Oceanic Influence on North Atlantic Climate Variability

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Abstract
The role of ocean dynamics in low-frequency climate variability in the North Atlantic is investigated using the Hadley Centre coupled climate model (HadCM3). An ensemble of three 100-yr experiments, in which low-frequency damping is applied below 500 m in the North Atlantic Ocean, is compared with the HadCM3 control integration that shows realistic variability in both SST and the North Atlantic Oscillation (NAO). All three damped runs have shown significantly reduced SST and NAO variability on decadal timescales, while interannual variability remains unchanged or even increased. A significance test of the ensemble mean low-frequency variance, against any three random 100-yr segments from the full HadCM3 control run, gives a confidence level above 95%. It is therefore concluded that ocean dynamics does play an active role in low-frequency variability of both the SST and the NAO at decadal and interdecadal timescales. By comparing mechanisms in the control and damped experiments it is also possible to find out how ocean dynamics affect the long-term variability of the SST and the NAO. The deep ocean damping leads to reduced variability of the meridional overturning and its associated northward heat transport, as well as constraining the position and strength changes of the Gulf Stream/North Atlantic Current (NAC) system. The study has also confirmed that the surface tripole mode is largely an ocean response to NAO forcing. Ocean dynamics comes into play through a Gulf Stream mode in the autumn forcing of the winter NAO. By constraining the Gulf Stream/NAC, it is possible to effectively reduce the low-frequency variability of SST and associated surface heat flux, and therefore cause the NAO to lose its red spectrum.