Observations of atmosphere-ocean coupling in the North Atlantic



Anindex of sea surface temperature (SST) variability, δT, is introduced that measures the difference in SST across the separated Gulf Stream in late winter. By analysing a long observational record of SST and sea-level pressure (SLP), it is shown that δT exhibits damped oscillations of decadal period, and covaries with the strength of a dipolar SLP anomaly reminiscent of the North Atlantic Oscillation (NAO). Analysis in the frequency domain shows a broad-band ‘peak’ at 10–20 years in δT, with a continuous decrease of power on longer time-scales. Similar spectral signatures are found in the northern part of the SLP dipole (the Greenland-Icelandic Low region) but not in its southern part (the subtropical High region), whose power increases on long time-scales.

The observations are interpreted in the framework of a delayed-oscillator model in which the ocean circulation introduces the delay, and modulates δT on decadal time-scales. The decrease of power seen on long time-scales (>25 years) in the δT index is captured by a model including wind-driven ocean circulation, and arises primarily as a passive response of the latter to the NAO forcing. Variability of the ocean's meridional overturning circulation could also play a role in modulating δT on decadal time-scales. If a small feedback of δT on the NAO pattern is introduced, the simple model can also reproduce the spectral structures seen in the SLP anomaly in the Greenland-Iceland region.