The physical processes determining the path of the Gulf Stream as it leaves the continental shelf of the USA are complex and involved. Understanding these processes is important to global climate studies because the Gulf Stream plays a major rôle in the distribution of heat in the northern hemisphere. Here we show that during the last 3 decades, the latitude of the Gulf Stream has been clearly correlated with the North Atlantic Oscillation (NAO); high values of the NAO index, which correspond to stronger westerly and trade winds, favouring more northerly paths of the Stream about 2 years later. The time‐delay may be associated with the adjustment time of this part of the ocean circulation. If allowance is made for persistence of the Gulf Stream position from one year to the next, more than half the interannual variation in the position can be predicted.

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The complexity of ecosystems can cause subtle and chaotic responses to changes in external forcing. Although ecosystems may not normally behave chaotically, sensitivity to external influences associated with nonlinearity can lead to amplification of climatic signals. Strong correlations between an El Niño index and rainfall and maize yield in Zimbabwe have been demonstrated; the correlation with maize yield was stronger than that with rainfall. A second example is the 100,000-year ice-age cycle, which may arise from a weak cycle in radiation through its influence on the concentration of atmospheric CO2 (ref. 5). Such integration of a weak climatic signal has yet to be demonstrated in a realistic theoretical system. Here we use a particular climatic phenomenon-the observed association between plankton populations around the UK and the position of the Gulf Stream-as a probe to demonstrate how a detailed marine ecosystem model extracts a weak signal that is spread across different meteorological variables. Biological systems may therefore respond to climatic signals other than those that dominate the driving variables.