Decadal Wind Forcing of the North Atlantic Subtropical Gyre

Abstract

In the central North Atlantic Ocean there are large decadal-scale fluctuations of sea level and of the depth of the thermocline. This variability can be explained by low-frequency Rossby waves forced by wind. The authors have used a simple model of ocean fluctuations driven by the COADS wind-stress curl and find that their model results and hydrographic data, to the limited extent they can be compared meaningfully, agree rather well. The variation in wind curl over the ocean leads to surprisingly large north?south variability in the computed oceanic response over the subtropical gyre. The peak-to-peak sea level differences are as great as 20 cm and persist for many years. These large variations could induce major errors in calculations of the mean ocean flow when hydrographic sections from many years are combined. It appears possible, however, to correct for these wind-induced effects to allow the lower-frequency signals to be determined. The westernmost edge of our oceanic calculations is at the offshore side of the Gulf Stream. These fluctuations show changes in the north?south slope of sea level that imply long intervals of transport into or out of the Gulf Stream and provide suggestive evidence for low-frequency variability in coastal sea level and in the transport of the stream. This variability has been found previously in more complex ocean models, but the simplicity of these calculations may make the forcing mechanism and the ocean?s response easier to understand.