Presenter: Ayako Yamamoto (Tokyo University of Marine Science and Technology)

Description:
In spite of its wide-ranging potential impacts, the exact cause of the Atlantic multidecadal oscillation/variability (AMO/AMV) is still under debate. While the emergence of the AMO sea surface temperature (SST) pattern has been conventionally attributed to the ocean heat transport, a more recent study showed that the atmospheric stochastic forcing is sufficient without any role for the ocean. In this study, we resolve this conundrum by decomposing the multidecadal SST tendency into a part that is caused by surface heat fluxes and another by ocean dynamics, using a MIROC6 preindustrial control simulation. We found that while the horizontal ocean heat advection primarily acts to warm the subpolar SST as in previous studies, when the vertical component is also considered, the ocean dynamics overall acts to cool the subpolar region. Alternatively, the heat flux term is primarily responsible for the subpolar North Atlantic SST warming, despite that the associated surface heat flux anomalies are directed upward as observed. Further decomposition of the heat flux term reveals that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling by increasing the upper ocean heat content, thus leading to relative warming by increasing the upper-ocean heat capacity. This role of the MLD variability in the AMO signature had not been addressed in previous studies. The MLD variability is primarily induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation, with turbulent fluxes playing a secondary role. Thus, depending on how we interpret the MLD variability, our results support the two previously suggested frameworks, yet slightly modifying the previous notions.  Further, by applying the same method to the MIROC6 historical and future simulations, we examine if the relationship we found holds for the future climate.