Gulf Stream Transport Variability at Periods of Decades

Abstract

Variability of sea level on the offshore side of the Gulf Stream has been estimated with a wind-forced numerical model. The difference in sea level between the model and coastal tide gauges therefore provides an estimate of variability of the Gulf Stream. These results can be compared with direct measurements of transport; the agreement is surprisingly good. Transport estimates are then made for sections offshore of four major tide stations along the U.S. East Coast. When data since World War II are used, the spectrum of sea level at the coast appears to peak at periods of ?150?250 mo. The difference signal (ocean minus coast), however, which the authors interpret as transport variability, has a weakly red spectrum. Power decreases at somewhat less than f?1 at periods just less than ?500 months but decreases strongly at periods less than ?150 months. The low-frequency variability arises primarily from the influx of open ocean Rossby waves. The large variance at low frequencies suggests that measurements of the transport of western boundary currents do not have many degrees of freedom; measurements made many years apart may vary substantially because of this localized variability. Sea level at the coast is coherent over long distances, but the incoming Rossby wave radiation from the open ocean has a relatively short north?south scale. These results emphasize that transport measured at one location along the coast may be incoherent with transport at locations only ?200 km away. As a result, measurements at one location will in general not be representative of transport along the entire coast.