Observations of atmosphere-ocean coupling in the North Atlantic

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Abstract

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.

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