skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


  • NSF Public Access
  • Search Results
  • 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
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. ; ; ; (, Journal of Climate)
    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
  2. ; ; ; ; ; (, Journal of Climate)
    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
  3. ; ; (, Journal of Geophysical Research: Atmospheres)
    Abstract Zonal extensions of the Western Pacific subtropical high (WPSH) strongly modulate extreme rainfall activity and tropical cyclone (TC) landfall over the Western North Pacific (WNP) region. These zonal extensions are primarily forced on seasonal timescales by inter‐basin zonal sea surface temperature (SST) gradients. However, despite the presence of large‐scale zonal SST gradients, the WPSH response to SSTs varies from year to year. In this study, we force the atmosphere‐only NCAR Community Earth System Model version 2 simulations with two real‐world SST patterns, both featuring the large‐scale zonal SST gradient characteristic of decaying El Niño‐developing La Niña summers. For each of these patterns, we performed four experimental sets that tested the relative contributions of the tropical Indian Ocean, Pacific, and Atlantic basin SSTs to simulated westward extensions over the WNP during June–August. Our results indicate that the subtle differences between the two SST anomaly patterns belie two different mechanisms forcing the WPSH's westward extensions. In one SST anomaly pattern, extratropical North Pacific SST forcing suppresses the tropical Pacific zonal SST gradient forcing, resulting in tropical Atlantic and Indian Ocean SSTs being the dominant driver. The second SST anomaly pattern drives a similar westward extension as the first pattern, but the underlying SST gradient driving the WPSH points to intra‐basin forcing mechanisms originating in the Pacific. The results of this study have implications for understanding and predicting the impact of the WPSH's zonal variability on tropical cyclones and extreme rainfall over the WNP. 
    more » « less
  4. ; (, Journal of Geophysical Research: Atmospheres)
    Abstract The El Niño—Southern Oscillation (ENSO) is an important mode of tropical Pacific atmosphere‐ocean variability that drives teleconnections with weather and climate globally. However, prior studies using state‐of‐the‐art climate models lack consensus regarding future ENSO projections and are often impacted by tropical Pacific sea‐surface temperature (SST) biases. We used 173 simulations from 29 climate models participating in the Coupled Model Intercomparison Project, version 6 (CMIP6) to analyze model biases and future ENSO projections. We analyzed two ENSO indices, namely the ENSO Longitude Index (ELI), which measures zonal shifts in tropical Pacific deep convection and accounts for changes in background SST, and the Niño 3.4 index, which measures SST anomalies in the central‐eastern equatorial Pacific. We found that the warm eastern tropical‐subtropical Pacific SST bias typical of previous generations of climate models persists into many of the CMIP6 models. Future projections of ENSO shift toward more El Niño‐like conditions based on ELI in 48% of simulations and 55% of models, in association with a future weakening of the zonal equatorial Pacific SST gradient. On the other hand, none of the models project a significant shift toward La Niña‐like conditions. The standard deviation of the Niño 3.4 index indicates a lack of consensus on whether an increase or decrease in ENSO variability is expected in the future. Finally, we found a possible relationship between historical SST and low‐level cloud cover biases in the ENSO region and future changes in ELI; however, this result may be impacted by limitations in data availability. 
    more » « less
  5. ; ; ; ; (, Journal of Climate)
    Abstract We examine the hypothesis that the observed connection between the stratospheric quasi-biennial oscillation (QBO) and the strength of the Madden–Julian oscillation (MJO) is modulated by the sea surface temperature (SST)—for example, by El Niño–Southern Oscillation (ENSO). A composite analysis shows that, globally, La Niña SSTs are remarkably similar to those that occur during the easterly phase of the QBO. A maximum covariance analysis suggests that MJO power and SST are strongly linked on both the ENSO time scale and the QBO time scale. We analyze simulations with a modified configuration of version 2 of the Community Earth System Model, with a high top and fine vertical resolution. The model is able to simulate ENSO, the QBO, and the MJO. The ocean-coupled version of the model simulates the QBO, ENSO, and MJO, but does not simulate the observed QBO–MJO connection. When driven with prescribed observed SST anomalies based on composites for QBO east and QBO west (QBOE and QBOW), however, the same atmospheric model produces a modest enhancement of MJO power during QBOE relative to QBOW, as observed. We explore the possibility that the SST anomalies are forced by the QBO itself. Indeed, composite Hovmöller diagrams based on observations show the propagation of QBO zonal wind anomalies all the way from the upper stratosphere to the surface. Also, subsurface ocean temperature composites reveal a similarity between the western Pacific and Indian Ocean subsurface signal between La Niña and QBOE. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email