//
you're reading...

Climate Change

Just How Permanent was El Niño in the Past?

ARTICLE:

Zhang, Y.G., Pagani, M., and Liu, Z. (2014). A 12-Million-Year temperature history of the Tropical Pacific Ocean. Science 344, 84. DOI: 10.1126/science.1246172.

BACKGROUND:

El Niño and La Niña, two opposing modes of the El Niño Southern Oscillation (ENSO), are notorious for altering both regional and global climate on short timescales (i.e. on the order of 2 to 7 years). This interannual climate variability arises from the gradient in sea surface temperatures (SST) between the western equatorial Pacific (WEP) and the eastern equatorial Pacific (EEP). The WEP is often referred to as the “warm pool”, due to the annual average SST being amongst the highest globally (near or exceeding 30°C). Conversely, the EEP is often called the “cold tongue”, as the SST of this region is anomalously cool (approximately 25°C).  The average difference in SST between the modern-day warm pool and cold tongue is approximately 4-5°C, with deviations from this average depending on the prevailing mode of ENSO (Figure 1).

Figure 1.  Mean annual modern-day sea surface temperature reconstruction.  Sediment core site locations labeled.  Centered on ODP 806, the western equatorial warm pool contains some of the warmest surface waters on Earth.  The eastern equatorial Pacific, often referred to as the cold tongue, is the narrow band of regionally cool surface waters centered on ODP 850.  The gradient between the warm pool and the cold tongue has major implications for global climate.

Figure 1. Mean annual modern-day sea surface temperature reconstruction. Sediment core site locations labeled. Centered on ODP 806, the western equatorial warm pool contains some of the warmest surface waters on Earth. The eastern equatorial Pacific, often referred to as the cold tongue, is the narrow band of regionally cool surface waters centered on ODP 850. The gradient between the warm pool and the cold tongue has major implications for global climate.

ENSO is responsible for interannual climate variability, including how heat is exported from the tropics and the distribution and frequency of of precipitation. El Niño conditions are characterized by a significant reduction in SST gradient between the warm pool and the cold tongue. The persistent easterly tradewinds, which are responsible for piling up water in the WEP warm pool, relax during El Niño, allowing warm pool water to slosh back towards the eastern Pacific. This is a measureable phenomenon, as the eastward intrusion of warm water reduces sea-level in the WEP and increases sea-level in the EEP. South America feels the strong effects of El Niño including a seasonal (typically April – October) increase in precipitation in the eastern Pacific in regions such as Peru , and drought conditions in the western Pacific near Indonesia and Australia. Additionally, the relaxed trade winds and warmer surface waters in the EEP greatly reduce upwelling, the oceanographic process responsible for transporting deep, nutrient-rich water to the surface. A lack of surface nutrients causes a domino-effect of collapses across marine ecosystems, leading to severely reduced South American fisheries.

La Niña is characterized increased easterly tradewinds and increased equatorial temperature gradients. The climatic effects are opposite of those experienced during El Niño. For example, reduced seasonal precipitation leads to drought conditions in South America.

 

PREVIOUS FINDINGS:

Considering the powerful effects the ENSO phases have on global climate, paleoceanographers (oceanographers who study the oceans on geologic timescales) and climate scientists have questioned the prevalence of ENSO in the past. Previous studies have reconstructed SST gradients between the WEP and EEP using various SST proxies; such as the use of Magnesium/Calcium (Mg/Ca) ratios found in the calcium carbonate shells of planktonic foraminifera (single celled, surface ocean dwelling protists) that are preserved in the ocean’s deep sediments. This Mg/Ca ratio, with proper calibration, is an indicator of SST during the time period in which the critters lived. For example, Mg/Ca records from foraminifera dating back to the Pliocene epoch [5.3 – 2.6 million years ago (Ma)], have shown that warm pool (WEP) SST were similar, if not slightly cooler than WEP temperatures recorded in modern-day, despite the global climate being 3-4°C warmer in the Pliocene. Findings from the EEP show SST cooling during the Pliocene, resulting a non-existent SST gradient from the WEP to the EEP, reminiscent of modern short-lived El Niño events. These findings lead to a prevalent hypothesis that permanent El Niño conditions were in place prior to 3 Ma.

The strength and reliability of the temperature records reconstructed from Mg/Ca ratios from the shells of foraminifera are largely dependent on assumptions that ocean chemistry and process have remained constant over millions of years, and that the foraminifera preserved in the ocean’s sediment have not been altered drastically. Additional analyses of foraminifera from modern day sediment at the same sediment core location failed to capture modern day SST measurements. This finding undermines the strength of Mg/Ca ratios for SST reconstructions, and thus the hypothesis of El Niño permanence in the past, necessitating a more robust method of reconstructing past SST.

 

THIS STUDY – METHODOLOGY:

The focus of the highlighted study was to reconstruct SST in the warm pool (WEP) and cold tongue (EEP) over the past 12 million years using a multi-proxy approach. Emphasis was placed on the strength and limitations of each proxy used.

