On the variability of Gulf Stream transport from seasonal to decadal timescales

Given the Gulf Stream's central role in the North Atlantic's wind-driven and meridional overturning circulation (MOC), there is considerable interest in measuring mass and heat flux to sufficient accuracy that their variability can be quantified with some degree of confidence. Here we combine high-resolution direct measurements of upper ocean transport from the last 17 years of Oleander ADCP data with previously published estimates of baroclinic transport to examine Gulf Stream transport variability over the last 80 years just downstream of where the current separates from the U.S. east coast. By far the greatest source of variability occurs on short time scales related to the meandering of the current and energetic eddy field to either side such that the inherent uncertainty of a single transport estimate is similar to 15% with respect to an annual mean. The annual cycle of layer transport at 55-m depth has a maximum increase of 4.3% of the mean in September while the annual cycle at 205 m reaches a maximum of only 1.5% in July. A running low-pass filter indicates transport variations of only a few percent of the mean on inter-annual and longer time scales although swings as large as 10-12% over a few years can occur. The length of the time series now reveals a significant correlation between the NAO index and near-surface transport in the Gulf Stream. No significant trend in transport can be detected from either the last 17 years of directly measured surface currents, or from hydrographic sections starting in the 1930's. It follows therefore that the upper branch of the MOC, the other major component of Gulf Stream transport at the Oleander line, must have been quite stable over the last 80 years.