Prominent precession-band variance in El Ni&ntilde;o&ndash;Southern Oscillation Intensity over the last 300,000 years

Zhengyao Lu

doi:https://doi.org/10.5194/egusphere-egu2020-11315

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[Back] [Session CL1.9]

EGU2020-11315

https://doi.org/10.5194/egusphere-egu2020-11315

EGU General Assembly 2020

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Prominent precession-band variance in El Niño–Southern Oscillation Intensity over the last 300,000 years

Zhengyao Lu

Lund University, Lund, Sweden (luzhengyao88@gmail.com)

It remains unclear how El Niño–Southern Oscillation (ENSO)—the prominent interannual anomalous climate mode—varied during the full glacial cycles. We study the evolution of ENSO of the last 300,000 years using continuous fully-coupled climate model simulations. How the slow time‐varying changes in insolation, greenhouse gases concentration, and continental ice sheets could influence the behaviours of El Niño are taken into account. The simulated ENSO variance and the tropical eastern Pacific annual cycle (AC) amplitude change in phase, and both have pronounced precession-band variance (~21,000 years) rather than the obliquity-band (~40,000 years). The precession‐modulated slow (orbital time scales) ENSO evolution is determined linearly by the change of the coupled ocean‐atmosphere instability, notably the Ekman upwelling feedback and thermocline feedback. In contrast, the greenhouse gases and ice sheet forcings (~100,000‐year cycles with sawtooth shapes) are opposed to each other as they influence ENSO variability through changes in AC amplitude via a common nonlinear frequency entrainment mechanism. The relatively long simulations which involve pronounced glacial‐interglacial forcing effects gives us more confidence in understanding ENSO forcing mechanisms, so they may shed light on ENSO dynamics and how ENSO will change in the future.

How to cite: Lu, Z.: Prominent precession-band variance in El Niño–Southern Oscillation Intensity over the last 300,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11315, https://doi.org/10.5194/egusphere-egu2020-11315, 2020

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Minor editings to correct typos.

CC1:
Comment on EGU2020-11315, Chris Brierley, 05 May 2020
Hello Zhengyao,
I was wondering if there is any interaction between the accelerated forcing (and the associated lack of equilibrium) and the ENSO signals that you're detecting? Would you expect the real system to display stronger signals?
Chris
- AC1: Reply to CC1, Zhengyao Lu, 05 May 2020
  Hi Chris,
  
  Thanks for your question! Yes, The acceleration could dampen and delay the precession signals below the surface ocean associated with ENSO intensity (surface climate is largely unaffected).
  
  For instance, in real time, it takes decades for the subtropical Pacific Ocean to affect the equatorial temperature through the subduction process; in the accelerated time, this subduction time will be equivalent to several thousands of years. The delayed forcing signals may cancel out the opposite anomalies between two successive precessional forcing conditions and further weaken precession‐scale oscillations in ENSO feedbacks.
  
  We further compared the accelerated simulations (ORB in this study) with the unaccelerated TRACE-ORB simulation for the last 21000 years. It is found that the amplitude (signal) of the BJ index is reduced by ~50% in the accelerated simulation.

CC2:
Comment on EGU2020-11315, John Bruun, 05 May 2020
Lu, this is very interesting work and great to see a strong enough spectral feature of ENSO with this record.
my question:Why does ENSO not have an obliquity spectral peak but SST does?
Your response: ENSO show only pronounced precession forcing signal, whereas SST has both obliquity and precession signals. We diagnosed the BJ index (ENSO feedbacks) and found no obliquity signal in it so the stratification is more sensitive to thermocline temperature signal changes.
Further discussion:
The ENSO processes is very much driven by the Pacific resonance effect, regulated by the equatorial wave guide and thethermocline variation:
Heartbeat of the Southern Oscillation:
A mechanism for Pacific interdecadal Resonances
I wonder if the absence (presence) of the obliquity in the ENSO (SST) spectral response can be explained by these ocean-atmosphere interaction effects? For SST is this a form of frequency mode entrainment?
This may be able to help us infer properties of the quasibiennial and quasiquadrennial modes, see for example: ENSO complexity
How these modes interact and their nature is still an open question - and perhaps your approach can help to progress this..
I would be very interested to follow this up. j.bruun@exeter.ac.uk
- AC2:
  Reply to CC2, Zhengyao Lu, 05 May 2020
  Hi John,
  
  Thanks for your questions and comments. In our model simulations, a large part of ENSO variability (mainly in precession band) can be explained by its linear growth rate (quantified by Bjerknes stability analysis). So I would argue that it seems the orbital forcing mechanisms do not involve nonlinear processes.
  
  On the other hand, for 100,000-year glacial cycles, we found the linear growth rate change is hard to explain the evolution of ENSO variability (note that now we have GHG and ice-sheet forcings). We notice an inverse relationship between ENSO amplitude and annual cycle amplitude, i.e., stronger annual cycle leading to weaker ENSO, and vice versa. We hypothesise that there can be frequency entrainment between these two oscillations.
  
  However, it is still not clear why the frequency entrainment works for the 100,000 years ENSO variance change (associated with GHG and ice-sheet forcings), but not for the precession ENSO variance change (associated with orbital forcings).
  
  If our simulation results can be used to study your research questions, please let me know and we can share the data. zhengyao.lu@nateko.lu.se
  - CC3: Reply to AC2, John Bruun, 05 May 2020
    Yes, please do share the data record: I look forwards to see what can be established together with this.

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