Instability analysis of the AMOC with varying freshwater input and sea water temperature in the Atlantic Ocean

The global climate system continues to change under the influence of human activities. Of particular concern is the difficulty of continuing human activities due to irreversible and long-term abrupt changes caused by the global climate system exceeding its tipping point. Climate systems that have the potential to exceed the tipping point are called tipping elements and are being studied. Among them, the Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the movement of materials through the ocean and is connected to many other tipping elements. While there is concern that the AMOC may decrease in strength due to rising temperatures in the Atlantic Ocean, freshwater inflow due to melting ice in the Arctic region has been investigated as a stabilizing factor. Therefore, it is important to comprehensively consider these influences when evaluating the AMOC tipping point.

 

In the AMOC modelling, Stommel’s two box model describes its nature well. Although, it does not treat freshwater input from multiple estuaries. We have applied three box model (TBM) [1] which divide the Atlantic Ocean into three elements with double estuaries. Freshwater inflows to the Arctic Ocean due to ice sheet thawing in Greenland and permafrost thawing in Siberia were calculated using AWI-ESM [2] and CMIP6 [3] data, respectively. In addition, temperature differences between the southern and northern Atlantic regions were calculated by MRI-ESM2.0 [4].

We also adopted the method of analyzing the time-series behavior of the AMOC as a stochastic process, as in Ditlevsen et al. (2010) [5]. Finally, we estimated the age of AMOC decay based on the analytical AMOC behavior by TBM and by identifying the parameters of the Langevin equation.

 

[1] E. Lambert, T. Eldevik, P.M. Haugan “How northern freshwater input can stabilise thermohaline circulation”, Tellus A: Dynamic Meteorology and Oceanography, 68 (1) (2016), p. 31051

[2] Ackermann, L., Danek, C., Gierz, P., and Lohmann, G. “AMOC Recovery in a multicentennial scenario using a coupled atmosphere-ocean-ice sheet model”, Geophys. Res. Lett., 47, 2020.

[3] Wang, S., Wang, Q., Wang, M., Lohmann, G., & Qiao, F. (2022). ”Arctic Ocean freshwater in CMIP6 coupled models” Earth’s Future, 10(9)

[4] Yukimoto, Seiji; Koshiro, Tsuyoshi; Kawai, Hideaki; et al. (2019) “MRI-ESM2.0 model output prepared for CMIP6 ScenarioMIP ssp585”

[5] Ditlevsen, P. D. and Johnsen, S. J.: Tipping points: Early warning and wishful thinking, Geophys. Res. Lett., 37, L19703, 2010