Japan Geoscience Union Meeting 2019

Presentation information

[E] Oral

A (Atmospheric and Hydrospheric Sciences ) » A-OS Ocean Sciences & Ocean Environment

[A-OS10] Atlantic climate variability, and its global impacts and predictability

Thu. May 30, 2019 9:00 AM - 10:30 AM 105 (1F)

convener:Ingo Richter(JAMSTEC Japan Agency for Marine-Earth Science and Technology), Hiroki Tokinaga(Research Institute for Applied Mechanics, Kyushu University), Noel S Keenlyside(Geophysical Institute Bergen), Carlos R Mechoso(University of California Los Angeles), Chairperson:Hiroki Tokinaga(京都大学白眉センター), Ingo Richter

10:00 AM - 10:15 AM

[AOS10-05] Atlantic Multidecadal SST Signal Modulated by the Low-Frequency Mixed Layer Depth Variability

*Ayako Yamamoto1, Hiroaki Tatebe1, Masami Nonaka1 (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:Atlantic Multidecadal Oscillation/Variability, Gulf Stream, mixed-layer depth

Atlantic Multidecadal Oscillation (AMO) has conventionally been attributed to the heat transport associated with the Atlantic Meridional Overturning Circulation. A recent study, however, challenged this paradigm and indicated that the North Atlantic basin-wide sea surface temperature (SST) pattern analogous to AMO can emerge owing to the atmospheric stochastic forcings in slab ocean models, without an active ocean. Subsequent studies have disputed this finding, arguing that the cause of the SST warming differ between the slab ocean models and fully-coupled models; however, a consensus is yet to be reached.

In this study, we take an approach to resolve this conundrum, by partitioning the multidecadal SST tendency into a part that is induced by the surface heat fluxes and another by the ocean, using a latest version of a fully-coupled climate model, MIROC6. We found that the heat flux term is primarily responsible for the North Atlantic subpolar SST warming, opposing the cooling caused by the oceanic term, despite that the associated heat flux anomalies are upward. Further decomposition of the heat flux term indicates that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling. This MLD variability is essentially induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation. The tropical North Atlantic SST signal, in contrast, arises due to the surface heat fluxes. Our study thus implies the key role of both air-sea fluxes and the ocean dynamics in the emergence of the AMO signal.