EGU2020-10881
https://doi.org/10.5194/egusphere-egu2020-10881
EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Sensitivity of the Atlantic meridional overturning circulation (AMOC) to the tropical Indian Ocean warming.

Brady Ferster1, Alexey Fedorov1,2, Juliette Mignot1, and Eric Guilyardi1
Brady Ferster et al.
  • 1Sorbonne Université, IPSL, LOCEAN, France
  • 2Department of Geology and Geophysics, Yale University, New Haven, CT, USA

The Arctic and North Atlantic Ocean play a fundamental role in Earth’s water cycle, distribution of energy (i.e. heat), and the formation of cold, dense waters. Through the Atlantic meridional overturning circulation (AMOC), heat is transported to the high-latitudes. Classically, the climate impact of AMOC variations has been investigated through hosing experiments, where anomalous freshwater is artificially added or removed from the North Atlantic to modulate deep water formation. However, such a protocol introduces artificial changes in the subpolar area, possibly masking the effect of the AMOC modulation. Here, we develope a protocol where AMOC intensity is modulated remotely through the teleconnection of the tropical Indian Ocean (TIO), so as to investigate more robustly the impact of the AMOC on climate. Warming in the TIO has recently been shown to strengthen the Walker circulation in the Atlantic through the propagation of Kelvin and Rossby waves, increasing and stabilizing the AMOC on longer timescales. Using the latest coupled-model from Insitut Pierre Simon Laplace (IPSL-CM6), we have designed a three-member ensemble experiment nudging the surface temperatures of the TIO by -2°C, +1°C, and +2°C for 100 years. The objectives are to better quantify the timescales of AMOC variability outside the use of hosing experiments and the TIO-AMOC relationship.  In each ensemble member, there are two distinct features compared to the control run. The initial changes in AMOC (≤20 years) are largely atmospherically driven, while on longer timescales is largely driven by the TIO teleconnection to the tropical Atlantic. In the northern North Atlantic, changes in sensible heat fluxes range from 15 to 20 W m-2 in all three members compared to the control run, larger than the natural variability. On the longer timescales, AMOC variability is strongly influenced from anomalies in the tropical Atlantic Ocean. The TIO teleconnection supports decreased precipitation in the tropical Atlantic Ocean during warming (opposite during TIO cooling) events, as well as positive salinity anomalies and negative temperature anomalies. Using lagged correlations, there are the strongest correlations on scales within one year and a delayed response of 30 years (in the -2°C ensembles). In comparing the last 20 years, nudging the TIO induces a 3.3 Sv response per 1°C change. In summary, we have designed an experiment to investigate the AMOC variability without directly changing the North Atlantic through hosing, making way for a more unbiased approach to analysing the AMOC variability in climate models.

How to cite: Ferster, B., Fedorov, A., Mignot, J., and Guilyardi, E.: Sensitivity of the Atlantic meridional overturning circulation (AMOC) to the tropical Indian Ocean warming., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10881, https://doi.org/10.5194/egusphere-egu2020-10881, 2020

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Presentation version 2 – uploaded on 02 May 2020
Version description: We have updated the presentation to include an additional slide on sea ice changes throughout our experiment.[...]
  • CC1: Comment on EGU2020-10881, David Docquier, 04 May 2020

    Hi Brady et al.,

    I have attended your display presentation today and asked you the question of the impact of domain size on your results.

    I find your study very interesting, and I have a couple of further questions:

    1) What's the difference between your control run and the IPSL CM6 piControl run in Slide 3?

    2) In Slide 3, you talk about SST change of 0.7, 1.4 and -1.4K. Are these average changes in the TIO domain or somewhere else?

    3) What do you think is the exact process that leads from TIO warming to Kelvin and Rossby waves? (Slide 4) And from these waves to AMOC strenghtening?

    4) Do you have an idea about how the OHT behaves in the North Atlantic in your experiments?

    5) Are you sure that what you see in Slide 8 is the impact of the AMOC on sea ice, as you may suggest, instead of a direct influence of TIO SST change?

    FYI, I have done kind of similar sensitivity experiments with EC-Earth using different domains, but focusing on the impact of OHT on Arctic sea ice. You can have a look at my display: https://meetingorganizer.copernicus.org/EGU2020/EGU2020-3352.html. And I've also put some results on a git page: https://git.smhi.se/david.docquier/oseaice/-/wikis/home.

    I'm happy to discuss this further with you: david.docquier@smhi.se

    Thanks for your feedback.

    David

    • AC1: Reply to CC1, Brady Ferster, 05 May 2020

      Hello David,

      Thank you for the interest in our work, these are very thoughtful questions.

       

      • 1) What's the difference between your control run and the IPSL CM6 piControl run in Slide 3?

