Timescale‐dependent AMOC–AMO relationship in an earth system model of intermediate complexity

The relationship between Atlantic multi‐decadal oscillation (AMO) and Atlantic meridional overturning circulation (AMOC) is examined with respect to two (inter‐ and multi‐decadal) different timescales using a long‐term unforced simulation of an earth system model of intermediate complexity. In the inter‐decadal timescale, the AMO and the AMOC establish a self‐sustaining oscillatory mode; the AMOC induces the positive AMO through meridional heat transport (MHT), but with the time delay of approximately 7 years as the AMOC anomalies propagate southward over time within the Atlantic basin. After then, the AMO reduces the density in the main sinking region and brings the negative phase of the AMOC, which results in the rest half of the cycle. On the other hand, in the multi‐decadal timescale, the AMO and the AMOC are almost in phase because the AMOC is spatially stationary, resulting in a pan‐Atlantic surface warming. In addition, the Arctic‐originated density fluctuations are required for the multi‐decadal AMOC to switch its phase. The results obtained in this study suggest that timescale dependency should be considered when investigating the AMOC–AMO relationship. Lead–lag correlation between the AMOC and AMO indices. (a) Without separation of inter‐ and multi‐decadal timescales. To exclude high‐frequency variations, a 15‐year low‐pass filter and an 11‐year running average were applied to the AMOC and AMO indices, respectively. (b) Inter‐decadal timescale, with a 10–40‐year band‐pass filter is applied to both indices. (c) Same as (b) but for the multi‐decadal timescale with a 40–100‐year band‐pass filter. Black and pink lines indicate the simulated and observational results, respectively. Thick lines indicate statistically significant values at the 95% confidence level. Note that the x‐axis range is extended in (c).