Effects of NAO variability on the North Atlantic Ocean circulation

The ocean response to an idealised North Atlantic
Oscillation-like wind stress is evaluated as a function of
the atmospheric forcing frequency. In order to elucidate
the relationship between internal and forced variability, the
NAO is modulated with two specific timescales, 10 and
50 years, which characterise the spectrum of the system’s
internal variability. Different timescales of atmospheric
variability select distinct sea surface temperature (SST)
and large scale circulation patterns. Under a 50 year NAO
forcing period, a lagged SST response is excited in the Gulf
Stream extension region, which is consistent with the spinup
of the gyre circulation. The thermohaline circulation
varies in phase with the NAO and shows a strong sensitivity
to the forcing frequency: a dipole mode of the overturning is
excited by a 10 year modulation of the NAO, while an
enhanced overturning response emerges under a 50 year
NAO. With low enough lateral mixing the ocean exhibits
an irregular response to a regular NAO-like forcing.