       

      The IPSL-CM6 is the latest coupled-model from Insitut Pierre Simon Laplace (IPSL-CM6), to which we used the most recently available pre-industrial control run (r1i2, dashed black). The control run members (solid black lines) we show in the figure are members from the same initial condition of the IPSL-CM6 piControl, but have added a white noise perturbation to the initial state of each control run member (as to make a new control run from an initial state). We have done this to:

       

      1) increase the control run data throughout the perturbation experiment

      2) compare the robustness and variability within the control run

      3) to potentially further investigate the centennial variability within the IPSL-CM6

       

      • 2) In Slide 3, you talk about SST change of 0.7, 1.4 and -1.4K. Are these average changes in the TIO domain or somewhere else?

       

      The SST changes referenced are to the TIO domain nudged (30°-100°E, 30°S-30°N) and averaged over years 1-100 for the experiment. The Restoring was set to 40 W m-2 K-1 within the surface mixed layer, resulting in the +1°C nudging to have a mean TIO SST increase of 0.7°C on average, with additional changes being found within the mixed layer and the surface atmospheric temperature. A similar result was found with both the +2°C (+1.4°C change in SST) and -2°C (-1.4°C change in SST) ensemble members.

       

      • 3) What do you think is the exact process that leads from TIO warming to Kelvin and Rossby waves? (Slide 4) And from these waves to AMOC strengthening?

       

      The suspected mechanisms and processes are detailed Hu and Fedorov, 2019:

      https://www.nature.com/articles/s41558-019-0566-x

       

      In short, perturbing the Indian Ocean amplifies the geostationary Rossby waves and Kelvin waves generated within the tropical Indian Ocean through changes in the surface heat fluxes. In a warming tropical Indian Ocean, there is increased latent heat release, triggering the waves. Our resulting mechanisms are slightly different than the above reference Hu and Fedorov (2019), where this experiment suggests a strong teleconnection directly with the Arctic and North Atlantic regions. On monthly and annual timescales, the geostationary Rossby waves drive changes in the westerlies (Ekman transport) and heat fluxes, which corresponds to changes in sea ice and surface density. The continued duration of the geostationary Rossby waves help drive changes to the North Atlantic sea ice and density profile (OHC, salinity), and ultimately influencing AMOC.

       

      The mechanism detailed in Hu and Fedorov (2019) is as follows: The Kelvin and Rossby waves drive changes in the Walker circulation, shifting precipitation northward throughout the tropical Atlantic and decreasing the salinity in this region on annual timescales. The resulting salinity anomalies are transported northward on timescales of 30-40 years, depending on the scenario, and can drive changes to AMOC as the anomalies reach the deep convective regions.

       

      • 4) Do you have an idea about how the OHT behaves in the North Atlantic in your experiments?

       

      We have not explicitly explored the OHT throughout the various Arctic pathways at this point, as detailed in the EGU abstract you shared. We have explored the global OHC and the top 300m OHC, as well as the OHT at ~48°N. These results are ongoing and we have not compared the OHT in the Arctic to sea ice extent in the new mean-state from our ensembles (years>150), but is an objective of our project.

       

      For the transient phase: The OHC (300m and depth integrated) signal is consistent with the changes in density in the North Atlantic within the first few decades and OHT transports change/behave similarly to AMOC. During a tropical Indian Ocean warming, there is increased OHT (~48°N) after a decade in the +2C members. On longer timescales, the nudging drives strong changes to the entire system, and OHC and OHT (~48°N) are anomalous compared to the control (either induced warming or cooling to the system). This is ongoing and preliminary work within our group and plan to expand further on the relationship of AMOC-Sea Ice-OHT in the near future.

       

      Thank you for sharing your abstract in the question, the work parallels our work and has insightful results!

       

      • 5) Are you sure that what you see in Slide 8 is the impact of the AMOC on sea ice, as you may suggest, instead of a direct influence of TIO SST change?

       

      The initial ~100 years are indeed probably strongly influenced by atmospheric teleconnections. Sea ice anomalies appear very clearly, together with anomalous atmospheric circulation. The exact mechanism is still under investigation. The later changes (>100years are probably more influenced by the AMOC, given the strong AMOC changes and associated OHT. These AMOC-Sea Ice relationship in the new mean-state (years >150) were not further discussed in this presentation, but are the focus of additional studies using this experiment for our group to explore AMOC-Sea Ice in different mean-states.

       

      Cordially,

      Brady S Ferster

      brady.ferster@locean-ipsl.upmc.fr

      • CC2: Reply to AC1, David Docquier, 05 May 2020

        Thanks Brady for your detailed replies.

        Good luck and happy to keep in touch,

        David

Presentation version 1 – uploaded on 30 Apr 2020 , no comments