1095
Subject(s) / Keyword(s):
Paleooceanography Pacific: Micropaleontology
Format(s):
Medium: X Size: 2 MB Other: pdfA
Size(s):
2 MB
Location:
San Francisco, CA
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Interactions between ocean basins affect El Niño–Southern Oscillation (ENSO), altering its impacts on society. Here, we explore the effect of Atlantic Multidecadal Variability (AMV) on ENSO behavior using idealized experiments performed with the NCAR‐CESM1 model. Comparing warm (AMV+) to cold (AMV−) AMV conditions, we find that ENSO sea surface temperature (SST) anomalies are reduced by ∼10% and ENSO precipitation anomalies are shifted to the west during El Niño and east during La Niña. Using the Bjerknes stability index, we attribute the reduction in ENSO variability to a weakened thermocline feedback in boreal autumn. In AMV+, the Walker circulation and trade winds strengthen over the tropical Pacific, increasing the background zonal SST gradient. The background changes shift ENSO anomalies westwards, with wind stress anomalies more confined to the west. We suggest the changes in ENSO‐wind stress decrease the strength of the thermocline feedback in the east, eventually reducing ENSO growth rate.

     
    more » « less
  2. null (Ed.)
    Abstract Many previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño–Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the interdecadal Pacific oscillation (IPO) is further investigated here to understand the nonstationary ENSO–IOBW relationship during ENSO decaying summer (July–September). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO–IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO–IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies. 
    more » « less
  3. Abstract

    The modern eastern equatorial Pacific Ocean (EEP) exhibits strong upwelling, producing pronounced gradients in sea surface temperature (SST), nutrient concentration, and biological productivity between 80° and 140°W. During the globally warmer late Pliocene (3.0–3.6 Ma), the EEP may have experienced permanent El Niño‐like conditions, supported by a reduced SST gradient across the equatorial Pacific. However, the weakened east‐west SST gradient has been controversial, with disparate results depending on the proxy used to monitor Western Warm Pool SSTs. We present new Pliocene alkenone‐based SST and paleoproductivity records from four Ocean Drilling Program (ODP) cores spanning an east‐west transect across the EEP, which present an internally consistent picture of SST and productivity gradients in the modern cold tongue, resolved at orbital‐scale variability. Strong agreement between core top reconstructions and satellite estimates indicates that alkenone paleotemperature and paleoproductivity proxies are appropriate for reconstructing Pliocene EEP conditions. The average SST gradient between 90° and 120°W was reduced from the modern 1.8°C gradient to 0.9°C in the late Pliocene. Despite the weakened SST gradient, the surface productivity gradient was stronger during the late Pliocene compared to modern, based on calibrated X‐ray fluorescence biogenic opal and alkenone average accumulation rates. Contrary to modern El Niño SST and productivity patterns, reduced Pliocene surface productivity did not accompany the weakened SST gradient. Instead, strong Pliocene biogenic opal and alkenone concentration accumulation gradients in the eastern EEP suggest that subsurface tilting of the nutricline and thermocline persisted to supply vigorous upwelling of warm but nutrient‐rich subsurface waters in a warmer climate.

     
    more » « less
  4. Abstract

    El Niño–Southern Oscillation (ENSO) exhibits highly asymmetric temporal evolutions between its warm and cold phases. While El Niño events usually terminate rapidly after their mature phase and show an already established transition into the cold phase by the following summer, many La Niña events tend to persist throughout the second year and even reintensify in the ensuing winter. While many mechanisms were proposed, no consensus has been reached yet and the essential physical processes responsible for the multiyear behavior of La Niña remain to be illustrated. Here, we show that a unique ocean physical process operates during multiyear La Niña events. It is characterized by rapid double reversals of zonal ocean current anomalies in the equatorial Pacific and exhibits a fairly regular near-annual periodicity. Mixed-layer heat budget analyses reveal comparable contributions of the thermocline and zonal advective feedbacks to the SST anomaly growth in the first year of multiyear La Niña events; however, the zonal advective feedback plays a dominant role in the reintensification of La Niña events. Furthermore, the unique ocean process is identified to be closely associated with the preconditioning heat content state in the central to eastern equatorial Pacific before the first year of La Niña, which has been shown in previous studies to play an active role in setting the stage for the future reintensification of La Niña. Despite systematic underestimation, the above oceanic process can be broadly reproduced by state-of-the-art climate models, providing a potential additional source of predictability for the multiyear La Niña events.

     
    more » « less
  5. Abstract

    The longitudinal location of precipitation anomalies over the equatorial Pacific shows a distinctive feature with the westernmost location for La Niña, the easternmost location for eastern Pacific (EP) El Niño, and somewhere between for central Pacific (CP) El Niño, even though the center of the sea surface temperature anomaly (SSTA) for La Niña is located slightly east of that of CP El Niño. The mechanisms for such a precipitation diversity were investigated through idealized model simulations and moisture and moist static energy budget analyses. It is revealed that the boundary layer convergence anomalies associated with the precipitation diversity are mainly induced by underlying SSTA through the Lindzen–Nigam mechanism, that is, their longitudinal locations are mainly controlled by the meridional and zonal distributions of the ENSO SSTA. The westward shift of the precipitation anomaly center during La Niña relative to that during CP El Niño is primarily caused by the combined effects of nonlinear zonal moist enthalpy advection anomalies and the Lindzen–Nigam mechanism mentioned above. Such a zonal diversity is further enhanced by the “convection–cloud–longwave radiation” feedback, the SST-induced latent heat flux anomalies, and the advection of mean moist enthalpy by anomalous winds. This diversity in the longitudinal location of precipitation anomalies has contributions to the diversities in the longitudinal locations of anomalous Walker circulation and western North Pacific anomalous anticyclone/cyclone among the three types of ENSO.

     
    more » « less