New SST reconstructions were made using the TEX86 temperature proxy, which is essentially an analysis of membrane lipids of single-celled organisms belonging to the Thaumarchaeota phylum. According to current calibrations, TEX86 has the tendency to underestimate temperatures in the tropics and for this reason, an additional temperature proxy, the Uk’37 proxy, was recruited from previous studies to reconstruct SST. Coccolithophorids (calcium carbonate shell producing, unicellular phytoplankton) produce two types of alkenones (long-chained organic molecules, resistant to degradation) with different levels of unsaturation (double bonds) depending on the temperature of the water in which these creatures live. By looking at the ratio at which these unsaturated molecules are produced, you can reconstruct temperature records from the time period which these organisms lived.. However, it is important to note that Uk’37 only remains accurate up to 28.5°C. The TEX86 records reveal that from 3 Ma and older, SST exceeded 28.5°C, and therefore Uk’37 would only be useful for reconstructing SST in the warm pool for 0 – 3 Ma. Beyond 3 Ma, the study relies on TEX86 which is not greatly influenced by high temperatures. In the cold tongue region, TEX86 and Uk’37 showed similar trends with only minor deviations. Since TEX86 tends to slightly underestimate tropic SST, and the temperature in this region stayed below 28.5°C, Uk’37 was favored for SST reconstruction in the cold tongue over the 12 million year record.

 

FINDINGS:

WEP warm pool

The warm pool (WEP) was found to have been warmer than modern-day and TEX86 records reveal 4°C cooling of SST over the 12 million year record, with 2°C occurring in the last 5 Ma (Figure 2).  This differs from previous findings from Mg/Ca paleotemperature reconstructions, which showed little to no significant change in warm pool SST.

EEP cold tongue

The cold tongue (EEP), like the warm pool, was warmer than modern-day and TEX86 and the favored Uk’37 both reveal 6°C cooling over the 12 million year record, with higher cooling rates in the last 5 million years (Figure 2).  This cooling is consistent with published Mg/Ca paleotemperature reconstructions.

Equatorial temperature gradients

The equatorial temperature gradient over the past 12 Ma was similar to the modern-day configuration, which is in disagreement with the hypothesis of permanent El Niño-like conditions >3 Ma (Figure 2).  Both the warm pool and cold tongue were warmer than modern-day.  These findings lead to the hypothesis that the oceanographic processes responsible for the equatorial temperature gradient similar to that of modern-day (and thus the existence of the cold tongue) must have been in operation over the past 12 million years.  It is therefore also hypothesized that the strong temperature asymmetry in the tropics would have been favorable for ENSO, including both the El Niño and La Niña modes.  This study helps to bring agreement between equatorial Pacific SST and other proxy records, which have long shown evidence of ENSO like behavior (e.g. equatorial biogenic opal accumulation, which arises from upwelling at the cold tongue).

Figure 2.  Reconstructions of the temperature history of the warm pool (WEP trend) and cold tongue (EEP Trend) from 12 Ma to present.  The WEP trend relies on reconstructions from TEX86 up until approximately 3 Ma due to temperatures exceeding the maximum calculable temperature of U_37^K' (~28.5°C).  The temperature history suggests that the eastern equatorial Pacific has been persistently cooler than the western equatorial Pacific, conditions that are necessary for ENSO.

Figure 2. Reconstructions of the temperature history of the warm pool (WEP trend) and cold tongue (EEP Trend) from 12 Ma to present. The WEP trend relies on reconstructions from TEX86 up until approximately 3 Ma due to temperatures exceeding the maximum calculable temperature of Uk’37 (~28.5°C). The temperature history suggests that the eastern equatorial Pacific has been persistently cooler than the western equatorial Pacific, conditions that are necessary for ENSO.

 

SIGNIFICANCE:

Any study that serves to remediate disagreeing data in the climate record should be considered particularly important.  The nature of good science is to limit where possible, or at least acknowledge possible sources of error.  Scientific progress is measured by the ability to test previous hypotheses and systematically reduce error as new methodologies and technologies become available.  This study accomplishes these very goals.  With the tendency of climate science to be highly politicized by mainstream media, it is of utmost importance to report successful scientific progress and properly communicate the remediation of data disagreement.  The ever-increasing understanding of our planet’s complicated climate system as it currently exists and in the past will hopefully underpin policy decisions regarding how society chooses to respond to the future societal challenges that have been predicted.

Discussion

Trackbacks/Pingbacks

  1. […] “Just How Permanent was El Niño in the Past?”  – Brian Caccioppoli   […]

Post a Comment

Instagram

  • by oceanbites 2 days ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 1 month ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 1 month ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 2 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 2 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 3 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 3 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 3 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 3 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 4 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 4 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 5 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 5 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 6 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 6 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 7 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 7 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 8 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
  • by oceanbites 8 months ago
    Feeling a bit flattened by the week? So are these summer flounder larvae. Fun fact: flounder larvae start out with their eyes set like normal fish, but as they grow one of their eyes migrates to meet the other and
  • by oceanbites 8 months ago
    Have you seen a remote working setup like this? This is a photo from one of our Oceanbites team members Anne Hartwell. “A view from inside the control can of an underwater robot we used to explore the deep parts
WP2Social Auto Publish Powered By : XYZScripts.com