Impacts of the strengthened Atlantic meridional overturning circulation on the North Atlantic sea surface temperature: mean state

Impacts of the AMOC on the mean state of the North Atlantic sea surface temperature (SST) are investigated by implementing two seawater equations of state (EOS), the standard EOS and the “stiffened” EOS, into an earth system model (ESM). The ESM using the “stiffened” EOS improves the performance of the simulated AMOC. Moreover, the “stiffened” EOS significantly increases the SST in the northern tropical Atlantic (NTA) and northern North Atlantic (NNA) regions. Singular value decomposition analyses reveal that the AMOC changes exert important influences on the SST changes in the NTA and NNA regions. The enhanced AMOC intensifies the volume-integrated meridional heat transport, which dominates the SST warming in the NNA region. Physical processes responsible for the NTA SST changes are more complicated. Regarding the net surface heat flux, it plays an inverse role in modulating SST changes in the NTA region. The net surface heat flux dampens SST changes in the northern NTA region and favors SST changes in the southern NTA region. Influenced by the local and remote SST heating, we find that an anomalous southeasterly appears over the tropical North Atlantic in the simulation with the “stiffened” EOS, which increases the SST in the NTA region through the wind-evaporation-SST mechanism. Concerning the oceanic processes, the enhanced AMOC weakens the subtropical cell in the North Atlantic. Moreover, the enhanced deep convection downward transports the warm surface water in the NNA region to the deep ocean; then the well-mixed warm deep water is southward transported. When reaching the tropical North Atlantic the warm deep water is upward convected by the diapycnal mixing and is eventually upwelled to the sea surface by the enhanced cyclonic winds. The mixed-layer heat budget analysis shows that the NTA SST changes are mainly attributed to the changes in the zonal advection and vertical convection of the seawater temperature.